Quantification of the effect of altering hematocrit and temperature on blood viscosity.

Rheological changes occurring with the conduct of cardiopulmonary bypass affect the distribution of blood throughout the cardiovascular system. The purpose of this study was to evaluate the effects of changing physical characteristics of fluid on the dynamics of blood flow in an in vitro model. An extracorporeal model simulating coronary vessel constriction was designed that consisted of tubing with varying internal diameters. Tubing sizes were selected as percentage reductions (11, 33, 56, and 78%) of a normal sized (3.6 mm) coronary artery. Flow rates were randomly varied between 150 and 300 mL min -1 temperatures of 6 and 37°C, and hematocrits of 0, 20, and 38%. Endpoints included viscosity, pressure drop, and volume distribution. As temperature fell from 37 to 6°C, viscosity increased with hematocrit as follows: 192% at 0%, 225% at 20%, and 249% at 38%, p <.001. Pressure drop increased significantly across each tubing size ranging from 173-351%, p <.01, as fluid was cooled from 37 to 6°C. However, intraconduit statistical differences in volumetric distribution of flow were not achieved. Although the induced hypothermia resulted in increases in resistance, statistical significance was only seen in the smallest lumen conduit. In conclusion, the effects of changing temperature has profound influence on fluid distribution secondary to changing blood viscosity in an in vitro model for fluid distribution. Knowledge of such flow alterations may aid in determining optimal perfusion strategies where vessel constrictions are encountered.