Panic Disorder Subtypes: Differential Responses to CO₂ Challenge

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.