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, cer ebral oxygen consumption (CMRO(2)), oxygen extraction, cerebral vascul ar resistance, and total body vascular resistance were examined in 27 pediatric patients during hypothermic cardiopulmonary bypass (hCPB). D uring steady state hCPB the extracorporeal flows were randomly adjuste d to a conventional PFR and a reduced PFR for each patient. The reduce d 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. Reduc tions in PFR of 45%-70% from conventional PFRs affect the brain differ ently during either moderate or deep hCPB. At moderate hCPB (26 degree s to 29 degrees C), reductions in PFRs of 45% to 70% resulted in a sig nificant decrease in cerebral blood flow and CMRO(2), whereas oxygen e xtraction increased in a compensatory manner. During deep hCFB (18 deg rees to 22 degrees C), PFR reductions of 45% to 70% of conventional PF R significantly reduced cerebral blood flow and CMRO(2) but did not in crease oxygen extraction, suggesting that at deep hypothermic temperat ures, cerebral blood now and CMRO(2) exceed cerebral metabolic needs. Cerebral vascular resistance increased significantly with decreasing t emperature but was not affected by pump now reductions. We have derive d indices for minimal acceptable low-flow cardiopulmonary bypass based on the known effects of temperature on cerebral metabolism and have s peculated on its utility based on our limited data and a literature re view.