Capillary oxygen transport during severe hypoxia: role of hemoglobin oxygen affinity

The efficacy of an increased hemoglobin oxygen affinity [decreased oxygen half-saturation pressure of hemoglobin (P50)] on capillary oxygen transport was evaluated in the hamster retractor muscle under conditions of a severely limited oxygen supply resulting from the combined effects of a 40% reduction in systemic hematocrit and hypoxic ventilation (inspired oxygen fraction 0.1). Two groups of hamsters were utilized: one with a normal oxygen affinity (untreated; P50 = 26.1 +/- 2.4 Torr) and one with an increased oxygen affinity (treated; P50 = 15.7 +/- 1.4 Torr) induced by the chronic short-term administration of sodium cyanate. Using in vivo video microscopy and image analysis techniques, we determined oxygen saturation and associated hemodynamics at both ends of the capillary network. During hypoxic ventilation, the decrease in oxygen saturation across the network was 3.6% for untreated animals compared with 9.9% for treated animals. During hypoxia, estimated end-capillary PO2 was significantly higher in the untreated animals. These data indicate that, at the capillary level, a decreased P50 is advantageous for tissue oxygenation when oxygen supply is severely compromised, because normal oxygen losses in capillaries are maintained in treated but not in untreated animals. The data are consistent with the presence of a diffusion limitation for oxygen during severe hypoxia in animals with a normal hemoglobin oxygen affinity.