Contrasting effects of isocapnic and hypocapnic hyperventilation on orthostatic circulatory control.

The effects of hyperventilation (HV) on mean arterial pressure (MAP) are variable. To identify factors affecting the MAP response to HV, we dissected the effects of hypocapnic HV (HHV) and isocapnic HV (IHV) and evaluated the effects of acute vs. prolonged HHV. In 11 healthy subjects the cardio- and cerebrovascular effects of HHV and IHV vs. normal ventilation were examined for 15 min in the supine position and also for 15 min during 60 degrees head-up tilt. The end-tidal CO(2) of the HHV condition was set at 15-20 mmHg. With HHV in the supine position, mean cerebral blood flow velocity (mCBFV) declined [95% confidence interval (CI) -43 to -34%], heart rate (HR) increased (95% CI 7 to 16 beats/min), but MAP did not change (95% CI -1 to 6 mmHg). However, an augmentation of the supine MAP was observed in the last 10 min of HHV compared with the first 5 min of HHV (95% CI 2 to 12 mmHg). During HHV in the tilted position mCBFV declined (95% CI -28 to -12%) and MAP increased (95% CI 3 to 11 mmHg) without changes in HR. With supine IHV, mCBFV decreased (95% CI -14 to -4%) and MAP increased (95% CI 1 to 13 mmHg) without changes in HR. During IHV in the tilted position MAP was further augmented (95% CI 11 to 20 mmHg) without changes in CBFV or HR. Preventing hypocapnia during HV resulted in a higher MAP, suggesting two contrasting effects of HV on MAP: hypocapnia causing vasodepression and hyperpnea without hypocapnia acting as a vasopressor.