How Breathing Influences the Brain and Nervous System

Title: Breathing Practices for Stress and Anxiety Reduction: Conceptual Framework of Implementation Guidelines Based on a Systematic Review of the Published Literature

Authors: Bentley TGK, D’Andrea-Penna G, Rakic M, Arce N, LaFaille M, Berman R, Cooley K, Sprimont P.

Journal: Brain Sciences. 2023; 13(12):1612. https://doi.org/10.3390/brainsci13121612

Link to full text: Breathing Practices for Stress and Anxiety Reduction: Conceptual Framework of Implementation Guidelines Based on a Systematic Review of the Published Literature

Abstract: Anxiety and stress plague populations worldwide. Voluntary regulated breathing practices offer a tool to address this epidemic. We reviewed the peer-reviewed published literature to evaluate practice components associated with stress reduction effectiveness. We identified 58 clinical studies, comprising 41 RCTs and 17 single-arm, pre-post, non-randomized trials; 5407 total participants were included, with a mean age of 35.9 years and 43.92% female. Among the 72 included interventions, breathing practices significantly reduced stress/anxiety in 54 interventions. Findings were used to develop a conceptual framework of breath practice effectiveness.

While commonly emphasized factors such as specific breath pace, population, and group versus individual practice were not found to be associated with effectiveness, five core components were: avoiding fast-only and <5 min practices; and incorporating human-guided training during initial sessions, multiple sessions, and long-term practice. Following this simple, evidence-based framework can help maximize the stress reduction benefits of breathing practices among broad populations.

Title: Slow-Breathing Curriculum for Stress Reduction in High School Students: Lessons Learned From a Feasibility Pilot

Authors: Bentley TGK, Seeber C, Hightower E, Mackenzie B, Wilson R, Velazquez A, Cheng A, Arce NN, Lorenz KA.

Journal: Front Rehabil Sci. 2022 Jul 1;3:864079. doi: 10.3389/fresc.2022.864079. PMID: 36189008; PMCID: PMC9397716.

Link to full text: Slow-Breathing Curriculum for Stress Reduction in High School Students: Lessons Learned From a Feasibility Pilot

Abstract: Purpose: Nearly one in three US adolescents meet the criteria for anxiety, an issue that has worsened with the COVID-19 pandemic. We developed a video-based slow diaphragmatic breathing stress-reduction curriculum for high school students and evaluated its feasibility, tolerability, and preliminary effectiveness.

Methods: This cluster-randomized feasibility pilot compared 5-min slow diaphragmatic breathing for 5 weeks with treatment-as-usual control among four 12th-grade public high school classes. Students individually participated after school during COVID-19-related hybrid teaching, with slow diaphragmatic breathing three times/week and breath science education once/week. Feasibility was based on completion of breathing exercises, breath science education, and preliminary effectiveness assessments, and ease/tolerability was based on qualitative assessments. Results: Forty-three students consented to participate. Breath practice compliance ranged from 29 to 83% across classes and weeks, and decreased on average over the 5 weeks. Compliance with the breath science videos ranged from 43 to 86%, and that with the weekly STAI-State and CO2TT measures varied from 36 to 86%. Compliance with ease/tolerability assessments ranged from 0 to 60%. Preliminary effectiveness assessments' compliance varied across classes from 83 to 89% during baseline, and 29 to 72% at follow-up. The curriculum was rated as somewhat-to-definitely useful/beneficial, and definitely-to-very easy/tolerable. Students reported enjoying the diaphragmatic breathing, CO2TT, and breath science education; some found the extended exhales challenging and the curriculum and assessments time-consuming. Preliminary effectiveness analyses indicated no significant changes in STAI or CO2TT from baseline to followup or from before to after breathing exercises (p > 0.05 for all).

Conclusions: Implementation of this 5-week slow breathing curriculum was feasible and tolerable to this cohort. Compliance, tolerability, and effectiveness may be improved with in-class participation. Future research on simple and accessible slow-breathing exercises is warranted to address today's adolescent stress-management crisis.

Title: Effects of conscious connected breathing on cortical brain activity, mood and state of consciousness in healthy adults

Authors: Bahi, C., Irrmischer, M., Franken, K. et al.

Journal: Curr Psychol 43, 10578–10589 (2024). https://doi.org/10.1007/s12144-023-05119-6

Link to full text: Effects of conscious connected breathing on cortical brain activity, mood and state of consciousness in healthy adults

Abstract: Breathwork as a means of inducing non-ordinary states of consciousness is gaining traction as a potential therapeutic modality. We examined the effects of breathwork (in the form of connected breathing) on electroencephalography (EEG) and mood in 20 healthy participants (aged between 23 and 39 years (female = 11, Mage = 29). In addition, to compare with other means of inducing non-ordinary states of consciousness, we assessed the subjective effects of breathwork using the 11 Dimension Altered State of Consciousness questionnaire. EEG spectral power analysis of eyes closed rest recordings before and after the breathwork session showed a decrease in delta (1–4 Hz) and theta (4–8 Hz) frequencies in frontotemporal and parietal regions, respectively no changes were seen in Alpha (9–12 Hz) and Beta (12–30 Hz) bands. However, after decomposing the beta waves in Beta 1 (12–15 Hz), Beta 2 (15–20 Hz), Beta 3 (20–30 Hz), decreases in power were observed across Beta1 and Beta 2 in parietotemporal regions. Notably, the spectral power in gamma increased in experienced practitioners. Scores on the Profile of Mood States questionnaire showed a reduction in negative affect (anger, tension, confusion, and depression) and an increase in esteem. Scores on the 11D-ASC scale indicated that subjective experiences during breathwork were similar to those after medium to high doses of psilocybin, suggesting the occurrence of experiences of mystical quality. Present results indicate that breathwork changes brain activity and mood, and induces mystical experiences. These results are promising and suggest that such techniques could be useful to improve mental well-being.

Title: Breathwork interventions for adults with clinically diagnosed anxiety disorders: A scoping review

Authors: Banushi B, Brendle M, Ragnhildstveit A, et al.

Journal: Brain Sci. 2023 Feb 2;13(2):256. doi: 10.3390/brainsci13020256. PMID: 36831799; PMCID: PMC9954474.

Link to full text: Breathwork interventions for adults with clinically diagnosed anxiety disorders: A scoping review

Abstract: Anxiety disorders are the most common group of mental disorders, but they are often underrecognized and undertreated in primary care. Dysfunctional breathing is a hallmark of anxiety disorders; however, mainstays of treatments do not tackle breathing in patients suffering anxiety. This scoping review aims to identify the nature and extent of the available research literature on the efficacy of breathwork interventions for adults with clinically diagnosed anxiety disorders using the DSM-5 classification system. Using the PRISMA extension for scoping reviews, a search of PubMed, Embase, and Scopus was conducted using terms related to anxiety disorders and breathwork interventions. Only clinical studies using breathwork (without the combination of other interventions) and performed on adult patients diagnosed with an anxiety disorder using the DSM-5 classification system were included. From 1081 articles identified across three databases, sixteen were included for the review. A range of breathwork interventions yielded significant improvements in anxiety symptoms in patients clinically diagnosed with anxiety disorders. The results around the role of hyperventilation in treatment of anxiety were contradictory in few of the examined studies. This evidence-based review supports the clinical utility of breathwork interventions and discusses effective treatment options and protocols that are feasible and accessible to patients suffering anxiety. Current gaps in knowledge for future research directions have also been identified.

Title: Transcutaneous Carbon Dioxide Improves Contractures After Spinal Cord Injury in Rats

Authors: Inoue S, Moriyama H, Yakuwa T, Mizuno E, Suzuki R, Nomura M, Sakai Y, Akisue T.

Journal: Clin Orthop Relat Res. 2019 Aug;477(8):1934-1946. doi: 10.1097/CORR.0000000000000808. PMID: 31135536; PMCID: PMC7000005.

Link to full text: Transcutaneous Carbon Dioxide Improves Contractures After Spinal Cord Injury in Rats

Abstract: Background: Joint contractures are a major complication in patients with spinal cord injuries. Positioning, stretching, and physical therapy are advocated to prevent and treat contractures; however, many patients still develop them. Joint motion (exercise) is crucial to correct contractures. Transcutaneous carbon dioxide (CO2) therapy was developed recently, and its effect is similar to that of exercise. This therapy may be an alternative or complementary approach to exercise.

Question/purposes: Using an established model of spinal cord injury in rats with knee flexion contractures, we sought to clarify whether transcutaneous CO2 altered (1) contracture, as measured by ROM; (2) muscular and articular factors contributing to the loss of ROM; (3) fibrosis and fibrosis-related gene expression in muscle; and (4) the morphology of and fibrosis-related protein expression in the joint capsule.

Methods: Thirty-six Wistar rats were divided into three equal groups: caged control, those untreated after spinal cord injury, and those treated with CO2 after spinal cord injury. The rats were treated with CO2 from either the first day (prevention) or 15th day (treatment) after spinal cord injury for 2 or 4 weeks. The hindlimbs of rats in the treated group were exposed to CO2 gas for 20 minutes once daily. Knee extension ROM was measured with a goniometer and was measured again after myotomy. We calculated the muscular and articular factors responsible for contractures by subtracting the post-myotomy ROM from that before myotomy. We also quantified histologic muscle fibrosis and evaluated fibrosis-related genes (collagen Type 1, α1 and transforming growth factor beta) in the biceps femoris muscle with real-time polymerase chain reaction. The synovial intima's length was measured, and the distribution of fibrosis-related proteins (Type I collagen and transforming growth factor beta) in the joint capsule was observed with immunohistochemistry. Knee flexion contractures developed in rats after spinal cord injuries at all timepoints.

Results: CO2 therapy improved limited-extension ROM in the prevention group at 2 weeks (22° ± 2°) and 4 weeks (29° ± 1°) and in the treatment group at 2 weeks (31° ± 1°) compared with untreated rats after spinal cord injuries (35° ± 2°, mean difference, 13°; 39° ± 1°, mean difference, 9°; and 38° ± 1°, mean difference, 7°, respectively) (95% CI, 10.50-14.86, 8.10-10.19, and 4.73-9.01, respectively; all p < 0.001). Muscular factors decreased in treated rats in the prevention group at 2 weeks (8° ± 2°) and 4 weeks (14°± 1°) and in the treatment group at 2 weeks (14 ± 1°) compared with untreated rats (15° ± 1°, 4.85-9.42; 16° ± 1°, 1.24-3.86; and 17° ± 2°, 1.16-5.34, respectively; all p < 0.05). The therapy improved articular factors in the prevention group at 2 weeks (4° ± 1°) and 4 weeks (6° ± 1°) and in the treatment group at 2 weeks (8° ± 1°) compared with untreated rats (10° ± 1°, 4.05-7.05; 12° ± 1°, 5.18-8.02; and 11° ± 2°, 1.73-5.50, respectively; all p < 0.05). CO2 therapy decreased muscle fibrosis in the prevention group at 2 weeks (p < 0.001). The expression of collagen Type 1, α1 mRNA in the biceps femoris decreased in treated rats in the prevention group at 2 and 4 weeks compared with untreated rat (p = 0.002 and p = 0.008, respectively), although there was little difference in the expression of transforming growth factor beta (p > 0.05). CO2 therapy did not improve shortening of the synovial intima at all timepoints (all p > 0.05). CO2 therapy decreased transforming growth factor beta immunolabeling in joint capsules in the rats in the prevention group at 2 weeks. The staining intensity and Type I collagen pattern showed no differences among all groups at all timepoints.

Conclusion: CO2 therapy may be useful for preventing and treating contractures after spinal cord injuries. CO2 therapy particularly appears to be more effective as a prevention and treatment strategy in early-stage contractures before irreversible degeneration occurs, as shown in a rat model.

Title: Role of the medullary raphe nuclei in the respiratory response to CO2

Authors: Dreshaj IA, Haxhiu MA, Martin RJ.

Journal: Respir Physiol. 1998 Jan;111(1):15-23. doi: 10.1016/s0034-5687(97)00110-2. PMID: 9496468.

Link to PubMed: Role of the medullary raphe nuclei in the respiratory response to CO2

Abstract: We characterized the role of neurons within the midline of the medulla oblongata on phrenic and hypoglossal nerve responses to hypercapnia during early-development. Studies were performed on decorticate or anesthetized; vagotomized and mechanically ventilated 14-20 day old piglets. Reversible withdrawal of midline neuronal activity was induced by microinjections of lidocaine (2%, 300 nl; n = 10) and lesioning was caused by microinjections of the neurotoxic agent, ibotenic acid (n = 12), at the same sites. At any given end-tidal CO2, peak phrenic and hypoglossal activities after lidocaine were significantly lower than in the control period (P < 0.01). Similarly, 1-2 h after injections of ibotenic acid, both phrenic and hypoglossal nerve responses to CO2 were significantly lower than in the control period (P < 0.01). The results indicate for the first time that the medullary midline neurons are required for full expression of ventilatory responses to hypercapnia and raise the possibility that dysfunction of these nuclei may contribute to respiratory instability during early postnatal life.

Title: Smaller amygdala is associated with anxiety in patients with panic disorder

Authors: Hayano F, Nakamura M, Asami T, Uehara K, Yoshida T, Roppongi T, Otsuka T, Inoue T, Hirayasu Y.

Journal: Psychiatry Clin Neurosci. 2009 Jun;63(3):266-76. doi: 10.1111/j.1440-1819.2009.01960.x. PMID: 19566756.

Link to full text: Smaller amygdala is associated with anxiety in patients with panic disorder

Abstract: Aims: Anxiety a core feature of panic disorder, is linked to function of the amygdala. Volume alterations in the brain of patients with panic disorder have previously been reported, but there has been no report of amygdala volume association with anxiety.

Methods: Volumes of hippocampus and amygdala were manually measured using magnetic resonance imaging obtained from 27 patients with panic disorder and 30 healthy comparison subjects. In addition the amygdala was focused on, applying small volume correction to optimized voxel-based morphometry (VBM). State-Trait Anxiety Inventory and the NEO Personality Inventory Revised were also used to evaluate anxiety.

Results: Amygdala volumes in both hemispheres were significantly smaller in patients with panic disorder compared with control subjects (left: t = -2.248, d.f. = 55, P = 0.029; right: t = -2.892, d.f. = 55, P = 0.005). VBM showed that structural alteration in the panic disorder group occurred on the corticomedial nuclear group within the right amygdala (coordinates [x,y,z (mm)]: [26,-6,-16], Z score = 3.92, family-wise error-corrected P = 0.002). The state anxiety was negatively correlated with the left amygdala volume in patients with panic disorder (r = -0.545, P = 0.016).

Conclusions: These findings suggested that the smaller volume of the amygdala may be associated with anxiety in panic disorder. Of note, the smaller subregion in the amygdala estimated on VBM could correspond to the corticomedial nuclear group including the central nucleus, which may play a crucial role in panic attack.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Amygdalar atrophy in panic disorder patients detected by volumetric magnetic resonance imaging

Authors: Massana G, Serra-Grabulosa JM, Salgado-Pineda P, Gastó C, Junqué C, Massana J, Mercader JM, Gómez B, Tobeña A, Salamero M.

Journal: Neuroimage. 2003 May;19(1):80-90. doi: 10.1016/s1053-8119(03)00036-3. PMID: 12781728.

Link to PubMed: Amygdalar atrophy in panic disorder patients detected by volumetric magnetic resonance imaging

Abstract: It has been suggested that the pathophysiology of panic disorder (PD) may involve abnormalities in several brain structures, including the amygdala. To date, however, no study has used quantitative structural neuroimaging techniques to examine amygdalar anatomy in this disorder. Volumetric magnetic resonance imaging (MRI) studies of the amygdalas, hippocampi, and temporal lobes were conducted in 12 drug-free, symptomatic PD patients (six females and six males), and 12 case-matched healthy comparison subjects. Volumetric MRI data were normalized for brain size. PD patients were found to have smaller left-sided and right-sided amygdalar volumes than controls. No differences were found in either hippocampi or temporal lobes. These findings provide new evidence of changes in amygdalar structure in PD and warrant further anatomical and MRI brain studies of patients with this disorder.

Title: Neural Correlates of Competing Fear Behaviors Evoked by an Innately Aversive Stimulus

Authors: Mongeau R, Miller GA, Chiang E, Anderson DJ.

Journal: J Neurosci. 2003 May 1;23(9):3855-68. doi: 10.1523/JNEUROSCI.23-09-03855.2003. PMID: 12736356; PMCID: PMC6742203.

Link to full text: Neural Correlates of Competing Fear Behaviors Evoked by an Innately Aversive Stimulus

Abstract: Environment and experience influence defensive behaviors, but the neural circuits mediating such effects are not well understood. We describe a new experimental model in which either flight or freezing reactions can be elicited from mice by innately aversive ultrasound. Flight and freezing are negatively correlated, suggesting a competition between fear motor systems. An unfamiliar environment or a previous aversive event, moreover, can alter the balance between these behaviors. To identify potential circuits controlling this competition, global activity patterns in the whole brain were surveyed in an unbiased manner by c-fos in situ hybridization, using novel experimental and analytical methods. Mice predominantly displaying freezing behavior had preferential neural activity in the lateral septum ventral and several medial and periventricular hypothalamic nuclei, whereas mice predominantly displaying flight had more activity in cortical, amygdalar, and striatal motor areas, the dorsolateral posterior zone of the hypothalamus, and the vertical limb of the diagonal band. These complementary patterns of c-fos induction, taken together with known connections between these structures, suggest ways in which the brain may mediate the balance between these opponent defensive behaviors.

Title: Panic and the Brainstem: Clues from Neuroimaging Studies

Authors: Perna G, Guerriero G, Brambilla P, Caldirola D.

Journal: CNS Neurol Disord Drug Targets. 2014;13(6):1049-56. doi: 10.2174/1871527313666140612112923. PMID: 24923341.

Link to PubMed: Panic and the Brainstem: Clues from Neuroimaging Studies

Abstract: One of the most influential theories has conceived unexpected panic attack (PA) as a primal defensive reaction to threat within the internal milieu of the body. This theory is based on findings suggesting the involvement of dysfunctional respiratory regulation and/or abnormally sensitive central neural network of carbon dioxide (CO2)/hydrogen ion (H+) chemoreception in PA. Thus, unexpected PA may be related to phylogenetically older brain structures, including the brainstem areas, which process basic functions related to the organism's internal milieu. The brainstem represents a crucial area for homeostatic regulation, including chemoreception and cardio-respiratory control. In addition, the midbrain dorsal periaqueductal gray may be involved in the unconditioned defense reactions to proximal threats, including internal physical stimuli. Our aim was to specifically consider the potential involvement of the brainstem in panic disorder (PD) by a comprehensive review of the available neuroimaging studies. Available data are limited and potentially affected by several limitations. However, preliminary evidence of a role of the brainstem in PD can be found and, secondly, the brainstem serotonergic system seems to be involved in panic modulation with indications of both altered serotonergic receptors and 5-HT transporter bindings. In conclusion, our review suggests that the brainstem may be involved in psychopathology of PD and supports the relevant role of subcortical serotonergic system in panic pathogenesis.

Title: fMRI of fearful facial affect recognition in panic disorder: the cingulate gyrus-amygdala connection

Authors: Pillay SS, Gruber SA, Rogowska J, Simpson N, Yurgelun-Todd DA.

Journal: J Affect Disord. 2006 Aug;94(1-3):173-81. doi: 10.1016/j.jad.2006.04.007. Epub 2006 Jun 16. PMID: 16782207.

Link to PubMed: fMRI of fearful facial affect recognition in panic disorder: the cingulate gyrus-amygdala connection

Abstract: Background: This study investigated cingulate cortex (CC) and amygdala response to fearful facial affect recognition in patients with panic disorder (PD) as measured by BOLD fMRI during the presentation of static facial images.

Methods: Eight patients with PD and eight controls were studied. Scanning was performed on a GE Signa 1.5-T scanner. Echo planar and high-resolution MR images were acquired.

Results: Controls produced greater CC activation compared to patients with PD in response to fearful faces. Furthermore, patients with PD produced less amygdala activation than controls in response to fearful faces. During the neutral face condition, overall activation for the CC was significantly greater in PD patients although anterior cingulate cortex (ACC) activation was not as markedly different between both groups. There were no between group differences in amygdala activation on exposure to the neutral face. Only left CC activation was significantly correlated negatively with HAM-A in PD patients in the fearful facial affect condition.

Limitations: Although comparable to similar studies, the sample size is small enough to warrant further investigation. Also, the effects of medication need to be considered when interpreting these results.

Conclusions: Patients with PD activate the ACC and amygdala significantly less than controls when asked to identify fearful facial affect during fMRI. The higher the anxiety, the lower the left CC activation. Thus, chronic hyperarousal in PD may diminish attentional resources and emotional response reflected in reduced ACC and amygdala activation. Even if these are medication effects, the differences from controls are clinically relevant.

Title: Neuroanatomical Correlates of a Lactate-Induced Anxiety Attack

Authors: Reiman EM, Raichle ME, Robins E, Mintun MA, Fusselman MJ, Fox PT, Price JL, Hackman KA.

Journal: Arch Gen Psychiatry. 1989 Jun;46(6):493-500. doi: 10.1001/archpsyc.1989.01810060013003. PMID: 2786401.

Link to PubMed: Neuroanatomical Correlates of a Lactate-Induced Anxiety Attack

Abstract: Positron emission tomographic measurements of regional blood flow were used to assess local neuronal activity in patients with panic disorder and in normal control subjects before and during the infusion of sodium lactate. A new technique for the analysis of positron emission tomographic data was employed to identify significant changes in regional blood flow associated with lactate infusion in the panicking patients, nonpanicking patients, and controls. Lactate-induced panic was associated with significant blood flow increases bilaterally in the temporal poles; bilaterally in insular cortex, claustrum, or lateral putamen; bilaterally in or near the superior colliculus; and in or near the left anterior cerebellar vermis. Lactate infusion was not associated with significant changes in regional blood flow in the nonpanicking patients or control subjects. Thus, the identified regions seemed to be involved in an anxiety attack.

Title: Amygdala circuitry mediating reversible and bidirectional control of anxiety

Authors: Tye KM, Prakash R, Kim SY, Fenno LE, Grosenick L, Zarabi H, Thompson KR, Gradinaru V, Ramakrishnan C, Deisseroth K.

Journal: Nature. 2011 Mar 17;471(7338):358-62. doi: 10.1038/nature09820. Epub 2011 Mar 9. PMID: 21389985; PMCID: PMC3154022.

Link to full text: Amygdala circuitry mediating reversible and bidirectional control of anxiety

Abstract: Anxiety--a sustained state of heightened apprehension in the absence of immediate threat--becomes severely debilitating in disease states. Anxiety disorders represent the most common of psychiatric diseases (28% lifetime prevalence) and contribute to the aetiology of major depression and substance abuse. Although it has been proposed that the amygdala, a brain region important for emotional processing, has a role in anxiety, the neural mechanisms that control anxiety remain unclear. Here we explore the neural circuits underlying anxiety-related behaviours by using optogenetics with two-photon microscopy, anxiety assays in freely moving mice, and electrophysiology. With the capability of optogenetics to control not only cell types but also specific connections between cells, we observed that temporally precise optogenetic stimulation of basolateral amygdala (BLA) terminals in the central nucleus of the amygdala (CeA)--achieved by viral transduction of the BLA with a codon-optimized channelrhodopsin followed by restricted illumination in the downstream CeA--exerted an acute, reversible anxiolytic effect. Conversely, selective optogenetic inhibition of the same projection with a third-generation halorhodopsin (eNpHR3.0) increased anxiety-related behaviours. Importantly, these effects were not observed with direct optogenetic control of BLA somata, possibly owing to recruitment of antagonistic downstream structures. Together, these results implicate specific BLA-CeA projections as critical circuit elements for acute anxiety control in the mammalian brain, and demonstrate the importance of optogenetically targeting defined projections, beyond simply targeting cell types, in the study of circuit function relevant to neuropsychiatric disease.

Title: Neurobiology of panic and pH chemosensation in the brain

Authors: Wemmie JA.

Journal: Dialogues Clin Neurosci. 2011;13(4):475-83. doi: 10.31887/DCNS.2011.13.4/jwemmie. PMID: 22275852; PMCID: PMC3263394.

Link to full text: Neurobiology of panic and pH chemosensation in the brain

Abstract: Panic disorder is a common and disabling illness for which treatments are too frequently ineffective. Greater knowledge of the underlying biology could aid the discovery of better therapies. Although panic attacks occur unpredictably, the ability to provoke them in the laboratory with challenge protocols provides an opportunity for crucial insight into the neurobiology of panic. Two of the most well-studied panic provocation challenges are CO(2) inhalation and lactate infusion. Although it remains unclear how these challenges provoke panic animal models of CO(2) and lactate action are beginning to emerge, and offer unprecedented opportunities to probe the molecules and circuits underlying panic attacks. Both CO(2) and lactate alter pH balance and may generate acidosis that can influence neuron function through a growing list of pH-sensitive receptors. These observations suggest that a key to better understanding of panic disorder may He in more knowledge of brain pH regulation and pH-sensitive receptors.

Keywords: CO2; chemo-sensation; lactate, brain pH; panic disorder; provocation challenge.

Title: Behavioral and cardiovascular effects of 7.5% CO2 in human volunteers

Authors: Bailey JE, Argyropoulos SV, Kendrick AH, Nutt DJ.

Journal: Depress Anxiety. 2005;21(1):18-25. doi: 10.1002/da.20048. PMID: 15782425.

Link to PubMed: Behavioral and cardiovascular effects of 7.5% CO2 in human volunteers

Abstract: The study of carbon dioxide (CO2) inhalation in psychiatry has a long and varied history, with recent interest in using inhaled CO2 as an experimental tool to explore the neurobiology and treatment of panic disorder. As a consequence, many studies have examined the panic-like response to the gas either using the single or double breath 35% CO2 inhalation or 5-7% CO2 inhaled for 15-20 min, or rebreathing 5% CO2 for a shorter time. However, this lower dose regime produces little physiological or psychological effects in normal volunteers. For this reason we have studied the effects of a higher concentration of CO2, 7.5%, given over 20 min. Twenty healthy volunteers were recruited to a double blind, placebo-controlled study where air and 7.5% CO2 were inhaled for 20 min. Cardiovascular measures and subjective ratings were obtained. When compared to air, inhaling 7.5% CO2 for 20 min increases systolic blood pressure and heart rate, indicating increased autonomic arousal. It also increases ratings of anxiety and fear and other subjective symptoms associated with an anxiety state. The inhalation of 7.5% CO2 for 20 min is safe for use in healthy volunteers and produces robust subjective and objective effects. It seems promising as an anxiety provocation test that could be beneficial in the study of the effects of anxiety on sustained performance, the discovery of novel anxiolytic agents, and the study of brain circuits and mechanisms of anxiety.

Title: The 35% CO2 challenge in panic disorder: optimization by receiver operating characteristic (ROC) analysis

Authors: Battaglia M, Perna G.

Journal: J Psychiatr Res. 1995 Mar-Apr;29(2):111-9. doi: 10.1016/0022-3956(94)00045-s. PMID: 7666379.

Link to PubMed: The 35% CO2 challenge in panic disorder: optimization by receiver operating characteristic (ROC) analysis

Abstract: To test the power of the 35% CO2 test as a challenge for panic disorder (PD), and to set an ideal threshold of discrimination between patients and controls, we analysed by receiver operating characteristic (ROC) analysis the responses of 91 out-patients with PD and 46 controls who inhaled a 35% CO2/65% O2 gas mixture. ROC analysis confirmed that the CO2 challenge discriminates well between PD patients and controls, with 86% probability to classify them correctly on the basis of subjective anxiety after the test. A relatively modest increment in subjective anxiety (i.e. an absolute increment of 20 units, or a relative increment of 26% of subjective anxiety) proved to be the ideal threshold to separate the two groups of our sample.

Title: On the psychotropic effects of carbon dioxide

Authors: Colasanti A, Esquivel G, Schruers KJ, Griez EJ.

Journal: Curr Pharm Des. 2012;18(35):5627-37. doi: 10.2174/138161212803530745. PMID: 22632467.

Link to PubMed: On the psychotropic effects of carbon dioxide

Abstract: It has been well established that the inhalation of Carbon Dioxide (CO2) can induce in humans an emotion closely replicating spontaneous panic attacks, as defined by current psychiatry nosology. The purpose of this review is to provide a critical summary of the data regarding CO2's psychopharmacological properties and underlying mechanisms. The authors review the literature on the human and animal response for the exposure of exogenous CO2 focusing on five points of interest: 1) the early history of the use of CO2 as an anesthetic and therapeutic agent, 2) the subjective effects of breathing CO2 at different concentrations in humans, 3) the use of CO2 in experimental psychiatric research as an experimental model of panic, 4) the pharmacological modulation of CO2-induced responses, and 5) the putative neurobiological mechanisms underlying the affective state induced by CO2. The authors conclude with an evolutionary-inspired notion that CO2 might act as an agent of a primal emotion serving a homeostatic function, in the control of respiration and acid-base balance.

Title: Body fluids after CO2 inhalation: insight into panic mechanisms?

Authors: Griez E, van den Hout MA, Verstappen F.

Journal: Eur Arch Psychiatry Neurol Sci. 1987;236(6):369-71. doi: 10.1007/BF00377427. PMID: 3119345.

Link to PubMed: Body fluids after CO2 inhalation: insight into panic mechanisms?

Abstract: Blood gases and electrolyte fluctuations were studied in arterial blood, after a single 35% CO2-65% O2 inhalation, which is known to trigger panic attacks in patients with panic disorder. The immediate effect of this maneuver was a brief hypercapnic acidosis followed by a slight alkalotic rebound, with shifts in Ca2+ and K+. The possible effect of these changes on neuronal membrane excitability is discussed, referring to recent experimental findings in panic provocation.

Title: A Long-Term Prospective Evaluation of First-Degree Relatives of Panic Patients Who Underwent the 35% CO2 Challenge

Authors: Perna G, Cocchi S, Allevi L, Bussi R, Bellodi L.

Journal: Biol Psychiatry. 1999 Feb 1;45(3):365-7. doi: 10.1016/s0006-3223(98)00030-4. PMID: 10023515.

