STROMA-FREE HEMOGLOBIN - ITS PRESENCE IN PLASMA DOES NOT IMPROVE OXYGEN-SUPPLY TO THE RESTING HINDLIMB VASCULAR BED OF HEMODILUTED DOGS

In hemodilution, red cell spacing in the microcirculation is increased, flow distribution may become more heterogeneous, and, as a result, oxygen supply to tissues may suffer. We tested the hypothesis that oxygen extraction from diluted blood may be enhanced by the presence of hemoglobin in the plasma phase in relatively low concentrations. In anesthetized dogs, the hindlimb vascular bed was isolated and perfused with the animal's own blood by a roller pump. One group of dogs (n = 6) was hemodiluted (hematocrit = 15.0 +/- 1.0%) with a 6% solution of dextran. A second group of dogs (n = 6) was similarly hemodiluted (hematocrit = 16.0 +/- 0.4%) with dextran containing stroma-free hemoglobin solution whereby plasma-phase hemoglobin concentration was raised to 1.1 +/- 0.1 g . dL-1. Systemic hemodynamic observations were made repeatedly over the subsequent 2.5 h, while blood flow to the hindlimb was progressively reduced in stepwise decrements. The hemoglobin-hemodiluted group showed increased systemic arterial blood pressure and total peripheral resistance when compared with the control (dextran diluted) group. The isolated hindlimb also showed evidence of increased vascular resistance in the hemoglobin-treated group. In each individual animal, critical oxygen delivery and extraction were determined by finding the intercept of the supply-independent and supply-dependent portions of the oxygen uptake/oxygen delivery relationship. Neither the critical oxygen delivery rates (5.75 +/- 0.83 vs. 6.41 +/- 0.53 mL . kg-1 . min-1) nor critical oxygen extraction ratios (0.75 +/- 0.03 vs. 0.76 +/- 0.04) were found to be significantly different in the two groups. We conclude that hemoglobin in solution in the plasma at a concentration of about 1 g . dL-1 induces significant increase in vascular resistance and fails to augment oxygen extraction from diluted blood by the resting hindlimb.