Letters to the Editor
Carbon Dioxide Test in Respiratory Panic Disorder Subtype
Dear Editor:
Inhaling high concentrations of carbon dioxide (CO2) has consistently
been shown to provoke panic attacks in patients with panic disorder
(PD) (1). Our objective was to verify the sensitivity to CO2 challenge
of PD patients with respiratory and nonrespiratory subtypes. We
randomly selected 20 PD patients (diagnosed according to DSM-IV
criteria) at the Laboratory of Panic and Respiration in Rio de Janeiro.
The subjects signed a voluntary written inform consent, and the
protocol was approved by our local ethical committee.
To participate, we required the subjects to be at least 18 years
old, to report at least 3 panic attacks in the last 2 weeks, and
to be free of psychotropic drugs for at least 1 week. Exclusion
criteria were any other current mental or major medical disorders,
pregnancy, and substance abuse within the prior 6 months. We assessed
the clinical symptoms of the most severe recent panic attack before
the initial test to classify the subjects as respiratory and nonrespiratory
subtypes (2).
The test was double-blind, and 2 mixtures were used: 1) 35% CO2
and 65% O2, and 2) 100% atmospheric compressed air, given 20 minutes
apart. Participants were asked to exhale as fully as possible, place
the mask on their face, and take a fast, vital-capacity breath,
inhaling either the 35% CO2 mixture or the atmospheric compressed
air. They were asked to hold their breath for 8 seconds. Immediately
after, they were asked to repeat the fast, vital-capacity breath
and hold it again for 8 seconds. After 20 minutes, this test was
repeated with the other gas mixture. We repeated the tests in 2
weeks. During that time, no participants received any psychotropic
drugs.
We defined a CO2-induced panic attack as follows: 1) 4 or more
DSM-IV panic attack symptoms, 2) at least 1 DSM-IV cognitive symptom
(for example, fear of dying or losing control), 3) the sensation
of a spontaneous panic attack, and 4) agreement of 2 investigators.
To compare the differences between the presence of panic attacks
after CO2 challenge, we used Fisher’s exact test.
The patients were 12 women and 8 men with a mean (SD) age of 35.8
(6.9) years. In the respiratory subtype there were 6 women and 5
men with a mean (SD) age of 33.4 (9.8) years, and in the nonrespiratory
subtype there were 6 women and 3 men with a mean (SD) age of 37.8
(4.3) years. There was no difference between the 2 groups in mean
age (t-test, P = 0.677).
In the first CO2 challenge test, 7/11 (63.6%) respiratory PD patients
and 3/9 (33.3%) nonrespiratory PD patients had a panic attack (Fisher’s
exact test, P = 0.024). In the second CO2 challenge (after 2 weeks),
9/11 (81.8%) respiratory PD patients and 3/9 (33.3%) nonrespiratory
PD patients had a panic attack (Fisher’s exact test, P = 0.011).
No patient had a panic attack with atmospheric air.
Our results are similar to those of Biber and Alkin (1). Klein’s
theory (3) could explain why the respiratory panic subtype patients
had higher rates of panic attacks than those of the nonrespiratory
subtype, indicating greater sensitivity to CO2. The characterization
of PD subtypes through CO2 challenge may be useful in elucidating
the biological features, course, and response to treatment of PD.
References
1. Biber B, Alkin T. Panic disorder subtypes: differential
responses to CO2 challenge. Am J Psychiatry 1999;156:739–44.
2. Briggs AC, Stretch DD, Brandon S. Subtyping of panic
disorder by symptom profile. Br J Psychiatry 1993;163:201–9.
3. Klein DF. False suffocation alarms, spontaneous
panics, and related conditions. An integrative hypothesis. Arch
Gen Psychiatry 1993;50:306–17.
Antonio E Nardi , MD
Alexandre M. Valença , MD
Isabella Nascimento , MD
Walter A Zin, MD
Márcio Versiani, MD
Rio de Janeiro, Brazil
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