Link to PubMed: A Long-Term Prospective Evaluation of First-Degree Relatives of Panic Patients Who Underwent the 35% CO2 Challenge

Abstract: This follow-up study investigated the potential priming effect of the 35% CO2 challenge on the development of anxiety disorders and/or panic attacks in healthy first-degree relatives of panic patients across a period of 3-4 years subsequent to the challenge. Thirty-one relatives who underwent the 35% CO2 challenge 3-4 years before and 14 relatives, free from psychiatric diagnoses in the same period, were directly reevaluated for the presence of anxiety disorders and panic attacks. None developed anxiety disorders and only 1, among relatives previously tested with the 35% CO2 challenge, reported sporadic panic attacks. The 35% CO2 challenge is a safe research paradigm in the investigation of healthy subjects with a familial vulnerability to panic, and CO2 hypersensitivity might be considered a trait marker of an underlying familial vulnerability to panic disorder.

Title: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Authors: Roberson-Nay R, Beadel JR, Gorlin EI, Latendresse SJ, Teachman BA.

Journal: J Behav Ther Exp Psychiatry. 2015 Jun;47:68-76. doi: 10.1016/j.jbtep.2014.10.013. Epub 2014 Nov 15. PMID: 25496936; PMCID: PMC4324118.

Link to full text: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Abstract: Carbon dioxide (CO2) hypersensitivity is hypothesized to be a robust endophenotypic marker of panic spectrum vulnerability. The goal of the current study was to explore the latent class trajectories of three primary response systems theoretically associated with CO2 hypersensitivity: subjective anxiety, panic symptoms, and respiratory rate (fR). Participants (n = 376; 56% female) underwent a maintained 7.5% CO2 breathing task that included three phases: baseline, CO2 air breathing, and recovery. Growth mixture modeling was used to compare response classes (1…n) to identify the best-fit model for each marker. Panic correlates also were examined to determine class differences in panic vulnerability. For subjective anxiety ratings, a three-class model was selected, with individuals in one class reporting an acute increase in anxiety during 7.5% CO2 breathing and a return to pre-CO2 levels during recovery. A second, smaller latent class was distinguished by elevated anxiety across all three phases. The third class reported low anxiety reported during room air, a mild increase in anxiety during 7.5% CO2 breathing, and a return to baseline during recovery. Latent class trajectories for fR yielded one class whereas panic symptom response yielded two classes. This study examined CO2 hypersensitivity in one of the largest samples to date, but did not ascertain a general population sample thereby limiting generalizability. Moreover, a true resting baseline measure of fR was not measured. Two classes potentially representing different risk pathways were observed. Implications of results will be discussed in the context of panic risk research.

Title: Five percent carbon dioxide challenge: valid analogue and marker of panic disorder?

Authors: Sanderson WC, Wetzler S.

Journal: Biol Psychiatry. 1990 Apr 1;27(7):689-701. doi: 10.1016/0006-3223(90)90584-o. PMID: 2109638.

Link to PubMed: Five percent carbon dioxide challenge: valid analogue and marker of panic disorder?

Abstract: The administration of 5% carbon dioxide (CO2) to patients with panic disorder (PD) induces a behavioral response similar to a naturally occurring panic attack. This article reviews the literature on the nature and incidence of this response. We conclude that the 5% CO2 challenge test is a valid and useful laboratory analogue of naturally occurring panic attacks, and shows promise as a marker to identify a subset of PD patients. Though further research on reliability, validity, and dose-response effects must be conducted, the CO2 challenge test provides important information regarding the phenomenology of panic states.

Title: Fear Reactivity to Bodily Sensations Among Heavy Smokers and Nonsmokers

Authors: Abrams K, Zvolensky MJ, Dorflinger L, Galatis A, Blank M, Eissenberg T.

Journal: Exp Clin Psychopharmacol. 2008 Jun;16(3):230-9. doi: 10.1037/1064-1297.16.3.230. PMID: 18540783.

Link to PubMed: Fear Reactivity to Bodily Sensations Among Heavy Smokers and Nonsmokers

Abstract: Individuals who smoke are more likely to experience panic attacks and develop panic disorder than those in the general population. One possible explanation is that smokers may experience a heightened fear response to somatic disturbances. To date, few laboratory studies have tested this hypothesis directly. The present study examined 24 adult heavy smokers (10 females) in 12-hr nicotine withdrawal and 24 adult nonsmokers (12 females) on subjective and physiological reactivity to a 4-min carbon dioxide rebreathing challenge. Results indicate that, despite an attenuated acceleration in respiration during the challenge, smokers experienced a significantly greater increase in self-reported panic symptoms than nonsmokers. In addition, smokers reported significantly greater trait levels of suffocation fear prior to the challenge. Findings are discussed with respect to the role of smoking in panic vulnerability.

Title: Interoceptive Fear Conditioning and Panic Disorder: The Role of Conditioned Stimulus–Unconditioned Stimulus Predictability

Authors: Acheson DT, Forsyth JP, Moses E.

Journal: Behav Ther. 2012 Mar;43(1):174-89. doi: 10.1016/j.beth.2011.06.001. Epub 2011 Jun 15. PMID: 22304889.

Link to PubMed: Interoceptive Fear Conditioning and Panic Disorder: The Role of Conditioned Stimulus–Unconditioned Stimulus Predictability

Abstract: Interoceptive fear conditioning is at the core of contemporary behavioral accounts of panic disorder. Yet, to date only one study has attempted to evaluate interoceptive fear conditioning in humans (see Acheson, Forsyth, Prenoveau, & Bouton, 2007). That study used brief (physiologically inert) and longer-duration (panicogenic) inhalations of 20% CO(2)-enriched air as an interoceptive conditioned (CS) and unconditioned (US) stimulus and evaluated fear learning in three conditions: CS only, CS-US paired, and CS-US unpaired. Results showed fear conditioning in the paired condition, and fearful responding and resistance to extinction in an unpaired condition. The authors speculated that such effects may be due to difficulty discriminating between the CS and the US. The aims of the present study are to (a) replicate and expand this line of work using an improved methodology, and (b) clarify the role of CS-US discrimination difficulties in either potentiating or depotentiating fear learning. Healthy participants (N=104) were randomly assigned to one of four conditions: (a) CS only, (b) contingent CS-US pairings, (c) unpaired CS and US presentations, or (d) an unpaired "discrimination" contingency, which included an exteroceptive discrimination cue concurrently with CS onset. Electrodermal and self-report ratings served as indices of conditioned responding. Consistent with expectation, the paired contingency and unpaired contingencies yielded elevated fearful responding to the CS alone. Moreover, adding a discrimination cue to the unpaired contingency effectively attenuated fearful responding. Overall, findings are consistent with modern learning theory accounts of panic and highlight the role of interoceptive conditioning and unpredictability in the etiology of panic disorder.

Title: A genetic study of the acute anxious response to carbon dioxide stimulation in man

Authors: Battaglia M, Ogliari A, Harris J, Spatola CA, Pesenti-Gritti P, Reichborn-Kjennerud T, Torgersen S, Kringlen E, Tambs K.

Journal: J Psychiatr Res. 2007 Dec;41(11):906-17. doi: 10.1016/j.jpsychires.2006.12.002. Epub 2007 Jan 24. PMID: 17254605.

Link to PubMed: A genetic study of the acute anxious response to carbon dioxide stimulation in man

Abstract: People with panic disorder-agoraphobia and their relatives often react anxiously to CO(2)-enriched gas mixtures. Available data are not suited to disentangle genetic from common environmental causes of familial aggregation of CO(2) reactivity, nor provide quantitative estimations of the sources of trait variation. Three-hundred-forty-six twin pairs belonging to the general population-based Norwegian NIPH Mental Health Study underwent self-assessments of anxiety and of DSM-IV panic symptoms after inhalation of a 35%CO(2)-65%O(2) mixture. Two thresholds were employed - at sample's 75th and 90th percentiles of responses - to define provoked panic attacks and to calculate polychoric correlations. Variance components were estimated by structural equation modelling (SEM). For definitions of responses based on the sum of all 13 panic symptoms, SEM could not discriminate between shared environmental versus genetic causes of familial resemblance for provoked attacks. For definitions of responses based on global anxiety, or on the sums of those symptoms (dyspnea, dizziness, palpitations) with highest variance post-CO(2), the best-fitting models indicated additive genetic factors as the sole causes for within-family resemblance. Best-fit heritability estimates ranged from 0.42 to 0.57. Genetic and idiosyncratic environmental factors explain most of individual differences in reactivity to hypercapnia. Within-family similarities for this trait are largely explained by genetic determinants.

Title: A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss

Authors: Battaglia M, Pesenti-Gritti P, Medland SE, Ogliari A, Tambs K, Spatola CA.

Journal: Arch Gen Psychiatry. 2009 Jan;66(1):64-71. doi: 10.1001/archgenpsychiatry.2008.513. PMID: 19124689.

Link to PubMed: A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss

Abstract: Context: Childhood separation anxiety disorder can predate panic disorder, which usually begins in early adulthood. Both disorders are associated with heightened sensitivity to inhaled CO(2) and can be influenced by childhood parental loss. Objectives: To find the sources of covariation between childhood separation anxiety disorder, hypersensitivity to CO(2), and panic disorder in adulthood and to measure the effect of childhood parental loss on such covariation. Design: Multivariate twin study. Participants: Seven hundred twelve young adults from the Norwegian Institute of Public Health Twin Panel, a general population cohort. Main outcome measures: Personal direct assessment of lifetime panic disorder through structured psychiatric interviews, history of childhood parental loss, and separation anxiety disorder symptoms. Subjective anxiety response to a 35% CO(2)/65% O(2) inhaled mixture compared with compressed air (placebo). Results: Our best-fitting solution yielded a common pathway model, implying that covariation between separation anxiety in childhood, hypersensitivity to CO(2), and panic disorder in adulthood can be explained by a single latent intervening variable influencing all phenotypes. The latent variable governing the 3 phenotypes' covariation was in turn largely (89%) influenced by genetic factors and childhood parental loss (treated as an identified element of risk acting at a family-wide level), which accounted for the remaining 11% of covariance. Residual variance was explained by 1 specific genetic variance component for separation anxiety disorder and variable-specific unique environmental variance components. Conclusions: Shared genetic determinants appear to be the major underlying cause of the developmental continuity of childhood separation anxiety disorder into adult panic disorder and the association of both disorders with heightened sensitivity to CO(2). Inasmuch as childhood parental loss is a truly environmental risk factor, it can account for a significant additional proportion of the covariation of these 3 developmentally related phenotypes.

Title: CO2-Induced Panic Attacks: A Twin Study

Authors: Bellodi L, Perna G, Caldirola D, Arancio C, Bertani A, Di Bella D.

Journal: Am J Psychiatry. 1998 Sep;155(9):1184-8. doi: 10.1176/ajp.155.9.1184. PMID: 9734540.

Link to PubMed: CO2-Induced Panic Attacks: A Twin Study

Abstract: Objective: The authors investigated the role of genetic factors in 35% CO2-induced panic attacks. Method: Ninety twins recruited from the general population were challenged with one-vital-capacity inhalations of 35% CO2-65% O2. Probandwise concordance rates were calculated and rates compared for monozygotic and for dizygotic twins. Results: A significantly higher concordance was found for 35% CO2-induced panic attacks among monozygotic than dizygotic twins (55.6% versus 12.5%). Conclusions: These results suggest a relevant role of genetic factors in 35% CO2-induced panic attacks.

Title: Panic disorder subtypes: differential responses to CO2 challenge

Authors: Biber B, Alkin T.

Journal: Am J Psychiatry. 1999 May;156(5):739-44. doi: 10.1176/ajp.156.5.739. PMID: 10327907.

Link to PubMed: Panic disorder subtypes: differential responses to CO2 challenge

Abstract: Objective: The purpose of this study was to investigate the possibility of a differential sensitivity to CO2 in patients diagnosed with panic disorder subtypes that were defined by the presence of prominent respiratory symptoms. Method: The authors used a 35% CO2 and 65% O2 mixture as a challenge agent. Fifty-one unmedicated subjects with DSM-III-R panic disorder, who were divided into respiratory (N = 28) and nonrespiratory (N = 23) subtypes by their symptom profiles, underwent a CO2 challenge procedure. Patients in the two groups were compared with regard to physiological and psychological measures, pulmonary function tests, panic rates, and smoking habits. Results: The patients in the respiratory group were significantly more sensitive to CO2 than were the patients in the nonrespiratory group. The respiratory group also had higher scores on the Panic and Agoraphobia Scale and had a longer duration of illness; both of these factors can be indicators of illness severity. In addition, the respiratory group's higher cigarette consumption (mean = 12.46 package-years, SD = 2.49) may have been a contributory factor not only for illness severity but also for the pathogenesis of panic disorder. Conclusions: The CO2 challenge procedure appears to be a good dissection tool in the understanding of different subtypes of panic disorder. Moreover, there may be a more specific association with prominent respiratory symptom subtype and CO2 hypersensitivity.

Title: Experiential, autonomic, and respiratory correlates of CO2 reactivity in individuals with high and low anxiety sensitivity

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Psychiatry Res. 2013 Oct 30;209(3):566-73. doi: 10.1016/j.psychres.2013.02.010. Epub 2013 Mar 13. PMID: 23489596.

Link to PubMed: Experiential, autonomic, and respiratory correlates of CO2 reactivity in individuals with high and low anxiety sensitivity

Abstract: Psychometric studies indicate that anxiety sensitivity (AS) is a risk factor for anxiety disorders such as panic disorder (PD). To better understand the psychophysiological basis of AS and its relation to clinical anxiety, we examined whether high-AS individuals show similarly elevated reactivity to inhalations of carbon dioxide (CO2) as previously reported for PD and social phobia in this task. Healthy individuals with high and low AS were exposed to eight standardized inhalations of 20% CO2-enriched air, preceded and followed by inhalations of room air. Anxiety and dyspnea, in addition to autonomic and respiratory responses were measured every 15 s. Throughout the task, high AS participants showed a respiratory pattern of faster, shallower breathing and reduced inhalation of CO2 indicative of anticipatory or contextual anxiety. In addition, they showed elevated dyspnea responses to the second set of air inhalations accompanied by elevated heart rate, which could be due to sensitization or conditioning. Respiratory abnormalities seem to be common to high AS individuals and PD patients when considering previous findings with this task. Similarly, sensitization or conditioning of anxious and dyspneic symptoms might be common to high AS and clinical anxiety. Respiratory conditionability deserves greater attention in anxiety disorder research.

Title: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Biol Psychol. 2010 Apr;84(1):104-11. doi: 10.1016/j.biopsycho.2010.01.002. Epub 2010 Jan 12. PMID: 20064582; PMCID: PMC2891596.

Link to full text: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Abstract: Inhalation of carbon dioxide (CO₂) enriched air triggers anxiety in panic disorder (PD) patients, which is often interpreted as a sign of biological vulnerability. However, most studies have not measured respiration in these tasks. We compared patients with PD (n=20) and social phobia (SP, n=19) to healthy controls (n=18) during eight inhalations of 20% CO₂, preceded and followed by two inhalations of room air, while continuously measuring subjective anxiety and dyspnea as well as autonomic and respiratory variables. PD patients showed increased reactivity and delayed recovery during CO₂ inhalations for most measures. Unlike both other groups, the PD group's tidal volume responses did not habituate across CO₂ inhalations. However, PD patients did not differ from SP patients on most other measures, supporting a continuum model of CO₂ sensitivity across anxiety disorders. Both patient groups showed continued reactivity during the last air inhalations, which is unlikely to be due to a biological sensitivity.

Title: The 35% CO2 challenge test in patients with social phobia

Authors: Caldirola D, Perna G, Arancio C, Bertani A, Bellodi L.

Journal: Psychiatry Res. 1997 Jun 16;71(1):41-8. doi: 10.1016/s0165-1781(97)00038-3. PMID: 9247980.

Link to PubMed: The 35% CO2 challenge test in patients with social phobia

Abstract: Panic disorder (PD) and social phobia (SP) share many clinical, demographic and biological characteristics. To investigate the relationships between the two disorders, the responses to inhalation of a 35% carbon-dioxide (CO2) and 65% oxygen (O2) gas mixture were assessed. Sixteen patients with PD, 16 patients with SP, 13 patients with both SP and PD, seven patients with SP who experienced sporadic unexpected panic attacks and 16 healthy control subjects inhaled one vital capacity of 35% CO2 or compressed air. A double-blind, randomized, crossover design was used. PD patients and SP patients showed similar anxiogenic reactions to 35% CO2, both stronger than seen in control subjects. Patients with both disorders and SP patients with sporadic unexpected panic attacks reacted similarly to subjects with PD or SP alone. These results suggest that PD and SP share a common hypersensitivity to CO2 and thus might belong to the same spectrum of vulnerability.

Title: Carbon Dioxide-Induced Emotion and Respiratory Symptoms in Healthy Volunteers

Authors: Colasanti A, Salamon E, Schruers K, van Diest R, van Duinen M, Griez EJ.

Journal: Neuropsychopharmacology. 2008 Dec;33(13):3103-10. doi: 10.1038/npp.2008.31. Epub 2008 Mar 19. PMID: 18354390.

Link to full text: Carbon Dioxide-Induced Emotion and Respiratory Symptoms in Healthy Volunteers

Abstract: A number of evidences have established that panic and respiration are closely related. Clinical studies indicated that respiratory sensations constitute a discrete cluster of panic symptoms and play a major role in the pathophysiology of panic. The aim of the present study was to explore the phenomenology of an experimental model of panic in healthy volunteers based on the hypothesis that: (1) we can isolate discrete clusters of panic symptoms, (2) respiratory symptoms represent a distinct cluster of panic symptoms, and (3) respiratory symptoms are the best predictor of the subjective feeling of panic, as defined in the DSM IV criteria. Sixty-four healthy volunteers received a double inhalation of four mixtures containing 0, 9, 17.5 and 35% CO(2,) respectively, in a double-blind, cross-over, random design. An electronic visual analog scale and the Panic Symptom List (PSL) were used to assess subjective 'fear/discomfort' and panic symptoms, respectively. Statistical analyses consisted of Spearman's correlations, a principal component factor analysis of the 13 PSL symptoms, and linear regressions analyses. The factor analysis extracted three clusters of panic symptoms: respiratory, cognitive, and neurovegetative (r(2)=0.65). Respiratory symptoms were highly related to subjective feeling of fear/discomfort specifically in the CO(2)-enriched condition. Moreover, the respiratory component was the most important predictor of the subjective feeling of 'fear/discomfort' (beta=0.54). The discrete clusters of symptoms observed in this study were similar to those elicited in panic attacks naturally occurring in patients affected by panic disorder. Consistent with the idea that respiration plays a crucial role in the pathophysiology of panic, we found that respiratory symptoms were the best predictors the subjective state defined in the DSM IV criteria for panic.

Title: Hypersensitivity to carbon dioxide as a disease-specific trait marker

Authors: Coryell W.

Journal: Biol Psychiatry. 1997 Feb 1;41(3):259-63. doi: 10.1016/s0006-3223(97)87457-4. PMID: 9024948.

Link to PubMed: Hypersensitivity to carbon dioxide as a disease-specific trait marker

Abstract: There is now substantial evidence that an abnormal threshold for suffocation alarm underlies panic disorder. Because this disorder is highly familial, evidence of an abnormal suffocation threshold may be apparent in high-risk individuals before they develop clinical illness. To explore this possibility, we used a single inhalation of 35% CO2 vs. air to evaluate 11 subjects who had at least one first-degree relative with DSM-III-R panic disorder, 13 who had at least two relatives treated for mania or for depression (HR-AD), and 15 low-risk controls who had no family history of panic disorder, affective disorder, or alcoholism (LR-C). All were aged 18-34 and had no history of panics or of any Research Diagnostic Criteria disorder. Five (45.5%) of the subjects at high risk for panic disorder, but none of the LR-C subjects (p = .007), nor any of the HR-AD subjects (p = .011), developed a panic attack following inhalation of the CO2 mixture.

Title: Anxiety responses to CO2 inhalation in subjects at high-risk for panic disorder

Authors: Coryell W, Pine D, Fyer A, Klein D.

Journal: J Affect Disord. 2006 May;92(1):63-70. doi: 10.1016/j.jad.2005.12.045. Epub 2006 Mar 9. PMID: 16527360.

Link to PubMed: Anxiety responses to CO2 inhalation in subjects at high-risk for panic disorder

Abstract: Background: A number of reports have shown that patients with panic disorder have greater anxiety responses to the inhalation of enhanced carbon dioxide mixtures than do well controls or patients with other psychiatric illnesses. Three earlier studies have shown that well individuals who have first-degree relatives with panic disorder also experience more anxiety following CO(2) than do controls without such a family history. The following was undertaken to confirm and extend these findings. Methods: Well subjects at high risk for panic disorder (

Title: The 35% CO2 Inhalation Procedure: Test–Retest Reliability

Authors: Coryell W, Arndt S.

Journal: Biol Psychiatry. 1999 Apr 1;45(7):923-7. doi: 10.1016/s0006-3223(98)00241-8. PMID: 10202581.

Link to PubMed: The 35% CO2 Inhalation Procedure: Test–Retest Reliability

Abstract: The subjective response to a single-breath, 35% carbon dioxide challenge test shows promise as a tool for the study of panic disorder and may comprise a trait marker for that disorder. Little has been done to measure the reliability of test results, however. Subjects took a single breath at 35% CO2 and completed a self-rating of anxiety symptoms immediately thereafter. This procedure was repeated after a mean interval of 29 days. One group, considered at high risk for panic disorder, consisted of well, first-degree relatives of individuals treated for panic disorder. The control group included well subjects at high risk for affective disorder and subjects who had family histories negative for both affective disorder and panic disorder. On both testing occasions, subjects at high risk for panic disorder had symptom scores that were significantly higher than those of control subjects. Group differences in the portions who experienced a panic attack were dependent on the symptom threshold used to define an attack. A lower threshold was optimal with the second testing and a single, positive test result appeared to be more meaningful than a single negative result. The majority of individual symptom ratings were highly correlated across tests. Ratings for "smothering sensations," in particular, correlated highly across tests and consistently discriminated high-risk from control subjects. The sources of test result variability are unclear and warrant more investigation before the tests can be clinically useful. Research efforts should seek optimal thresholds to define positive test results within given data sets.

Title: Restoring Acid-Sensing Ion Channel-1a in the Amygdala of Knock-Out Mice Rescues Fear Memory But Not Unconditioned Fear Responses

Authors: Coryell MW, Wunsch AM, Haenfler JM, Allen JE, McBride JL, Davidson BL, Wemmie JA.

Journal: J Neurosci. 2008 Dec 17;28(51):13738-41. doi: 10.1523/JNEUROSCI.3907-08.2008. PMID: 19091964; PMCID: PMC2651157.

Link to full text: Restoring Acid-Sensing Ion Channel-1a in the Amygdala of Knock-Out Mice Rescues Fear Memory But Not Unconditioned Fear Responses

Abstract: Acid-sensing ion channel-1a (ASIC1a) contributes to multiple fear behaviors, however the site of ASIC1a action in behavior is not known. To explore a specific location of ASIC1a action, we expressed ASIC1a in the basolateral amygdala of ASIC1a-/- mice using viral vector-mediated gene transfer. This rescued context-dependent fear memory, but not the freezing deficit during training or the unconditioned fear response to predator odor. These data pinpoint the basolateral amygdala as the site where ASIC1a contributes to fear memory. They also discriminate fear memory from fear expressed during training and from unconditioned fear. Furthermore, this work illustrates a strategy for identifying discrete brain regions where specific genes contribute to complex behaviors.

Title: The effects of acceptance versus control contexts on avoidance of panic-related symptoms

Authors: Eifert GH, Heffner M.

Journal: J Behav Ther Exp Psychiatry. 2003 Sep-Dec;34(3-4):293-312. doi: 10.1016/j.jbtep.2003.11.001. PMID: 14972675.

Link to full text: The effects of acceptance versus control contexts on avoidance of panic-related symptoms

Abstract: The present study compared the effects of creating an acceptance versus a control treatment context on the avoidance of aversive interoceptive stimulation. Sixty high anxiety sensitive females were exposed to two 10-min periods of 10% carbon dioxide enriched air, an anxiogenic stimulus. Before each inhalation period, participants underwent a training procedure aimed at encouraging them either to mindfully observe (acceptance context) or to control symptoms via diaphragmatic breathing (control context). A third group was given no particular training or instructions. We hypothesized that an acceptance rather than control context would be more useful in the reduction of anxious avoidance. Compared to control context and no-instruction participants, acceptance context participants were less avoidant behaviorally and reported less intense fear and cognitive symptoms and fewer catastrophic thoughts during the CO(2) inhalations. We discuss the implications of our findings for an acceptance-focused vs. control-focused context when conducting clinical interventions for panic and other anxiety disorders.

Title: The effects of acute exercise and high lactate levels on 35% CO2 challenge in healthy volunteers

Authors: Esquivel G, Schruers K, Kuipers H, Griez E.

Journal: Acta Psychiatr Scand. 2002 Nov;106(5):394-7. doi: 10.1034/j.1600-0447.2002.01333.x. PMID: 12366475.

Link to PubMed: The effects of acute exercise and high lactate levels on 35% CO2 challenge in healthy volunteers

Abstract: Objective: To test the possible antipanic effects of acute exercise in healthy volunteers exposed to an inhalation of 35% CO2 challenge. Method: Twenty healthy subjects in a randomized separate group design, performed exercise in a bicycle ergometer reaching >6 mm of blood lactate and a control condition of minimal activity in the same fashion with no lactate elevation. Immediately afterwards an inhalation of a vital capacity using a mixture of 35% CO2/65% O2 through a mask was given on both conditions. Results: Subjects under the exercise condition reported less panic symptoms than controls after a CO2 challenge on the diagnostic statistical manual-IV (DSM-IV) Panic Symptom List but no difference on the Visual Analogue Anxiety Scale. Conclusion: Subjects under the exertion condition had lactate levels comparable with those of lactate infusions but an inhibitory rather than accumulative effect was seen when combined with a CO2 challenge.

Title: To inhale or not to inhale: Conditioned avoidance in breathing behavior in an odor—20% CO2 paradigm

Authors: Fannes S, Van Diest I, Meulders A, De Peuter S, Vansteenwegen D, Van den Bergh O.

Journal: Biol Psychol. 2008 Apr;78(1):87-92. doi: 10.1016/j.biopsycho.2008.01.003. Epub 2008 Jan 31. PMID: 18316154.

Link to PubMed: To inhale or not to inhale: Conditioned avoidance in breathing behavior in an odor—20% CO2 paradigm

Abstract: This study investigated breathing behavior in an odor-CO2-inhalation fear conditioning paradigm. A differential conditioning paradigm was applied in 55 participants. Both acquisition and extinction consisted of three CS+ and three CS- trials. Diluted ammonia and butyric acid served as conditional odor cues (CSs); inhalation of 20% CO2-enriched as US. The US was presented 10s after CS+ onset and both stimuli co-terminated 30s later. Subjective anxiety and US-expectancy were measured online upon presentation of the CSs. Respiratory behavior showed a biphasic pattern during CS+ acquisition trials. Participants paradoxically lowered their ventilation first; an increased ventilation was observed only towards the end of the trial. Extinction of this breathing inhibition was found. Participants avoiding the CO2 during acquisition did not show a reduction in fear from acquisition to extinction, whereas Non-avoiders did. We conclude that paradoxical decreases in ventilation constitute a relevant behavioral index of fear in CO2-inhalation paradigms.

Title: Emotional avoidance: an experimental test of individual differences and response suppression using biological challenge

Authors: Feldner MT, Zvolensky MJ, Eifert GH, Spira AP.

Journal: Behav Res Ther. 2003 Apr;41(4):403-11. doi: 10.1016/s0005-7967(02)00020-7. PMID: 12643964.

Link to PubMed: Emotional avoidance: an experimental test of individual differences and response suppression using biological challenge

Abstract: The present study examined the affective consequences of response inhibition during a state of anxiety-related physical stress. Forty-eight non-clinical participants were selected on the basis of pre-experimental differences in emotional avoidance (high versus low) and subjected to four inhalations of 20% carbon dioxide-enriched air. Half of the participants were instructed to inhibit the challenge-induced aversive emotional state, whereas the other half was instructed to simply observe their emotional response. Participants high in emotional avoidance compared to those low in emotional avoidance responded with greater levels of anxiety and affective distress but not physiological arousal. Individuals high in emotional avoidance also reported greater levels of anxiety relative to the low emotional avoidance group when suppressing compared to observing bodily sensations. These findings are discussed in terms of the significance of emotional avoidance processes during physical stress, with implications for better understanding the nature of panic disorder.

Title: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Authors: Forsyth JP, Lejuez CW, Finlay C.

Journal: J Behav Ther Exp Psychiatry. 2000 Jun;31(2):103-21. doi: 10.1016/s0005-7916(00)00014-8. PMID: 11132115.

Link to PubMed: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Abstract: Increasingly carbon dioxide-enriched air is being used as an aversive unconditioned stimulus in laboratory examinations of anxiety. Yet, little is known about the stability of the autonomic and subjective effects of this stimulus across repeated inhalations and sessions. We examined whether repeated administrations of high concentrations of CO2-enriched air produced either habituation, stability, or sensitization across several autonomic and self-report indices within one session (Experiment 1) and then several sessions (Experiment 2) of exposure. Results suggest that non-clinical participants do not habituate to CO2 within sessions, but do show habituation on cardiac and subjective report of anxiety across sessions. Individual difference factors such as anxiety sensitivity and suffocation fear seem to moderate some of these effects, including self-reported distress and anxiety in response to the challenge. These results support the use of CO2 as a panicogenic aversive stimulus in laboratory models of fear onset and in clinical settings for interoceptive exposure treatments of panic.

Title: The absence of relation between anxiety sensitivity and fear conditioning using 20% versus 13% CO2-enriched air as unconditioned stimuli

Authors: Forsyth JP, Palav A, Duff K.

Journal: Behav Res Ther. 1999 Feb;37(2):143-53. doi: 10.1016/s0005-7967(98)00113-2. PMID: 9990745.

