The 35% CO2 inhalation procedure: test-retest reliability

Individual differences in rat sensitivity to CO2

Feelings of fear, anxiety, dyspnea and panic when inhaling carbon dioxide (CO2) are variable among humans, in part due to differences in CO2 sensitivity. Rat aversion to CO2 consistently varies between individuals; this variation in aversion may reflect CO2 sensitivity, but other personality traits could also account for individual differences in aversion. The aims of this study were to 1) assess the stability of individual differences in rat aversion to CO2, 2) determine if individual differences in sweet reward motivation are associated with variation in aversion to CO2, and 3) assess whether variation in aversion to CO2 is related to individual differences in motivation to approach gains (promotion focus) or maintain safety (prevention focus). Twelve female Sprague Dawley rats were exposed multiple times at three different ages (3, 9 and 16 months old) to CO2 in approach-avoidance testing to assess motivation to avoid CO2 against motivation to gain sweet rewards. Rats were also tested for motivation to find hidden sweet rewards, and for their motivation to approach rewards or darkness. Tolerance to CO2 increased with repeated exposures and was higher at older ages. Individual differences in aversion to CO2 were highly repeatable but unrelated to motivation for sweet rewards or the strength of promotion and prevention focus. These results indicate that individual differences in aversion to CO2 reflect variation in CO2 sensitivity.

Diagnostic Utility of Sodium Lactate Infusion and CO2-35% Inhalation for Panic Disorder

Laboratory measures have played an integral role in diagnosing pathology; however, compared to traditional medicine, psychiatric medicine has lagged behind in using such measures. A growing body of literature has begun to examine the viability and development of different laboratory measures in order to diagnose psychopathologies. The present review examines the current state of development of both sodium lactate infusion and CO2-35% inhalation as potential ancillary measures to diagnose panic disorder (PD). A previously established 3-step approach to identifying laboratory-based diagnostic tests was applied to available literature assessing the ability of both sodium lactate infusion or CO2-35% inhalation to induce panic attacks in PD patients, healthy controls, and individuals with other psychiatric conditions. Results suggest that across the literature reviewed, individuals with PD were more likely to exhibit panic attacks following administration of sodium lactate or CO2-35% compared to control participants. The majority of the studies examined only compared individuals with PD to healthy controls, suggesting that these ancillary measures are underdeveloped. In order to further determine the utility of these ancillary measures, research is needed to determine if panic attacks following administration of these chemical agents are unique to PD, or if individuals with related pathologies also respond, which may be indicative of transdiagnostic characteristics found across disorders.

Evidence for consistent individual differences in rat sensitivity to carbon dioxide

Carbon dioxide (CO₂) gradual-fill is commonly used to kill laboratory rats, but this use remains controversial due to a lack of agreement between studies. Inconsistencies may arise from differences in behaviors measured (e.g. active versus passive behaviors), in how rats cope with threats, or in rat sensitivity to CO₂. The aims of the current study were to 1) describe active and passive responses during CO₂ forced exposure, 2) determine if these responses are consistent within individuals and across aversive stimuli, 3) assess individual differences in aversion to CO₂ in aversion-avoidance and approach-avoidance tests and 4) determine how responses in aversion tests relate to individual differences in behavior during forced exposure. Twelve Sprague Dawley female rats were exposed twice to three treatments: CO₂, oxygen (O₂), and fox scent, and were exposed to CO₂ twice in each aversion test. The change in behavior from baseline was higher for rearing and locomotion when rats were exposed to CO₂ than when exposed to O₂ and fox scent. Responses varied among rats but were consistent across multiple tests within rats. For example, rearing was consistent within individuals between two exposures to CO₂. Similarly, the strength of aversion was consistent within individuals across multiple exposures to CO₂ in aversion-avoidance and approach-avoidance testing. Latency to avoid CO₂ in aversion-avoidance tests was negatively correlated with rearing during CO₂ forced exposure. Collectively, these results indicate that rat responses to CO₂ vary between (but are consistent within) individuals, suggesting that rats vary in CO₂ sensitivity. However, even the less responsive rats avoided CO₂ concentrations far below those necessary to achieve unconsciousness, indicating that all rats likely experience negative states when euthanized with CO₂.

