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

Abstract

Changes in cerebral blood flow (CBF) in response to changes in Pa(CO2) were measured by intraaortic injection of 133Xe in 12 patients during hypothermic (23-30° C) cardiopulmonary bypass. In each patient, CBF was determined at two randomly ordered levels of Pa(CO2) 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 Pa(CO2) (uncorrected for body temperature) of 36 ± 4 mmHg (mean ± SD) was associated with a CBF of 13 ± 5 ml·100 g-1·min-1, while a Pa(CO2) of 42 ± 4 mmHg was associated with a CBF of 19 ± 10 ml·100 g-1·min-1. In group II, a Pa(CO2) of 47 ± 3 mmHg was associated with a CBF of 20 ± 8 ml·100 g-1·min-1, and a Pa(CO2) of 53 ± 3 mmHg was associated with a CBF of 26 ± 9 ml·100 g-1·min-1. Within group I, the difference in CBF was significant (P < 0.05); within group II, the difference in CBF was significant at the P < 0.002 level. All CBF measurements were lower than those reported for normothermic, unanesthetized subjects of similar age. The response of the cerebral circulation to changes in CO2 tension was well-maintained during hypothermic cardiopulmonary bypass. CBF increased by an average of 1.07 ± 1.19 (SD) ml·100 g-1·min-1·mmHg-1 increased in temperature-uncorrected Pa(CO2) in group I, and by 1.05 ± 0.54 ml·100 g-1·min-1·mmHg-1 increase in group II.