Acetazolamide and exercise in sojourners to 6,300 meters--a preliminary study.

To examine the effect of acetazolamide on resting acid-base balance and on exercise performance at extreme altitude, we studied four members of the American Medical Research Expedition to Mount Everest at an altitude of 6,300 meters. After an initial progressive exercise test to exhaustion on a bicycle ergometer, subjects were re-studied after taking acetazolamide 250 mg every 8 h for three doses. We measured venous blood during rest for determination of hemoglobin, hematocrit, 2,3-diphosphoglycerate (DPG), bicarbonate, pH, P50, and arterial oxygen saturation by ear oximeter. The results showed that pH, bicarbonate, and DPG:hemoglobin ratio were lower on acetazolamide, whereas P50 at in vivo conditions was unchanged. Exercise ventilation and oxygen consumption for the same workload were slightly higher after acetazolamide, whereas VCO2/VO2 respiratory exchange ratio (R) was lower, and oxygen saturation was unchanged. Two of four subjects had decreased time at maximum workload on acetazolamide; none had an increased performance. The results of this study show that partial carbonic anhydrase inhibition in individuals sojourning to very high altitude produces a further base deficit and a metabolic acidosis, stimulates ventilation, and may impair maximum exercise performance. Although acetazolamide effectively prevents acute mountain sickness, it does not improve performance, and may even impair exercise performance at extreme altitude.