Cardiopulmonary bypass temperature, hematocrit, and cerebral oxygen delivery in humans

Background.: The neurologic effects of warm heart operations is a subject of popular interest. The purpose of this study was to examine the adequacy of cerebral oxygenation during normothermic cardiopulmonary bypass and better define the relationship between hematocrit, temperature, and brain oxygen delivery. Methods.: Cerebral blood flow, metabolic rate, and oxygen delivery were measured in 60 patients randomized to normothermic (37°C) or hypothermic (27°C) cardiopulmonary bypass. The nitrous oxide saturation technique of Kety and Schmidt was used for cerebral blood flow determinations. Both temperature groups underwent moderate (31%) hemodilution. Results.: During normothermic cardiopulmonary bypass, cerebral blood flow increased secondary to hemodilution and decreased cerebral vascular resistance; a normal matching of oxygen demand and delivery was maintained. During hypothermic bypass, hemodilution and hypothermia had essentially equal, opposing effects on cerebral vascular resistance and blood flow. With hypothermia, brain oxygen demand and delivery were both reduced but not closely coupled. Conclusions.: From the standpoint of global cerebral perfusion and oxygenation, our data support the practice of "warm" heart operations. It clarifies the marked influence of hematocrit on cerebral blood flow and delineates the interaction of temperature and hematocrit on cerebral oxygen delivery. It also suggests that additional investigation to better define "temperature-appropriate" hemodilution during cardiopulmonary bypass is indicated.