The relationship of exercise performance to variations in blood acid-base status induced by changes in the inspired oxygen fraction

This study was undertaken to investigate two possible limiting factors in human exercise performance. Six subjects rode a bicycle ergometer on three different occasions breathing either a 17%, 21%, or 60% oxygen mixture. In addition to rest and recovery periods, each subject worked for 10 minutes at 55% of maximal oxygen uptake and then to exhaustion at about 9l% of maximal oxygen uptake. The values measured included performance time, inspired and expired gas fractions, minute ventilation, heart rate, and "arterialized" venous P0₂, PCO₂, and pH. Oxygen uptake, carbon dioxide output, respiratory exchange ratio, [H⁺] and, [HCo₃^-] were calculated.

The results indicated that performance times were longer in hyperoxia than in normoxia or hypoxia. However, contrary to other published literature, oxygen uptake was not different at exhaustion in normoxia as compared to hypoxia (p>.1) or hyperoxia (p = .09). Also, during exercise the higher inspired oxygen fractions were generally associated with decreased HLa and increased [H⁺], PCO₂, and [HCO₃^-]. However, at exhaustion [H⁺] was not different under any inspired oxygen fraction (p = .8).

These results support the contention that oxygen is not a limiting factor for exercise of the intensity and duration used in this study. The results also suggest that [H⁺] is limiting for such exercise and that the effect of oxygen on performance is related to control of [H⁺].