Effects of low pulmonary CO2 on panic anxiety

Baseline respiratory parameters in panic disorder: a meta-analysis

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.

Acids in the brain: a factor in panic?

Several methods to experimentally induce panic cause profound acid-base disturbances. Evidence suggests that CO(2) inhalations, lactate infusions and, to a certain extent, voluntary hyperventilation can conceivably lead to a common scenario of brain acidosis in the face of disparate intravascular pH alterations. The importance of this event is reflected in data that support a model in which experimental panic attacks, as proxy to those occurring spontaneously, constitute a response to acute brain acidosis. Given that central CO(2)/H(+) chemoreception is an important drive for ventilation, and many chemosensitive neurons are related to respiration and arousal, this model can explain much of the connection between panic and respiration. We propose that the shared characteristics of CO(2)/H(+) sensing neurons overlap to a point where threatening disturbances in brain pH homeostasis, such as those produced by CO(2) inhalations, elicit a primal emotion that can range from breathlessness to panic.

Panic disorder: from respiration to the homeostatic brain

There is some experimental evidence to support the existence of a connection between panic and respiration. However, only recent studies investigating the complexity of respiratory physiology have revealed consistent irregularities in respiratory pattern, suggesting that these abnormalities might be a vulnerability factor to panic attacks. The source of the high irregularity observed, together with unpleasant respiratory sensations in patients with panic disorder (PD), is still unclear and different underlying mechanisms might be hypothesized. It could be the result of compensatory responses to abnormal respiratory inputs or an intrinsic deranged activity in the brainstem network shaping the respiratory rhythm. Moreover, since basic physiological functions in the organism are strictly interrelated, with reciprocal modulations and abnormalities in cardiac and balance system function having been described in PD, the respiratory findings might arise from perturbations of these other basic systems or a more general dysfunction of the homeostatic brain. Phylogenetically ancient brain circuits process physiological perceptions/sensations linked to homeostatic functions, such as respiration, and the parabrachial nucleus might filter and integrate interoceptive information from the basic homeostatic functions. These physiological processes take place continuously and subconsciously and only occasionally do they pervade the conscious awareness as 'primal emotions'. Panic attacks could be the expression of primal emotion arising from an abnormal modulation of the respiratory/homeostatic functions.

Central and peripheral chemoreflexes in panic disorder

Klein (Arch Gen Psychiatry, 50, 1993, 306-317) has suggested that panic disorder patients have a false suffocation alarm that may be associated with a lowered threshold for carbon dioxide detection. We compared the thresholds and sensitivities of the central and peripheral chemoreflexes between panic disorder patients and age- and sex-matched healthy volunteers to test this aspect of the hypothesis. We used a modified version of Read's rebreathing technique in 11 panic disorder patients and 10 healthy volunteers to examine the peripheral and central chemoreflex characteristics in these two populations. Subjects were examined during three rebreathing tests: training, hyperoxic (central chemoreflex alone) and hypoxic (combined central and peripheral chemoreflex). Panic symptoms were retrospectively assessed between groups using a DSM-IV derived Panic Symptom Scale. Comparisons of panic disorder patients with agoraphobia and healthy volunteers showed no significant differences in sensitivities or thresholds. Klein's hypothesis is not supported by these data. If a false suffocation alarm exists, its triggering may not be implemented within the respiratory chemoreflexes.

Respiratory biofeedback-assisted therapy in panic disorder

The authors describe a new methodologically improved behavioral treatment for panic patients using respiratory biofeedback from a handheld capnometry device. The treatment rationale is based on the assumption that sustained hypocapnia resulting from hyperventilation is a key mechanism in the production and maintenance of panic. The brief 4-week biofeedback therapy is aimed at voluntarily increasing self-monitored end-tidal partial pressure of carbon dioxide (PCO2) and reducing respiratory rate and instability through breathing exercises in patients' environment. Preliminary results from 4 patients indicate that the therapy was successful in reducing panic symptoms and other psychological characteristics associated with panic disorder. Physiological data obtained from home training, 24-hour ambulatory monitoring pretherapy and posttherapy, and laboratory assessment at follow-up indicate that patients started out with low resting PCO2 levels, increased those levels during therapy, and maintained those levels at posttherapy and/or follow-up. Partial dissociation between PCO2 and respiratory rate questions whether respiratory rate should be the main focus of breathing training in panic disorder.