Link to PubMed: The absence of relation between anxiety sensitivity and fear conditioning using 20% versus 13% CO2-enriched air as unconditioned stimuli

Abstract: Anxiety sensitivity has been implicated as a risk factor in the development and maintenance of anxiety and fear-related disorders. Indeed, persons who score high on the anxiety sensitivity index (ASI) are generally more responsive to biological challenge procedures such as CO2-inhalation that directly evoke the feared bodily events. One would expect, therefore, that persons high on anxiety sensitivity should be more conditionable and hence more likely to acquire fears, than persons low on anxiety sensitivity when CO2-enriched air is used as an unconditioned stimulus (UCS). Undergraduates (N = 96), scoring high, medium and low on the ASI received 8 repeated 20-s inhalations of either 20 or 13% CO2-enriched air (UCSs) paired with one of three CSs differing in fear-relevance (snake, heart and flowers). Several autonomic and self-report measures were assessed. Contrary to expectation, electrodermal and cardiac conditioned responses failed to discriminate between ASI groups. Yet, SUDS and severity and frequency of DSM-IV panic symptoms varied reliably as a function of anxiety sensitivity. Overall, the findings suggest that anxiety sensitivity is related to subjective fear-related complaints, but not autonomic responding and conditionability. We discuss clinical and theoretical implications for understanding the place fo anxiety sensitivity in fear onset.

Title: High-Dose Carbon Dioxide Challenge Test in Anxiety Disorder Patients

Authors: Gorman JM, Papp LA, Martinez J, Goetz RR, Hollander E, Liebowitz MR, Jordan F.

Journal: Biol Psychiatry. 1990 Nov 1;28(9):743-57. doi: 10.1016/0006-3223(90)90510-9. PMID: 2124151.

Link to full text: High-Dose Carbon Dioxide Challenge Test in Anxiety Disorder Patients

Abstract: Many investigators have shown that panic disorder patients and possibly social phobics are hypersensitive to the anxiogenic effects of inhaled carbon dioxide (CO2). In this study we administered double-breath inhalation of 35% CO2 and 65% oxygen (O2) to panic disorder patients, social phobics, and normal controls. At baseline, panic disorder patients were characterized by higher pulse, anxiety score, and evidence of hyperventilation. Panic patients and social phobics panicked more often to 35% CO2 than to room air; normal controls did not have a higher rate of panic to CO2 than to room air. However, we did not find significant group differences in anxiety level, physiological measures, or biochemical measures in response to CO2 breathing compared with room air breathing. These results confirm earlier reports of baseline hyperventilation in panic disorder patients. However, 35% CO2 may be too high a dose to differentiate respiratory responses of patients compared with normals.

Title: Anxiogenic effects of CO2 and hyperventilation in patients with panic disorder

Authors: Gorman JM, Papp LA, Coplan JD, Martinez JM, Lennon S, Goetz RR, Ross D, Klein DF.

Journal: Am J Psychiatry. 1994 Apr;151(4):547-53. doi: 10.1176/ajp.151.4.547. PMID: 8147452.

Link to PubMed: Anxiogenic effects of CO2 and hyperventilation in patients with panic disorder

Abstract: Objective: Previous studies have indicated that patients with panic disorder are more likely than normal subjects to have acute panic attacks during inhalation of CO2, but methodological objections have been raised. In this study the authors attempted to address three of these methodological problems by ensuring that raters who assessed whether panic attacks occurred were blind to subjects' diagnoses, by randomizing the order of administration of 5% CO2 and hyperventilation, and by challenging a greater number of subjects with 7% CO2. Method: Patients with panic disorder and normal subjects underwent 20-minute inhalations of 5% CO2 and 7% CO2 and 15 minutes of room-air hyperventilation. Ratings of panic/no panic during each condition were made separately by an assessor blind to diagnosis and by the subject. Scores on four panic rating scales were also recorded before and after each intervention. Results: Room-air hyperventilation caused panic attacks in a small number of patients; the difference in panic rate between patients and comparison subjects was statistically significant by the subjects' but not by the raters' assessment. Panic rates during 5% CO2 and 7% CO2 were significantly greater among the patients by both assessments; the panic rate was greatest during 7% CO2. Order of administration did not significantly affect panic rates for hyperventilation and 5% CO2. Conclusions: Panic patients were clearly more sensitive to the anxiogenic effects of CO2 than comparison subjects, and CO2 was a more potent anxiogenic stimulus than room-air hyperventilation. Seven percent CO2 discriminated best between patients and comparison subjects and should be the focus of further research.

Title: Response to hyperventilation in a group of patients with panic disorder

Authors: Gorman JM, Askanazi J, Liebowitz MR, Fyer AJ, Stein J, Kinney JM, Klein DF.

Journal: Am J Psychiatry. 1984 Jul;141(7):857-61. doi: 10.1176/ajp.141.7.857. PMID: 6428243.

Link to PubMed: Response to hyperventilation in a group of patients with panic disorder

Abstract: Twelve patients with DSM-III diagnoses of panic disorder and four normal volunteers were studied during hyperventilation of room air and increased ventilation stimulated by 5% CO2. Patients also underwent sodium lactate infusion. Eight of the patients had a panic attack during sodium lactate infusion, seven during CO2 inhalation, and three during room-air hyperventilation. Since CO2 increases the firing of the locus ceruleus, and since sodium lactate infusion is known to increase cerebral CO2, these findings are consistent with other evidence implicating the locus ceruleus in panic anxiety.

Title: The Effect of Successful Treatment on the Emotional and Physiological Response to Carbon Dioxide Inhalation in Patients with Panic Disorder

Authors: Gorman JM, Martinez J, Coplan JD, Kent J, Kleber M.

Journal: Biol Psychiatry. 2004 Dec 1;56(11):862-7. doi: 10.1016/j.biopsych.2004.08.016. PMID: 15576063.

Link to PubMed: The Effect of Successful Treatment on the Emotional and Physiological Response to Carbon Dioxide Inhalation in Patients with Panic Disorder

Abstract: Background: A number of studies have shown that patients with panic disorder are more likely to have panic attacks during carbon dioxide inhalation than are normal comparison subjects. Some studies have shown that antipanic medications can reduce the anxiogenic response to carbon dioxide, but none have shown if this is the case for cognitive behavioral therapy or if successful treatment reduces the respiratory physiologic response to carbon dioxide. Methods: Twenty-five patients with panic disorder and 13 normal comparison subjects underwent baseline testing with 5% and 7% carbon dioxide inhalation. The patients were then retested after at least 12 weeks of treatment with either antipanic medication or cognitive behavioral therapy. Comparison subjects were retested after a similar interval. Results: Successful treatment resulted in lower panic rates, and reduced anxiogenic response. Treatment had no effect, however, on the respiratory physiologic response. Conclusions: There is dissociation in treatment response between the subjective and objective responses to carbon dioxide inhalation in panic disorder patients, with the former but not the latter showing positive change. We hypothesize that the strengthening of higher cortical control over subcortical fear-related structures, whether via medication or cognitive behavioral therapy treatment, results in less anxiety and fear in response to provoked symptoms reminiscent of naturally occurring panic.

Title: Ventilatory physiology of patients with panic disorder

Authors: Gorman JM, Fyer MR, Goetz R, Askanazi J, Liebowitz MR, Fyer AJ, Kinney J, Klein DF.

Journal: Arch Gen Psychiatry. 1988 Jan;45(1):31-9. doi: 10.1001/archpsyc.1988.01800250035006. Erratum in: Arch Gen Psychiatry 1991 Feb;48(2):181. PMID: 2827599.

Link to PubMed: Ventilatory physiology of patients with panic disorder

Abstract: Thirty-one patients with DSM-III panic disorder or agoraphobia with panic attacks, 13 normal controls, and 12 patients with other anxiety disorders were studied during ventilatory challenge with room air hyperventilation and 5% carbon dioxide inhalation. Patients also underwent sodium lactate infusion. Among the patients with panic disorder, 58% panicked with sodium lactate, 39% with 5% CO2, and 23% with room air hyperventilation. Of the other patients, four panicked with sodium lactate, none with 5% CO2, and one with room air hyperventilation. One normal control panicked with both sodium lactate and 5% CO2. Panic with CO2 was associated with an exaggerated ventilatory response and increases in plasma norepinephrine level and diastolic blood pressure. Patients with panic disorder may have hypersensitive CO2 receptors that, when triggered, evoke a subjective panic associated with an exaggerated ventilatory response and consequent hypocapnic alkalosis.

Title: CO2 Vulnerability in Panic Disorder

Authors: Griez EJ, Lousberg H, van den Hout MA, van der Molen GM.

Journal: Psychiatry Res. 1987 Feb;20(2):87-95. doi: 10.1016/0165-1781(87)90001-1. PMID: 3107009.

Link to PubMed: CO2 Vulnerability in Panic Disorder

Abstract: The immediate effects of a single inhalation of a 35% CO2 mixture in oxygen were examined in 12 patients with panic disorders and 11 normal control subjects. Compared to a placebo air inhalation, the CO2 inhalation provoked short-lived autonomic panic symptoms in both patients and normals; it also elicited high subjective anxiety in patients with panic disorders. The latter rated the overall CO2-induced state as very similar to a real-life panic attack.

Title: Carbon Dioxide Inhalation Induces Dose-Dependent and Age-Related Negative Affectivity

Authors: Griez EJ, Colasanti A, van Diest R, Salamon E, Schruers K.

Journal: PLoS One. 2007 Oct 3;2(10):e987. doi: 10.1371/journal.pone.0000987. PMID: 17912364; PMCID: PMC1991589.

Link to full text: Carbon Dioxide Inhalation Induces Dose-Dependent and Age-Related Negative Affectivity

Abstract: Background: Carbon dioxide inhalation is known to induce an emotion similar to spontaneous panic in Panic Disorder patients. The affective response to carbon dioxide in healthy subjects was not clearly characterized yet. Methodology/principal findings: Sixty-four healthy subjects underwent a double inhalation of four mixtures containing respectively 0, 9, 17.5 and 35% CO(2) in compressed air, following a double blind, cross-over, randomized design. Affective responses were assessed according to DSM IV criteria for panic, using an Electronic Visual Analogue Scale and the Panic Symptom List. It was demonstrated that carbon dioxide challenges induced a dose dependent negative affect (p<0.0001). This affect was semantically identical to the DSM IV definition of panic. Older individuals were subjectively less sensitive to Carbon Dioxide (p<0.05). Conclusions/significance: CO(2) induced affectivity may lay on a continuum with pathological panic attacks. Consistent with earlier suggestions that panic is a false biological alarm, the affective response to CO(2) may be part of a protective system triggered by suffocation and acute metabolic distress.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Neural Pathways Underlying Lactate-Induced Panic

Authors: Johnson PL, Truitt WA, Fitz SD, Lowry CA, Shekhar A.

Journal: Neuropsychopharmacology. 2008 Aug;33(9):2093-107. doi: 10.1038/sj.npp.1301621. Epub 2007 Dec 5. PMID: 18059441; PMCID: PMC3065200.

Link to full text: Neural Pathways Underlying Lactate-Induced Panic

Abstract: Panic disorder is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as 0.5 M sodium lactate infusions. Although studied for four decades, the mechanism of lactate sensitivity in panic disorder has not been understood. The dorsomedial hypothalamus/perifornical region (DMH/PeF) coordinates rapid mobilization of behavioral, autonomic, respiratory and endocrine responses to stress, and rats with disrupted GABA inhibition in the DMH/PeF exhibit panic-like responses to lactate, similar to panic disorder patients. Utilizing a variety of anatomical and pharmacological methods, we provide evidence that lactate, via osmosensitive periventricular pathways, activates neurons in the compromised DMH/PeF, which relays this signal to forebrain limbic structures such as the bed nucleus of the stria terminalis to mediate anxiety responses, and specific brainstem sympathetic and parasympathetic pathways to mediate the respiratory and cardiovascular components of the panic-like response. Acutely restoring local GABAergic tone in the DMH/PeF blocked lactate-induced panic-like responses. Autonomic panic-like responses appear to be a result of DMH/PeF-mediated mobilization of sympathetic responses (verified with atenolol) and resetting of the parasympathetically mediated baroreflex. Based on our findings, DMH/PeF efferent targets such as the C1 adrenergic neurons, paraventricular hypothalamus, and the central amygdala are implicated in sympathetic mobilization; the nucleus of the solitary tract is implicated in baroreflex resetting; and the parabrachial nucleus is implicated in respiratory responses. These results elucidate neural circuits underlying lactate-induced panic-like responses and the involvement of both sympathetic and parasympathetic systems.

Title: Sex differences in response to a panicogenic challenge procedure: an experimental evaluation of panic vulnerability in a non-clinical sample

Authors: Kelly MM, Forsyth JP, Karekla M.

Journal: Behav Res Ther. 2006 Oct;44(10):1421-30. doi: 10.1016/j.brat.2005.10.012. Epub 2005 Dec 20. PMID: 16364237.

Link to PubMed: Sex differences in response to a panicogenic challenge procedure: an experimental evaluation of panic vulnerability in a non-clinical sample

Abstract: Epidemiological studies show that women are twice as likely as men to develop panic disorder (PD) during their lifetimes. Data from retrospective studies also suggest that women are more distressed by panic and other negative emotional states than men, and that this tendency may precede the development of PD. The present prospective study sought to expand this work by evaluating sex differences in the predisposition to panic in individuals without PD or other forms of psychopathology. Participants (N=96; 52 women) were exposed to 12 panicogenic inhalations of 20% CO2-enriched air while physiological (e.g., heart rate, electrodermal response, Frontalis EMG) and self-report (e.g., subject distress, fear, frequency and severity of DSM-IV panic symptoms) response domains were monitored. As expected, magnitude of autonomic responding failed to reliably discriminate between women and men. Yet, women reported more fear and panic immediately following the challenge procedure relative to men, and this sex difference persisted when assessed again 30 min post-challenge. Implications of these results are discussed broadly in the context of biopsychosocial accounts of sex differences in the clinical presentation of PD.

Title: Carbon dioxide inhalation as a human experimental model of panic: The relationship between emotions and cardiovascular physiology

Authors: Leibold NK, Viechtbauer W, Goossens L, De Cort K, Griez EJ, Myin-Germeys I, Steinbusch HW, van den Hove DL, Schruers KR.

Journal: Biol Psychol. 2013 Oct;94(2):331-40. doi: 10.1016/j.biopsycho.2013.06.004. Epub 2013 Jun 28. PMID: 23816952.

Link to PubMed: Carbon dioxide inhalation as a human experimental model of panic: The relationship between emotions and cardiovascular physiology

Abstract: Inhaling carbon dioxide (CO2)-enriched air induces fear and panic symptoms resembling real-life panic attacks, the hallmark of panic disorder. The present study aimed to describe the emotional and cardiovascular effects evoked by inhaling CO2, taking shortcomings of previous studies into account. Healthy volunteers underwent a double inhalation of 0, 9, 17.5, and 35% CO2, according to a randomized, cross-over design. In addition to fear, discomfort, and panic symptom ratings, blood pressure and heart rate were continuously monitored. Results showed a dose-dependent increase in all self-reports. Systolic and diastolic blood pressure rose with increasing CO2 concentration, whereas heart rate results were less consistent. Diastolic blood pressure and heart rate variation correlated with fear and discomfort. Based on this relationship and the observation that the diastolic blood pressure most accurately mimicked the degree of self-reported emotions, it might serve as a putative biomarker to assess the CO2-reactivity in the future.

Title: Avoidance of 20% carbon dioxide-enriched air with humans

Authors: Lejuez CW, O'Donnell J, Wirth O, Zvolensky MJ, Eifert GH.

Journal: J Exp Anal Behav. 1998 Jul;70(1):79-86. doi: 10.1901/jeab.1998.70-79. PMID: 9684345; PMCID: PMC1284669.

Link to full text: Avoidance of 20% carbon dioxide-enriched air with humans

Abstract: Four college students were exposed to a Sidman avoidance procedure to determine if an avoidance contingency involving 20% carbon dioxide-enriched air (CO2) would produce and maintain responding. In Phase 1, two conditions (contingent and noncontingent) were conducted each day. These conditions were distinguished by the presence or absence of a blue or green box on a computer screen. In the contingent condition, CO2 presentation were delivered every 3 s unless a subject pulled a plunger. Each plunger pull postponed CO2 presentations for 10 s. In the noncontingent condition, CO2 presentations occurred on the average of every 5 min independent of responding. Following stable responding in Phase 1, condition-correlated stimuli were reversed. In both conditions, plunger response rate was high during the contingent condition and low or zero during the noncontingent condition. Furthermore, subjects avoided most CO2 presentations. However, CO2 presentations did not increase verbal reports of fear. Overall, the results from the present study suggest that CO2 can be used effectively in basic studies of aversive control and in laboratory analogues of response patterns commonly referred to as anxiety.

Title: Examination of mask disturbance behavior during a carbon dioxide-enriched air challenge

Authors: Marshall-Berenz EC, Gonzalez A, Leyro TM, Zvolensky MJ.

Journal: J Behav Ther Exp Psychiatry. 2011 Sep;42(3):253-7. doi: 10.1016/j.jbtep.2011.01.004. Epub 2011 Jan 12. PMID: 21349246; PMCID: PMC3086949.

Link to full text: Examination of mask disturbance behavior during a carbon dioxide-enriched air challenge

Abstract: Background and objectives: The present investigation examined mask disturbance behavior in the context of a biological challenge. Method: Participants included 128 adults (63.3% women; M(age) = 23.2, SD = 8.9) who underwent a 10% carbon dioxide-enriched air challenge. Results: Mask disturbance behavior, including complete or partial mask removal, was common during the challenge. Moreover, mask removal behavior during the challenge was significantly related to a greater increase in anxiety pre- to post-challenge, as well as greater levels of self-reported avoidance of a future challenge administration. However, mask touching and lifting behaviors were not related to other challenge criterion variables. Limitations: The current study consisted primarily of young adults and did not include individuals with a panic disorder diagnosis. Replication and extension of the current findings is recommended. Conclusions: Results suggest that mask removal behavior during the challenge may represent a possible behavioral marker of panic-relevant risk, although further work is necessary to determine its usefulness as a challenge measure.

Title: Shared Variance Among Self-Report and Behavioral Measures of Distress Intolerance

Authors: McHugh RK, Daughters SB, Lejuez CW, Murray HW, Hearon BA, Gorka SM, Otto MW.

Journal: Cognit Ther Res. 2011 Jun;35(3):266-275. doi: 10.1007/s10608-010-9295-1. PMID: 23894216; PMCID: PMC3721199.

Link to full text: Shared Variance Among Self-Report and Behavioral Measures of Distress Intolerance

Abstract: Distress intolerance may be an important individual difference variable in understanding maladaptive coping responses across diagnostic categories. However, the measurement of distress intolerance remains inconsistent across studies and little evidence for convergent validity among existing measures is available. This study evaluated the overlap among self-report and behavioral measures of distress intolerance in four samples, including an unselected sample, a sample of patients with drug dependence, and two samples of cigarette smokers. Results suggested that the self-report measures were highly correlated, as were the behavioral measures; however, behavioral and self-report measures did not exhibit significant associations with each other. There was some evidence of domain specificity, with anxiety sensitivity demonstrating strong associations with somatic distress intolerance, and a lack of association between behavioral measures that elicit affective distress and those that elicit somatic distress. These findings highlight a potential divergence in the literature relative to the conceptualization of distress intolerance as either sensitivity to distress or as the inability to persist at a task when distressed. Further research is needed to elucidate the conceptualization and measurement of distress intolerance to facilitate future clinical and research applications of this construct.

Title: History of suffocation, state-trait anxiety, and anxiety sensitivity in predicting 35% carbon dioxide-induced panic

Authors: Monkul ES, Onur E, Tural U, Hatch JP, Alkın T, Yücel B, Fidaner H.

Journal: Psychiatry Res. 2010 Sep 30;179(2):194-7. doi: 10.1016/j.psychres.2009.06.015. Epub 2010 May 16. PMID: 20478634.

Link to PubMed: History of suffocation, state-trait anxiety, and anxiety sensitivity in predicting 35% carbon dioxide-induced panic

Abstract: The aim of this study was to examine the effects of history of suffocation, state-trait anxiety, and anxiety sensitivity on response to a 35% carbon dioxide (CO₂) challenge in panic disorder patients, their healthy first-degree relatives and healthy comparisons. Thirty-two patients with panic disorder, 32 first-degree relatives, and 34 healthy volunteers underwent the 35% CO₂ challenge. We assessed baseline anxiety with the Anxiety Sensitivity Index (ASI) and State-Trait Anxiety Inventory (STAI1), and panic symptoms with the Panic Symptom List (PSL III-R). A history of suffocation was associated with greater risk of CO₂ reactivity in the combined sample. Patients had more anxiety sensitivity and state and trait anxiety than relatives and healthy comparisons; the difference between relatives and healthy comparisons was not significant. In female patients, trait anxiety predicted CO₂-induced panic. Having a CO₂-sensitive panic disorder patient as a first-degree relative did not predict CO₂-induced panic in a healthy relative. History of suffocation may be an important predictor of CO₂-induced panic. Trait anxiety may have a gender-specific relation to CO₂ reactivity.

Title: Acute panicogenic, anxiogenic and dissociative effects of carbon dioxide inhalation in patients with post-traumatic stress disorder (PTSD)

Authors: Muhtz C, Yassouridis A, Daneshi J, Braun M, Kellner M.

Journal: J Psychiatr Res. 2011 Jul;45(7):989-93. doi: 10.1016/j.jpsychires.2011.01.009. Epub 2011 Feb 15. PMID: 21324483.

Link to PubMed: Acute panicogenic, anxiogenic and dissociative effects of carbon dioxide inhalation in patients with post-traumatic stress disorder (PTSD)

Abstract: Background: Increased anxiety and panic to inhalation of carbon dioxide (CO(2)) has been described in patients with anxiety disorders, especially panic disorder, compared to healthy subjects. Post-traumatic stress disorder (PTSD) has been hypothesised to resemble panic disorder and is currently classified as an anxiety disorder in DSM-IV. However, there are only very few data available about the sensitivity of patients with PTSD to CO(2). Methods: In 10 patients with PTSD, 10 sex- and age-matched healthy subjects and 8 patients with panic disorder we assessed anxiety, panic, dissociative and PTSD symptoms before and after a single vital capacity inhalation of 35% CO(2). Results: Patients with PTSD showed an increased anxiety, panic and dissociative reaction to the inhalation of 35% CO(2) compared to healthy participants. PTSD subjects' responses were indistinguishable from those of panic patients. Additionally, PTSD-typical symptoms like post-traumatic flashbacks were provoked in patients with PTSD after the inhalation of CO(2). Conclusions: In our sample, PTSD was associated with an increased CO(2) reactivity, pointing to an increased susceptibility of PTSD patients to CO(2) challenge.

Title: Panicogens in patients with Post-Traumatic Stress Disorder (PTSD)

Authors: Muhtz C, Wiedemann K, Kellner M.

Journal: Curr Pharm Des. 2012;18(35):5608-18. doi: 10.2174/138161212803530817. PMID: 22632476.

Link to PubMed: Panicogens in patients with Post-Traumatic Stress Disorder (PTSD)

Abstract: Symptom provocation has proved its worth for understanding the pathophysiology of diseases and in general for the development of new therapeutic approaches in the medical field. In the research of anxiety disorders, investigations using experimentally induced panic attacks by various agents, such as sodium lactate, carbon dioxide, cholezystokinine-tetrapetid etc., have a long tradition and allow the exploration of usually naturally occuring spontaneous psychopathological phenomena under controlled conditions. Post-Traumatic Stress Disorder (PTSD) is a prevalent disorder that can develop following exposure to an extreme traumatic event. In DSM-IV it is currently classified as an anxiety disorder and shares phenomenological similarities with panic disorder. The use of panicogenic challenge tests is also an interesting neurobiological approach to learn more about the nature of PTSD and may be a possibility to develop new therapeutic strategies for the treatment of PTSD symptoms. Not only panic anxiety, but also flashbacks and other dissociative symptoms can be provoked by several panicogens in PTSD. The purpose of this review is to evaluate studies using panicogens in PTSD. Methodological short-comings of current studies and needed directions of further research are discussed.

Title: 35% Carbon dioxide and breath-holding challenge tests in panic disorder: a comparison with spontaneous panic attacks

Authors: Nardi AE, Valença AM, Mezzasalma MA, Lopes FL, Nascimento I, Veras AB, Freire RC, de-Melo-Neto VL, Zin WA.

Journal: Depress Anxiety. 2006;23(4):236-44. doi: 10.1002/da.20165. PMID: 16528718.

Link to PubMed: 35% Carbon dioxide and breath-holding challenge tests in panic disorder: a comparison with spontaneous panic attacks

Abstract: Respiration and its control mechanisms may represent an important system involved in abnormal anxiety. Our aim was to compare the demographic and clinical features of patients with panic disorder (PD) with agoraphobia (DSM-IV) who had a panic attack after both the 35% carbon dioxide (CO(2)) test and the breath-holding test (CPA group), and compare them with PD patients who did not have a panic attack after both tests (NPA group). We examined 76 patients with PD who were administered a 35% CO(2)test and a breath-holding test within a 1-week interval. Anxiety scales were applied before and after each test. A panic attack was induced in 50 (65.8%) patients during the CO(2)test (chi(2) = 28.44, df = 1, P<.001) and in 40 (52.6%) patients during the breath-holding test (chi(2) = 15.35, df = 1, P = .036). All patients who had a panic attack during the breath-holding test also had a panic attack during the CO(2)test (n = 40; CPA group). Twenty-six (34.2%) patients with PD did not have a panic attack after both respiratory tests (NPA group). The CPA group had more (chi(2) = 21.67, df = 1, P = .011) respiratory PD subtype. In the CPA group, the disorder started earlier (Mann-Whitney, P<.001), had a higher familial prevalence of PD (chi(2) = 18.34, df = 1, P = .028), and had more previous depressive episodes (chi(2) = 23.59, df = 1, P<.001). Our data suggest that there is an association between respiratory PD subtype and the response to respiratory challenge tests: CO(2)and breath-holding. The CPA may be confirmed as a subgroup of respiratory PD subtype.

Title: Sex differences in panic-relevant responding to a 10% carbon dioxide-enriched air biological challenge

Authors: Nillni YI, Berenz EC, Rohan KJ, Zvolensky MJ.

Journal: J Anxiety Disord. 2012 Jan;26(1):165-72. doi: 10.1016/j.janxdis.2011.10.008. Epub 2011 Nov 6. PMID: 22115836; PMCID: PMC3254702.

Link to full text: Sex differences in panic-relevant responding to a 10% carbon dioxide-enriched air biological challenge

Abstract: The current study examined sex differences in psychological (i.e., self-reported anxiety, panic symptoms, and avoidance) and physiological (i.e., heart rate and skin conductance level) response to, and recovery from, a laboratory biological challenge. Participants were a community-recruited sample of 128 adults (63.3% women; M(age)=23.2 years, SD=8.9) who underwent a 4-min 10% CO(2)-enriched air biological challenge. As predicted, women reported more severe physical panic symptoms and avoidance (i.e., less willingness to participate in another challenge) and demonstrated increased heart rate as compared to men above and beyond the variance accounted for by other theoretically relevant variables (recent panic attack history, neuroticism, and anxiety sensitivity). Additionally, women demonstrated a faster rate of recovery with respect to heart rate compared to men. These results are in line with literature documenting sex-specific differences in panic psychopathology, and results are discussed in the context of possible mechanisms underlying sex differences in panic vulnerability.

Title: The Relationships between Adverse Events, Early Antecedents, and Carbon Dioxide Reactivity as an Intermediate Phenotype of Panic Disorder

Authors: Ogliari A, Tambs K, Harris JR, Scaini S, Maffei C, Reichborn-Kjennerud T, Battaglia M.

Journal: Psychother Psychosom. 2010;79(1):48-55. doi: 10.1159/000259417. Epub 2009 Nov 18. PMID: 19923875.

Link to PubMed: The Relationships between Adverse Events, Early Antecedents, and Carbon Dioxide Reactivity as an Intermediate Phenotype of Panic Disorder

Abstract: Background: Although adverse events have been consistently described to precede and potentially precipitate the onset of panic disorder, there is no information about their ability to alter the individual reactivity to inhaled carbon dioxide, a putative intermediate phenotype of susceptibility to panic disorder. Method: Seven-hundred twelve subjects belonging to the general population-based Norwegian Institute of Public Health Twin Panel underwent a 35% CO(2)/65% O(2) inhalation challenge test and interview-based lifetime assessments of DSM-IV panic disorder, separation anxiety disorder, childhood parental separation/loss, major life events, adverse events of suffocative nature and common stressful life events. Regression models were applied to predict global subjective anxiety and DSM-IV panic symptoms after 35% CO(2)/65% O(2) inhalation. Results: The responses to the challenge measured as semicontinuous variables were predicted by symptoms of childhood separation anxiety, childhood parental loss, common stressful events, major life events, suffocative events and the female gender. The role of most of these predictors was confirmed and held true after the exclusion of subjects with lifetime panic attacks/disorder from the analyses. Conclusions: Several factors which have been reported by previous clinical studies to influence the individual susceptibility to develop panic disorder seem to affect the individual reactivity to inhaled carbon dioxide in people from the general population. Some elements of risk may impact simultaneously upon the individual liability to panic and exaggerated sensitivity to hypercapnia.

Title: Respiratory Psychophysiology of Panic Disorder: Three Respiratory Challenges in 98 Subjects

Authors: Papp LA, Martinez JM, Klein DF, Coplan JD, Norman RG, Cole R, de Jesus MJ, Ross D, Goetz R, Gorman JM.

Journal: Am J Psychiatry. 1997 Nov;154(11):1557-65. doi: 10.1176/ajp.154.11.1557. PMID: 9356564.

