Effect of pump flow rate on cerebral blood flow during hypothermic cardiopulmonary bypass in adults

Objective: The purpose of this study was to examine the effect of cardiopulmonary bypass flow rate on cerebral blood flow and cerebral metabolic rate for oxygen during hypothermic (27°C) cardiopulmonary bypass. Design, Setting, and Participants: The investigation was a prospective, randomized study in a tertiary care hospital setting. The 30 participants were volunteer adult cardiac surgical patients at a single institution. Interventions: The N₂O saturation method of Kety and Schmidt was used to determine global cerebral blood flow and metabolic rate during four periods: prebypass, cardiopulmonary bypass (CPB) (27°C) flow rates of 2.3 and 1.2 L/min/m², and 30 minutes post-CPB. Anesthesia consisted of fentanyl end midazolam; pH management was α-star, and mean arterial pressure was maintained at 50 to 70 mmHg throughout CPB. Measurements and Main Results: In the context of an unchanged mean arterial pressure, the pump flow did not affect cerebral blood flow or metabolic rate during hypothermic CPB. Systemic venous oxygen saturation was also maintained during reduced flow at 27°C. Hemodilution during hypothermic CPB maintained cerebral blood flow at prebypass levels. In the postbypass period, persistent hemodilution resulted in an elevated cerebral blood flow. Conclusions: Brain oxygenation is well maintained at lower than conventional pump flow levels during CPB. There may be practical advantages to reduced flows during hypothermia, and flow reductions do not appear to adversely affect cerebral blood flow or metabolism.