The effect of pump flow on cerebral oxygen metabolism during cardiopulmonary bypass

Abstract

We evaluated effects of pump flow on cerebral metabolism using transcranial Doppler (TCD) during cardiopulmonary bypass (CPB) in 22 adult patients undergoing coronary artery bypass grafting. All the patients were anesthetized with high dose fentanyl. The pump flow was controlled with non-pulsatile roller pump at 2.2-2.5 L/min/m2 in group L and 2.7-3.0 L/min/m2 in group H under alpha-stat acid-base regulation. Pharyngeal temperature was kept at 31 degrees C in steady CPB state. Mean velocity of middle cerebral artery (MCAV) was monitored with TCD fixed on the temple continuously. Cerebral oxygen consumption was estimated by relating the difference in oxygen content between arterial and venous (jugular bulb) blood (AVDO2) to flow velocity. In group L, blood oxygen saturation of jugular bulb (SjO2) was stable during hypothermic period, but decreased significantly during rewarming period. In group H, SjO2 was significantly increased with cooling, but went down to preoperative level during rewarming period. Significant difference of SjO2 between two groups was noticed in rewarming period (52.9 +/- 10.0% in group L and 65.6 +/- 11.8% in group H, p = 0.0133). MCAV tended to decrease with cooling and increase with rewarming, but which was not significant change respectively. Relative cerebral metabolic rate for oxygen (rCMRO2) was defined as the percent change of the product AVDO2 and MCAV. In each group, rCMRO2 was decreased with cooling and increased with rewarming significantly. Especially, rCMRO2 right after CPB discontinued was increased 1.7 times in L group and 2.0 times in group H as much as that of steady state of CPB. It is suggested that cerebral metabolism should be decreased during cooling to 31 degrees C of pharyngeal temperature, 2.2-2.5 l/min/m2 of pump flow was adequate to keep SjO2 stable. On the other hand, it is necessary to increase pump flow to 2.7-3.0 l/min/m2 during rewarming period as cerebral oxygen metabolic demand becomes greater.