Link to PubMed: Respiratory Psychophysiology of Panic Disorder: Three Respiratory Challenges in 98 Subjects

Abstract: Objective: Respiratory abnormalities may play a central role in the pathophysiology of panic disorder. The current study was undertaken to examine the respiratory response in the largest series of subjects to date during three respiratory challenges that used improved methodology. Method: Fifty-nine patients with DSM-III-R panic disorder and 39 normal volunteers were challenged with 5% and 7% CO2 inhalation and room air hyperventilation separated by room air breathing with continuous spirometry. Results: Patients with panic disorder were more sensitive to the anxiogenic effects of CO2 than were normal subjects, and CO2 was a more potent stimulus to panic than hyperventilation. Patients increased their respiratory rate more quickly during CO2 inhalation than did comparison subjects, and this increase preceded the panic attacks. Patients who panicked in response to 5% CO2 demonstrated continued rise in end-tidal CO2, while the end-tidal CO2 of the comparison groups stabilized. Low end-tidal CO2 and high variance in minute ventilation at baseline predicted panic attacks during CO2 inhalation. Following CO2 or hyperventilation challenges, respiratory rate dropped sharply, while tidal volume remained elevated longer in patients than in comparison subjects. Conclusions: The findings confirm the greater behavioral and physiological sensitivity of patients with panic disorder to CO2 inhalation and identify a series of respiratory abnormalities. Panic attacks in panic disorder may be explained by inefficient compensatory mechanisms, primarily of respiratory rate.

Title: Diagnostic and substance specificity of carbon-dioxide-induced panic

Authors: Papp LA, Klein DF, Martinez J, Schneier F, Cole R, Liebowitz MR, Hollander E, Fyer AJ, Jordan F, Gorman JM.

Journal: Am J Psychiatry. 1993 Feb;150(2):250-7. doi: 10.1176/ajp.150.2.250. PMID: 8123056.

Link to PubMed: Diagnostic and substance specificity of carbon-dioxide-induced panic

Abstract: Objective: The authors assessed the substance and diagnostic specificity of carbon-dioxide-induced panic since, in addition to the specific biochemical effects of inhaled carbon dioxide (CO2), simple physiologic distress is also frequently implicated as a panicogenic factor during respiratory challenge studies with CO2 in patients with anxiety disorders. Method: Eighteen patients with panic disorder, 20 with social phobia, and 23 psychiatrically normal subjects inhaled a mixture of 35% CO2 and 65% O2 for 30 seconds through a face mask. They also breathed for 30 seconds through a valve reducing the diameter of the airway. A double-blind, counterbalanced, randomized design was used. Results: In spite of important similarities between the two interventions, including the induction of equal amounts of subjective respiratory distress, carbon dioxide inhalation was significantly more potent than increased airway resistance in provoking panic in the anxiety disorder patients. The patients with panic disorder were significantly more sensitive to CO2 than were the patients with social phobia or the normal subjects. Conclusions: Carbon dioxide inhalation appears to have a specific panicogenic effect in panic patients that goes beyond simple breathlessness.

Title: Psychophysiological responses to CO2 inhalation

Authors: Pappens M, De Peuter S, Vansteenwegen D, Van den Bergh O, Van Diest I.

Journal: Int J Psychophysiol. 2012 Apr;84(1):45-50. doi: 10.1016/j.ijpsycho.2012.01.008. Epub 2012 Jan 20. PMID: 22265938.

Link to PubMed: Psychophysiological responses to CO2 inhalation

Abstract: Inhalation of CO(2)-enriched air has been used as a laboratory model for a number of anxiety disorders, such as general anxiety disorder and panic disorder. Because studies describing psychophysiological responses to this challenge are scarce, the present studies investigated skin conductance level, eyeblink startle, self-reported anxiety and fractional end-tidal carbon dioxide during inhalation of CO(2)-enriched air. In study 1, thirty-five healthy volunteers inhaled 7.5% CO(2) for 2min. In study 2, twenty healthy volunteers inhaled 20% CO(2) for 30s. Control groups (N=20 in each study) inhaled room air during the same time periods. Compared to room air breathing, both CO(2)-mixtures were associated with increases in skin conductance levels, self-reported anxiety and fractional end-tidal CO(2.) Eyeblink startles were inhibited during CO(2) compared to room air breathing in both experiments. Our findings suggest that inhalation of CO(2)-enriched air is associated with a circa-strike defensive response pattern, corroborating its application as an interoceptive, panic-relevant stimulus in fear research.

Title: Carbon dioxide/oxygen challenge test in panic disorder

Authors: Perna G, Battaglia M, Garberi A, Arancio C, Bertani A, Bellodi L.

Journal: Psychiatry Res. 1994 May;52(2):159-71. doi: 10.1016/0165-1781(94)90085-x. PMID: 7972572.

Link to PubMed: Carbon dioxide/oxygen challenge test in panic disorder

Abstract: The effects of a single inhalation of a 35% CO2/65% O2 gas mixture were examined in 71 patients with panic disorder with or without agoraphobia and 44 normal control subjects. Compared with the placebo condition, inhalation of air, the CO2/O2 mixture elicited a clear anxiety reaction only in panic disorder patients, who experienced a sudden rise of subjective anxiety as well as of several panic symptoms. Respiratory symptoms and the fear of dying best distinguished the patients from the control subjects. Baseline anxiety was not the key factor in explaining this differential reaction. The clinical features of panic disorder (namely, frequency of panic attacks, agoraphobia, anticipatory anxiety, and duration of illness) were not significantly related to the response to the challenge test, suggesting that CO2 reactivity might be a trait marker of panic disorder.

Title: 35% CO2 challenge in panic and mood disorders

Authors: Perna G, Barbini B, Cocchi S, Bertani A, Gasperini M.

Journal: J Affect Disord. 1995 Mar 14;33(3):189-94. doi: 10.1016/0165-0327(94)00088-q. PMID: 7790671.

Link to PubMed: 35% CO2 challenge in panic and mood disorders

Abstract: 20 patients with Panic Disorder (PD), 19 patients with Mood Disorder (MD) and 20 healthy controls inhaled one vital capacity of 35% CO2-65% O2 gas mixture and of compressed air in a double-blind, random, cross-over design. Only PD patients showed a strong reaction to 35% CO2 while MD patients and controls did not react significantly. These results support the specificity of the 35% CO2 challenge in PD patients and suggest that PD and MD are separate disorders.

Title: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Authors: Perna G, Bertani A, Caldirola D, Gabriele A, Cocchi S, Bellodi L.

Journal: J Clin Psychopharmacol. 2002 Jun;22(3):300-8. doi: 10.1097/00004714-200206000-00011. PMID: 12006901.

Link to PubMed: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Abstract: Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

Title: Sensitivity to 35% CO2 in healthy first-degree relatives of patients with panic disorder

Authors: Perna G, Cocchi S, Bertani A, Arancio C, Bellodi L.

Journal: Am J Psychiatry. 1995 Apr;152(4):623-5. doi: 10.1176/ajp.152.4.623. PMID: 7694916.

Link to PubMed: Sensitivity to 35% CO2 in healthy first-degree relatives of patients with panic disorder

Abstract: Objective: The authors tested the hypothesis that hyperreactivity to CO2 in healthy subjects represents an underlying familial vulnerability to panic disorder. Method: One vital-capacity inhalation of 35% CO2 and 65% O2 was administered to each of 84 patients with panic disorder, 23 healthy first-degree relatives of probands with panic disorder, and 44 healthy subjects with no family history of panic disorder. Results: The first-degree relatives of the probands with panic disorder reacted significantly more than the healthy subjects and significantly less than the probands. Conclusions: These findings suggest an association between family history of panic disorder and hyperreactivity to 35% CO2 in healthy subjects.

Title: Differential carbon dioxide sensitivity in childhood anxiety disorders and nonill comparison group

Authors: Pine DS, Klein RG, Coplan JD, Papp LA, Hoven CW, Martinez J, Kovalenko P, Mandell DJ, Moreau D, Klein DF, Gorman JM.

Journal: Arch Gen Psychiatry. 2000 Oct;57(10):960-7. doi: 10.1001/archpsyc.57.10.960. PMID: 11015814.

Link to PubMed: Differential carbon dioxide sensitivity in childhood anxiety disorders and nonill comparison group

Abstract: Background: To examine the relationship between respiratory regulation and childhood anxiety disorders, this study considered the relationship between anxiety disorders and symptoms during carbon dioxide (CO(2)) exposure, CO(2) sensitivity in specific childhood anxiety disorders, and the relationship between symptomatic and physiological responses to CO(2). Methods: Following procedures established in adults, 104 children (aged 9-17 years), including 25 from a previous study, underwent 5% CO(2) inhalation. The sample included 57 probands with an anxiety disorder (social phobia, generalized anxiety disorder, separation anxiety disorder, and panic disorder) and 47 nonill comparison subjects. Symptoms of anxiety were assessed before, during, and after CO(2) inhalation. Results: All children tolerated the procedure well, experiencing transient or no increases in anxiety symptoms. Children with an anxiety disorder, particularly separation anxiety disorder, exhibited greater changes in somatic symptoms during inhalation of CO(2)-enriched air, relative to the comparison group. During CO(2) inhalation, symptom ratings were positively correlated with respiratory rate increases, as well as with levels of tidal volume, minute ventilation, end-tidal CO(2), and irregularity in respiratory rate during room-air breathing. Conclusions: Childhood anxiety disorders, particularly separation anxiety disorder, are associated with CO(2) hypersensitivity, as defined by symptom reports. Carbon dioxide hypersensitivity is associated with physiological changes similar to those found in panic disorder. These and other data suggest that certain childhood anxiety disorders may share pathophysiological features with adult panic disorder.

Title: Response to 5% carbon dioxide in children and adolescents: relationship to panic disorder in parents and anxiety disorders in subjects

Authors: Pine DS, Klein RG, Roberson-Nay R, Mannuzza S, Moulton JL 3rd, Woldehawariat G, Guardino M.

Journal: Arch Gen Psychiatry. 2005 Jan;62(1):73-80. doi: 10.1001/archpsyc.62.1.73. PMID: 15630075.

Link to PubMed: Response to 5% carbon dioxide in children and adolescents: relationship to panic disorder in parents and anxiety disorders in subjects

Abstract: Background: Carbon dioxide (CO(2)) sensitivity is postulated to be a familial risk marker of panic disorder (PD). Exaggerated responses to CO(2) inhalation have been reported in adults with PD and their unaffected adult relatives, as well as in clinic-referred children with anxiety disorders.

Objective: To test in a family-based design whether CO(2) hypersensitivity is a familial risk marker for PD and associated with current anxiety disorders in children and adolescents.

Setting and participants: One hundred forty-two offspring (aged 9-19 years) of parents with PD, major depressive disorder, or no disorder. Forty-five (32%) had a current anxiety disorder, excluding specific phobia.

Design and main outcome measures: Parents and offspring received diagnostic assessments. Offspring underwent 5% CO(2) inhalation at home. Panic symptoms and panic attacks were rated with the Acute Panic Inventory at baseline, while anticipating CO(2) delivery ("threat"), and during CO(2) inhalation. Respiratory rate and volume were measured with spirometry.

Results: No group differences were found in Acute Panic Inventory ratings at baseline or in respiratory measures during threat. Risk for PD was not associated with CO(2) sensitivity (panic symptoms and respiratory physiologic response). During CO(2) inhalation, offspring with anxiety disorders, relative to offspring without anxiety disorders, experienced significantly more panic symptoms and panic attacks, as well as elevated respiratory rates. During threat, panic symptoms were significantly and independently associated with both parental PD and offspring anxiety disorders.

Conclusions: No support was obtained for CO(2) hypersensitivity as a familial risk marker for PD in children and adolescents. Links between childhood anxiety disorders and CO(2) sensitivity were replicated. Familial risk for PD in children and adolescents may be associated with vulnerability to anticipatory anxiety.

Title: Cognitive mediation in the affective component of spontaneous panic attacks

Authors: Rapee R, Mattick R, Murrell E.

Journal: J Behav Ther Exp Psychiatry. 1986 Dec;17(4):245-53. doi: 10.1016/0005-7916(86)90059-5. PMID: 3100580.

Link to PubMed: Cognitive mediation in the affective component of spontaneous panic attacks

Abstract: Single inhalations of a 50% carbon dioxide/50% oxygen gas mixture were administered to 16 subjects with spontaneous panic attacks and to 16 social phobics who did not experience such attacks. Half of each diagnostic category was randomly allocated to either a no explanation condition in which minimal instructions on expected outcome were provided, or an explanation condition in which all possible sensations were described and attributed to the effects of the gas. Subjects with panic attacks who were given no explanation reported a greater proportion of catastrophic cognitions, greater panic, and a greater similarity of the overall experience to a naturally-occurring panic attack than those with panic attacks who received a full explanation. In contrast, both groups of social phobics reported similar effects to each other, regardless of the explanation given. The results provide support for cognitive mediation in the "panic" component of spontaneous panic attacks.

Title: Response to hyperventilation and inhalation of 5.5% carbon dioxide-enriched air across the DSM-III-R anxiety disorders

Authors: Rapee RM, Brown TA, Antony MM, Barlow DH.

Journal: J Abnorm Psychol. 1992 Aug;101(3):538-52. doi: 10.1037//0021-843x.101.3.538. PMID: 1500611.

Link to PubMed: Response to hyperventilation and inhalation of 5.5% carbon dioxide-enriched air across the DSM-III-R anxiety disorders

Abstract: Anxiety disorder patients (n = 198; under criteria of the Diagnostic and Statistical Manual of Mental Disorders; rev. 3rd ed.; American Psychiatric Association, 1987) and nonanxious control subjects (n = 25) underwent challenges of 90 s of voluntary hyperventilation and 15 min of 5.5% carbon dioxide in air. Panic disorder subjects showed a greater subjective response to both challenges than did subjects with other anxiety disorders, who in turn responded more than did control subjects. Furthermore, subjects with panic disorder as an additional diagnosis tended to report more subjective response than did anxiety disorder subjects without panic disorder. The best prechallenge predictor of response to each procedure was a measure of fear of physical symptoms. The findings support previous results that have pointed to a greater fear or anxiety-inducing effect of these challenge procedures in panic disorder patients, as compared with other subjects.

Title: Evidence for distinct genetic effects associated with response to 35% CO₂

Authors: Roberson-Nay R, Moruzzi S, Ogliari A, Pezzica E, Tambs K, Kendler KS, Battaglia M.

Journal: Depress Anxiety. 2013 Mar;30(3):259-66. doi: 10.1002/da.22038. Epub 2013 Jan 24. PMID: 23349098; PMCID: PMC4096694.

Link to full text: Evidence for distinct genetic effects associated with response to 35% CO₂

Abstract: Background: Carbon dioxide (CO2 ) hypersensitivity represents an individual difference response to breathing CO2 enriched air. People with a history of panic attacks or panic disorder are particularly prone to anxious response, suggesting that CO2 hypersensitivity is a robust risk marker of panic spectrum vulnerability.

Methods: Twin pairs (n = 346) from the general population-based Norwegian NIPH Mental Health Study completed a measure of anxiety before and after vital capacity inhalation of 35% CO2 air and before and after inhalation of regular air. Three hypotheses regarding genetic factors for CO2 hypersensitivity were examined: (1) a single set of genetic risk factors impacts anxiety before exposure to CO2 and these same genes constitute the only genetic influences on anxiety in response to CO2 , (2) the genetic effects on pre-CO2 anxiety are entirely different from the genetic effects on anxiety in response to exposure to CO2 (i.e., new genetic effects), and (3) pre-CO2 anxiety influences anxiety in response to CO2 as well as unique genetic factors that become activated by respiratory stimulation.

Results: Our results support the latter hypothesis for response to 35% CO2 , with additive genetic and unique environmental factors best fitting the data. Evidence of new genetic effects was observed, accounting for 20% unique variance in post 35% CO2 anxiety response. New genetic effects were not observed for anxiety ratings made post regular air where only preregular air anxiety ratings explained significant variance in this outcome.

Conclusions: These data suggest that there are distinct genetic factors associated with responsivity to respiratory stimulation via 35% CO2 .

Title: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Authors: Roberson-Nay R, Beadel JR, Gorlin EI, Latendresse SJ, Teachman BA.

Journal: J Behav Ther Exp Psychiatry. 2015 Jun;47:68-76. doi: 10.1016/j.jbtep.2014.10.013. Epub 2014 Nov 15. PMID: 25496936; PMCID: PMC4324118.

Link to full text: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Abstract: Background and objectives: Carbon dioxide (CO2) hypersensitivity is hypothesized to be a robust endophenotypic marker of panic spectrum vulnerability. The goal of the current study was to explore the latent class trajectories of three primary response systems theoretically associated with CO2 hypersensitivity: subjective anxiety, panic symptoms, and respiratory rate (fR).

Methods: Participants (n = 376; 56% female) underwent a maintained 7.5% CO2 breathing task that included three phases: baseline, CO2 air breathing, and recovery. Growth mixture modeling was used to compare response classes (1…n) to identify the best-fit model for each marker. Panic correlates also were examined to determine class differences in panic vulnerability.

Results: For subjective anxiety ratings, a three-class model was selected, with individuals in one class reporting an acute increase in anxiety during 7.5% CO2 breathing and a return to pre-CO2 levels during recovery. A second, smaller latent class was distinguished by elevated anxiety across all three phases. The third class reported low anxiety reported during room air, a mild increase in anxiety during 7.5% CO2 breathing, and a return to baseline during recovery. Latent class trajectories for fR yielded one class whereas panic symptom response yielded two classes.

Limitations: This study examined CO2 hypersensitivity in one of the largest samples to date, but did not ascertain a general population sample thereby limiting generalizability. Moreover, a true resting baseline measure of fR was not measured.

Conclusions: Two classes potentially representing different risk pathways were observed. Implications of results will be discussed in the context of panic risk research.

Keywords: Anxiety; Carbon dioxide hypersensitivity; Latent class; Panic; Respiratory; Risk.

Title: The influence of an illusion of control on panic attacks induced via inhalation of 5.5% carbon dioxide-enriched air

Authors: Sanderson WC, Rapee RM, Barlow DH.

Journal: Arch Gen Psychiatry. 1989 Feb;46(2):157-62. doi: 10.1001/archpsyc.1989.01810020059010. PMID: 2492423.

Link to PubMed: The influence of an illusion of control on panic attacks induced via inhalation of 5.5% carbon dioxide-enriched air

Abstract: The current study tested the notion that a sense of control can mitigate anxiety and panic attacks caused by the inhalation of 5.5% carbon dioxide (CO2)-enriched air. Twenty patients with panic disorder inhaled a mixture of 5.5% CO2-enriched air for 15 minutes. All patients were instructed that illumination of a light directly in front of them would signal that they could decrease the amount of CO2 that they were receiving, if desired, by turning a dial attached to their chair. For ten patients, the light was illuminated during the entire administration of CO2. For the remaining ten patients, the light was never illuminated. In fact, all patients experienced the full CO2 mixture, and the dial was ineffective. When compared with patients who believed they had control, patients who believed they could not control the CO2 administration (1) reported a greater number of DSM-III-revised panic attack symptoms, (2) rated the symptoms as more intense, (3) reported greater subjective anxiety, (4) reported a greater number of catastrophic cognitions, (5) reported a greater resemblance of the overall inhalation experience to a naturally occurring panic attack, and (6) were significantly more likely to report panic attacks. These data illustrate the contribution of psychologic factors to laboratory induction of panic attacks through inhalation of 5.5% CO2-enriched air.

Title: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Authors: Schmidt NB, Trakowski JH, Staab JP.

Journal: J Abnorm Psychol. 1997 Nov;106(4):630-8. doi: 10.1037//0021-843x.106.4.630. PMID: 9358693.

Link to PubMed: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Abstract: Inhalations of high concentrations of carbon dioxide (CO2) reliably produce panic attacks in patients with panic disorder. The present study evaluated whether cognitive-behavioral treatment (CBT) for panic disorder would extinguish CO2-induced panic and whether changes in panic and arousal-related cognitions were associated with the induction of panic. Patients with panic disorder (N = 54) were assigned to 1 of 3 experimental conditions: CBT with respiratory training (CBT-R), CBT without respiratory training (CBT), or delayed treatment. Participants received 5 repeated vital-capacity inhalations of 35% CO2/65% O2 prior to and following either 12 treatment sessions or a 12-week waiting period. During pretreatment assessments, 74% of patients experienced a panic attack during at least 1 inhalation. At posttreatment, only 20% of treated participants (CBT-R = 19%, CBT = 22%), compared with 64% of untreated participants, panicked. Forty-four percent of treated participants, compared with 0% of untreated participants, reported no anxiety during all posttreatment inhalations. Anxiety sensitivity as well as panic appraisals regarding the likelihood of panic and self-efficacy with coping with panic were significantly related to fearful responding to the CO2 challenge.

Title: Biological Challenge Manipulation of PCO2 Levels: A Test of Klein's (1993) Suffocation Alarm Theory of Panic

Authors: Schmidt NB, Telch MJ, Jaimez TL.

Journal: J Abnorm Psychol. 1996 Aug;105(3):446-54. doi: 10.1037//0021-843x.105.3.446. PMID: 8772015.

Link to PubMed: Biological Challenge Manipulation of PCO2 Levels: A Test of Klein's (1993) Suffocation Alarm Theory of Panic

Abstract: D.F. Klein (1993) proposed that patients with panic disorder (PD) have a hypersensitive suffocation monitor that predisposes them to experience panic attacks under certain conditions. The suffocation alarm theory predicts differential emotional responding to biological challenges that affect arterial partial pressure of carbon dioxide (PCO2). These PD patients should exhibit (a) lower fear and less likelihood of panic in response to biological challenges that lower PCO2 levels (e.g., hyperventilation), and (b) increased fear and greater likelihood of panic in response to biological challenges that raise PCO2 levels (e.g., inhalation of 35% CO2 gas). The following indicators of the suffocation monitor were assessed: (a) severity of dyspnea symptoms, (b) frequency of dyspnea symptoms, (c) heightened respiration rate, and (d) lowered PCO2 levels. Ratings of physiological and subjective responding, as well as panic, were obtained during both a hyperventilation and a 35% CO2 challenge. None of the classification methods predicted differential emotional responding to hyperventilation versus 35% CO2 challenge.

Title: Genetic moderation of CO2-induced fear by 5-HTTLPR genotype

Authors: Schruers K, Esquivel G, van Duinen M, Wichers M, Kenis G, Colasanti A, Knuts I, Goossens L, Jacobs N, van Rozendaal J, Smeets H, van Os J, Griez E.

Journal: J Psychopharmacol. 2011 Jan;25(1):37-42. doi: 10.1177/0269881110372543. Epub 2010 Jun 28. PMID: 20584994.

Link to PubMed: Genetic moderation of CO2-induced fear by 5-HTTLPR genotype

Abstract: Inhalation of an increased concentration of carbon dioxide (CO(2)) has been shown to induce a state of negative affect in healthy subjects that is closely related to the clinical phenomenon of panic. It has been suggested that the vulnerability to CO(2) is moderated by differences in serotonin (5-HT) activity, caused by a functional polymorphism in the promoter region of the 5-HT transporter (5-HTTLPR) gene. Our aim was to examine the relationship between bi- and tri-allelic 5-HTTLPR genotype and the affective response to different dosages of inhaled CO(2) in healthy volunteers. Ninety-six subjects performed a double inhalation of four mixtures containing, respectively, 0%, 9%, 17.5% and 35% CO(2), following a double-blind, cross-over, randomized design. Affective responses were measured with a visual analogue scale for fear and the Panic Symptom List. 5-HTTLPR genotype was expressed as LL, SL and SS. Subjects with the SL and SS genotype reported less fear than LL subjects. A significant interaction effect was found between genotype and CO(2) dosage: the SS genotype showed lower fear scores than the LL genotype, particularly in the 17.5% CO(2) dose condition. The present study suggests that the dose-dependent fear reaction to CO(2) is moderated by a polymorphism in the 5-HT transporter gene, particularly at intermediate CO(2) dosages. It also underscores the usefulness of the introduction of an intermediate phenotype related to panic to reveal an underlying genetic vulnerability otherwise staying elusive. These results are in line with current theories on the role of 5-HT in both panic and respiration.

Title: Symptom profiles of natural and laboratory panic attacks

Authors: Schruers KR, van de Mortel H, Overbeek T, Griez E.

Journal: Acta Neuropsychiatr. 2004 Apr;16(2):101-6. doi: 10.1111/j.0924-2708.2004.0084.x. PMID: 26984003.

Link to PubMed: Symptom profiles of natural and laboratory panic attacks

Abstract: Background: Little accurate information is available about the symptomatology of real-life panic attacks and about how well they are reproduced by an experimental model such as the 35% CO2 challenge.

Method: Real-life panic symptoms were assessed in a group of 67 panic disorder patients, using daily life monitoring. Panic symptoms elicited by a 35% CO2 challenge were assessed in 61 panic disorder patients, and their frequency was compared with the real-life symptoms.

Results: The most frequent real-life symptoms were palpitations, dizziness and trembling. The 35% CO2 challenge reproduced well the majority of real-life symptoms.

Conclusion: The findings suggest that the 35% CO2 challenge is a marker for spontaneous panic attacks, which are considered the core of panic disorder.

Title: Effects of 7.5% CO2 challenge in generalized anxiety disorder

Authors: Seddon K, Morris K, Bailey J, Potokar J, Rich A, Wilson S, Bettica P, Nutt DJ.

Journal: J Psychopharmacol. 2011 Jan;25(1):43-51. doi: 10.1177/0269881110364270. Epub 2010 Mar 16. PMID: 20233897.

Link to PubMed: Effects of 7.5% CO2 challenge in generalized anxiety disorder

Abstract: We have previously developed a putative model of generalized anxiety disorder in healthy volunteers using a 20-minute 7.5% carbon dioxide (CO(2)) inhalation challenge. The aim of this study was to validate the 7.5% CO(2) paradigm by assessing its effects in patients with generalized anxiety disorder in a test-retest design. Twelve medication-free generalized anxiety disorder patients attended our lab for two study days. On each study day placebo (compressed air) and 7.5% CO(2) mixture were randomly administered over 20 min, at least 30 min apart, in a single blind, randomized, placebo-controlled cross-over design. Subjective ratings, cardiovascular measures and cortisol levels were collected throughout. CO(2) challenge significantly increased ratings for anxiety and other subjective symptoms associated with generalized anxiety disorder, compared with air. It also significantly increased systolic blood pressure on day 2, indicating increased autonomic arousal. There was no change between the two test days in mean anxiety rating scores, and there also appeared to be a correlation for individual scores on a number of the subjective measures. In conclusion, 20 min of 7.5% CO(2) gas inhalation increases anxiety responses in patients with generalized anxiety disorder, and this is reliable over time.

Title: Respiratory sensations in subjects who lack a ventilatory response to CO2

Authors: Shea SA, Andres LP, Shannon DC, Guz A, Banzett RB.

Journal: Respir Physiol. 1993 Aug;93(2):203-19. doi: 10.1016/0034-5687(93)90006-v. PMID: 8210759.

Link to PubMed: Respiratory sensations in subjects who lack a ventilatory response to CO2

Abstract: An urge to breath is perceived during breath hold and hypercapnia (termed 'air hunger') and during heavy exercise (often termed 'shortness of breath'). To better understand the neural mechanisms responsible for these sensations we studied five patients (8-17 years old) with congenital central hypoventilation syndrome (CCHS) who lack ventilatory response to CO2. CCHS patients reported no respiratory discomfort during CO2 inhalation or during maximal breath hold which was of much longer duration than age-matched controls. However, all 3 CCHS patients who exercised heavily reported some sensations akin to shortness of breath (they increased breathing nearly as much as controls). Our results are consistent with two possibilities. First, the air hunger of hypercapnia and breath hold is caused by projection to the forebrain of respiratory chemoreceptor afferents which bypass the respiratory centers, while exercise shortness of breath is caused by direct projections of limb afferents or locomotory center activity. Second, air hunger and shortness of breath share the same origin--projection of increased brain stem respiratory center motor activity (corollary discharge) to the forebrain.

Title: The interplay between physical activity and anxiety sensitivity in fearful responding to carbon dioxide challenge

Authors: Smits JA, Tart CD, Rosenfield D, Zvolensky MJ.

Journal: Psychosom Med. 2011 Jul-Aug;73(6):498-503. doi: 10.1097/PSY.0b013e3182223b28. Epub 2011 Jun 23. PMID: 21700713; PMCID: PMC3131468.

Link to full text: The interplay between physical activity and anxiety sensitivity in fearful responding to carbon dioxide challenge

Abstract: Objective: Physical activity may confer protective effects in the development of anxiety and its disorders. These effects may be particularly strong among individuals who have elevated levels of anxiety sensitivity (AS; i.e., the fear of somatic arousal), an established cognitive-based risk factor for anxiety and its disorders. The present study performed a laboratory test of the interplay between physical activity and AS.

Methods: The participants were adults free of Axis I psychopathology (n = 145) who completed measures of physical activity and AS before undergoing a recurrent 20% carbon dioxide-enriched air (CO(2)) challenge.

Results: Consistent with the hypothesis, physical activity was significantly related to CO(2) challenge reactivity among persons with elevated levels of AS, at high levels of physical activity (p < .001) but not at low levels of physical activity (p = .90). Also consistent with hypothesis, irrespective of the level of physical activity, physical activity did not relate significantly to CO(2) challenge reactivity among persons with normative levels of AS (p = .28).

Conclusions: These findings provide novel empirical insight into the role that physical activity may play in terms of resiliency for the development of anxiety disorders. Specifically, the protective effects of physical activity may only be evident at higher doses and among persons who are at increased risk of developing anxiety disorders because they have elevated AS.

Title: Unexpected arousal, anxiety sensitivity, and their interaction on CO₂-induced panic: further evidence for the context-sensitivity vulnerability model

Authors: Telch MJ, Harrington PJ, Smits JA, Powers MB.

Journal: J Anxiety Disord. 2011 Jun;25(5):645-53. doi: 10.1016/j.janxdis.2011.02.005. Epub 2011 Mar 3. PMID: 21474277.

Link to PubMed: Unexpected arousal, anxiety sensitivity, and their interaction on CO₂-induced panic: further evidence for the context-sensitivity vulnerability model

Abstract: The present experiment tested several predictions derived from the context-sensitivity vulnerability model of panic. Participants (N=79) scoring either high or low in anxiety sensitivity (AS) and with no history of unexpected panic were randomly assigned to one of two instructional sets: expected arousal (EA) or expected relaxation (ER). All participants were administered inhalation of room air and 35% CO(2) in a counterbalanced order. Consistent with theoretical predictions, High-AS participants who received ER instructions showed greater emotional responding compared to High-AS participants who received EA instructions, while instructional set did not affect responding among Low-AS participants. Panic attacks were observed in 52% of the High-AS-ER group compared to 17%, 5%, and 5% in the High-AS-EA, Low-AS-ER, and Low-AS-EA groups respectively. These findings are consistent with the theory's assertion that dispositional tendencies, such as anxiety sensitivity potentiate the panicogenic effects of threat-relevant context variables.

