Oxygen uptake, acid-base status, and performance with varied inspired oxygen fractions

Six subjects rode a bicycle ergometer on three occasions breathing 17, 21, or 60% oxygen. In addition to rest and recovery periods, each subject worked for 10 min at 55% of maximal oxygen uptake (VO2 max) and then to exhaustion at approximately 90% VO2 max. Performance time, inspired and expired gas fractions, ventilation, and arterialized venous oxygen tension (PO2), carbon dioxide tension (PCO2), lactate, and pH were measured. VO2, carbon dioxide output, [H+]a, and [HCO3-]a were calculated. Performance times were longer in hyperoxia than in normoxia or hypoxia. However, VO2 was not different at exhaustion in normoxia compared with hypoxia or hyperoxia. During exercise, hypoxia was associated with increased lactate levels and decreased [H+]a, PCO2, and [HCO3-]a. The opposite trends were generally associated with hyperoxia. At exhaustion, [H+]a was not different under any inspired oxygen fraction. These results support the contention that oxygen is not limiting for exercise of this intensity and duration. The results also suggest that [H+] is a possible limiting factor and that the effect of oxygen on performance is perhaps related to control of [H+].