Experimental hypervolemic hemodilution: physiological correlations of cortical blood flow, cardiac output, and intracranial pressure with fresh blood viscosity and plasma volume.

Rheological, cerebrovascular, and cardiovascular alterations induced by serial plasma volume (PV) expansion were evaluated in splenectomized dogs. Seven dogs received two infusions of autologous plasma within 120 minutes; each infusion equaled 20% of the respective dog's total blood volume (TBV). The PV increased 31% and then another 26% after the two respective infusions, and the hematocrit (Hct) was obligatorily decreased by 22% during the experiment. The fresh blood viscosity at the shear rate of 10 sec-1 varied inversely with the TBV and the PV and correlated directly with the Hct after these plasma infusions. Cardiac output (CO) increased 71% after the two infusions without significant alterations in mean arterial blood pressure. Additionally, CO was inversely related to both Hct and blood viscosity. Although the 15% rise in regional cortical blood flow (rCoBF) in the territory of the middle cerebral artery did not reach statistical significance, the rCoBF was related inversely to both Hct and blood viscosity and directly to TBV, PV, and CO after the plasma infusions. Cortical vascular resistance (CVR) decreased 18% after the two infusions. The CVR correlated inversely with PV and directly with Hct and blood viscosity. Our data suggest that hypervolemic hemodilution with expansion of PV increases CO more than cerebral blood flow in normal brain. Fresh blood viscosity seems to be a major factor determining CO and cerebral perfusion after IV expansion. This study adds support to the hypothesis that reductions of blood viscosity account for the direct relationship between cerebral blood flow and CO observed after intravascular volume expansion with hemodiluting agents. Hypervolemic hemodilution with plasma reduces CVR, possibly secondary to its effect on blood viscosity, and also raises intracranial pressure.