EFFECT OF ALTERING PUMP FLOW-RATE ON CEREBRAL BLOOD-FLOW AND METABOLISM IN INFANTS AND CHILDREN

The effects of reduced pump now rate (PFR) on cerebral blood flow, cerebral oxygen consumption (CMRO(2)), oxygen extraction, cerebral vascular resistance, and total body vascular resistance were examined in 27 pediatric patients during hypothermic cardiopulmonary bypass (hCPB). During steady state hCPB the extracorporeal flows were randomly adjusted to a conventional PFR and a reduced PFR for each patient. The reduced pump flow rates were dictated by surgical needs. Cerebral blood now measured using Xenon 133 clearance, and CMRO(2) and oxygen extraction were calculated. Our results demonstrated that cerebral blood now and CMRO(2) are unchanged if pump flow rates are reduced by 35% to 45% of conventional PFRs at moderate and deep hypothermic temperatures. Reductions in PFR of 45%-70% from conventional PFRs affect the brain differently during either moderate or deep hCPB. At moderate hCPB (26 degrees to 29 degrees C), reductions in PFRs of 45% to 70% resulted in a significant decrease in cerebral blood flow and CMRO(2), whereas oxygen extraction increased in a compensatory manner. During deep hCFB (18 degrees to 22 degrees C), PFR reductions of 45% to 70% of conventional PFR significantly reduced cerebral blood flow and CMRO(2) but did not increase oxygen extraction, suggesting that at deep hypothermic temperatures, cerebral blood now and CMRO(2) exceed cerebral metabolic needs. Cerebral vascular resistance increased significantly with decreasing temperature but was not affected by pump now reductions. We have derived indices for minimal acceptable low-flow cardiopulmonary bypass based on the known effects of temperature on cerebral metabolism and have speculated on its utility based on our limited data and a literature review.