MAXIMAL O2 UPTAKE OF INSITU DOG MUSCLE DURING ACUTE HYPOXEMIA WITH CONSTANT PERFUSION

We investigated the relationships among maximal O2 uptake (V̇02(max)), effluent venous PO2 (Pv(O2)), and calculated mean capillary PO2 (P̄c(O2)) in isolated dog gastrocnemius in situ as arterial PO2 (Pa(O2)) was progressively reduced with muscle blood flow held constant. The hypothesis that V̇O2(max) is determined in part by peripheral tissue O2 diffusion predicts proportional declines in V̇O2(max) and P̄c(O2) if the diffusing capacity of the muscle remains constant. The inspired O2 fraction was altered in each of six dogs to produce four different levels of Pa(O2) [22 ± 2, 29 ± 1, 38 ± 1, and 79 ± 4 (SE) Torr]. Muscle blood flow, wih the circulation isolated, was held constant at 122 ± 15 ml·100 g-1·min-1 while the muscle worked maximally (isometric twitches at 5-7 Hz) at each of the four different values of Pa(O2). Arterial and venous samples were taken to measure lactate, pH, PO2, PCO2, and muscle V̇O2. P̄c(O2) was calculated using Fick's law of diffusion and a Bohr integration procedure. V̇O2(max) fell progressively (P < 0.01) with decreasing Pa(O2). The decline in V̇O2(max) was proportional (R = 0.99) to the fall in both muscle Pv(O2) and calculated P̄c(O2), although lactate output from the muscle was not different among the four conditions. Fatigue developed more rapidly with lower (Pa(O2), although lactate output from the muscle was not different among conditions. These results are consistent with the hypothesis that resistance to O2 diffusion in the peripheral tissue may be a principal determinant of V̇O2(max).