Experimental pathophysiology of panic

In this article, we review how the knowledge of the pathophysiology of panic disorder has expanded, with special emphasis on laboratory models using lactate and carbon dioxide challenges. Experiments in the late 1960s revealed that lactate infusion can induce panic attacks. A prominent feature of these attacks is hyperventilation. Because lactate infusion induces a metabolic alkalosis, one would rather expect a compensatory hypoventilation. For years hyperventilation was thought to be causally linked to panic, but it has since been proven to be a symptom rather than a cause of panic attacks. Similarly, it is not hypocapnia but hypercapnia that has proven to be capable of provoking panic attacks. Carbon dioxide challenges are comparable to lactate infusion in the degree to which they meet the criteria for an ideal model of panic disorder. Experiments with carbon dioxide in first-degree relatives of panic disorder patients and in monozygotic twins support the idea of a constitutional predisposition to panic disorder. Of the various other agents that have been used to trigger panic attacks, cholecystokinin seems particularly promising as a valid laboratory model of panic disorder and may provide valuable data regarding the mechanism of panic attacks. The false suffocation alarm theory, proposed by Klein, is an integrative hypothesis that may account for a large number of the laboratory as well as clinical observations.

Is CO2 inhalation a specific marker for panic disorder? An experimental panic model

Patients with panic disorder (PD) report a transient feeling of anxiety upon inhalation of CO2 enriched air. They rate this experience as similar to symptoms of a panic attack. This hypersensitivity has been proposed as an experimental disease model. This paper examines the validity of single breath inhalation of 35% CO2 as a model for PD.

Voluntary hyperventilation: the influence of duration and depth on the development of symptoms

Hyperventilation is considered an important factor in the development of somatic symptoms or even panic attacks, though its role has recently been disputed. Arguments are often based on findings from the so-called Hyperventilation Provocation Test (HVPT), which is a procedure consisting of voluntarily overbreathing. The HVPT has been widely used for diagnosing Hyperventilation Syndrome and for experimentally eliciting panic attacks. Almost no attention, however, has been paid to standardizing the test and determining critical values with respect to depth and duration of hyperventilation. In the present study, symptom development was examined in 16 healthy subjects who underwent four HVPTs that differed in depth of hyperventilation (end-tidal PCO2 < 2.4 kPa or < 1.9 kPa), as well as duration of hyperventilation (2 or 5 min). Both depth and duration appeared to have an independent effect on the development of symptoms. In the 5-min condition, symptoms appeared mainly within the first 3 min. To be sure that the HVPT is long enough and deep enough to elicit symptoms in most people, a minimum duration of 3 min is advised, with end-tidal PCO2 decreasing to at least 1.9 kPa or dropping well over 50% of baseline.

No chronic hyperventilation in panic disorder patients

Arterial blood gases were measured and base excess calculated in 18 nonpanicking panic disorder (PD) patients, 12 subjects suffering from other anxiety disorders, and 18 normal control subjects. There was neither chronic nor clinically significant acute hyperventilation in either group.

Asthma and emotion: a review

This review of the empirical literature on the relationship between asthma and emotion presents an explanatory model of the connection between them. Asthmatics tend to report and display a high level of negative emotion, and asthma exacerbations have been linked temporally to periods of heightened emotionality. Causality may be bidirectional. Hypothesized mediators for the relationship between asthma and emotionality include vagal and alpha-sympathetic hyperreactivity, predominant obstruction in the larger airways, individual response stereotypy, direct effects of emotion-related facial muscle tension on the airways, the emotional effects of asthma medications, heightened respiratory drive, and hyperventilation. Predictions are presented for research on this model of asthma and emotion, and for the psychological treatment of asthma.

