This study was designed to demonstrate the changes in brain metabolism/function, and to clarify the reason for assessing brain dysfunction during cardiopulmonary bypass (CPB) with systemic hypothermia. Fifteen patients, who received high-dose fentanyl anesthesia, for cardiac surgery under CPB were analysed concerning systemic hemodynamics, brain oxygen extraction, internal jugular venous oxygen saturation (SjVO2), glucose uptake, and compressed spectral array EEG for brain function. Internal jugular venous temperature decreased from 35.4 +/- 1.0 degrees C (mean +/- SD) at pre-CPB, to 22.3 +/- 1.6 degrees C during CPB. Systemic oxygen delivery decreased by 43% (P < 0.01) with coincided decrease in brain oxygen extraction by 31% (P < 0.01) during CPB. SjVO2 rose from 64.5 +/- 6.3% of pre-CPB level to 78.7 +/- 2.2% during CPB (P < 0.01). Glucose uptake decreased 71% during CPB (P < 0.01). EEG frequency slowed down from 9.0 +/- 0.6 Hz of pre-CPB level to 3.5 +/- 0.3 Hz during CPB (P < 0.01). No patient showed neurological complications during and after surgery. The predictable changes in the variables during brain ischemia/hypoxia may be increase in oxygen extraction, decrease in SjVO2, inhibition of metabolism and low EEG activity. During CPB, EEG activity exhibited identical trend as during ischemia/hypoxia, while the changes in brain oxygen extraction and SjVO2 did not. In conclusion, detection of brain dysfunction by monitoring brain oxygen metabolism or EEG is controversial during CPB using hypothermia.