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We sought to examine what factors, including cerebrovascular carbon dioxide (CO(2)) reactivity, are related to a decrease in internal jugular venous oxygen saturation (SjvO(2)) during normothermic cardiopulmonary bypass (CPB) in patients with diabetes mellitus.Twenty-three diabetic patients scheduled to undergo elective coronary artery bypass grafting were studied. As a control, 27 age-matched control patients without diabetes mellitus were also examined. After the induction of anesthesia, a fiberoptic oximetry oxygen saturation catheter was inserted into the right jugular bulb to continuously monitor SjvO(2). Arterial and jugular venous blood gases were measured during CPB. The cerebrovascular CO(2) reactivity was measured after the induction of anesthesia and before the start of surgery using a 2.5-MHz pulsed transcranial Doppler probe.The SjvO(2) values in the diabetic group were lower than those in the control group at the initiation of CPB and at 20, 40, and 60 min after the start of CPB. The values for pre- and post-CO(2) reactivity in the control group did not significantly differ (pre-CPB: 4.8% +/- 2.3% mmHg(-1); post-CPB: 5.9% +/- 4.4% mmHg(-1)). In contrast, the values for CO(2) reactivity were lower post CPB than pre-CPB in the diabetic group (Pre-CPB: 6.3% +/- 2.9% mmHg(-1); post-CPB: 4.7% +/- 2.6% mmHg(-1); P0.05). In the diabetic group, glycosylated hemoglobin A1c (HbA1c) is considered to be a factor related to a decrease in SjvO(2) during CPB.Cerebrovascular CO(2) reactivity in diabetic patients decreased after the cessation of CPB but not in the control patients. In addition, HbA1c is also thought to be a factor related to a decrease in SjvO(2) in diabetic patients.