Effect of hemodilution on cerebral hemodynamics and oxygen metabolism

To evaluate the effects of hemodilution on cerebral hemodynamics and oxygen metabolism in the normal human brain, we measured regional cerebral blood flow, oxygen extraction fraction, oxygen metabolic rate, and regional cerebral blood volume in eight normal volunteers before and after hemodilution using positron emission tomography and oxygen-15-labeled gas inhalation technique.

Hemodilution was accomplished by phlebotomy of 400 ml and drip infusion of 500 ml low molecular weight dextran, which reduced hematocrit from 42.5% to 37.2% and arterial oxygen content from 19.1 to 16.9 ml/dl (both p less than 0.005). It also increased mean cerebral blood flow from 45.2 to 47.7 ml/100 ml/min (p less than 0.025), but decreased tissue oxygen delivery from 8.7 to 8.0 ml/100 ml/min (p less than 0.05) and cerebral blood volume from 4.9% to 4.6% (p less than 0.025) in the overall cortical gray matter.

Our results indicate that hemodilution in the tested range does not improve oxygen transport or tissue oxygenation in the normal human brain, although it increases cerebral blood flow.