Title: Emotional reactivity to a single inhalation of 35% carbon dioxide and its association with later symptoms of posttraumatic stress disorder and anxiety in soldiers deployed to Iraq

Authors: Telch MJ, Rosenfield D, Lee HJ, Pai A.

Journal: Arch Gen Psychiatry. 2012 Nov;69(11):1161-8. doi: 10.1001/archgenpsychiatry.2012.8. PMID: 23117637.

Link to PubMed: Emotional reactivity to a single inhalation of 35% carbon dioxide and its association with later symptoms of posttraumatic stress disorder and anxiety in soldiers deployed to Iraq

Abstract: Context: The identification of modifiable predeployment vulnerability factors that increase the risk of combat stress reactions among soldiers once deployed to a war zone offers significant potential for the prevention of posttraumatic stress disorder (PTSD) and other combat-related stress disorders. Adults with anxiety disorders display heightened emotional reactivity to a single inhalation of 35% carbon dioxide (CO(2)); however, data investigating prospective linkages between emotional reactivity to CO(2) and susceptibility to war-zone stress reactions are lacking.

Objective: To investigate the association of soldiers' predeployment emotional reactivity to 35% CO(2) challenge with several indices of subsequent war-zone stress symptoms assessed monthly while deployed in Iraq.

Design, setting, and participants: Prospective cohort study of 158 soldiers with no history of deployment to a war zone were recruited from the Texas Combat Stress Risk Study between April 2, 2007, and August 28, 2009.

Main outcome measures: Multilevel regression models were used to investigate the association between emotional reactivity to 35% CO(2) challenge (assessed before deployment) and soldiers' reported symptoms of general anxiety/stress, PTSD, and depression while deployed to Iraq.

Results: Growth curves of PTSD, depression, and general anxiety/stress symptoms showed a significant curvilinear relationship during the 16-month deployment period. War-zone stressors reported in theater were associated with symptoms of general anxiety/stress, PTSD, and depression. Consistent with the prediction, soldiers' emotional reactivity to a single inhalation of 35% CO(2)-enriched air before deployment significantly potentiated the effects of war-zone stressors on the subsequent development of PTSD symptoms and general anxiety/stress symptoms but not on the development of depression, even after accounting for the effects of trait anxiety and the presence of past or current Axis I mental disorders.

Conclusion: Soldiers' emotional reactivity to a 35% CO(2) challenge may serve as a vulnerability factor for increasing soldiers' risk for PTSD and general anxiety/stress symptoms in response to war-zone stressors.

Title: Objective and subjective measures in recovery from a 35% carbon dioxide challenge

Authors: Niccolai V, van Duinen MA, Griez EJ.

Journal: Can J Psychiatry. 2008 Nov;53(11):737-44. doi: 10.1177/070674370805301105. PMID: 19087467.

Link to PubMed: Objective and subjective measures in recovery from a 35% carbon dioxide challenge

Abstract: Objectives: Because hyperventilation, dyspnea, and a feeling of choking are often core features of a panic attack, respiration has been one of the most widely studied physiological parameters in panic disorder (PD) patients. A respiratory subgroup of PD, with distinct etiological pathways, has also been suggested. Investigation of the recovery phase following a respiratory challenge may be a reliable way to establish respiratory impairment in PD patients. The objective of the present study was to investigate the recovery phase from a 35% carbon dioxide challenge in PD patients and in healthy controls, and to test the hypothesis of a different respiratory pattern in patients, compared to control subjects.

Methods: Eleven nonmedicated PD patients with or without agoraphobia, 11 medicated PD patients, and 11 control subjects took part in a 35% carbon dioxide and 65% oxygen inhalation challenge. Respiratory rate, partial pressure of carbon dioxide, heart rate, and blood pressure were recorded during the baseline phase (10 minutes) and the recovery phase (10 minutes). Visual Analogue Scale of Anxiety and Panic Symptom List scores were collected pre- and post-challenge.

Results: Nonmedicated patients had increased variability in respiratory rate and partial pressure of carbon dioxide during recovery, compared with control subjects and medicated PD patients. Also, PD patients tended to have higher heart rates and to need more time to recover from the challenge than control subjects.

Conclusions: Results suggest that PD patients have less effective homeostatic control after their physiological equilibrium has been disrupted by a respiratory stressor.

Title: Reactivity to a 35% CO2 challenge in healthy first-degree relatives of patients with panic disorder

Authors: Van Beek N, Griez E.

Journal: Biol Psychiatry. 2000 May 1;47(9):830-5. doi: 10.1016/s0006-3223(99)00265-6. PMID: 10812042.

Link to PubMed: Reactivity to a 35% CO2 challenge in healthy first-degree relatives of patients with panic disorder

Abstract: Background: The effects of a 35% CO2 challenge were examined in healthy first-degree relatives of panic disorder patients and in healthy control subjects matched for age and gender.

Methods: One single inhalation of a 35% CO2/65% O2 challenge was administered to 50 first-degree relatives of panic disorder patients and 50 control subjects.

Results: The first-degree relatives were more reactive to the 35% CO2 challenge than the control subjects.

Conclusions: These findings indicate that being a member of a family with a panic disorder patient is, in itself an important factor in CO2 hypersensitivity among subjects who have never experienced a panic attack. Both panic disorder patients and their first-degree relatives have a tendency to be more reactive to the CO2 challenge.

Title: Reliability of the 35% carbon dioxide panic provocation challenge

Authors: Verburg K, Pols H, de Leeuw M, Griez E.

Journal: Psychiatry Res. 1998 May 8;78(3):207-14. doi: 10.1016/s0165-1781(98)00009-2. PMID: 9657425.

Link to PubMed: Reliability of the 35% carbon dioxide panic provocation challenge

Abstract: The objective of this study was to determine the test-retest reliability of the 35% carbon dioxide (CO2) panic provocation challenge. Thirty patients with panic disorder were included in this study. Twenty-four patients were challenged twice, with 1 week between the two challenges. Six patients dropped out after the first test. The 35% CO2 challenge appeared to have a good test-retest reliability; both on induced subjective anxiety, measured on a Visual Analogue Scale for Anxiety (VAS-A), and induced panic symptoms, measured with a Panic Symptom List. Assessing the state of anxiety immediately after the challenge gave the most reliable results. Calculating increase in anxiety from the pre- and post-scores on the VAS-A rendered less reliable scores. This study completes a series of studies in which the criteria for an ideal model of panic are tested for the 35% CO2 challenge. Apart from an absolute specificity for panic disorder, the challenge meets these criteria.

Title: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Authors: Ziemann AE, Allen JE, Dahdaleh NS, Drebot II, Coryell MW, Wunsch AM, Lynch CM, Faraci FM, Howard MA 3rd, Welsh MJ, Wemmie JA.

Journal: Cell. 2009 Nov 25;139(5):1012-21. doi: 10.1016/j.cell.2009.10.029. PMID: 19945383; PMCID: PMC2808123.

Link to full text: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Abstract: The amygdala processes and directs inputs and outputs that are key to fear behavior. However, whether it directly senses fear-evoking stimuli is unknown. Because the amygdala expresses acid-sensing ion channel-1a (ASIC1a), and ASIC1a is required for normal fear responses, we hypothesized that the amygdala might detect a reduced pH. We found that inhaled CO(2) reduced brain pH and evoked fear behavior in mice. Eliminating or inhibiting ASIC1a markedly impaired this activity, and localized ASIC1a expression in the amygdala rescued the CO(2)-induced fear deficit of ASIC1a null animals. Buffering pH attenuated fear behavior, whereas directly reducing pH with amygdala microinjections reproduced the effect of CO(2). These data identify the amygdala as an important chemosensor that detects hypercarbia and acidosis and initiates behavioral responses. They also give a molecular explanation for how rising CO(2) concentrations elicit intense fear and provide a foundation for dissecting the bases of anxiety and panic disorders.

Title: The effects of offset control over 20% carbon-dioxide-enriched air on anxious responding

Authors: Zvolensky MJ, Eifert GH, Lejuez CW, McNeil DW.

Journal: J Abnorm Psychol. 1999 Nov;108(4):624-32. doi: 10.1037//0021-843x.108.4.624. PMID: 10609427.

Link to PubMed: The effects of offset control over 20% carbon-dioxide-enriched air on anxious responding

Abstract: Control over the offset of repeated administrations of 20% carbon-dioxide-enriched air was assessed in nonclinical participants (n = 30) reporting elevated levels of anxiety sensitivity--a population at an increased risk for experiencing panic attacks and possibly developing panic disorder. In Phase I, participants were randomly assigned to 1 of 2 conditions: one that permitted offset control over gas inhalation and one that did not. These conditions were reversed in Phase II. Across phases, a lack of offset control resulted in greater self-reported anxiety compared with having control, although no significant differences were observed for heart rate. Whereas all participants demonstrated a Stroop interference effect for general (e.g., coffin) compared with specific (e.g., dizzy) physical threat word types prior to the first experimental phase, this effect persisted only for participants who had offset control in Phase I. We discuss these results in relation to the differential effects of offset control, with implications for better understanding anxious responding during elevated bodily arousal.

Title: Offset control during recurrent 20% carbon dioxide-enriched air induction: relation to individual difference variables

Authors: Zvolensky MJ, Eifert GH, Lejuez CW.

Journal: Emotion. 2001 Jun;1(2):148-65. doi: 10.1037/1528-3542.1.2.148. PMID: 12899194.

Link to PubMed: Offset control during recurrent 20% carbon dioxide-enriched air induction: relation to individual difference variables

Abstract: Although control over aversive events maintains a central role in contemporary models of anxiety pathology, particularly panic disorder, there is little understanding about the emotional consequences of specific types of control processes. In the present study, offset control over 8 20% carbon dioxide-enriched air administrations was experimentally manipulated in a large nonclinical population (n = 96) varying in anxiety sensitivity (high or low) and gender. Dependent measures included self-reported anxiety, affective reports of valence, arousal, emotional control, and physiological indices of heart rate and skin conductance. High anxiety-sensitive participants who lacked offset control reported significantly greater elevations in self-reported anxiety, emotional displeasure, arousal, and dyscontrol relative to their yoked counterparts with offset control. In contrast, low anxiety-sensitive individuals responded with similar levels of cognitive and affective distress regardless of the offset control manipulation. Although the provocation procedure reliably produced bodily arousal relative to baseline, at a physiological level of analysis, no significant differences emerged across conditions. These findings are discussed in relation to offset control during recurrent interoceptive arousal, with implications for better understanding anxiety about abrupt bodily sensations.

Title: Cognitive therapy versus interoceptive exposure as treatment of panic disorder without agoraphobia

Authors: Arntz A.

Journal: Behav Res Ther. 2002 Mar;40(3):325-41. doi: 10.1016/s0005-7967(01)00014-6. PMID: 11863242.

Link to PubMed: Cognitive therapy versus interoceptive exposure as treatment of panic disorder without agoraphobia

Abstract: Cognitive therapy (CT) and interoceptive exposure (IE) as treatments of panic disorder without agoraphobia were compared in a sample of 69 patients, randomly allocated to condition. There were no significant differences between treatments as to reductions in panic frequency, daily anxiety levels and a composite questionnaire score, at posttest after the 12-session treatment, and at both follow-ups (4 weeks, 6 months). In both conditions, high percentages of patients were panic free at post and follow-up tests (range 75-92%). Although the reduction in idiosyncratic beliefs about the catastrophic nature of bodily sensations was equally strong in both conditions, post-treatment beliefs correlated strongly with symptoms at post and follow-up tests in the CT condition, but not in the IE condition. Reduction of beliefs may be essential in CT, but not in IE. This suggests that the two treatments utilize different change mechanisms.

Title: Anxiety sensitivity and interoceptive exposure: a transdiagnostic construct and change strategy

Authors: Boswell JF, Farchione TJ, Sauer-Zavala S, Murray HW, Fortune MR, Barlow DH.

Journal: Behav Ther. 2013 Sep;44(3):417-31. doi: 10.1016/j.beth.2013.03.006. Epub 2013 Apr 2. PMID: 23768669; PMCID: PMC3727659.

Link to full text: Anxiety sensitivity and interoceptive exposure: a transdiagnostic construct and change strategy

Abstract: Recent findings support the relevance of anxiety sensitivity (AS) and interoceptive exposure (IE) across emotional disorders. This study (a) evaluated levels of AS across different anxiety disorders, (b) examined change in AS over the course of transdiagnostic psychological intervention, and its relationship with outcome, and (c) described the implementation of IE to address AS with patients with different anxiety disorders. Participants (N=54) were patients who received treatment with the Unified Protocol for Transdiagnostic Treatment of Emotional Disorders (UP) in two consecutive treatment trials. Participants completed a measure of AS at pre- and posttreatment, and multiple occasions during treatment. Symptom severity was assessed at pre- and posttreatment, and clinical information related to physical symptoms and IE were collected as part of routine clinical practice. Elevated AS was observed at pretreatment across diagnoses and decreases in AS were observed from pre- to posttreatment. Similar changes occurred across the diagnostic categories, notably coinciding with the introduction of IE. Change in AS was correlated with reduced symptom levels at posttreatment and 6-month follow-up. Patients with different anxiety disorders endorsed similar physical symptoms and practiced similar IE exercises with similar effects. Results provide preliminary support for the usefulness of IE as a treatment strategy across the spectrum of anxiety disorders, and additional support for the transdiagnostic relevance of AS.

Title: Efficacy of interoceptive exposure therapy combined with trauma-related exposure therapy for posttraumatic stress disorder: a pilot study

Authors: Wald J, Taylor S.

Journal: J Anxiety Disord. 2007;21(8):1050-60. doi: 10.1016/j.janxdis.2006.10.010. Epub 2007 Jan 10. PMID: 17270392.

Link to PubMed: Efficacy of interoceptive exposure therapy combined with trauma-related exposure therapy for posttraumatic stress disorder: a pilot study

Abstract: The aim of this case series was to examine efficacy of interoceptive exposure (IE) combined with trauma-related exposure therapy (TRE) for posttraumatic stress disorder (PTSD). Seven participants completed treatment consisting of four weekly sessions of IE followed by eight weekly sessions of TRE (four sessions of imaginal exposure and four sessions of in vivo exposure). Assessments were conducted at pretreatment, posttreatment, 1- and 3-month follow-up. Outcome measures included PTSD symptoms, anxiety sensitivity, posttraumatic cognitions, anxiety, and depression. Five of the seven participants showed pre- to posttreatment improvements on these measures, and two participants showed less symptom reduction. Results at the 1-month follow-up showed that treatment gains were generally maintained in five (of the seven) participants and four of these individuals no longer met PTSD diagnostic criteria. Four individuals completed the 3-month follow-up and their symptoms and diagnostic status remained unchanged. These preliminary findings are promising. The next step in this line of research is to conduct a randomized, controlled trial to further to examine the efficacy, tolerability, and mechanisms of using IE in the treatment of PTSD.

Title: Responses to interoceptive exposure in people with posttraumatic stress disorder (PTSD): a preliminary analysis of induced anxiety reactions and trauma memories and their relationship to anxiety sensitivity and PTSD symptom severity

Authors: Wald J, Taylor S.

Journal: Cogn Behav Ther. 2008;37(2):90-100. doi: 10.1080/16506070801969054. PMID: 18470740.

Link to PubMed: Responses to interoceptive exposure in people with posttraumatic stress disorder (PTSD): a preliminary analysis of induced anxiety reactions and trauma memories and their relationship to anxiety sensitivity and PTSD symptom severity

Abstract: A growing body of evidence suggests that anxiety sensitivity (AS; fear of arousal-related sensations) plays a role in posttraumatic stress disorder (PTSD). Consistent with this, evidence indicates that interoceptive exposure (IE), which is a method for reducing AS, reduces PTSD symptoms. Clinical observations from our treatment studies indicate that IE triggers both anxiety and trauma memories in people with PTSD. The primary aim of this study was to describe the anxiety responses to a series of IE exercises and to examine whether or not trauma memories were activated. A secondary aim was to explore the relationships among AS, PTSD symptom severity, and IE responses. Data were collected from 23 people with PTSD who completed measures of PTSD symptoms and AS and a standardized battery of 10 IE exercises. Elevated anxiety and strong arousal responses were frequently elicited by the exercises, and trauma memories were also frequently triggered. AS and IE-triggered trauma memories significantly predicted IE-induced peak anxiety. The implications of the findings are discussed in terms of how IE might exert its therapeutic effects in the treatment of PTSD.

Title: Response to repeated CO2 in individuals with elevated anxiety sensitivity: replication with 20% CO2

Authors: Beck JG, Wolf MS.

Journal: J Behav Ther Exp Psychiatry. 2001 Mar;32(1):1-16. doi: 10.1016/s0005-7916(01)00018-0. PMID: 11729942.

Link to PubMed: Response to repeated CO2 in individuals with elevated anxiety sensitivity: replication with 20% CO2

Abstract: The present report replicates and extends previous work examining response patterns to repeated presentation of CO2. In previous studies, two distinct response patterns to repeated presentation of 35% CO2 were noted, representing habituation and nonhabituation of anxiety. In this report, 21 participants who had never experienced a panic attack but who reported high levels of anxiety sensitivity were presented with 12 trials of 20% CO2, followed by a trial involving inhalation of room air (to examine dishabituation) and two more trials of 20% CO2. Results indicated that 67% of the sample reported habituation of anxiety. Reductions in anxiety across inhalations were paralleled by changes in tidal volume, perceived panic symptom severity, and feelings of panic. Notable dishabituation was observed in the nonhabituation sample. Results are discussed in the light of basic learning processes underlying the treatment of Panic Disorder (PD).

Title: How does interoceptive exposure for panic disorder work? An uncontrolled case study

Authors: Beck JG, Shipherd JC, Zebb BJ.

Journal: Beck JG, Shipherd JC, Zebb BJ.

Link to PubMed: How does interoceptive exposure for panic disorder work? An uncontrolled case study

Abstract: To examine the influence of interoceptive exposure (IE) when used alone in the treatment of Panic Disorder (PD), 17 PD patients were presented with six IE sessions, using 35% CO2 as the exposure medium. The data indicate that IE alone is effective in reducing panic, panic-related fears, and general anxiety. However, the positive effects of IE do not appear to extend to agoraphobia, related fears, or depressed mood. Two distinct within-session patterns of fear response to IE were noted, one indicating habituation and the other indicating a lack of fear reduction. Although both patterns were associated with reductions in panic and anxiety following IE, the Habituators appeared to have a more positive outcome, which occurred more rapidly. These data suggest that IE may operate via two different pathways. Implications for understanding fear reduction are discussed, along with directions for future study.

Title: Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? A preliminary report

Authors: Beck JG, Shipherd JC.

Journal: Behav Res Ther. 1997 Jun;35(6):551-7. doi: 10.1016/s0005-7967(97)00010-7. PMID: 9159979.

Link to PubMed: Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? A preliminary report

Abstract: In order to explore the mechanism of action of interoceptive exposure, 12 Panic Disorder (PD) patients were presented with two sessions of repeated CO2 inhalation. Two distinct patterns of responding were noted. The first pattern was described as habituation of fear (n = 6). These patients showed decrements in pre- and post-inhalation anxiety during both sessions (with more rapid decline during session 2), as well as spontaneous recovery of fear at the onset of session 2. The second pattern indicated fear sensitization. These patients showed relatively low levels of anticipatory anxiety preceding CO2 inhalation during both sessions but reported robust increases in fear following gas inhalation. The extent of this increase was slightly less during session 2 relative to session I and did not appear to be mediated by cardiovascular arousal, as both groups showed rapid HR habituation during both sessions. Results are discussed in light of current theories of PD and its treatment.

Title: Fearful responding to repeated CO2 inhalation: a preliminary investigation

Authors: Beck JG, Shipherd JC, Zebb BJ.

Journal: Behav Res Ther. 1996 Aug;34(8):609-20. doi: 10.1016/0005-7967(96)00039-3. PMID: 8870287.

Link to PubMed: Fearful responding to repeated CO2 inhalation: a preliminary investigation

Abstract: In an effort to explore factors which maintain fear of physical sensations, repeated administration of 35% CO2 was used with college students scoring high and low on the Anxiety Sensitivity Index. Half of each group was administered 12 CO2 trials, while the other half received 9 CO2 trials, followed by a dishabituation trial (Trial 10) and 2 more CO2 administrations (Trials 11 and 12). Measures included subjective anxiety, heart rate, skin conductance, and number of panic symptoms reported. Results indicated a nonsignificant trend for the High ASI group to show increased pre-inhalation anxiety across trials, while the Low ASI group showed a rapid reduction in pre-inhalation anxiety. Post-inhalation skin conductance mirrored this pattern, although rapid reduction in post-inhalation heart rate was observed. Overall, the High ASI participants showed a notable lack of fear reduction across trials. Results are discussed in light of sensitization as a factor contributing to anticipatory anxiety, with implications for understanding the etiology and maintenance of Panic Disorder.

Title: Response patterns to repeated CO2 inhalation in individuals with high anxiety sensitivity

Authors: Beck JG, Shipherd JC, Read J.

Journal: Behav Res Ther. 1999 Nov;37(11):1073-89. doi: 10.1016/s0005-7967(98)00196-x. PMID: 10500321.

Link to PubMed: Response patterns to repeated CO2 inhalation in individuals with high anxiety sensitivity

Abstract: The present report extends previous work which has documented two distinct response patterns to repeated presentation of interoceptive cues (using CO2 inhalation) in PD patients [Beck, J. G. & Shipherd, J. C. (1997). Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? Behaviour Research and Therapy, 35, 551-557]. We were interested in determining if these two patterns of fear habituation and sensitization would be noted in panic-naive individuals who reported high levels of Anxiety Sensitivity. A second aspect of this report examined whether attention to bodily sensations versus to neutral material would impact fear habituation and sensitization. Participants included 43 panic-naive individuals who scored at least 1 standard deviation above norms on the Anxiety Sensitivity Index. Results indicated that 37% of the sample reported habituation of fear, 47% reported fear sensitization and 16% demonstrated relatively stable fear levels across 12 inhalations of CO2 during session 1. The attentional manipulation did not exert a pronounced influence on anxiety, panic symptom severity, skin conductance, or heart rate in either Habituators or Sensitizers during session 2. These results are discussed in light of their relevance in understanding fundamental psychopathological processes underlying Panic Disorder.

Title: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Biol Psychol. 2010 Apr;84(1):104-11. doi: 10.1016/j.biopsycho.2010.01.002. Epub 2010 Jan 12. PMID: 20064582; PMCID: PMC2891596.

Link to full text: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Abstract: Inhalation of carbon dioxide (CO₂) enriched air triggers anxiety in panic disorder (PD) patients, which is often interpreted as a sign of biological vulnerability. However, most studies have not measured respiration in these tasks. We compared patients with PD (n=20) and social phobia (SP, n=19) to healthy controls (n=18) during eight inhalations of 20% CO₂, preceded and followed by two inhalations of room air, while continuously measuring subjective anxiety and dyspnea as well as autonomic and respiratory variables. PD patients showed increased reactivity and delayed recovery during CO₂ inhalations for most measures. Unlike both other groups, the PD group's tidal volume responses did not habituate across CO₂ inhalations. However, PD patients did not differ from SP patients on most other measures, supporting a continuum model of CO₂ sensitivity across anxiety disorders. Both patient groups showed continued reactivity during the last air inhalations, which is unlikely to be due to a biological sensitivity.

Keywords: anxiety disorder; biological challenge; carbon dioxide; habituation; psychophysiology; sensitization.

Title: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Authors: Forsyth JP, Lejuez CW, Finlay C.

Journal: J Behav Ther Exp Psychiatry. 2000 Jun;31(2):103-21. doi: 10.1016/s0005-7916(00)00014-8. PMID: 11132115.

Link to PubMed: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Abstract: Increasingly carbon dioxide-enriched air is being used as an aversive unconditioned stimulus in laboratory examinations of anxiety. Yet, little is known about the stability of the autonomic and subjective effects of this stimulus across repeated inhalations and sessions. We examined whether repeated administrations of high concentrations of CO2-enriched air produced either habituation, stability, or sensitization across several autonomic and self-report indices within one session (Experiment 1) and then several sessions (Experiment 2) of exposure. Results suggest that non-clinical participants do not habituate to CO2 within sessions, but do show habituation on cardiac and subjective report of anxiety across sessions. Individual difference factors such as anxiety sensitivity and suffocation fear seem to moderate some of these effects, including self-reported distress and anxiety in response to the challenge. These results support the use of CO2 as a panicogenic aversive stimulus in laboratory models of fear onset and in clinical settings for interoceptive exposure treatments of panic.

Title: The effect of successful treatment on the emotional and physiological response to carbon dioxide inhalation in patients with panic disorder

Authors: Gorman JM, Martinez J, Coplan JD, Kent J, Kleber M.

Journal: Biol Psychiatry. 2004 Dec 1;56(11):862-7. doi: 10.1016/j.biopsych.2004.08.016. PMID: 15576063.

Link to PubMed: The effect of successful treatment on the emotional and physiological response to carbon dioxide inhalation in patients with panic disorder

Abstract: Background: A number of studies have shown that patients with panic disorder are more likely to have panic attacks during carbon dioxide inhalation than are normal comparison subjects. Some studies have shown that antipanic medications can reduce the anxiogenic response to carbon dioxide, but none have shown if this is the case for cognitive behavioral therapy or if successful treatment reduces the respiratory physiologic response to carbon dioxide.

Methods: Twenty-five patients with panic disorder and 13 normal comparison subjects underwent baseline testing with 5% and 7% carbon dioxide inhalation. The patients were then retested after at least 12 weeks of treatment with either antipanic medication or cognitive behavioral therapy. Comparison subjects were retested after a similar interval.

Results: Successful treatment resulted in lower panic rates, and reduced anxiogenic response. Treatment had no effect, however, on the respiratory physiologic response.

Conclusions: There is dissociation in treatment response between the subjective and objective responses to carbon dioxide inhalation in panic disorder patients, with the former but not the latter showing positive change. We hypothesize that the strengthening of higher cortical control over subcortical fear-related structures, whether via medication or cognitive behavioral therapy treatment, results in less anxiety and fear in response to provoked symptoms reminiscent of naturally occurring panic.

Title: Effect of Antipanic Treatment on Response to Carbon Dioxide

Authors: Gorman JM, Browne ST, Papp LA, Martinez J, Welkowitz L, Coplan JD, Goetz RR, Kent J, Klein DF.

Journal: Biol Psychiatry. 1997 Dec 1;42(11):982-91. doi: 10.1016/s0006-3223(97)00160-1. PMID: 9386849.

Link to full text: Effect of Antipanic Treatment on Response to Carbon Dioxide

Abstract: Background: Disordered breathing among patients with panic disorder, including hyperventilation during attacks and increased anxiogenic response to carbon dioxide (CO2) inhalation, is well established. We wished to assess whether there is a change in the physiological response to CO2 after patients have undergone antipanic therapy with either tricyclic antidepressants or cognitive behavioral therapy (CBT).

Methods: Twenty-nine patients with panic disorder underwent baseline CO2 sensitivity testing using the traditional Read rebreathing method and then received either antidepressant treatment (n = 21) or CBT (n = 8). After completing treatment, CO2 testing was repeated. A comparison sample of 14 normal volunteers also had two CO2 sensitivity tests, separated by an average of 21.6 (SD = 8.8) weeks.

Results: Using a liberal standard, in which all CO2 sensitivity tests whose correlations between minute ventilation and end-tidal CO2 were at least .75 were used, patients, but not controls, demonstrated a significant reduction in CO2 sensitivity between the first and second test. Using a more conservative .90 correlation standard reduced the sample size available and resulted in trend reduction in patients but no significant change in controls. There was a suggestion that the change was most pronounced in treatment responders, although the number of patient nonresponders is extremely small in this sample.

Conclusions: These data indicate that treatment reduces CO2 sensitivity in patients with panic disorder. We speculate that manipulation of the serotonergic and noradrenergic neurotransmission systems, both known to play a role in the control of respiration, may have a specific effect in reducing respiratory hyperactivity in panic disorder.

Title: CO2 inhalation in the treatment of panic attacks

Authors: Griez E, van den Hout MA.

Journal: Behav Res Ther. 1986;24(2):145-50. doi: 10.1016/0005-7967(86)90085-9. PMID: 3083806.

Link to PubMed: CO2 inhalation in the treatment of panic attacks

Abstract: If panic disorder is conceptualized as a fear of interoceptive sensations, it can be treated by exposure to the autonomic symptoms that are induced by CO2 inhalation. In a cross-over study of 14 patients, propranolol was used as a control treatment. CO2 produced a significant reduction of fear of autonomic panic sensations, state and trait anxiety, agoraphobic anxiety and avoidance, and of the frequency of panicking. Propranolol resulted in improvements of state and of agoraphobic anxiety. Except for Trait Anxiety, CO2 was superior on all measures, most clearly on the Fear of Autonomic Sensations. At the 6-month follow-up, 3 patients required further treatment. Except for State Anxiety, the improvements were, on average, maintained. The clinical and theoretical implications are discussed.

Title: The relationship between the effect of lactate infusion on anxiety states, and their amelioration by carbon dioxide inhalation

Authors: Haslam MT.

Journal: Br J Psychiatry. 1974 Jul;125(0):88-90. doi: 10.1192/bjp.125.1.88. PMID: 4855036.

Link to PubMed: The relationship between the effect of lactate infusion on anxiety states, and their amelioration by carbon dioxide inhalation

Abstract: Anxiety is perhaps one of the commonest and most disabling symptoms in medicine, yet its satisfactory alleviation has been elusive. Work by Kluver and Bucy (1939) suggested that the amygdala generated anxiety, the function of the hippocampus being to decrease it. The hypothalamus appears to organize the level of arousal through the amygdala.

Title: Augmented respiratory relief. A new use of CO2 therapy in the treatment of phobic conditions: a preliminary report on two cases

Authors: Orwin A.

Journal: Br J Psychiatry. 1973 Feb;122(567):171-3. doi: 10.1192/bjp.122.2.171. PMID: 4714831.

