Physiologic instability in panic disorder and generalized anxiety disorder

Background: Because panic attacks can be accompanied by surges in physiologic activation, we tested the hypothesis that panic disorder is characterized by fluctuations of physiologic variables in the absence of external triggers. Methods: Sixteen patients with panic disorder, 15 with generalized anxiety disorder, and 19 normal control subjects were asked to sit quietly for 30 min. Electrodermal, cardiovascular, and respiratory measures were analyzed using complex demodulation to quantify variability in physiologic indices. Results: Both patient groups reported equally more anxiety and cardiac symptoms than control subjects, but certain other somatic symptoms, including breathlessness, were elevated only in panic disorder patients. Mean end-tidal pCO2 and respiratory rates were lower, and tidal volume and the number of sighs were higher in panic disorder patients than control subjects. Neither cardiovascular (heart rate, arterial pressure, cardiac output), nor electrodermal instability including sighs distinguished the groups; however, tidal volume instability was greater in panic disorder than generalized anxiety disorder patients or control subjects. Several other respiratory measures (pCO2, respiratory rate, minute volume, duty cycle) showed greater instability in both patient groups than in control subjects. Conclusions: Respiration is particularly unstable in panic disorder, underlining the importance of respiratory physiology in understanding this disorder. Whether our findings represent state or trait characteristics is discussed.