Temporal stability of multiple response systems to 7.5% carbon dioxide challenge

Self-reported anxiety, and potentially physiological response, to maintained inhalation of carbon dioxide (CO₂) enriched air shows promise as a putative marker of panic reactivity and vulnerability. Temporal stability of response systems during low-dose, steady-state CO₂ breathing challenge is lacking. Outcomes on multiple levels were measured two times, one week apart, in 93 individuals. Stability was highest during the CO₂ breathing phase compared to pre-CO₂ and recovery phases, with anxiety ratings, respiratory rate, skin conductance level, and heart rate demonstrating good to excellent temporal stability (ICCs≥0.71). Cognitive symptoms tied to panic were somewhat less stable (ICC=0.58) than physical symptoms (ICC=0.74) during CO₂ breathing. Escape/avoidance behaviors and DSM-5 panic attacks were not stable. Large effect sizes between task phases also were observed. Overall, results suggest good-excellent levels of temporal stability for multiple outcomes during respiratory stimulation via 7.5% CO₂.

A study of myocardial perfusion in patients with panic disorder and low risk coronary artery disease after 35% CO2 challenge

BACKGROUND

We have previously reported that 35% CO2 challenge induced myocardial ischemia in 81% of coronary artery disease (CAD) patients with comorbid panic disorder (PD) and previous positive nuclear exercise stress tests. However, it is yet unclear whether this is the case among CAD patients with PD and normal nuclear exercise stress test results. We hypothesized that a potent mental stressor such as a panic challenge among CAD patients with PD would also induce ischemia in patients with normal exercise stress tests.

METHODS

Forty-one coronary artery disease patients with normal nuclear exercise stress tests (21 patients with PD and 20 without PD) were submitted to a well-established panic challenge test (with 1 vital capacity inhalation of a gas mixture containing 35% CO2 and 65% O2) and injected with Tc-99m-tetrofosmin (Myoview), upon inhalation. Single photon emission computed tomography imaging was used to assess per-panic challenge reversible myocardial ischemia and HR, BP, and a 12 lead ECG was continuously measured during the procedure.

RESULTS

Fifty-eight percent of panic disorder patients (12/21) had a panic attack during the panic challenge vs 15% (3/20) of controls (p=0.005). Only 10% of patients in each group displayed myocardial ischemia per panic challenge.

CONCLUSIONS

These findings suggest that panic attacks among panic disorder patients with lower-risk coronary artery disease may not confer a risk for myocardial ischemia.

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

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 .

Hypersensitivity to hypercapnia: definition/(s)

Empirical evidence indicates that panic disorder (PD) patients experience hypersensitivity to hypercapnia, a condition in which the blood level of carbon dioxide exceeds the normal value. The importance of this research line is substantial and indeed, hypercapnic hypersensitivity has been advanced as a possible endophenotype of panic. Definitions of "hypersensitivity," however, have varied. The purpose of this brief review is to delineate and critique different definitions of hypercapnic hypersensitivity. Several definitions - panic attack rate, panic symptoms including dyspnea, subjective anxiety, and respiratory disturbance - are explored. The review concludes that although no ideal definition has emerged, marked anxiety post-hypercapnia has substantial support as a putative trait marker of PD. The term "subjective hypersensitivity" (Coryell et al., 2001) is re-introduced to denote pronounced anxiety post-hypercapnia and recommended for use along with its previous definition: increased self-reported anxiety measured on a continuous visual analog scale, already widely in use. Due to the well-established link between panic and respiration, definitional candidates focusing on aberrant respiratory response - less investigated as trait markers of PD in high risk studies - warrant scrutiny as well. Several reasons why definitional clarity might be beneficial are presented, along with ideas for future research.

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

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.

