The effect of temperature on cerebral metabolism and blood flow in adults during cardiopulmonary bypass

The effect of temperature on cerebral blood flow and metabolism was studied in 41 adult patients scheduled for operations requiring cardiopulmonary bypass. Plasma levels of midazolam and fentanyl were kept constant by a pharmacokinetic model-driven infusion system. Cerebral blood flow was measured by xenon 133 clearance (initial slope index) methods. Cerebral blood flow determinations were made at 27° C (hypothermia) and 37° C (normothermia) at constant cardiopulmonary bypass pump flows of 2 L/min/m². Blood gas management was conducted to maintain arterial carbon dioxide tension (not corrected for temperature) 35 to 40 mm Hg and arterial oxygen tension of 150 to 250 mm Hg. Blood gas samples were taken from the radial artery and the jugular bulb. With decreased temperature there was a significant (p < 0.0001) decrease in the arterial venous-oxygen content difference, suggesting brain flow in excess of metabolic need. For each patient, the cerebral metabolic rate of oxygen consumption at 37° C and 27° C was calculated from the two measured points at normothermia and hypothermia with the use of a linear relationship between the logarithm of cerebral metabolic rate of oxygen consumption and temperature. The temperature coefficient was then computed as the ratio of cerebral metabolic rate of oxygen consumption at 37° C to that at 27° C. The median temperature coefficient for man on nonpulsatile cardiopulmonary bypass is 2.8. Thus reducing the temperature from 37° to 27° C reduces cerebral metabolic rate of oxygen consumption by 64%.