Effects of Hematocrit and Blood Viscosity on Myocardial Blood Flow During Temporary Coronary Occlusions in Dogs

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Abstract
Normal and collateral coronary blood flow was studied in dogs with the Kr85 clearance technique before and after variations in hematocrit and blood viscosity produced by low molecular weight dextran in glucose and water (LMD, mean molecular weight 40,000) and by high molecular weight dextran in glucose and water (HMD, mean molecular weight about 1 million). Following hemodilution with LMD the intact coronary vascular bed reacted by an adequate vasodilatation even for profound reductions in hematocrit. Accordingly, the observed decrease in flow resistance was a combined effect of this dilator response and a lowered blood viscosity. Hemodilution also increased the collateral blood flow to the ischemic muscle areas, but rarely by more than 50 per cent. Hematocrit was found to be more important than blood viscosity in determining the collateral flow resistance. HMD administration raised blood viscosity by 50–80 per cent, resulting in a reduction of the intact myo-cardial blood flow and in the collateral blood supply to ischemic region. After HMD infusion, blood viscosity was the most important determinant of the flow resistance. The effects of variations in the rheological properties of the blood on the normally and collaterally perfused myocardium are discussed.