The effects of single dose anxiolytic medication on the CO2 models of anxiety: differentiation of subjective and objective measures

This was a double blind, placebo-controlled, 4-way cross-over study in 12 healthy volunteer subjects of the acute effects of three drugs each of which are used in the clinic to treat some forms of anxiety: propranolol 40 mg, hydroxyzine 25 mg, flupentixol 0.5 mg and placebo. Each test session consisted of inhalation of air for 20 min, 10-min rest, inhalation of CO2 7.5% for 20 min, 10-min rest, followed by a single vital capacity inhalation of 35% CO2. The CO2 7.5% was administered at peak drug effect. Subjective effects were measured using Visual Analogue Scales (VAS), the Panic Symptom Inventory and the Generalised Anxiety Disorder Assessment inventory. Twelve subjects participated (eight men), with a mean age of 25.9 years. The expected subjective effects of CO2 were seen and these were significantly different from effects of peak air. However, there were no statistically significant differences between the drugs or between drugs and placebo, indeed there was a trend for some VAS anxiety scores to be higher than placebo in the drug groups. There were some significant differences in cardiovascular responses to CO2, with propranolol significantly decreasing heart rate and flupentixol increasing blood pressure when compared with placebo. The lack of subjective anxiolytic actions of the three drugs contrasts with our previous findings with acute benzodiazepines and chronic selective serotonin reuptake inhibitor administration. It may be that prolonged treatment with these agents would be required to show anxiolytic effects, although it may also be that their efficacy is insufficient to be demonstrated in this model. The lack of anxiolytic actions of propranolol, despite a significant reduction in heart rate, is a further support for a central action of CO2 to produce anxiety.

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

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.

Myocardial perfusion study of panic attacks in patients with coronary artery disease

Panic disorder (PD) and panic-like anxiety have been associated with an increased risk of cardiovascular death. No study has specifically examined the association between panic attacks and ischemia in patients who have coronary artery disease (CAD). We hypothesized that panic attacks would induce myocardial perfusion defects in patients who have CAD and PD. Sixty-five patients who had CAD and positive results with nuclear exercise stress testing (35 with PD and 30 without PD served as controls) underwent a well-established panic challenge test (1 vital capacity inhalation of a gas mixture containing 35% carbon dioxide and 65% oxygen) and were injected with technetium-99m sestamibi at inhalation. Single-photon emission computed tomography was used to assess per-panic challenge perfusion defects, and heart rate, blood pressure, and 12-lead electrocardiogram were continuously measured during the procedure. Patients were not withdrawn from their cardiac medications. Patients who had PD were significantly younger than the controls; otherwise groups did not differ with respect to gender, cardiac medications, nuclear exercise test results, and baseline heart rate and blood pressure. Seventy-four percent of patients (26 of 35) who had PD had a panic attack at inhalation versus 6.7% of controls (2 of 30, p <0.001). As hypothesized, patients who had PD and demonstrated a panic attack were more likely to develop a reversible myocardial perfusion defect than were controls who did not have an attack (80.9% vs 46.4% p = 0.009). Thus, despite being on their cardiac medications, panic attacks preferentially induced significant perfusion defects in patients who had CAD and PD. In conclusion, panic attacks in patients who have CAD appear to be bad for the heart.

Beyond the usual suspects: a cholinergic route for panic attacks

For unknown reasons and through poorly understood mechanisms, people at risk of panic attacks are hypersensitive to suffocative stimuli and experience hyperventilation and anxiety after exposure to heightened concentrations of carbon dioxide. Similarly to the physiological reflex response to hypercapnia in animals and man, the anxious response to carbon dioxide in people with panic disorder is at least partially controlled by the central muscarinic receptors. It is suggested here that some modifications of the cholinergic functions could underlie human individual differences in carbon dioxide sensitivity and proneness to experience panic attacks. The hypothesis is based upon experimental evidence that stressful and potentially harmful stimuli prime relatively long-lasting changes in cholinergic genes expression and cholinergic receptors' regulation. The adaptive sequels of these modifications include protection of the brain from overstimulation, and, at the level of the corticolimbic circuitries, promotion of passive avoidance and learning after stress. The extension of the same modifications to the cholinergic receptors involved in chemoception, however, could lower the threshold for reaction to suffocative stimuli, including carbon dioxide. The exaggerated sensitivity to carbon dioxide observed in humans suffering from panic attacks could then be thought of as an evolutionary cost of the involvement of the cholinergic system in shaping otherwise adaptive responses to stress and threatening stimuli.

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

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.