Peak oxygen intake and hypoxia: influence of physical fitness.

Eight men and eight women each performed peak oxygen intake tests on a cycle ergometer breathing ambient air and a mixture of 12% oxygen in nitrogen (equivalent to an altitude of 4400 m) in the two experiments. Hypoxia induced an average 28% decrease of peak oxygen intake, with a somewhat smaller decrease of power output. There were also small decreases in peak heart rate, peak blood pressure, peak ventilation, and peak blood lactate concentration. The major part of the impairment in oxygen transport was due to a reduction of arterial oxygen saturation, with small contributions from the decrease in heart rate and the decrease of ventilation. Subjects in good physical condition suffered a larger decrement of oxygen transport than their more sedentary colleagues, probably due to an unfavorable ratio of peak diffusing capacity to peak cardiac output. However, in the short term, this handicap could be countered by hyperventilation, and such a tactic could probably improve athletic performance over moderate distances.