The effect of altering the inspired oxygen fraction on lactate accumulation during progressive exercise

These experiments were designed to investigate differences in lactate threshold and blood lactate accumulation during progressive bicycle ergometer exercise while breathing hypoxic, normoxic, or hyperoxic gas mixtures. Six subjects pedaled a stationary cycle ergometer to the point of exhaustion on three different occasions while breathing mixtures of 17%, 21%, or 60% O₂. After a 10-minute rest period, the subject rode for 3 minutes at a work rate of 60 W followed by 15 W increases every 3 minutes until exhaustion. Inspired and expired gas fractions, ventilation, heart rate, and arterialized venous lactate were measured. Oxygen uptake (V˙O₂) and carbon dioxide output were calculated for the last minute of each work rate while lactate samples were drawn during the last 10 seconds. V˙O₂ was not significantly different at any work rate among the three different conditions. V˙O₂ max did not differ significantly, but approached significance, among the three treatments. The rate of lactate accumulation was significantly greater during hypoxia as compared to either normoxia or hyperoxia. Lactate concentrations were significantly lower during hyperoxia and significantly higher during hypoxia when compared to normoxia. But when lactate levels were plotted against percent of V˙O₂ max for each treatment, no significant difference was found among treatments. A breaking point at which lactate increased above resting levels was determined. This occurred 15 W higher in hyperoxia than normoxia or hypoxia. Maximal lactate values at exhaustion were not significantly different among the three treatments. Four subjects were able to work for a longer period of time during hyperoxic breathing. These results indicate that variations in lactate threshold and accumulation as reported in this study cannot be explained on the basis of differences in V˙O₂. Differences in transient tissue hypoxia, excess pyruvate and other factors must be considered in an explanation of these data. This research also supports the hypothesis that decreases in lactate and H+ accumulation during hyperoxic breathing result in the increases reported in performance times while breathing hyperoxic mixtures.