Fig. 1.

Respiratory data and cerebral blood flow velocity (CBFv) are shown for all subjects before [baseline (Base)] and following head-up tilt measured at the time epochs shown. The increase in expiratory minute volume (V̇e) resulted from an increase in tidal volume (TV) in all groups compared with control (P < 0.001). Respiratory rate (RR) only increased significantly (P < 0.001) in the Panic group. End tidal CO₂ (ETCO₂) decreased similarly and significantly for all groups (P < 0.001), whereas CBFv decreased in parallel, as expected. HV, hyperventilation; Panic, panic disorder; POTS, postural tachycardia syndrome.