Effect of hematocrit and viscosity on coronary circulation and myocardial oxygen utilization.

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Abstract
The elevation of hematocrit is known to provoke myocardial ischemia in various clinical conditions. In order to clarify the effect of hematocrit and blood viscosity on coronary hemodynamics and oxygen utilization, isovolemic exchanges of blood with plasma or packed red cell were performed in ten open chest anesthetized dogs. Hematocrit was varied in the range of 20 to 60%. Coronary blood flow and cardiac output changed inversely with hematocrit. Coronary vascular resistance and blood viscosity elevated in logarithmic relation to hematocrit. Coronary vascular resistance and blood viscosity revealed linear relationship, the coronary vascular hindrance (resistance/viscosity) remaining constant for the hematocrit range of 20 to 60%. The arterial oxygen content increased linearly with the elevation of hematocrit, and the myocardial oxygen extraction rate was unchanged. The systemic oxygen transport rate increased with hematocrit. However, the left ventricular oxygen transport rate varied little, although the left ventricular oxygen transport at medium hematocrit was slightly higher than at low or high hematocrit. The myocardial oxygen consumption remained constant at each hematocrit, and the efficiency was high at low hematocrit in the face of the elevated cardiac output. The increase in hematocrit elevated the blood viscosity and played a significant role in determining the coronary vascular resistance. This could induce a disturbance in coronary microcirculation especially in pathological conditions.