Link to PubMed: Augmented respiratory relief. A new use of CO2 therapy in the treatment of phobic conditions: a preliminary report on two cases

Abstract: Respiratory relief (R.R.) appeared to be a simple and rapid treatment for specific phobias and for the phobic components of more complex neurotic conditions (Orwin, 1971). The basic hypothesis was that the R.R., satisfying an intense need to breathe, if simultaneous with the presentation of a phobic stimulus, would inhibit the degree of anxiety previously provoked and that this inhibition would persist. The need to breathe was induced by voluntary restriction up to maximum voluntary respiratory arrest (M.V.R.A.). As in conventional desensitization (Wolpe, 1958), a hierarchy was drawn up and the patients were desensitized using R.R. instead of relaxation. Carbon dioxide-oxygen mixtures were introduced because CO2 is known to stimulate respiration, and it was expected that by intensifying the need to breathe the speed and effectiveness of treatment would be increased by the augmented respiratory relief (A.R.R.).

Title: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Authors: Perna G, Bertani A, Caldirola D, Gabriele A, Cocchi S, Bellodi L.

Journal: J Clin Psychopharmacol. 2002 Jun;22(3):300-8. doi: 10.1097/00004714-200206000-00011. PMID: 12006901.

Link to PubMed: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Abstract: Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

Title: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Authors: Schmidt NB, Trakowski JH, Staab JP.

Journal: J Abnorm Psychol. 1997 Nov;106(4):630-8. doi: 10.1037//0021-843x.106.4.630. PMID: 9358693.

Link to PubMed: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Abstract: Inhalations of high concentrations of carbon dioxide (CO2) reliably produce panic attacks in patients with panic disorder. The present study evaluated whether cognitive-behavioral treatment (CBT) for panic disorder would extinguish CO2-induced panic and whether changes in panic and arousal-related cognitions were associated with the induction of panic. Patients with panic disorder (N = 54) were assigned to 1 of 3 experimental conditions: CBT with respiratory training (CBT-R), CBT without respiratory training (CBT), or delayed treatment. Participants received 5 repeated vital-capacity inhalations of 35% CO2/65% O2 prior to and following either 12 treatment sessions or a 12-week waiting period. During pretreatment assessments, 74% of patients experienced a panic attack during at least 1 inhalation. At posttreatment, only 20% of treated participants (CBT-R = 19%, CBT = 22%), compared with 64% of untreated participants, panicked. Forty-four percent of treated participants, compared with 0% of untreated participants, reported no anxiety during all posttreatment inhalations. Anxiety sensitivity as well as panic appraisals regarding the likelihood of panic and self-efficacy with coping with panic were significantly related to fearful responding to the CO2 challenge.

Title: The effects of inhaling a 35 per cent CO2-65 per cent O2 mixture upon anxiety level in neurotic patients

Authors: Slater SL, Leavy A.

Journal: Behav Res Ther. 1966 Nov;4(4):309-16. doi: 10.1016/0005-7967(66)90027-1. PMID: 5978686.

Link to PubMed: The effects of inhaling a 35 per cent CO2-65 per cent O2 mixture upon anxiety level in neurotic patients

Abstract: Wolpe has reported the use of a 35 per cent CO2-65 per cent O2 mixture to relax patients who cannot otherwise be relaxed sufficiently to allow desensitization. In this investigation, the anxiety-reducing effects of single inhalations of a 35 per cent CO2-65 per cent O2 were studied on twelve patients, all of whom had been diagnosed by their doctors as having a high level of anxiety. On the basis of the subjects' ratings, this mixture was more effective than was an equal volume of air which was breathed through the same apparatus as was the experimental mixture. Evidence is offered which suggests that the decrement in anxiety which followed administration of the 35 per cent CO2-65 per cent O2 mixture was due to some effect of the gas other than the hyperventilation which it induces.

Title: Mechanism of change in cognitive-behavioral treatment of panic disorder: evidence for the fear of fear mediational hypothesis

Authors: Smits JA, Powers MB, Cho Y, Telch MJ.

Journal: J Consult Clin Psychol. 2004 Aug;72(4):646-52. doi: 10.1037/0022-006X.72.4.646. PMID: 15301649.

Link to PubMed: Mechanism of change in cognitive-behavioral treatment of panic disorder: evidence for the fear of fear mediational hypothesis

Abstract: Numerous clinical trials have demonstrated the efficacy of cognitive-behavioral treatment (CBT) for panic disorder. However, studies investigating the mechanisms responsible for improvement with CBT are lacking. The authors used regression analyses outlined by R. M. Baron and D. A. Kenny (1986) to test whether a reduction in fear of fear (FOF) underlies improvement resulting from CBT. Pre- and posttreatment measures were collected from 90 CBT-treated patients and 40 wait-list control participants. Overall, treatment accounted for 31% of the variance in symptom reduction. The potency of FOF as a mediator varied as a function of symptom facet, as full mediation was observed for the change in global disability, whereas the effects of CBT on agoraphobia, anxiety, and panic frequency were partially accounted for by reductions in FOF. Clinical implications and future research directions are discussed.

Title: Reduction of CO2-induced anxiety in patients with panic attacks after repeated CO2 exposure

Authors: van den Hout MA, van der Molen GM, Griez E, Lousberg H, Nansen A.

Journal: Am J Psychiatry. 1987 Jun;144(6):788-91. doi: 10.1176/ajp.144.6.788. PMID: 3109265.

Link to PubMed: Reduction of CO2-induced anxiety in patients with panic attacks after repeated CO2 exposure

Abstract: The authors compared the subjective reaction of 13 panic patients and eight control subjects to a 35% CO2 challenge, a treatment known to produce physical symptoms comparable to those of natural or lactate-induced panic, and to placebo treatment (inhalation of air). They found that patients had higher placebo scores than control subjects, patients tended to get highly anxious on CO2 and control subjects did not, and CO2-induced subjective anxiety in patients decreased as the number of CO2-induced exposures to interoceptive anxiety symptoms increased. The data support a behavioral account of the effects of anxiogenics.

Title: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Authors: Wolpe J.

Journal: J Nerv Ment Dis. 1987 Mar;175(3):129-33. doi: 10.1097/00005053-198703000-00001. PMID: 3102688.

Link to PubMed: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Abstract: A lucky chance more than 30 years ago revealed the remarkable efficacy of single inhalations of high concentrations of carbon dioxide in eliminating or markedly reducing free-floating anxiety. The reduction of anxiety lasts for days, weeks, or longer--well beyond the persistence of carbon dioxide in the body. The effects are explicable on the hypothesis that free-floating anxiety is anxiety conditioned to continuously present sources of stimulation, such as background noise or the awareness of space or time, and that the anxiety response habit is weakened when the anxiety is inhibited by the competition of responses that carbon dioxide induces. More recently, it has become apparent that inhalations of carbon dioxide, applied in a different manner, are effective in overcoming maladaptive anxiety responses to specific stimuli, e.g., social stimuli. The substance is also proving to be a valuable resource in the treatment of the common variety of panic attacks.

Title: Comparison of the panicogenic effect of cholecystokinin 30-33 and carbon dioxide in panic disorder

Authors: Bradwejn J, Koszycki D.

Journal: Prog Neuropsychopharmacol Biol Psychiatry. 1991;15(2):237-9. doi: 10.1016/0278-5846(91)90086-g. PMID: 1908105.

Link to PubMed: Comparison of the panicogenic effect of cholecystokinin 30-33 and carbon dioxide in panic disorder

Abstract: 1. Twenty-two patients who met DSM-III-R criteria for panic disorder received either cholecystokinin 30-33 (25 micrograms i.v.) or 33% carbon dioxide. 2. The principal outcome measures of the study included the number and sum intensity of panic symptoms and the incidence of panic attacks. 3. The incidence of panic attacks tended (P = .07) to be higher with cholecystokinin 30-33 than with carbon dioxide. Nevertheless, patients who panicked within each group did not differ significantly with regard to the number and sum intensity of symptoms or symptom profile. 4. That cholecystokinin 30-33 and 35% carbon dioxide induced panic attacks which were qualitatively and quantitatively similar suggest that these agents might act on distinct systems that have a final common target or a final common mechanism of action.

Title: Restoring Acid-sensing ion channel-1a in the amygdala of knock-out mice rescues fear memory but not unconditioned fear responses

Authors: Coryell MW, Wunsch AM, Haenfler JM, Allen JE, McBride JL, Davidson BL, Wemmie JA.

Journal: J Neurosci. 2008 Dec 17;28(51):13738-41. doi: 10.1523/JNEUROSCI.3907-08.2008. PMID: 19091964; PMCID: PMC2651157.

Link to full text: Restoring Acid-sensing ion channel-1a in the amygdala of knock-out mice rescues fear memory but not unconditioned fear responses

Abstract: Acid-sensing ion channel-1a (ASIC1a) contributes to multiple fear behaviors, however the site of ASIC1a action in behavior is not known. To explore a specific location of ASIC1a action, we expressed ASIC1a in the basolateral amygdala of ASIC1a-/- mice using viral vector-mediated gene transfer. This rescued context-dependent fear memory, but not the freezing deficit during training or the unconditioned fear response to predator odor. These data pinpoint the basolateral amygdala as the site where ASIC1a contributes to fear memory. They also discriminate fear memory from fear expressed during training and from unconditioned fear. Furthermore, this work illustrates a strategy for identifying discrete brain regions where specific genes contribute to complex behaviors.

Title: Baseline respiratory parameters in panic disorder: a meta-analysis

Authors: Grassi M, Caldirola D, Vanni G, Guerriero G, Piccinni M, Valchera A, Perna G.

Journal: J Affect Disord. 2013 Apr 5;146(2):158-73. doi: 10.1016/j.jad.2012.08.034. Epub 2012 Oct 27. PMID: 23107756.

Link to PubMed: Baseline respiratory parameters in panic disorder: a meta-analysis

Abstract: Background: The presence of abnormalities in baseline respiratory function of subjects with panic disorder (PD) is expected according to PD respiratory theories. We aimed to meta-analyze results from studies comparing baseline respiratory and hematic parameters related to respiration between subjects with PD and controls.

Methods: A literature research in bibliographic databases was performed. Fixed-effects models were applied for all parameters while random-effects models only when suitable (at least 10 independent studies). Several moderator analyses and publication bias diagnostics were performed.

Results: We found significantly higher mean minute ventilation and lower et-pCO(2) in subjects with PD than controls. Moreover we also found evidences of reduced HCO(3)(-) and PO(4)(-) hematic concentrations, higher indexes of respiratory variability/irregularity and higher rate of sighs and apneas. Evidence of heterogeneity was partly explained by moderator analyses. No relevant publication bias was found.

Limitations: Several shortcomings affected the included studies, such as over-inclusive recruitment criteria, samples unbalanced for socio-demographic characteristics, lack of statistical details and small number of studies available for several parameters.

Discussion: Our results support the idea of abnormalities in respiratory function of subjects with PD. Compared to controls, they showed baseline hyperventilation; the results from hematic parameters suggest that hyperventilation may be chronic and not simply caused by their high anxiety levels during respiratory assessment. Evidences of higher variability and irregularity in respiratory patterns of subjects with PD were also found. It is unclear to what extent the higher rate of sighs and apneas may explain the other baseline respiratory abnormalities found in PD.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Neural pathways underlying lactate-induced panic

Authors: Johnson PL, Truitt WA, Fitz SD, Lowry CA, Shekhar A.

Journal: Neuropsychopharmacology. 2008 Aug;33(9):2093-107. doi: 10.1038/sj.npp.1301621. Epub 2007 Dec 5. PMID: 18059441; PMCID: PMC3065200.

Link to full text: Neural pathways underlying lactate-induced panic

Abstract: Panic disorder is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as 0.5 M sodium lactate infusions. Although studied for four decades, the mechanism of lactate sensitivity in panic disorder has not been understood. The dorsomedial hypothalamus/perifornical region (DMH/PeF) coordinates rapid mobilization of behavioral, autonomic, respiratory and endocrine responses to stress, and rats with disrupted GABA inhibition in the DMH/PeF exhibit panic-like responses to lactate, similar to panic disorder patients. Utilizing a variety of anatomical and pharmacological methods, we provide evidence that lactate, via osmosensitive periventricular pathways, activates neurons in the compromised DMH/PeF, which relays this signal to forebrain limbic structures such as the bed nucleus of the stria terminalis to mediate anxiety responses, and specific brainstem sympathetic and parasympathetic pathways to mediate the respiratory and cardiovascular components of the panic-like response. Acutely restoring local GABAergic tone in the DMH/PeF blocked lactate-induced panic-like responses. Autonomic panic-like responses appear to be a result of DMH/PeF-mediated mobilization of sympathetic responses (verified with atenolol) and resetting of the parasympathetically mediated baroreflex. Based on our findings, DMH/PeF efferent targets such as the C1 adrenergic neurons, paraventricular hypothalamus, and the central amygdala are implicated in sympathetic mobilization; the nucleus of the solitary tract is implicated in baroreflex resetting; and the parabrachial nucleus is implicated in respiratory responses. These results elucidate neural circuits underlying lactate-induced panic-like responses and the involvement of both sympathetic and parasympathetic systems.

Title: A key role for orexin in panic anxiety

Authors: Johnson PL, Truitt W, Fitz SD, Minick PE, Dietrich A, Sanghani S, Träskman-Bendz L, Goddard AW, Brundin L, Shekhar A.

Journal: Nat Med. 2010 Jan;16(1):111-5. doi: 10.1038/nm.2075. Epub 2009 Dec 27. PMID: 20037593; PMCID: PMC2832844.

Link to full text: A key role for orexin in panic anxiety

Abstract: Panic disorder is a severe anxiety disorder with recurrent, debilitating panic attacks. In individuals with panic disorder there is evidence of decreased central gamma-aminobutyric acid (GABA) activity as well as marked increases in autonomic and respiratory responses after intravenous infusions of hypertonic sodium lactate. In a rat model of panic disorder, chronic inhibition of GABA synthesis in the dorsomedial-perifornical hypothalamus of rats produces anxiety-like states and a similar vulnerability to sodium lactate-induced cardioexcitatory responses. The dorsomedial-perifornical hypothalamus is enriched in neurons containing orexin (ORX, also known as hypocretin), which have a crucial role in arousal, vigilance and central autonomic mobilization, all of which are key components of panic. Here we show that activation of ORX-synthesizing neurons is necessary for developing a panic-prone state in the rat panic model, and either silencing of the hypothalamic gene encoding ORX (Hcrt) with RNAi or systemic ORX-1 receptor antagonists blocks the panic responses. Moreover, we show that human subjects with panic anxiety have elevated levels of ORX in the cerebrospinal fluid compared to subjects without panic anxiety. Taken together, our results suggest that the ORX system may be involved in the pathophysiology of panic anxiety and that ORX antagonists constitute a potential new treatment strategy for panic disorder.

Title: Respiratory Feedback for Treating Panic Disorder

Authors: Meuret AE, Wilhelm FH, Roth WT.

Journal: J Clin Psychol. 2004 Feb;60(2):197-207. doi: 10.1002/jclp.10245. PMID: 14724927.

Link to PubMed: Respiratory Feedback for Treating Panic Disorder

Abstract: Panic disorder patients often complain of shortness of breath or other respiratory complaints, which has been used as evidence for both hyperventilation and false suffocation alarm theories of panic. Training patients to change their breathing patterns is a common intervention, but breathing rarely has been measured objectively in assessing the patient or monitoring therapy results. We report a new breathing training method that makes use of respiratory biofeedback to teach individuals to modify four respiratory characteristics: increased ventilation (Respiratory Rate x Tidal Volume), breath-to-breath irregularity in rate and depth, and chest breathing. As illustrated by a composite case, feedback of respiratory rate and end-tidal pCO2 can facilitate voluntary control of respiration and reduce symptoms. Respiratory monitoring may provide relevant diagnostic, prognostic, and outcome information.

Title: Changes in respiration mediate changes in fear of bodily sensations in panic disorder

Authors: Meuret AE, Rosenfield D, Hofmann SG, Suvak MK, Roth WT.

Journal: J Psychiatr Res. 2009 Mar;43(6):634-41. doi: 10.1016/j.jpsychires.2008.08.003. Epub 2008 Oct 5. PMID: 18835608; PMCID: PMC3327292.

Link to full text: Changes in respiration mediate changes in fear of bodily sensations in panic disorder

Abstract: The purpose of the study was to examine whether changes in pCO(2) mediate changes in fear of bodily sensation (as indexed by anxiety sensitivity) in a bio-behavioral treatment for panic disorder that targets changes in end-tidal pCO(2). Thirty-five panic patients underwent 4 weeks of capnometry-assisted breathing training targeting respiratory dysregulation. Longitudinal mediation analyses of the changes in fear of bodily symptoms over time demonstrated that pCO(2), but not respiration rate, was a partial mediator of the changes in anxiety sensitivity. Results were supported by cross lag panel analyses, which indicated that earlier pCO(2) levels predicted later levels of anxiety sensitivity, but not vice versa. PCO(2) changes also led to changes in respiration rate, questioning the importance of respiration rate in breathing training. The results provide little support for changes in fear of bodily sensations leading to changes in respiration, but rather suggest that breathing training targeting pCO(2) reduced fear of bodily sensations in panic disorder.

Title: Feedback of end-tidal pCO2 as a therapeutic approach for panic disorder

Authors: Meuret AE, Wilhelm FH, Ritz T, Roth WT.

Journal: J Psychiatr Res. 2008 Jun;42(7):560-8. doi: 10.1016/j.jpsychires.2007.06.005. Epub 2007 Aug 3. PMID: 17681544; PMCID: PMC2890048.

Link to full text: Feedback of end-tidal pCO2 as a therapeutic approach for panic disorder

Abstract: Background: Given growing evidence that respiratory dysregulation is a central feature of panic disorder (PD) interventions for panic that specifically target respiratory functions could prove clinically useful and scientifically informative. We tested the effectiveness of a new, brief, capnometry-assisted breathing therapy (BRT) on clinical and respiratory measures in PD.

Methods: Thirty-seven participants with PD with or without agoraphobia were randomly assigned to BRT or to a delayed-treatment control group. Clinical status, respiration rate, and end-tidal pCO(2) were assessed at baseline, post-treatment, 2-month and 12-month follow-up. Respiratory measures were also assessed during homework exercises using a portable capnometer as a feedback device.

Results: Significant improvements (in PD severity, agoraphobic avoidance, anxiety sensitivity, disability, and respiratory measures) were seen in treated, but not untreated patients, with moderate to large effect sizes. Improvements were maintained at follow-up. Treatment compliance was high for session attendance and homework exercises; dropouts were few.

Conclusions: The data provide preliminary evidence that raising end-tidal pCO(2) by means of capnometry feedback is therapeutically beneficial for panic patients. Replication and extension will be needed to verify this new treatment's efficacy and determine its mechanisms.

Title: Does fear reactivity during exposure predict panic symptom reduction?

Authors: Meuret AE, Seidel A, Rosenfield B, Hofmann SG, Rosenfield D.

Journal: J Consult Clin Psychol. 2012 Oct;80(5):773-85. doi: 10.1037/a0028032. Epub 2012 Apr 9. PMID: 22486408; PMCID: PMC3404244.

Link to full text: Does fear reactivity during exposure predict panic symptom reduction?

Abstract: Objective: Fear reactivity during exposure is a commonly used indicator of learning and overall therapy outcome. The objective of this study was to assess the predictive value of fear reactivity during exposure using multimodal indicators and an advanced analytical design. We also investigated the degree to which treatment condition (cognitive training vs. respiratory skill training) moderated fear reactivity and therapeutic outcome.

Method: Thirty-four patients with panic disorder and agoraphobia completed a total of 123 in-vivo exposure sessions, comprising 3 weekly sessions and a 4th session 2 months following therapy completion. Sessions varied in length and phobic stimuli. Cardiorespiratory physiology (heart rate, carbon dioxide partial pressure [PCO2], respiration rate) and experiential symptoms (panic symptoms and anxiety) were assessed repeatedly throughout exposure sessions, in addition to weekly assessments of panic cognitions, avoidance, and functioning.

Results: Panic symptomatology decreased substantially in both treatment conditions during therapy and follow-up. Significant cardiorespiratory and experiential reactivity was observed during all exposures, characterized by activation followed by reduction. Greater within-session activation of anxiety and panic symptoms was inversely related to improvement in panic symptoms severity, but neither physiological activation nor within- or between-session reduction of either physiological or experiential variables was predictive of outcome. No moderating effects of treatment condition were found.

Conclusions: Fear activation and reduction during exposure are weak predictors of corrective learning and fear extinction. Clinical implications for exposure therapy and directions for future research are discussed.

Title: Do unexpected panic attacks occur spontaneously?

Authors: Meuret AE, Rosenfield D, Wilhelm FH, Zhou E, Conrad A, Ritz T, Roth WT.

Journal: Biol Psychiatry. 2011 Nov 15;70(10):985-91. doi: 10.1016/j.biopsych.2011.05.027. Epub 2011 Jul 23. PMID: 21783179; PMCID: PMC3327298.

Link to full text: Do unexpected panic attacks occur spontaneously?

Abstract: Background: Spontaneous or unexpected panic attacks, per definition, occur "out of the blue," in the absence of cues or triggers. Accordingly, physiological arousal or instability should occur at the onset of, or during, the attack, but not preceding it. To test this hypothesis, we examined if points of significant autonomic changes preceded the onset of spontaneous panic attacks.

Methods: Forty-three panic disorder patients underwent repeated 24-hour ambulatory monitoring. Thirteen natural panic attacks were recorded during 1960 hours of monitoring. Minute-by-minute epochs beginning 60 minutes before and continuing to 10 minutes after the onset of individual attacks were examined for respiration, heart rate, and skin conductance level. Measures were controlled for physical activity and vocalization and compared with time matched control periods within the same person.

Results: Significant patterns of instability across a number of autonomic and respiratory variables were detected as early as 47 minutes before panic onset. The final minutes before onset were dominated by respiratory changes, with significant decreases in tidal volume followed by abrupt carbon dioxide partial pressure increases. Panic attack onset was characterized by heart rate and tidal volume increases and a drop in carbon dioxide partial pressure. Symptom report was consistent with these changes. Skin conductance levels were generally elevated in the hour before, and during, the attacks. Changes in the matched control periods were largely absent.

Conclusions: Significant autonomic irregularities preceded the onset of attacks that were reported as abrupt and unexpected. The findings invite reconsideration of the current diagnostic distinction between uncued and cued panic attacks.

Title: Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia

Authors: van Hulst RA, Lameris TW, Haitsma JJ, Klein J, Lachmann B.

Journal: Clin Physiol Funct Imaging. 2004 Jul;24(4):243-8. doi: 10.1111/j.1475-097X.2004.00557.x. PMID: 15233840.

Link to PubMed: Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia

Abstract: Objective: Levels of glucose and lactate were measured in the brain by means of microdialysis in order to evaluate the effects of ventilator-induced hypocapnia and hypercapnia on brain metabolism in healthy non-brain-traumatized animals.

Design and setting: Prospective animal study in a university laboratory.

Subjects: Eight adult Landrace/Yorkshire pigs.

Interventions: The microdialysis probe was inserted in the brain along with a multiparameter sensor and intracranial pressure (ICP) probe. The animals were ventilated in a pressure-controlled mode according to the open lung concept with an inspired oxygen fraction of 0.4/1.0. Starting at normoventilation (PaCO(2) +/-40 mmHg) two steps of both hypercapnia (PCO(2) +/- 70 and 100 mmHg) and hypocapnia (PaCO(2) +/- 20 and 30 mmHg) were performed. Under these conditions, brain glucose and lactate levels as well as brain oxygen (PbrO(2)), brain carbon dioxide (PbrCO(2)), brain pH (brpH), brain temperature and ICP were measured.

Results: At hypercapnia (PaCO(2) = 102.7 mmHg) there were no significant changes in brain glucose and lactate but there was a significant increase in PbrCO(2), PbrO(2) and ICP. In contrast, at hypocapnia (PCO(2) = 19.8 mmHg) there was a significant increase in brain lactate and a significant decrease in both brain glucose and PbrCO(2).

Conclusions: Hypocapnia decreases brain glucose and increases brain lactate concentration, indicating anaerobic metabolism, whereas hypercapnia has no influence on levels of brain glucose and brain lactate.

Title: Voluntary hyperventilation in the treatment of panic disorder--functions of hyperventilation, their implications for breathing training, and recommendations for standardization

Authors: Meuret AE, Ritz T, Wilhelm FH, Roth WT.

Journal: Clin Psychol Rev. 2005 May;25(3):285-306. doi: 10.1016/j.cpr.2005.01.002. PMID: 15792851.

Link to PubMed: Voluntary hyperventilation in the treatment of panic disorder--functions of hyperventilation, their implications for breathing training, and recommendations for standardization

Abstract: Hyperventilation has numerous theoretical and empirical links to anxiety and panic. Voluntary hyperventilation (VH) tests have been applied experimentally to understand psychological and physiological mechanisms that produce and maintain anxiety, and therapeutically in the treatment of anxiety disorders. From the theoretical perspective of hyperventilation theories of anxiety, VH is useful diagnostically to the clinician and educationally to the patient. From the theoretical perspective of cognitive-behavior therapy, VH is a way to expose patients with panic disorder to sensations associated with panic and to activate catastrophic cognitions that need restructuring. Here we review panic disorder treatment studies using breathing training that have included VH. We differentiate the roles of VH in diagnosis, education about symptoms, training of breathing strategies, interoceptive exposure, and outcome measurement--discussing methodological issues specific to these roles and VH test reliability and validity. We propose how VH procedures might be standardized in future studies.

Title: Carbon dioxide hypersensitivity, hyperventilation, and panic disorder

Authors: Papp LA, Klein DF, Gorman JM.

Journal: Am J Psychiatry. 1993 Aug;150(8):1149-57. doi: 10.1176/ajp.150.8.1149. PMID: 8392296.

Link to PubMed: Carbon dioxide hypersensitivity, hyperventilation, and panic disorder

Abstract: Objective: The purpose of this article is to offer a comprehensive, data-based explanation of the relationship between hyperventilation and panic disorder linking CO2 hypersensitivity, cognitive/behavioral factors, and the respiratory effects of antipanic pharmacologic and psychological treatments.

Method: The authors conducted a computerized search of MEDLINE for relevant articles.

Results: Some panic patients have a chronic, subtle respiratory disturbance. Acute hyperventilation is neither necessary nor sufficient for panic to occur. Respiratory abnormalities in panic patients may adaptively aim at coping with a hypersensitive CO2 chemoreceptor system. Pharmacologic panicogens also stimulate the respiratory system, causing hyperventilation. Triggering this hypersensitive respiratory control mechanism may incite panic. Antipanic medications may reset the receptor threshold. Misattribution and catastrophic interpretation of somatic symptoms or the sense of loss of control may contribute to panic symptoms. Behavioral interventions such as desensitization or breathing retraining may block the full-blown attack. Cognitive strategies through cognitive control of respiration may supplement and accentuate these interventions.

Conclusions: Panic disorder may be due to an inherently unstable autonomic nervous system, coupled with cognitive distress.

Title: Respiratory muscle tension as symptom generator in individuals with high anxiety sensitivity

Authors: Ritz T, Meuret AE, Bhaskara L, Petersen S.

Journal: Psychosom Med. 2013 Feb;75(2):187-95. doi: 10.1097/PSY.0b013e31827d1072. Epub 2013 Jan 16. PMID: 23324873.

Link to PubMed: Respiratory muscle tension as symptom generator in individuals with high anxiety sensitivity

Abstract: Objective: Anxiety and panic are associated with the experience of a range of bodily symptoms, in particular unpleasant breathing sensations (dyspnea). Respiratory theories of panic disorder have focused on disturbances in blood gas regulation, but respiratory muscle tension as a source of dyspnea has not been considered. We therefore examined the potential of intercostal muscle tension to elicit dyspnea in individuals with high anxiety sensitivity, a risk factor for developing panic disorder.

Methods: Individuals high and low in anxiety sensitivity (total N=62) completed four tasks: electromyogram biofeedback for tensing intercostal muscle, electromyogram biofeedback for tensing leg muscles, paced breathing at three different speeds, and a fine motor task. Global dyspnea, individual respiratory sensations, nonrespiratory sensations, and discomfort were assessed after each task, whereas respiratory pattern (respiratory inductance plethysmography) and end-tidal carbon dioxide (capnography) were measured continuously.

Results: In individuals with high compared to low anxiety sensitivity, intercostal muscle tension elicited a particularly strong report of obstruction (M=5.1, SD=3.6 versus M=2.5, SD=3.0), air hunger (M=1.9, SD=2.1 versus M=0.4, SD=0.8), hyperventilation symptoms (M=0.6, SD=0.6 versus M=0.1, SD=0.1), and discomfort (M=5.1, SD=3.2 versus M=2.2, SD=2.1) (all p values<.05). This effect was not explained by site-unspecific muscle tension, voluntary manipulation of respiration, or sustained task-related attention. Nonrespiratory control sensations were not significantly affected by tasks (F<1), and respiratory variables did not reflect any specific responding of high-Anxiety Sensitivity Index participants to intercostal muscle tension.

Conclusions: Respiratory muscle tension may contribute to the respiratory sensations experienced by panic-prone individuals. Theories and treatments for panic disorder should consider this potential source of symptoms.

Title: Autonomic function in panic disorder: cardiorespiratory and plasma catecholamine responsivity to multiple challenges of the autonomic nervous system

Authors: Stein MB, Asmundson GJ.

Journal: Biol Psychiatry. 1994 Oct 15;36(8):548-58. doi: 10.1016/0006-3223(94)90619-x. PMID: 7827218.

Link to PubMed: Autonomic function in panic disorder: cardiorespiratory and plasma catecholamine responsivity to multiple challenges of the autonomic nervous system

Abstract: Panic disorder has been widely hypothesized to be associated with dysfunction of the autonomic nervous system. In this study, 24 patients with panic disorder and 26 healthy control subjects took part in a broad battery of autonomic function tests, each designed to stress the autonomic nervous system in a particular fashion. Testing consisted of postural challenge, isometric exercise, cold pressor, and Valsalva maneuver. Dependent measures included heart rate, vagal tone, blood pressure, respiratory frequency, end-tidal CO2 levels, and plasma norepinephrine and epinephrine levels. The testing procedures reliably produced changes in autonomic output in the expected directions, but patients with panic disorder were not found to differ from healthy controls in their cardiorespiratory or plasma catecholaminergic responses. This pattern of normal autonomic responsivity in the patients with panic disorder was evident across multiple test conditions with varying autonomic demand characteristics, thereby supporting the integrity of autonomic regulatory systems in this illness. These data run counter to a simple notion of autonomic dysfunction in panic disorder.

