Effect of hematocrit variations on coronary hemodynamics and oxygen utilization

Twenty-five closed-chest pentobarbitalized dogs were used for studying coronary flow dynamics and myocardial oxygen utilization following variations of hematocrit (Hct) by isovolumetric exchange of blood with plasma or packed red cells. Coronary blood flow (133Xe washout) and cardiac output varied inversely with Hct. Coronary systemic, and pulmonary flow resistances varied in the same direction with Hct. Blood viscosity played a significant role in determining the flow resistances in these three regions. Analysis of vascular hindrance (vascular resistance/blood viscosity) suggested that coronary vasodilation occurred following Hct changes beyond the range of 20-60%. In systemic and pulmonary circulations, however, there was vasoconstriction following hemodilution. The range of optimum Hct for maximum O2 transport (blood flow X arterial O2 content) was much wider in coronary (20-60% Hct) than in systemic circulation (40-60% Hct). The O2 consumptions in total body and in myocardium were essentially constant over a wide range of Hct (20-60%). The maintenance of total body O2 consumption over the Hct range of 20-40% was attributable to an increase in A-V O2 extraction. The O2 extraction ratio in the coronary circulation was constant over the entire range of Hct studied, suggesting that the myocardial O2 consumption was primarily determined by the coronary O2 transport.