Carbon dioxide/oxygen challenge test in panic disorder.

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.