Mathematical analysis of haemodilution's direct effect on rate of brain cooling during cardiopulmonary bypass.

Cerebral hypothermia is the principal means of providing neurologic protection during cardiac surgery. Better understanding is needed of ways to improve brain cooling during bypass. The goal of this study is to find whether haemodilution has a significant direct effect on the rate of brain cooling, from changes in the blood's thermal properties. The brain is cooled during hypothermic cardiopulmonary bypass, almost exclusively, by the colder blood. We use the corresponding component of the bioheat transport model to predict the proportional direct effect of changing blood density and specific heat on the amplitude of the rate of brain cooling. We find that haemodilution can significantly change blood density and specific heat. For example, haemodilution with the fluorocarbon emulsion AF0104 from a haematocrit at 45% to a haematocrit at 22% increases blood density by 18%, and decreases specific heat by 21%. Nevertheless, the mathematical model predicts that the direct effect of haemodilution on the rate of brain cooling by the cold blood is small; +7%, +6% and -7% for normal saline, 5 g dl-1 albumin in normal saline, and AF0104 fluorocarbon emulsion, respectively. We conclude that, within the haemotocrit range used clinically during bypass, haemodilution with these substances has only a small direct effect on the rate of brain cooling.