Microvascular oxygen transport: impact of a left-shifted dissociation curve

The impact of an increased hemoglobin oxygen affinity (decreased P50) on oxygen transport was evaluated in capillaries of the retractor muscle under nonhypoxic (FIo2 = 0.30 and 0.21) and hypoxic (FIo2 = 0.10) conditions in hamsters with normal oxygen affinity [control; P50 = 26.1 +/- 1.0 (SD) mmHg, n = 12] and in hamsters with an increased oxygen affinity [treated; P50 = 16.2 +/- 1.6 (SD) mmHg, n = 7] induced by chronic short-term administration of sodium cyanate. Using in vivo video microscopy and computer-aided image analysis, we determined oxygen saturation (SO2) and associated hemodynamic parameters in both arteriolar (n = 30 control, 18 treated) and venular (n = 25 control, 17 treated) capillaries. In response to hypoxia, systemic arterial PO2 decreased to 29.6 +/- 6.0 (SD) mmHg in control animals and 24.7 +/- 3.8 (SD) mmHg in treated animals associated with abrupt decreases in systemic arterial blood pressure and increases in respiratory rate. The decrease in SO2 across the capillary network during nonhypoxic ventilation was 13.3% SO2 for control animals and 11.0% SO2 for treated animals. During hypoxic ventilation, the decrease in SO2 was 9.1% SO2 in control animals and 8.7% SO2 in treated animals. Hemodynamic parameters were not significantly different in the two groups during hypoxia. Estimated end-capillary PO2 was significantly lower in the treated animals. These data indicate that an increased oxygen affinity does not provide an obvious advantage for oxygen transport during hypoxia at the level of the capillary network in resting striated muscle; however, such an advantage might become apparent in the presence of an increased metabolic rate or a more severe hypoxic challenge.