Protective effects of acetazolamide and hyperbaric oxygenation on experimentally induced syncope

The protective effects of acetazolamide and hyperbaric oxygenation on experimentally induced syncope were evaluated in seven healthy male subjects. Syncope was induced by vigorous hyperventilation and Valsalva maneuver. Each subject performed these procedures three times in each of the following conditions: (1) breathing room air at normal atmospheric pressure, (2) breathing 100% oxygen at 2.36 atmospheric pressure, (3) breathing 9% oxygen at 2.36 atmospheres and (4) after intravenous injection of 500 mg of acetazolamide while breathing 100% oxygen at 2.36 atmospheres. With comparable changes of arterial pCO 2 and blood pressure during the hyperventilation-Valsalva maneuver, syncope occurred in 19 of 21 (91%) hyperventilation-Valsalva maneuvers performed at ambient environment, in 18 of 21 (86%) when subject was breathing 9% oxygen at 2.36 atmospheres, in 14 of 21 (67%) when 2.36 atmospheres of 100% oxygen was used, and in only 7 of 21 (33%) when acetazolamide was used in conjunction with hyperbaric oxygen. Syncope was completely prevented by hyperbaric oxygenation in one subject and by the combination of acetazolamide and hyperbaric oxygen in four subjects. These studies demonstrate that cerebral vasodilation induced by acetazolamide combined with increased oxygen delivery to the brain resulting from hyperbaric oxygenation may preserve cerebral function during the period of hypotension and hypocapnia produced by hyperventilation and Valsalva maneuver.