BACKGROUND: Substantial alterations in cerebral blood flow(CBF) are known to occur during cardiopulmonary bypass(CPB). Many investigators have speculated that these changes may be responsible for both minor and major cerebral damages after CPB. More recently, these changes in CBF have been observed to be intimately related to the arterial carbon dioxide tension(PaCO2) maintained during CPB. The present study was prospectively designed to investigate the clinical effects of normocapnic and hypercapnic CPB on the cerebral oxygen metabolism in cardiac surgery. MATERIAL AND METHOD: Thirty-six adult patients scheduled for elective cardiac surgery were randomized to either normocapnic group (PaCO2 35~40 mmHg, n=18) or hypercapnic group(PaCO2 45~55 mmHg, n=18) with moderately hypothermic nonpulsatile CPB(nasopharyngeal temperature of 29~30degrees C). In each patient, middle cerebral artery blood flow velocity(VMCA), cerebral arteriovenous oxygen content difference (C(a-v)O2), cerebral oxygen extraction(COE), cerebral metabolic rate for oxygen(CMRO2), cerebral oxygen transport(TEO2), TEO2/CMRO2 ratio, cerebral desaturation(internal jugular bulb blood oxygen saturation Adult ; Carbon Dioxide ; Cardiopulmonary Bypass* ; Humans ; Hypercapnia* ; Incidence ; Metabolism* ; Middle Cerebral Artery ; Oxygen* ; Prospective Studies ; Research Personnel ; Thoracic Surgery