Regional cerebral oxygenation during cardiopulmonary bypass

Abstract

A significant number of patients suffer transient neuropsychological dysfunction after coronary artery bypass graft (CABG) surgery. Recent studies have implicated reduced levels of O₂ supply/demand ratio during the rewarming phase of cardiopulmonary bypass (CPB). Using a near infrared spectroscopy (NIRS) system that permits continuous noninvasive monitoring of regional cerebral O₂ saturation (rSO₂), we investigated rSO₂ during CPB.

Following Institution Review Board approval and informed consent, 10 patients undergoing elective CABG were studied. Nonpulsatile CPB with a membrane oxygenator, haemodilution and alpha-stat pH management was used. The NIRS system consisting of two low-power laser diode sources (780 nm and 810 nm) with a photodetector placed on the frontal cortex was used to measure continuously haemoglobin, oxyhaemoglobin and cerebral blood volume. Continuous rSO₂ measurements were obtained before, during moderate hypothermia, and during the rewarming phase of CPB. A rSO₂ < 50% was defined as abnormal.

Onset of rSO₂ < 50% was at 32°C, and the highest incidence occurred during the late stages of rewarming (35-37°C). On rewarming from hypothermic CPB, 70% of our patients sustained a rSO₂ < 50% for an average total duration of 9.3 minutes, primarily at the end of the rewarming period. This suggests a transient global imbalance in cerebral O₂ supply and demand in a large percentage of patients during the rewarming phase of CPB. However, it has not been determined how long a rSO₂ < 50% can be permitted before neuropsychological dysfunction is induced. Additional studies correlating psychometric testing with rSO₂ measurements should prove useful in detecting episodes of impaired cerebral oxygenation, and help define rewarming protocols.