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 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.