High blood O2 affinity and relationship of O2 uptake and delivery in resting muscle

To clarify the effects of high oxygen (O2) affinity of blood under stagnant hypoxic conditions, we studied the relationship between VO2 and O2 delivery (CaO2.flow) in isolated dog gracilis muscles perfused with low and normal P50 blood. The high affinity blood was prepared by either short-term incubation (P50 = 24.8 +/- 1.0 Torr) or long-term refrigerated storage (P50 = 19.5 +/- 1.5 Torr) with sodium cyanate. VO2 of the resting muscle was independent of O2 delivery above a critical level (0.45 ml.min-1.100 g-1), but was dependent on it below this level. The critical O2 delivery (0.45 ml.min-1.100 g-1), the maximal O2 extraction ratio (0.62), and the plateau VO2 (0.28 ml.min-1.100 g-1) in the present perfusions with low P50 blood were similar to those with normal blood. "Critical PvO2" (i.e. PvO2 at the critical O2 delivery) was 33 Torr in the perfusions with normal blood but only 19 Torr in the low P50 perfusions. The perfusion pressure-blood flow relationship was not influenced by the affinity change. These results suggest that the true critical PvO2 in resting muscle is lower than 19 Torr and hence that the decrease in VO2 in resting skeletal muscle under stagnant flow conditions is not caused by a decrease in PO2 driving pressure, but by a decrease in the surface area available for diffusion due to a closure of capillaries induced by the low perfusion pressure.