The hyperventilation provocation test in panic disorder

Forty-eight patients with DSM-III-R Panic Disorder underwent a hyperventilation provocation Test (HVPT). Twenty-four patients rated the symptoms induced during the HVPT as similar to those occurring during panic attacks in daily life. Contrary to the classical hyperventilation model of panic, no differences were found in respiratory physiology between recognizers and non-recognizers before and during voluntary hyperventilation. Moreover, recognizers and non-recognizers reported comparable levels of panic and hyperventilation symptoms and state anxiety during panic attacks in daily life. Ten of the recognizers also had a panic attack during the HVPT, independent of any differential CO2 alterations. Compared to non-panickers, panickers obtained higher scores for agoraphobia and depression. On the basis of these results, it is concluded that recognizers or panickers do not show a tendency towards hyperventilation, but that reports of severe panic and hyperventilation symptoms are more closely related to the level of anxiety. These results are more consistent with the cognitive model of panic, which emphasizes the patient's tendency to interpret somatic symptoms catastrophically.

An analysis of panic symptoms during hypercarbia compared to hypocarbia in patients with panic attacks

Twenty panic disorder patients underwent a 35% CO2 challenge test and a hyperventilation provocation test. CO2-induced anxiety proved to correlate significantly with respiratory symptoms. These symptoms appeared to be considerably more severe during CO2 inhalation than during the hyperventilation provocation test, which induced no significant anxiety.

Elevated serum lactate associated with panic attacks induced by hyperventilation

Several lines of evidence suggest that lactate metabolism may be altered in panic disorder. We recently reported exaggerated increases in serum lactate in panic patients following hyperventilation during glucose infusion. In the current study, lactate metabolism was stimulated by hyperventilation following glucose ingestion in 12 panic patients and 12 controls. The seven patients who panicked during hyperventilation exhibited larger increases in serum lactate levels than nonpanicking patients or controls. The lactate response was significantly correlated with peak ratings of anxiety and panic symptoms, but not correlated with insulin or cortisol levels, heart rate, pCO2, adiposity, exercise habits, or diet. Hyperventilation-induced panic appears to be associated with metabolic changes leading to elevated serum lactate.

Hyperventilation-induced panic attacks in panic disorder with agoraphobia

Eight minutes of hyperventilation to an end-tidal PCO2 of less than 20 mmHg led to a panic attack in 7 of 12 patients with panic disorder with agoraphobia and only 1 of 12 normal controls. Patients experienced greater increases in panic symptoms than controls during hyperventilation. Patients who reported more distress from somatic symptoms of hyperventilation during the preceding week were more likely to panic during hyperventilation. Patients who panicked during hyperventilation exhibited a delayed recovery of normocapnia following hyperventilation. Hyperventilation by this protocol is an effective means of inducing panic attacks in the laboratory. A hyperventilation challenge may identify a subgroup of patients for whom hyperventilation symptoms are frequently associated with panic.

Specific sensitivity of patients with panic attacks to carbon dioxide inhalation

One inhalation of 35% CO2 in oxygen was administered to 36 patients with anxiety disorders and 14 healthy controls. Eighteen patients had a diagnosis of panic disorder (PD) and 18 of obsessive-compulsive disorder (OCD). As a placebo control for CO2, compressed air was administered in a double-blind design. Immediately before and after the inhalation, levels of anxiety and DSM-III-R symptoms of panic were assessed. CO2 elicited high levels of subjective anxiety in the PD group. Patients with OCD were hardly affected by the inhalation, and did not differ from healthy controls. These results suggest that CO2 challenge should be considered as a specific probe for subjects with panic-anxiety. It is speculated that CO2 may trigger some as yet undefined mechanisms, possibly linked to ventilation control, which demarcate panic from other types of pathological anxiety.

Hypercarbia versus hypocarbia in panic disorder

In order to compare the panicogenic effects of hypercarbia and hypocarbia in panic disorder (PD), 12 PD patients and 11 healthy controls underwent a 35% CO2 challenge as well as a hyperventilation provocation test in a random cross-over design. Both anxiety and anxiety symptoms proved to be significantly higher during the 35% CO2 challenge in PD patients as compared to the response during 35% CO2 in normals and during hyperventilation in both patients and normals. The results suggest that PD patients are specifically hypersensitive to an increase in pCO2.