Cerebral blood flow determinants and their clinical implications during cardiopulmonary bypass

Abstract

Cerebral blood flow (CBF) was measured in 58 patients undergoing elective coronary artery surgery (CABS) prior to, during and following cardiopulmonary bypass. CBF fell significantly during hypothermic bypass and returned to prebypass levels following the end of bypass. At all times CBF, measured as the initial slope index (ISI), was significantly correlated to arterial carbon dioxide tension (PaCO ₂) and cerebral oxygen consumption (CMRO₂). There was no significant correlation between CBF and changes in arterial blood pressure or, when on bypass, perfusion flow. This supports the notion that CBF autoregulation is maintained during bypass under the conditions of this study.The index of cerebral oxygen supply to demand (CERO₂), which reflects the ratio of oxygen consumed by the brain to that supplied by the cerebral circulation, was appropriately matched prior to and following bypass. During bypass the index fell significantly (p<0.001) to levels consistent with a mismatch of flow and demand indicating relative cerebral hyperperfusion. Acid base maintenance during bypass was consistent with a 'pH stat' protocol with pH on bypass approximating, following temperature adjustment, to 7.40. The maintenance of pH stat requires a higher PaCO₂ than does the maintenance of alphastat. It may be that the adoption of an alphastat acid base management protocol, as seen in poikilotherms rather than hibernating mammals, and the resultant reduction in absolute PaCO₂ for any given temperature may result in a lowering of CBF and a return to appropriate matching of CBF and demand during bypass. This may be particularly important in relation to the generation of perfusion related, microembolic, cerebral damage during cardiopulmonary bypass.