Effects of hypercapnia and hyperoxia on metabolism during exercise.

It has been postulated that the respiratory acidosis associated with hyperoxia (HO) may mediate some of the metabolic effects that are traditionally attributed to the elevation in PaO2. Five subjects performed 30 min of steady-state exercise (65% VO2max) on eight occasions while inspiring either 21 or 60% O2 in combination with 0, 2, 4, or 6% CO2. Statistical significance was accepted if P less than 0.05. The four HO tests were associated with increased VO2 and lower R and blood lactate. However, when compared to the four normoxic tests, all of the hypercapnic (HC) conditions (independent of the inspired O2 percent) had statistically lower blood lactate. Hypercapnia was associated with lower R values and increased blood H+. Regression analysis demonstrated relationships between H+ and R, as well as between H+ and blood lactate. These findings are independent of whether 21 or 60% O2 was inspired, and support the hypothesis that acidosis, not PO2, mediates the effects related to HO.