Effect of blood flow reduction on maximal O2 uptake in canine gastrocnemius muscle in situ.

Abstract

The purpose of this study was to decrease O2 delivery to maximally working muscle by reductions in muscle blood flow (Q), while maintaining hemoglobin concentration and the arterial PO2 (PaO2) constant, to investigate how the decreases in maximal O2 uptake (VO2max) that occur with ischemia are related to changes in the estimated effective muscle O2 diffusing capacity (DO2). Additionally, the relationships among Q, DO2, O2 uptake (VO2), and effluent venous PO2 (PVO2) were used to infer whether the reductions in Q occur uniformly throughout the muscle or whether a nonuniform (greater heterogeneity of Q to VO2) pattern develops. Isolated dog gastrocnemius muscle (n = 6) was stimulated maximally at three levels of muscle blood flow (controlled by pump perfusion): control [C; 119 +/- 3 ml.100 g-1.min-1 (SE)], moderate ischemia (MI; 80 +/- 6), and severe ischemia (SI; 45 +/- 6) in random order. Arterial and venous samples were taken to measure blood gases, O2 concentration, and lactate concentration, whereas a Bohr integration technique using a model based on Fick's law of diffusion was used to estimate mean capillary PO2 and DO2 for each Q condition. VO2max fell progressively (P < 0.05) with Q, even though the O2 extraction ratio (VO2/O2 delivery) increased significantly (C = 67%, MI = 84%, SI = 90%). PVO2 and VO2max fell in proportion to each other from C to MI, but there was not a significant fall in PVO2 from MI to SI. Thus the calculated DO2 did not change between C and MI but fell in proportion to Q between MI and SI. These results suggest that with moderate Q reduction, perfusion falls relatively uniformly throughout the muscle, whereas more severe ischemia leads to nonuniform changes in Q distribution with some areas being poorly perfused to allow more adequate perfusion to other areas.