Title: The bed nucleus of the stria terminalis is critical for anxiety-related behavior evoked by CO2 and acidosis

Authors: Taugher RJ, Lu Y, Wang Y, Kreple CJ, Ghobbeh A, Fan R, Sowers LP, Wemmie JA.

Journal: J Neurosci. 2014 Jul 30;34(31):10247-55. doi: 10.1523/JNEUROSCI.1680-14.2014. PMID: 25080586; PMCID: PMC4115136.

Link to full text: The bed nucleus of the stria terminalis is critical for anxiety-related behavior evoked by CO2 and acidosis

Abstract: Carbon dioxide (CO2) inhalation lowers brain pH and induces anxiety, fear, and panic responses in humans. In mice, CO2 produces freezing and avoidance behavior that has been suggested to depend on the amygdala. However, a recent study in humans with bilateral amygdala lesions revealed that CO2 can trigger fear and panic even in the absence of amygdalae, suggesting the importance of extra-amygdalar brain structures. Because the bed nucleus of the stria terminalis (BNST) contributes to fear- and anxiety-related behaviors and expresses acid-sensing ion channel-1A (ASIC1A), we hypothesized that the BNST plays an important role in CO2-evoked fear-related behaviors in mice. We found that BNST lesions decreased both CO2-evoked freezing and CO2-conditioned place avoidance. In addition, we found that CO2 inhalation caused BNST acidosis and that acidosis was sufficient to depolarize BNST neurons and induce freezing behavior; both responses depended on ASIC1A. Finally, disrupting Asic1a specifically in the BNST reduced CO2-evoked freezing, whereas virus-vector-mediated expression of ASIC1A in the BNST of Asic1a(-/-) and Asic1a(+/+) mice increased CO2-evoked freezing. Together, these findings identify the BNST as an extra-amygdalar fear circuit structure important in CO2-evoked fear-related behavior.

Keywords: ASIC1A; CO2; anxiety; bed nucleus of the stria terminalis; pH; panic.

Title: Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Authors: Vollmer LL, Strawn JR, Sah R.

Journal: Transl Psychiatry. 2015 May 26;5(5):e572. doi: 10.1038/tp.2015.67. PMID: 26080089; PMCID: PMC4471296.

Link to full text: Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Abstract: Panic disorder (PD), a complex anxiety disorder characterized by recurrent panic attacks, represents a poorly understood psychiatric condition which is associated with significant morbidity and an increased risk of suicide attempts and completed suicide. Recently however, neuroimaging and panic provocation challenge studies have provided insights into the pathoetiology of panic phenomena and have begun to elucidate potential neural mechanisms that may underlie panic attacks. In this regard, accumulating evidence suggests that acidosis may be a contributing factor in induction of panic. Challenge studies in patients with PD reveal that panic attacks may be reliably provoked by agents that lead to acid-base dysbalance such as CO2 inhalation and sodium lactate infusion. Chemosensory mechanisms that translate pH into panic-relevant fear, autonomic, and respiratory responses are therefore of high relevance to the understanding of panic pathophysiology. Herein, we provide a current update on clinical and preclinical studies supporting how acid-base imbalance and diverse chemosensory mechanisms may be associated with PD and discuss future implications of these findings.

Title: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Authors: Ziemann AE, Allen JE, Dahdaleh NS, Drebot II, Coryell MW, Wunsch AM, Lynch CM, Faraci FM, Howard MA 3rd, Welsh MJ, Wemmie JA.

Journal: Cell. 2009 Nov 25;139(5):1012-21. doi: 10.1016/j.cell.2009.10.029. PMID: 19945383; PMCID: PMC2808123.

Link to full text: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Abstract: The amygdala processes and directs inputs and outputs that are key to fear behavior. However, whether it directly senses fear-evoking stimuli is unknown. Because the amygdala expresses acid-sensing ion channel-1a (ASIC1a), and ASIC1a is required for normal fear responses, we hypothesized that the amygdala might detect a reduced pH. We found that inhaled CO(2) reduced brain pH and evoked fear behavior in mice. Eliminating or inhibiting ASIC1a markedly impaired this activity, and localized ASIC1a expression in the amygdala rescued the CO(2)-induced fear deficit of ASIC1a null animals. Buffering pH attenuated fear behavior, whereas directly reducing pH with amygdala microinjections reproduced the effect of CO(2). These data identify the amygdala as an important chemosensor that detects hypercarbia and acidosis and initiates behavioral responses. They also give a molecular explanation for how rising CO(2) concentrations elicit intense fear and provide a foundation for dissecting the bases of anxiety and panic disorders.

Title: Carbon dioxide for neurogenic orthostatic hypotension in adults: a novel therapy

Authors: Baker JR, Ranada SI, Incognito AV, Sheldon RS, Morillo CA, Wilson RJA, Phillips AA, Raj SR.

Journal: Eur Heart J. 2024 Oct 7:ehae653. doi: 10.1093/eurheartj/ehae653. Epub ahead of print. PMID: 39374345.

Link to PubMed: Carbon dioxide for neurogenic orthostatic hypotension in adults: a novel therapy

Abstract: Neurogenic orthostatic hypotension (nOH) is a hallmark feature of autonomic nervous system failure. On standing, people with nOH experience large reductions in blood pressure (≥20/10 mmHg) and debilitating symptoms (e.g. light-headedness, blurred vision, and syncope). Current blood pressure therapies have limited efficacy and potentially serious side effects, including supine hypertension, creating a strong clinical need for new therapeutic approaches. Previous studies examining the effects of hypocapnia [i.e. low arterial carbon dioxide (CO2)] on blood pressure have shown that hyperventilation-induced hypocapnia decreases supine blood pressure in nOH patients and can be readily prevented with exogenous CO2. Whether exogenous CO2 can prevent OH in these patients is unknown. In this proof-of-concept study, we tested the hypothesis that increased inspired CO2 increases standing blood pressure in patients with nOH.

Title: Hyperventilation syndrome after general anesthesia: Our experience

Authors: Bansal T, Hooda S.

Journal: J Anaesthesiol Clin Pharmacol. 2016 Oct-Dec;32(4):536-537. doi: 10.4103/0970-9185.168192. PMID: 28096595; PMCID: PMC5187629.

Link to full text: Hyperventilation syndrome after general anesthesia: Our experience

Abstract: We read with interest the article published in J Anaesthesiol Clin Pharmacol 2015;31:284-5. Tomioka et al. have reported two cases of hyperventilation syndrome after general anesthesia. These authors have described that hyperventilation syndrome reflects the enhancement of sympathoadrenal tone. The first case was managed with midazolam. The second case was managed with beta-adrenergic blocker, propranolol hydrochloride. Hyperventilation syndrome often occurs under stressful conditions and has been reported before, during or after anesthesia and surgery. Hyperventilation is associated with multiple somatic symptoms due to hypocalcemia induced by respiratory alkalosis. We wish to share our experience of a 30 years old full term parturient presenting in labor in a highly anxious state with symptoms and signs of hypocalcemia in the form of circumoral numbness and severe carpopedal spasm. The patient was managed successfully with reassurance and infusion of 10 ml of 10% calcium gluconate over 10 min. The blood sample taken for ionised serum calcium before administering calcium gluconate showed a low level of 0.7 mmol/L (normal range 1–1.5 mmol/L). We did not use anxiolytic or beta blocker for fear of any untoward effect on the fetus as the patient was in labor.

Title: Carbon dioxide influence on nitric oxide production in endothelial cells and astrocytes: cellular mechanisms

Authors: Fathi AR, Yang C, Bakhtian KD, Qi M, Lonser RR, Pluta RM.

Journal: Brain Res. 2011 Apr 22;1386:50-7. doi: 10.1016/j.brainres.2011.02.066. Epub 2011 Feb 26. PMID: 21362408; PMCID: PMC3073030.

Link to full text: Carbon dioxide influence on nitric oxide production in endothelial cells and astrocytes: cellular mechanisms

Abstract: Cerebral vessels may regulate cerebral blood flow by responding to changes in carbon dioxide (CO(2)) through nitric oxide (NO) production. To better determine the role of NO production by human adult cerebral microvascular endothelial cells and human fetal astrocytes under different CO(2) conditions, we studied endothelial cell and astrocyte production of NO under hypo-, normo- and hypercapnic conditions. Human cerebral endothelial cell and fetal astrocyte cultures were exposed to hypocapnic (pCO(2) 21.7±6.7mmHg), normocapnic (pCO(2) 40.1±0.9mmHg) and hypercapnic (pCO(2) 56.3±8.7mmHg) conditions. NO production was recorded continuously over 24hours with stable pH. N-nitro-l-arginine [NLA; a nitric oxide synthase (NOS) inhibitor] and l-arginine (substrate for NO production via NOS) were used to further define the role of NOS in chemoregulation. NO levels in endothelial cells increased during hypercapnia by 36% in 8hours and remained 25% above baseline. NO increase in astrocytes was 30% after 1hour but returned to baseline at 8hours. NLA blocked NO increase in endothelial cells under hypercapnia. During hypocapnia, NO levels in the endothelial cells decreased by 30% at 8hours but were unchanged in astrocytes. l-arginine prevented NO decrease in endothelial cells under hypocapnia. NO changes in the endothelial cells correlated with changes in pCO(2) (R=0.99) and were independent of pH. This study suggests that cerebral endothelial cells and astrocytes release NO under normocapnic conditions and NO production is increased during hypercapnia and decreased during hypocapnia independent of pH. Further, this demonstrates that endothelial cells may play a pivotal role in chemoregulation by modulating NOS activity.

Title: The influence of carbon dioxide on brain activity and metabolism in conscious humans

Authors: Xu F, Uh J, Brier MR, Hart J Jr, Yezhuvath US, Gu H, Yang Y, Lu H.

Journal: J Cereb Blood Flow Metab. 2011 Jan;31(1):58-67. doi: 10.1038/jcbfm.2010.153. Epub 2010 Sep 15. PMID: 20842164; PMCID: PMC3049465.

Link to full text: The influence of carbon dioxide on brain activity and metabolism in conscious humans

Abstract: A better understanding of carbon dioxide (CO(2)) effect on brain activity may have a profound impact on clinical studies using CO(2) manipulation to assess cerebrovascular reserve and on the use of hypercapnia as a means to calibrate functional magnetic resonance imaging (fMRI) signal. This study investigates how an increase in blood CO(2), via inhalation of 5% CO(2), may alter brain activity in humans. Dynamic measurement of brain metabolism revealed that mild hypercapnia resulted in a suppression of cerebral metabolic rate of oxygen (CMRO(2)) by 13.4% ± 2.3% (N=14) and, furthermore, the CMRO(2) change was proportional to the subject's end-tidal CO(2) (Et-CO(2)) change. When using functional connectivity MRI (fcMRI) to assess the changes in resting-state neural activity, it was found that hypercapnia resulted in a reduction in all fcMRI indices assessed including cluster volume, cross-correlation coefficient, and amplitude of the fcMRI signal in the default-mode network (DMN). The extent of the reduction was more pronounced than similar indices obtained in visual-evoked fMRI, suggesting a selective suppression effect on resting-state neural activity. Scalp electroencephalogram (EEG) studies comparing hypercapnia with normocapnia conditions showed a relative increase in low frequency power in the EEG spectra, suggesting that the brain is entering a low arousal state on CO(2) inhalation.

Title: High Behavioral Sensitivity to Carbon Dioxide Associates with Enhanced Fear Memory and Altered Forebrain Neuronal Activation

Authors: McMurray KMJ, Gray A, Horn P, Sah R.

Journal: Neuroscience. 2020 Mar 1;429:92-105. doi: 10.1016/j.neuroscience.2019.12.009. Epub 2020 Jan 10. PMID: 31930959; PMCID: PMC8418542.

Link to full text: High Behavioral Sensitivity to Carbon Dioxide Associates with Enhanced Fear Memory and Altered Forebrain Neuronal Activation

Abstract: There is considerable interest in pre-trauma individual differences that may contribute to increased risk for developing post-traumatic stress disorder (PTSD). Identification of underlying vulnerability factors that predict differential responses to traumatic experiences is important. Recently, the relevance of homeostatic perturbations in shaping long-term behavior has been recognized. Sensitivity to CO2 inhalation, a homeostatic threat to survival, was shown to associate with the later development of PTSD symptoms in veterans. Here, we investigated whether behavioral sensitivity to CO2 associates with PTSD-relevant behaviors and alters forebrain fear circuitry in mice. Mice were exposed to 5% CO2 or air inhalation and tested one week later on acoustic startle and footshock contextual fear conditioning, extinction and reinstatement. CO2 inhalation evoked heterogenous freezing behaviors (high freezing CO2-H and low freezing CO2-L) that significantly associated with fear conditioning and extinction behaviors. CO2-H mice elicited potentiated conditioned fear and delayed extinction while behavioral responses in CO2-L mice were similar to the air group. Persistent neuronal activation marker ΔFosB immunostaining revealed altered regional neuronal activation within the hippocampus, amygdala and medial pre-frontal cortex that correlated with conditioned fear and extinction. Inter-regional co-activation mapping revealed disruptions in the coordinated activity of hippocampal dentate-amygdala-infralimbic regions and infralimbic-prelimbic associations in CO2-H mice that may explain their enhanced fear phenotype. In conclusion, our data support an association of behavioral sensitivity to interoceptive threats such as CO2 with altered fear responding to exteroceptive threats and suggest that "CO2-sensitive" individuals may be susceptible to developing PTSD.

Title: Respiratory relief: a new and rapid method for the treatment of phobic states

Authors: Orwin A.

Journal: Br J Psychiatry. 1971 Dec;119(553):635-7. doi: 10.1192/bjp.119.553.635. PMID: 5143663.

Link to PubMed: Respiratory relief: a new and rapid method for the treatment of phobic states

Abstract: The treatment of phobic states by desensitization is tedious and not always completely successful (Marks, 1969). Recently newer methods claiming improvement in speed and efficacy have been introduced, such as flooding or implosion (for example, Boulougouris et al., 1971) and prolonged exposure (Watson et al., 1971). An approach based on respira tory control which seems to be as effective as other methods and even more rapid in action is described in this short paper. Since anxiety caused by maximum voluntary respiratory arrest (M.V.R.A.) is abolished on breathing, it was decided to determine whether voluntary manipulation of respiratory function could lead to the use of this respiratory relief (R.R.) as an antagonist to the anxiety induced by specific phobias. It was hypothesized that the temporal pairing of R.R. with the anxiety induced by presentation of the feared object, the timing of which could be accurate to within less than a second, should result in the extinc tion of the anxiety response. The methods described are simple, but more complex variants have been used (Orwin, 1971).

Title: Metabolism and physiological effects of carbon dioxide. Implications in anaesthetic management

Authors: Victoria R.

Journal: The Moldovan Medical Journal 60.3 (2017): 42-45.

Link to full text: Metabolism and physiological effects of carbon dioxide. Implications in anaesthetic management

Abstract: Background: Carbon dioxide is a normal by-product of aerobic metabolism that maintains the equilibrium of respiratory act, being eliminated from the lungs. Despite of an increasing number of researches concerning carbon dioxide metabolism and its effects on human homeostasis, there are still discussions about carbon dioxide metabolism, physiology and its implication in anaesthetic management, ICU, critically ill patient. The use of mild to moderate hypercapnia during general anaesthesia and in mechanically ventilated patients is growing, based on scientifical researches of last years. Material and methods: There has been scientifically evaluated data from PubMed, 2002-2017. Key words used in search are: "carbon dioxide", "ventilation", "metabolism", "gas change". There were selected articles, taking in consideration their title, and chosen abstracts. The article contains a qualitative analysis and synthesis of the recommendation, concerning anaesthetic management and metabolism of carbon dioxide. Conclusions: Carbon dioxide metabolism and its anaesthetic management, represents a challenge that will be actual for many years in future. The amount of controversial studies about effects of carbon dioxide on patients under general anaesthesia or mechanical ventilation, determined us to perform a review of literature, and evaluate it. There are known facts about carbon dioxide metabolism, such as normal values, how it is produced in human body, how it is evacuated, effects on cardiovascular, nervous systems, and still there are many controversial studies on that topic, that determine to study it more and find new research results. Key words: carbon dioxide, ventilation, metabolism, gas change.

Title: The Effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men

Authors: Kety SS, Schmidt CF.

Journal: J Clin Invest. 1948 Jul;27(4):484-92. doi: 10.1172/JCI101995. PMID: 16695569; PMCID: PMC439519.

Link to full text: The Effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men

Abstract: A method for measuring quantitatively the volume of cerebral blood flow in man by inhalation of nitrous oxide (1) found its first application in a study of the cerebral circulatory effects of low CO2 tension achieved by hyperventilation; of high CO2 tension, and of high and low 02 tensions obtained by inhalation of appropriate gas mixtures (2). Only the first part of this study, the effects of active and passive hyperventilation, has been published in detail (3). The purpose of the present paper is to present the remainder of these findings and to derive from them, together with those of the hyperventilation experiments, evidence bearing on the intrinsic control of the human cerebral circulation as revealed by quantitative measurements.

Title: Five percent CO₂ is a potent, fast-acting inhalation anticonvulsant

Authors: Tolner EA, Hochman DW, Hassinen P, Otáhal J, Gaily E, Haglund MM, Kubová H, Schuchmann S, Vanhatalo S, Kaila K.

Journal: 2011 Jan;52(1):104-14. doi: 10.1111/j.1528-1167.2010.02731.x. Epub 2010 Sep 30. PMID: 20887367; PMCID: PMC3017646.

Link to full text: Five percent CO₂ is a potent, fast-acting inhalation anticonvulsant

Abstract: Purpose: CO₂ has been long recognized for its anticonvulsant properties. We aimed to determine whether inhaling 5% CO₂ can be used to suppress seizures in epilepsy patients. The effect of CO₂ on cortical epileptic activity accompanying behavioral seizures was studied in rats and nonhuman primates, and based on these data, preliminary tests were carried out in humans.

Methods: In freely moving rats, cortical afterdischarges paralleled by myoclonic convulsions were evoked by sensorimotor cortex stimulation. Five percent CO₂ was applied for 5 min, 3 min before stimulation. In macaque monkeys, hypercarbia was induced by hypoventilation while seizure activity was electrically or chemically evoked in the sensorimotor cortex. Seven patients with drug-resistant partial epilepsy were examined with video-EEG (electroencephalography) and received 5% CO₂ in medical carbogen shortly after electrographic seizure onset.

Results: In rats, 5% CO₂ strongly suppressed cortical afterdischarges, by approximately 75%, whereas responses to single-pulse stimulation were reduced by about 15% only. In macaques, increasing pCO₂) from 37 to 44-45 mm Hg (corresponding to inhalation of 5% CO₂ or less) suppressed stimulation-induced cortical afterdischarges by about 70% and single, bicuculline-induced epileptiform spikes by approximately 25%. In a pilot trial carried out in seven patients, a rapid termination of electrographic seizures was seen despite the fact that the application of 5% CO₂ was started after seizure generalization.

Conclusions: Five percent CO₂ has a fast and potent anticonvulsant action. The present data suggest that medical carbogen with 5% CO₂ can be used for acute treatment to suppress seizures in epilepsy patients.

Title: Carbon dioxide influence on nitric oxide production in endothelial cells and astrocytes: Cellular mechanisms

Authors: Fathi AR, Yang C, Bakhtian KD, Qi M, Lonser RR, Pluta RM.

Journal: Brain Res. 2011 Apr 22;1386:50-7. doi: 10.1016/j.brainres.2011.02.066. Epub 2011 Feb 26. PMID: 21362408; PMCID: PMC3073030.

Link to full text: Carbon dioxide influence on nitric oxide production in endothelial cells and astrocytes: Cellular mechanisms

Abstract: Cerebral vessels may regulate cerebral blood flow by responding to changes in carbon dioxide (CO(2)) through nitric oxide (NO) production. To better determine the role of NO production by human adult cerebral microvascular endothelial cells and human fetal astrocytes under different CO(2) conditions, we studied endothelial cell and astrocyte production of NO under hypo-, normo- and hypercapnic conditions. Human cerebral endothelial cell and fetal astrocyte cultures were exposed to hypocapnic (pCO(2) 21.7±6.7mmHg), normocapnic (pCO(2) 40.1±0.9mmHg) and hypercapnic (pCO(2) 56.3±8.7mmHg) conditions. NO production was recorded continuously over 24hours with stable pH. N-nitro-l-arginine [NLA; a nitric oxide synthase (NOS) inhibitor] and l-arginine (substrate for NO production via NOS) were used to further define the role of NOS in chemoregulation. NO levels in endothelial cells increased during hypercapnia by 36% in 8hours and remained 25% above baseline. NO increase in astrocytes was 30% after 1hour but returned to baseline at 8hours. NLA blocked NO increase in endothelial cells under hypercapnia. During hypocapnia, NO levels in the endothelial cells decreased by 30% at 8hours but were unchanged in astrocytes. l-arginine prevented NO decrease in endothelial cells under hypocapnia. NO changes in the endothelial cells correlated with changes in pCO(2) (R=0.99) and were independent of pH. This study suggests that cerebral endothelial cells and astrocytes release NO under normocapnic conditions and NO production is increased during hypercapnia and decreased during hypocapnia independent of pH. Further, this demonstrates that endothelial cells may play a pivotal role in chemoregulation by modulating NOS activity.

Title: Out of thin air: Hyperventilation-triggered seizures

Authors: Salvati KA, Beenhakker MP.

Journal: Brain Res. 2019 Jan 15;1703:41-52. doi: 10.1016/j.brainres.2017.12.037. Epub 2017 Dec 27. PMID: 29288644; PMCID: PMC6546426.

Link to full text: Out of thin air: Hyperventilation-triggered seizures

Abstract: Voluntary hyperventilation triggers seizures in the vast majority of people with absence epilepsy. The mechanisms that underlie this phenomenon remain unknown. Herein, we review observations - many made long ago - that provide insight into the relationship between breathing and absence seizures.

Keywords: Circuit; Epilepsy; Respiration; Seizure; Thalamus; pH.

Title: Hyperventilation syndromes in medicine and psychiatry: a review.

Authors: Lum LC.

Journal: J R Soc Med. 1987 Apr;80(4):229-31. doi: 10.1177/014107688708000413. PMID: 3295237; PMCID: PMC1290766.

Link to full text: Hyperventilation syndromes in medicine and psychiatry: a review.

Abstract: In 1940 Lewis' described DaCosta's syndrome (a frequent presentation of hyperventilation) as 'one of the commonest afflictions of sedentary town dwellers'. In this issue ofthe JRSM(p. 216), the paper 'Role of the nurse counsellor in managing patients with the hyperventilation syndrome' prompts a review of recent advances in this field. The 1980s have seen a remarkable upsurge of interest in the role of hyperventilation in functional illness, anxiety disorder, panic and phobic states2-4. In a previous review5 the author traced the changing concepts over the last 50 years, from the early realization6 that overbreathing was the physiological mechanism whereby anxiety produces somatic disturbance, to the later view7 that anxiety disorders arose from the persistence of symptoms due to unrecognized hyperventilation. Yet, even in 1982, a Lancet editorial8 commented on how rarely this diagnosis was even considered. Snaith9, reviewing panic disorder, failed to mention it

Title: Hyperventilation syndrome: a diagnosis begging for recognition.

Authors: Magarian GJ, Middaugh DA, Linz DH.

Journal: West J Med. 1983 May;138(5):733-6. PMID: 6880192; PMCID: PMC1010816.

Link to full text: Hyperventilation syndrome: a diagnosis begging for recognition.

Abstract: Beginning with the American Civil War, military physicians seeing soldiers under the stress of combat have described a syndrome characterized by breathlessness, lightheadedness or dizziness, pronounced fatigue and exercise intolerance, numbness and paresthesias and chest pain. Rarely have organic diseases been found to account for the symptoms in such cases, yet despite reassurance, symptoms commonly persist for prolonged periods despite removal from the apparent stress setting. This syndrome has been given many names including irritable heart, soldier's heart, Da Costa's syndrome, effort syndrome, neurocirculatory asthenia and, more recently, hyperventilation syndrome. Since the original descriptions in soldiers, it is now recognized that hyperventilation occurs in many persons under stresses of daily living. It is manifest not only in those overtly stressed, anxious and depressed but also in those who appear outwardly calm as they "bottle up" their feelings, often because of undeveloped or lack of acceptable emotional outlets. Physicians and lay persons alike readily recognize acute hyperventilatory attacks occurring under acute stress. However, chronic or recurrent hyperventilation problems often are unrecognized probably for a variety of reasons, including the frequent lack of obvious overbreathing, a tendency to focus on one or two complaints that alone are not particularly suggestive of hyperventilation, minimal discussion of the topic in medical school and cursory coverage in medical textbooks.

Title: Carbon dioxide flooding of the pericardium-An old practice revisited

Authors: Bhaskar, J.

Journal: Indian Journal of Thoracic and Cardiovascular Surgery 24 (2008): 37-43.

Link to Springer: Carbon dioxide flooding of the pericardium-An old practice revisited

Abstract: Background
Cerebral air embolism is a known complication of open-heart surgery with devastating consequences. Neurological injury after cardio-pulmonary bypass can be broadly divided into 2 categories:
Type I: Focal injury
Type II: Intellectual deterioration/neurocognitive impairment.
Increasing evidence points to cerebral embolization as the principal culprit. Since the early days of modern cardiac surgery, surgeons have attempted to reduce embolization by various means. One of the early techniques propounded was carbon dioxide insufflation of the pericardial cavity. Carbon dioxide, was uniquely positioned for this, being denser than air, more soluble than air, and easily managed perioperatively if the blood levels became unphysiological.. The technique did not find favor, since there was no instrumentation, in the early days of cardiopulmonary bypass to measure and establish its impact on the actual outcomes.
Methods
A literature search was done using the key words carbon dioxide, open heart, cardiac across Pubmed, Google and Cochrane databases in an attempt to find out the role of carbon dioxide flooding in modern cardiac surgery. Our effort yielded over 244 articles, of which 48 were deemed to be relevant in answering our question.
Conclusions
Reviewing all the information we conclude that with proper technique, carbon dioxide flooding does significantly reduce embolic counts. However, there are no studies at this point in time, which show a direct correlation between the practice and improved neurocognitive outcome.

Title: Regulation of the Cerebral Circulation by Arterial Carbon Dioxide

Authors: Hoiland RL, Fisher JA, Ainslie PN.

Journal: Compr Physiol. 2019 Jun 12;9(3):1101-1154. doi: 10.1002/cphy.c180021. PMID: 31187899.

Link to PubMed: Regulation of the Cerebral Circulation by Arterial Carbon Dioxide

Abstract: Intact, coordinated, and precisely regulated cerebrovascular responses are required for the maintenance of cerebral metabolic homeostasis, adequate perfusion, oxygen delivery, and acid-base balance during deviations from homeostasis. Increases and decreases in the partial pressure of arterial carbon dioxide (PaCO2 ) lead to robust and rapid increases and decreases in cerebral blood flow (CBF). In awake and healthy humans, PaCO2 is the most potent regulator of CBF, and even small fluctuations can result in large changes in CBF. Alterations in the responsiveness of the cerebral vasculature can be detected with carefully controlled stimulus-response paradigms and hold relevance for cerebrovascular risk in steno-occlusive disease. As changes in PaCO2 do not typically occur in isolation, the integrative influence of physiological factors such as intracranial pressure, arterial oxygen content, cerebral perfusion pressure, and sympathetic nervous activity must be considered. Further, age and sex, as well as vascular pathologies are also important to consider. Following a brief summary of key historical events in the development of our understanding of cerebrovascular physiology and an overview of the measurement techniques to index CBF this review provides an in-depth description of CBF regulation in response to alterations in PaCO2 . Cerebrovascular reactivity and regional flow distribution are described, with further consideration of how differences in reactivity of parallel networks can lead to the "steal" phenomenon. Factors that influence cerebrovascular reactivity are discussed and the mechanisms and regulatory pathways mediating the exquisite sensitivity of the cerebral vasculature to changes in PaCO2 are outlined. Finally, topical avenues for future research are proposed. © 2019 American Physiological Society. Compr Physiol 9:1101-1154, 2019.

Title: Carbon Dioxide for the treatment of Febrile seizures: rationale, feasibility, and design of the CARDIF-study

Authors: Ohlraun S, Wollersheim T, Weiß C, Martus P, Weber-Carstens S, Schmitz D, Schuelke M.

Journal: J Transl Med. 2013 Jun 27;11:157. doi: 10.1186/1479-5876-11-157. PMID: 23806032; PMCID: PMC3700755.

Link to full text: Carbon Dioxide for the treatment of Febrile seizures: rationale, feasibility, and design of the CARDIF-study

Abstract: Background: 2-8% of all children aged between 6 months and 5 years have febrile seizures. Often these seizures cease spontaneously, however depending on different national guidelines, 20-40% of the patients would need therapeutic intervention. For seizures longer than 3-5 minutes application of rectal diazepam, buccal midazolam or sublingual lorazepam is recommended. Benzodiazepines may be ineffective in some patients or cause prolonged sedation and fatigue. Preclinical investigations in a rat model provided evidence that febrile seizures may be triggered by respiratory alkalosis, which was subsequently confirmed by a retrospective clinical observation. Further, individual therapeutic interventions demonstrated that a pCO2-elevation via re-breathing or inhalation of 5% CO2 instantly stopped the febrile seizures. Here, we present the protocol for an interventional clinical trial to test the hypothesis that the application of 5% CO2 is effective and safe to suppress febrile seizures in children.

