EFFECTS OF HYPEROXIA ON MAXIMAL LEG O-2 SUPPLY AND UTILIZATION IN MEN

We studied O-2 transport in the leg to determine if hyperoxia will inc rease the maximal rate of O-2, uptake (VO2max) in exercising muscle. A n increase in inspired O-2 fraction (FIO2) from 0.21 to 1.00 was postu lated to have the following effects: 1) increase the leg VO2max by sim ilar to 5-10%, 2) increase the maximal O-2 delivery [arterial O-2 conc entration.flow (Ca-O2.Q)] by similar to 10%, and 3) raise the leg VO2m ax in proportion to both the femoral venous PO2 and mean leg capillary PO2. To test these hypotheses, 11 men performed cycle exercise to the highest work rates (WR(max)) they could achieve while breathing 100% O-2 (hyperoxia), 21% O-2 (normoxia), and 12% O-2 (hypoxia). Leg VO2, w as derived from duplicate measurements of femoral venous blood flow an d Ca-O2 and femoral venous blood O-2, concentrations (Cv(O2)) at 20, 3 5, 50, 92, and 100% WR(max) in each FIO2. Femoral venous leg Q (Q(leg) ) was measured by the constant-infusion thermodilution technique, and leg O-2 uptake (VO2) was determined by the Fick principle [V-O2 = Q(le g)(CaO2 - Cv(O2))]. Leg VO2max was the mean of duplicate values of VO2 at 100% WR(max) for each FIO2. Hyperoxia increased leg VO2max by 8.1% (P = 0.016) and maximal O-2 delivery by 10.9% (P = 0.005) without cha nging Q(leg). There was a significant increase in femoral venous PO2 ( P <0.001) that was proportionally greater than the increase in leg VO2 max. The results support our first and second maw hypotheses, providin g direct evidence that in normal subjects leg VO2max is limited by O-2 supply during normoxia. Contrary to the third hypothesis, leg VO2max did not increase in proportion to either the femoral venous PO2 or mea n leg capillary PO2 during hyperoxia.