Response of cerebral blood flow to changes in carbon dioxide tension during hypothermic cardiopulmonary bypass

Changes in cerebral blood flow (CBF) in response to changes in PaCO2 were measured by intraaortic injection of 133Xe in 12 patients during hypothermic (23-30 degrees C) cardiopulmonary bypass. In each patient, CBF was determined at two randomly ordered levels of PaCO2 obtained by varying the rate of gas inflow into the pump oxygenator (Group I, n = 6) or by varying the percentage of CO2 added to the gas inflow (Group II, n = 6). Nasopharyngeal temperature, mean arterial pressure, pump-oxygenator flow, and hematocrit were maintained within a narrow range. In group I, a PaCO2 (uncorrected for body temperature) of 36 +/- 4 mmHg (mean +/- SD) was associated with a CBF of 13 +/- 5 ml X 100 g-1 X min-1, while a PaCO2 of 42 +/- 4 mmHg was associated with a CBF of 19 +/- 10 ml X 100 g-1 X min-1. In group II, a PaCO2 of 47 +/- 3 mmHg was associated with a CBF of 20 +/- 8 ml X 100 g-1 X min-1, and a PaCO2 of 53 +/- 3 mmHg was associated with a CBF of 26 +/- 9 ml X 100 g-1 X min-1. Within group I, the difference in CBF was significant (P less than 0.05); within group II, the difference in CBF was significant at the P less than 0.002 level. All CBF measurements were lower than those reported for normothermic, unanesthetized subjects of similar age.(ABSTRACT TRUNCATED AT 250 WORDS)