Methods: The CARDIF (CARbon DIoxide against Febrile seizures) trial is a monocentric, prospective, double-blind, placebo-controlled, randomized study. A total of 288 patients with a life history of at least one febrile seizure will be randomized to receive either carbogen (5% CO2 plus 95% O2) or placebo (100% O2). As recurrences of febrile seizures mainly occur at home, the study medication will be administered by the parents through a low-pressure can fitted with a respiratory mask. The primary outcome measure is the efficacy of carbogen to interrupt febrile seizures. As secondary outcome parameters we assess safety, practicability to use the can, quality of life, contentedness, anxiousness and mobility of the parents.

Prospect: The CARDIF trial has the potential to develop a new therapy for the suppression of febrile seizures by redressing the normal physiological state. This would offer an alternative to the currently suggested treatment with benzodiazepines. This study is an example of academic translational research from the study of animal physiology to a new therapy.

Title: New breathing therapy reduces panic and anxiety by reversing hyperventilation

Authors: Rosenfield, D., Seidel, A., Bhaskara, L., & Hofmann, S.

Journal: J. Med. Sci 11, no. 8 (2011): 327-328.

Link to full text: New breathing therapy reduces panic and anxiety by reversing hyperventilation

Abstract: A new treatment that helps people with panic disorder to normalize their breathing works better to reduce panic symptoms and hyperventilation than traditional cognitive therapy, according to a new study. The new 'CART' treatment was found to be better than traditional cognitive therapy at altering hyperventilation and panic symptoms.

Title: Spinal cord transection inhibits HR reduction in anesthetized rats immersed in an artificial CO2-hot spring bath

Authors: Yamamoto N, Hashimoto M.

Journal: Int J Biometeorol. 2007 Jan;51(3):201-8. doi: 10.1007/s00484-006-0055-6. Epub 2006 Sep 7. PMID: 16957940.

Link to PubMed: Spinal cord transection inhibits HR reduction in anesthetized rats immersed in an artificial CO2-hot spring bath

Abstract: Like humans, the heart rate (HR) of anesthetized rats immersed in CO(2)-water is lower than that when immersed in tap water at the same temperature. To investigate the afferent signal pathway in the mechanism of HR reduction, Wistar rats were anesthetized with urethane and then the spinal cord was transected between T(4) and T(5). The animals were immersed up to the axilla in a bathtub of tap-water (CO(2) contents: 10-20 mg.l(-1)) or of CO(2)-water (965-1,400 mg.l(-1)) at 35 degrees C while recording HR, arterial blood pressure, and arterial blood gas parameters (PaCO(2), PaO(2), pH). Arterial blood gas parameters did not change during immersion, irrespective of CO(2) concentration of the bath water, whereas the HR was reduced in the CO(2)-water bath. The inhalation of CO(2)-mixed gas (5% CO(2), 20% O(2), 75% N(2)) resulted in increased levels of blood gases and an increased HR during immersion in all types of water tested. The HR reduction observed in sham transected control animals immersed in CO(2)-water disappeared after subsequent spinal cord transection. These results show that the dominant afferent signal pathway to the brain, which is involved in inducing the reduced HR during immersion in CO(2)-water, is located in the neuronal route and not in the bloodstream.

Title: Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Authors: Terekhin P, Forster C.

Journal: Neurosci Lett. 2006 May 29;400(1-2):110-4. doi: 10.1016/j.neulet.2006.02.040. Epub 2006 Mar 6. PMID: 16517071.

Link to PubMed: Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Abstract: Stress, acute pain and chronic pain may often result in hyperventilation (HV) which produces hypocapnia. The aim of this fMRI-study was to investigate the influence of hypocapnia on cortical activation during noxious stimulation in 14 healthy volunteers. The intensity of voluntary HV was controlled by capnometry Three tasks were performed in the fMRI sessions: (I) three 3-min HV periods with 7-min periods of recovery in between; (II) mechanically induced phasic pain stimulation--pain task (PT); (III) tapping--motor task (MT). The last two of these protocols were performed under normocapnic and hypocapnic conditions. HV decreased the fMRI signal by 3-7% in all regions of the cortex and subcortical nuclei. This decrease was most prominent in the opercular, frontal and temporal areas. When the PT was performed during hypocapnia a strong reduction in cluster sizes and lower t-values in S1 and insular cortex were found. In contrast MT was accompanied by an increase in cluster sizes and higher t-values. From this we conclude that hypocapnia significantly influences the BOLD signal in nociceptive and motor systems, indicating that either the coupling between the BOLD effect and neuronal processing changed or that the activity in the cortical network which represents the pain processing is decreased. These effects should be considered for functional brain imaging studies on the nociceptive system.

Title: Adenosine and ATP link PCO2 to cortical excitability via pH

Authors: Dulla CG, Dobelis P, Pearson T, Frenguelli BG, Staley KJ, Masino SA.

Journal: Neuron. 2005 Dec 22;48(6):1011-23. doi: 10.1016/j.neuron.2005.11.009. PMID: 16364904; PMCID: PMC1924599.

Link to full text: Adenosine and ATP link PCO2 to cortical excitability via pH

Abstract: In addition to affecting respiration and vascular tone, deviations from normal CO(2) alter pH, consciousness, and seizure propensity. Outside the brainstem, however, the mechanisms by which CO(2) levels modify neuronal function are unknown. In the hippocampal slice preparation, increasing CO(2), and thus decreasing pH, increased the extracellular concentration of the endogenous neuromodulator adenosine and inhibited excitatory synaptic transmission. These effects involve adenosine A(1) and ATP receptors and depend on decreased extracellular pH. In contrast, decreasing CO(2) levels reduced extracellular adenosine concentration and increased neuronal excitability via adenosine A(1) receptors, ATP receptors, and ecto-ATPase. Based on these studies, we propose that CO(2)-induced changes in neuronal function arise from a pH-dependent modulation of adenosine and ATP levels. These findings demonstrate a mechanism for the bidirectional effects of CO(2) on neuronal excitability in the forebrain.

Title: Application of carbon-based nanomaterials in Alzheimer's disease

Authors: Bai M, Shao X, Wang C, Wang J, Wang X, Guan P, Hu X.

Journal: Mater Horiz. 2025 Feb 3;12(3):673-693. doi: 10.1039/d4mh01256a. Erratum in: Mater Horiz. 2025 Feb 3;12(3):1018. doi: 10.1039/d4mh90125h. PMID: 39526325.

Link to PubMed: Application of carbon-based nanomaterials in Alzheimer's disease

Abstract: Alzheimer's disease (AD) is a chronic, progressive neurodegenerative disorder marked by permanent impairment of brain function across the whole brain. This condition results in a progressive deterioration of cognitive function in patients and is frequently associated with psychological symptoms such as agitation and anxiety, imposing a significant burden on both patients and their families. Nanomaterials possess numerous distinctive physical and chemical features that render them extensively utilized. In the biomedical domain, nanomaterials can be utilized for disease prevention and therapy, including medication delivery systems, biosensors, and tissue engineering. This article explores the etiology and potential molecular processes of AD, as well as the application of carbon-based nanomaterials in the diagnosis and treatment of AD. Some of such nanomaterials are carbon quantum dots, carbon nanotubes, and graphene, among others. These materials possess distinctive physicochemical features that render them highly promising for applications in biosensing, drug delivery, neuroprotection, and photothermal treatment. In addition, this review explored various therapeutic approaches for AD in terms of reducing inflammation, preventing oxidative damage, and inhibiting Aβ aggregation. The advent of carbon nanomaterials in nanotechnology has facilitated the development of novel treatment approaches for Alzheimer's disease. These strategies provide promising approaches for early diagnosis, effective intervention and neuroprotection of the disease.

Title: Association of Hypocapnia in Children with Febrile Seizures

Authors: Sachan D, Goyal S.

Journal: J Pediatr Neurosci. 2018 Oct-Dec;13(4):388-391. doi: 10.4103/JPN.JPN_73_18. PMID: 30937077; PMCID: PMC6413613.

Link to full text: Association of Hypocapnia in Children with Febrile Seizures

Abstract: Introduction: Febrile seizure is a benign condition in children. Susceptibility genes associated with febrile convulsions have been identified, but the precise pathophysiologic mechanism that triggers febrile seizure is unclear. Using animal models, it has been demonstrated that hyperthermia causes respiratory alkalosis with consequent brain alkalosis and seizures. This study was conducted to find out any association of febrile seizures with fever induced hypocapnia.

Methods: We conducted a cross sectional observational study and enrolled 45 children presenting with febrile seizures. Axillary temperature was measured and venous blood gas analysis was done soon after admission and within 24 hour of seizure onset. Mean pH and pCO2 from venous blood gas analysis was measured and compared with standard normal values. Data was analyzed using SPSS software version 17.0 software.

Results: The mean pCO2 (27.95 ± 5.31mmHg) was much below normal range, and 91% of children had hypocapnia (pCO2 <35) after the febrile seizures. However alkalosis (pH > 7.45) was demonstrated in only 20% of children. Also pCO2 levels in samples drawn before 2 hours were significantly less than those taken after 2 hours (23.24 ± 3.44 vs 29.29 ± 4.99 respectively; p = 0.001).

Conclusion: Our data indicates that febrile seizures may be associated with fever induced hyperventilation and ensuing hypocapnia may be one of the precipitating factor in inducing seizures. However, well-structured human trials are needed to demonstrate the same.

Keywords: Febrile seizures; hypocapnia; respiratory alkalosis; venous blood gas.

Title: Cerebrospinal fluid lipidomic fingerprint of obstructive sleep apnoea in Alzheimer's disease

Authors: Dakterzada F, Benítez ID, Targa A, Carnes A, Pujol M, Jové M, Mínguez O, Vaca R, Sánchez-de-la-Torre M, Barbé F, Pamplona R, Piñol-Ripoll G.

Journal: Alzheimers Res Ther. 2023 Aug 7;15(1):134. doi: 10.1186/s13195-023-01278-7. PMID: 37550750; PMCID: PMC10408111.

Link to full text: Cerebrospinal fluid lipidomic fingerprint of obstructive sleep apnoea in Alzheimer's disease

Abstract: Background: Obstructive sleep apnoea (OSA) has a high prevalence in patients with Alzheimer's disease (AD). Both conditions have been shown to be associated with lipid dysregulation. However, the relationship between OSA severity and alterations in lipid metabolism in the brains of patients with AD has yet to be fully elucidated. In this context, we examined the cerebrospinal fluid (CSF) lipidome of patients with suspected OSA to identify potential diagnostic biomarkers and to provide insights into the pathophysiological mechanisms underlying the effect of OSA on AD.

Methods: The study included 91 consecutive AD patients who underwent overnight polysomnography (PSG) to diagnose severe OSA (apnoea-hypopnea index ≥ 30/h). The next morning, CSF samples were collected and analysed by liquid chromatography coupled to mass spectrometry in an LC-ESI-QTOF-MS/MS platform.

Results: The CSF levels of 11 lipid species were significantly different between AD patients with (N = 38) and without (N = 58) severe OSA. Five lipids (including oxidized triglyceride OxTG(57:2) and four unknown lipids) were significantly correlated with specific PSG measures of OSA severity related to sleep fragmentation and hypoxemia. Our analyses revealed a 4-lipid signature (including oxidized ceramide OxCer(40:6) and three unknown lipids) that provided an accuracy of 0.80 (95% CI: 0.71-0.89) in the detection of severe OSA. These lipids increased the discriminative power of the STOP-Bang questionnaire in terms of the area under the curve (AUC) from 0.61 (0.50-0.74) to 0.85 (0.71-0.93).

Conclusions: Our results reveal a CSF lipidomic fingerprint that allows the identification of AD patients with severe OSA. Our findings suggest that an increase in central nervous system lipoxidation may be the principal mechanism underlying the association between OSA and AD.

Keywords: Alzheimer's disease; Biomarker; Cerebrospinal fluid; Diagnosis; Lipidomics; Obstructive sleep apnoea; STOP-Bang questionnaire.

Title: Cerebrovascular reactivity to carbon dioxide in Alzheimer's disease

Authors: Glodzik L, Randall C, Rusinek H, de Leon MJ.

Journal: J Alzheimers Dis. 2013;35(3):427-40. doi: 10.3233/JAD-122011. PMID: 23478306; PMCID: PMC3776495.

Link to full text: Cerebrovascular reactivity to carbon dioxide in Alzheimer's disease

Abstract: There is growing evidence that cerebrovascular reactivity to carbon dioxide (CVRCO2) is impaired in Alzheimer's disease (AD). Preclinical and animal studies suggest chronic hypercontractility in brain vessels in AD. We review (a) preclinical studies of mechanisms for impaired CVRCO2 in AD; (b) clinical studies of cerebrovascular function in subjects with AD dementia, mild cognitive impairment (MCI), and normal cognition. Although results of clinical studies are inconclusive, an increasing number of reports reveal an impairment of vascular reactivity to carbon dioxide in subjects with AD, and possibly also in MCI. Thus, CVRCO2 may be an attractive means to detect an early vascular dysfunction in subjects at risk.

Title: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Authors: Wolpe J.

Journal: J Nerv Ment Dis. 1987 Mar;175(3):129-33. doi: 10.1097/00005053-198703000-00001. PMID: 3102688.

Link to PubMed: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Abstract: A lucky chance more than 30 years ago revealed the remarkable efficacy of single inhalations of high concentrations of carbon dioxide in eliminating or markedly reducing free-floating anxiety. The reduction of anxiety lasts for days, weeks, or longer--well beyond the persistence of carbon dioxide in the body. The effects are explicable on the hypothesis that free-floating anxiety is anxiety conditioned to continuously present sources of stimulation, such as background noise or the awareness of space or time, and that the anxiety response habit is weakened when the anxiety is inhibited by the competition of responses that carbon dioxide induces. More recently, it has become apparent that inhalations of carbon dioxide, applied in a different manner, are effective in overcoming maladaptive anxiety responses to specific stimuli, e.g., social stimuli. The substance is also proving to be a valuable resource in the treatment of the common variety of panic attacks.

Title: Corticospinal excitability is associated with hypocapnia but not changes in cerebral blood flow

Authors: Hartley GL, Watson CL, Ainslie PN, Tokuno CD, Greenway MJ, Gabriel DA, O'Leary DD, Cheung SS.

Journal: J Physiol. 2016 Jun 15;594(12):3423-37. doi: 10.1113/JP271914. Epub 2016 Feb 24. PMID: 26836470; PMCID: PMC4908026.

Link to full text: Corticospinal excitability is associated with hypocapnia but not changes in cerebral blood flow

Abstract: Key points: Reductions in cerebral blood flow (CBF) may be implicated in the development of neuromuscular fatigue; however, the contribution from hypocapnic-induced reductions (i.e. P ETC O2) in CBF versus reductions in CBF per se has yet to be isolated. We assessed neuromuscular function while using indomethacin to selectively reduce CBF without changes in P ETC O2 and controlled hyperventilation-induced hypocapnia to reduce both CBF and P ETC O2. Increased corticospinal excitability appears to be exclusive to reductions in P ETC O2 but not reductions in CBF, whereas sub-optimal voluntary output from the motor cortex is moderately associated with decreased CBF independent of changes in P ETC O2. These findings suggest that changes in CBF and P ETC O2 have distinct roles in modulating neuromuscular function.

Abstract: Although reductions in cerebral blood flow (CBF) may be involved in central fatigue, the contribution from hypocapnia-induced reductions in CBF versus reductions in CBF per se has not been isolated. This study examined whether reduced arterial PCO2 (P aC O2), independent of concomitant reductions in CBF, impairs neuromuscular function. Neuromuscular function, as indicated by motor-evoked potentials (MEPs), maximal M-wave (Mmax ) and cortical voluntary activation (cVA) of the flexor carpi radialis muscle during isometric wrist flexion, was assessed in ten males (29 ± 10 years) during three separate conditions: (1) cyclooxygenase inhibition using indomethacin (Indomethacin, 1.2 mg kg(-1) ) to selectively reduce CBF by 28.8 ± 10.3% (estimated using transcranial Doppler ultrasound) without changes in end-tidal PCO2 (P ETC O2); (2) controlled iso-oxic hyperventilation-induced reductions in P aC O2 (Hypocapnia), P ETC O2 = 30.1 ± 4.5 mmHg with related reductions in CBF (21.7 ± 6.3%); and (3) isocapnic hyperventilation (Isocapnia) to examine the potential direct influence of hyperventilation-mediated activation of respiratory control centres on CBF and changes in neuromuscular function. Change in MEP amplitude (%Mmax ) from baseline was greater in Hypocapnia tha in Isocapnia (11.7 ± 9.8%, 95% confidence interval (CI) [2.6, 20.7], P = 0.01) and Indomethacin (13.3 ± 11.3%, 95% CI [2.8, 23.7], P = 0.01) with a large Cohen's effect size (d ≥ 1.17). Although not statistically significant, cVA was reduced with a moderate effect size in Indomethacin (d = 0.7) and Hypocapnia (d = 0.9) compared to Isocapnia. In summary, increased corticospinal excitability - as reflected by larger MEP amplitude - appears to be exclusive to reduced P aC O2, but not reductions in CBF per se. Sub-optimal voluntary output from the motor cortex is moderately associated with decreased CBF, independent of reduced P aC O2.

Title: Intermittent hypoxia can aggravate motor neuronal loss and cognitive dysfunction in ALS mice

Authors: Kim SM, Kim H, Lee JS, Park KS, Jeon GS, Shon J, Ahn SW, Kim SH, Lee KM, Sung JJ, Lee KW.

Journal: PLoS One. 2013 Nov 26;8(11):e81808. doi: 10.1371/journal.pone.0081808. PMID: 24303073; PMCID: PMC3841127.

Link to full text: Intermittent hypoxia can aggravate motor neuronal loss and cognitive dysfunction in ALS mice

Abstract: Background: Patients with ALS may be exposed to variable degrees of chronic intermittent hypoxia. However, all previous experimental studies on the effects of hypoxia in ALS have only used a sustained hypoxia model and it is possible that chronic intermittent hypoxia exerts effects via a different molecular mechanism from that of sustained hypoxia. No study has yet shown that hypoxia (either chronic intermittent or sustained) can affect the loss of motor neurons or cognitive function in an in vivo model of ALS.

Objective: To evaluate the effects of chronic intermittent hypoxia on motor and cognitive function in ALS mice.

Methods: Sixteen ALS mice and 16 wild-type mice were divided into 2 groups and subjected to either chronic intermittent hypoxia or normoxia for 2 weeks. The effects of chronic intermittent hypoxia on ALS mice were evaluated using the rotarod, Y-maze, and wire-hanging tests. In addition, numbers of motor neurons in the ventral horn of the spinal cord were counted and western blot analyses were performed for markers of oxidative stress and inflammatory pathway activation.

Results: Compared to ALS mice kept in normoxic conditions, ALS mice that experienced chronic intermittent hypoxia had poorer motor learning on the rotarod test, poorer spatial memory on the Y-maze test, shorter wire hanging time, and fewer motor neurons in the ventral spinal cord. Compared to ALS-normoxic and wild-type mice, ALS mice that experienced chronic intermittent hypoxia had higher levels of oxidative stress and inflammation.

Conclusions: Chronic intermittent hypoxia can aggravate motor neuronal death, neuromuscular weakness, and probably cognitive dysfunction in ALS mice. The generation of oxidative stress with activation of inflammatory pathways may be associated with this mechanism. Our study will provide insight into the association of hypoxia with disease progression, and in turn, the rationale for an early non-invasive ventilation treatment in patients with ALS.

Title: Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Authors: Chew H, Solomon VA, Fonteh AN.

Journal: Front Physiol. 2020 Jun 9;11:598. doi: 10.3389/fphys.2020.00598. PMID: 32581851; PMCID: PMC7296164.

Link to full text: Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Abstract: Lipids constitute the bulk of the dry mass of the brain and have been associated with healthy function as well as the most common pathological conditions of the brain. Demographic factors, genetics, and lifestyles are the major factors that influence lipid metabolism and are also the key components of lipid disruption in Alzheimer's disease (AD). Additionally, the most common genetic risk factor of AD, APOE ϵ4 genotype, is involved in lipid transport and metabolism. We propose that lipids are at the center of Alzheimer's disease pathology based on their involvement in the blood-brain barrier function, amyloid precursor protein (APP) processing, myelination, membrane remodeling, receptor signaling, inflammation, oxidation, and energy balance. Under healthy conditions, lipid homeostasis bestows a balanced cellular environment that enables the proper functioning of brain cells. However, under pathological conditions, dyshomeostasis of brain lipid composition can result in disturbed BBB, abnormal processing of APP, dysfunction in endocytosis/exocytosis/autophagocytosis, altered myelination, disturbed signaling, unbalanced energy metabolism, and enhanced inflammation. These lipid disturbances may contribute to abnormalities in brain function that are the hallmark of AD. The wide variance of lipid disturbances associated with brain function suggest that AD pathology may present as a complex interaction between several metabolic pathways that are augmented by risk factors such as age, genetics, and lifestyles. Herewith, we examine factors that influence brain lipid composition, review the association of lipids with all known facets of AD pathology, and offer pointers for potential therapies that target lipid pathways.

Keywords: amyloid precursor protein; apolipoproteins; blood-brain barrier; energy metabolism; inflammation; late-onset Alzheimer's disease; mitochondria; myelination.

Title: One Small Step for Mouse: High CO2 Inhalation as a New Therapeutic Strategy for Parkinson's Disease

Authors: Nadeev AD, Kritskaya KA, Fedotova EI, Berezhnov AV.

Journal: Biomedicines. 2022 Nov 6;10(11):2832. doi: 10.3390/biomedicines10112832. PMID: 36359351; PMCID: PMC9687253.

Link to full text: One Small Step for Mouse: High CO2 Inhalation as a New Therapeutic Strategy for Parkinson's Disease

Abstract: Parkinson's disease (PD) is a ubiquitous neurodegenerative disorder for which no effective treatment strategies are available. Existing pharmacotherapy is aimed only at correcting symptoms and slowing the progression of the disease, mainly by replenishing dopamine deficiency. It is assumed that mitochondrial dysfunction plays a key role in the pathogenesis of PD. It has been suggested that activation of specific degradation of damaged mitochondria (mitophagy) may prevent cell death. An almost exclusive way to initiate mitophagy is acidification of intracellular pH. We attempted to implement transient brain acidification using two experimental therapy strategies: forced moderate physical activity and high CO2 inhalation. The beneficial effects of CO2 supplementation on behavioral aspects were demonstrated in a rotenone-induced PD model. Mice treated with CO2 restored their exploratory behavior and total locomotor activity lost after rotenone administration. Additionally, this treatment enabled the removal of impaired coordination. We have illustrated this therapeutic strategy using histological studies of brain sections to confirm the survival of nigrostriatal areas. These findings suggest that high CO2 inhalation presumably initiates mitophagy via transient brain acidification, and can treat PD-like symptoms in a rodent rotenone model of PD.

Keywords: Parkinson's disease; experimental therapy; hypercapnia; neurodegeneration; neuroprotection.

Title: Oxidative stress and cellular pathologies in Parkinson's disease

Authors: Puspita L, Chung SY, Shim JW.

Journal: Mol Brain. 2017 Nov 28;10(1):53. doi: 10.1186/s13041-017-0340-9. PMID: 29183391; PMCID: PMC5706368.

Link to full text: Oxidative stress and cellular pathologies in Parkinson's disease

Abstract: Parkinson's disease (PD) is a chronic and progressive neurodegeneration of dopamine neurons in the substantia nigra. The reason for the death of these neurons is unclear; however, studies have demonstrated the potential involvement of mitochondria, endoplasmic reticulum, α-synuclein or dopamine levels in contributing to cellular oxidative stress as well as PD symptoms. Even though those papers had separately described the individual roles of each element leading to neurodegeneration, recent publications suggest that neurodegeneration is the product of various cellular interactions. This review discusses the role of oxidative stress in mediating separate pathological events that together, ultimately result in cell death in PD. Understanding the multi-faceted relationships between these events, with oxidative stress as a common denominator underlying these processes, is needed for developing better therapeutic strategies.

Keywords: Alpha-synuclein; Dopamine neurons; Mitochondria; Oxidative stress; Parkinson's disease; Reactive oxygen species; Unfolded protein response.

Title: An essay on the shaking palsy. 1817

Authors: Parkinson J.

Journal: 1817. J Neuropsychiatry Clin Neurosci. 2002 Spring;14(2):223-36; discussion 222. doi: 10.1176/jnp.14.2.223. PMID: 11983801.

Link to full text: An essay on the shaking palsy. 1817

Abstract: The advantages which have been derived from the caution with which hypothetical statements are admitted, are in no instance more obvious than in those sciences which more particularly belong to the healing art. It therefore is necessary, that some conciliatory explanation should be offered for the present publication: in which, it is acknowledged, that mere conjecture takes the place of experiment; and, that analogy is the substitute for anatomical examination, the only sure foundation for pathological knowledge.
When, however, the nature of the subject, and the circumstances under which it has been here taken up, are considered, it is hoped that the offering of the following pages to the attention of the medical public, will not be severely censured. The disease, respecting which the present inquiry is made, is of a nature highly afflictive. Notwithstanding which, it has not yet obtained a place in the classification of nosologists; some have regarded its characteristic symptoms as distinct and different diseases, and others have given its name to diseases differing essentially from it; whilst the unhappy sufferer has considered it as an evil, from the domination of which he had no prospect of escape.
The disease is of long duration: to connect, therefore, the symptoms which occur in its later stages with those which mark its commencement, requires a continuance of observation of the same case, or at least a correct history of its symptoms, even for several years. Of both these advantages the writer has had the opportunities of availing himself, and has hence been led particularly to observe several other cases in which the disease existed in different stages of its progress. By these repeated observations, he hoped that he had been led to a probable conjecture as to the nature of the malady, and that analogy had suggested such means as might be productive of relief, and perhaps even of cure, if employed before the disease had been too long established. He therefore considered it to be a duty to submit his opinions to the examination of others, even in their present state of immaturity and imperfection.
To delay their publication did not, indeed, appear to be warrantable. The disease had escaped particular notice; and the task of ascertaining its nature and cause by anatomical investigation, did not seem likely to be taken up by those who, from their abilities and opportunities, were most likely to accomplish it. That these friends to humanity and medical science, who have already unveiled to us many of the morbid processes by which health and life is abridged, might be excited to extend their researches to this malady, was much desired; and it was hoped, that this might be procured by the publication of these remarks.
Should the necessary information be thus obtained, the writer will repine at no censure which the precipitate publication of mere conjectural suggestions may incur: but shall think himself fully rewarded by having excited the attention of those, who may point out the most appropriate means of relieving a tedious and most distressing malady.

Title: Panic anxiety, dyspnea, and respiratory disease. Theoretical and clinical considerations

Authors: Smoller JW, Pollack MH, Otto MW, Rosenbaum JF, Kradin RL.

Journal: Am J Respir Crit Care Med. 1996 Jul;154(1):6-17. doi: 10.1164/ajrccm.154.1.8680700. PMID: 8680700.

Link to PubMed: Panic anxiety, dyspnea, and respiratory disease. Theoretical and clinical considerations

Abstract: There is intriguing evidence suggesting pathophysiologic relationships among dyspnea, hyperventilation, and panic anxiety. The symptoms of panic attacks and pulmonary disease overlap, so that panic anxiety can reflect underlying cardiopulmonary disease and dyspnea can reflect an underlying anxiety disorder. The pathogenesis of panic may be related to respiratory physiology by several mechanisms: the anxiogenic effects of hyperventilation, the catastrophic misinterpretation of respiratory symptoms, and/or a neurobiologic sensitivity to CO2, lactate, or other signals of suffocation. In a subset of patients with PD, incipient pulmonary dysfunction may also contribute to their anxiety symptoms. Patients with pulmonary disease, particularly those with obstructive lung disease, have a high rate of panic symptoms and PD. There is reason to believe that pulmonary disease constitutes a risk factor for the development of panic related to repeated experiences with dyspnea and life-threatening exacerbations of pulmonary dysfunction, repeated episodes of hypercapnia or hyperventilation, the use of anxiogenic medications, and the stress of coping with chronic disease. Panic in pulmonary patients may carry significant morbidity, including phobic avoidance of activity, overly aggressive treatment with anxiogenic medications, and more prolonged and frequent hospitalization. Successful treatment of panic in these patients can improve functional status and quality of life by relieving anxiety and dyspnea. Nonpharmacologic treatment of panic, including cognitive-behavioral approaches, can be useful in patients with concomitant respiratory disease. Sedating medications such as benzodiazepines should be used with caution in patients with pulmonary disease to avoid respiratory depression. Serotonergic antidepressants (SSRIs) and anxiolytics (buspirone) may be effective treatments for panic or generalized anxiety in pulmonary patients and have relatively little potential for significant adverse effects.

Title: The Effects of Hypocapnia on Brain Tissue Pulsations

Authors: Alharbi M, Turner P, Ince J, Oura M, Ebirim KU, Almudayni A, Lecchini-Visintini A, Minhas JS, Chung EML.

Journal: Brain Sci. 2020 Sep 6;10(9):614. doi: 10.3390/brainsci10090614. PMID: 32899967; PMCID: PMC7565182.

Link to full text: The Effects of Hypocapnia on Brain Tissue Pulsations

Abstract: Hypocapnia is known to affect patients with acute stroke and plays a key role in governing cerebral autoregulation. However, the impact of hypocapnia on brain tissue pulsations (BTPs) is relatively unexplored. As BTPs are hypothesised to result from cerebrovascular resistance to the inflow of pulsatile arterial blood, it has also been hypothesised that cerebral autoregulation changes mediated by hypocapnia will alter BTP amplitude. This healthy volunteer study reports measurements of BTPs obtained using transcranial tissue Doppler (TCTD). Thirty participants underwent hyperventilation to induce mild hypocapnia. BTP amplitude, EtCO2, blood pressure, and heart rate were then analysed to explore the impact of hypocapnia on BTP amplitude. Significant changes in BTP amplitude were noted during recovery from hypocapnia, but not during the hyperventilation manoeuvre itself. However, a significant increase in heart rate and pulse pressure and decrease in mean arterial pressure were also observed to accompany hypocapnia, which may have confounded our findings. Whilst further investigation is required, the results of this study provide a starting point for better understanding of the effects of carbon dioxide levels on BTPs. Further research in this area is needed to identify the major physiological drivers of BTPs and quantify their interactions with other aspects of cerebral haemodynamics.

Keywords: BTP; brain tissue pulsations; cerebral autoregulation; hypocapnia.