Cardiopulmonary bypass (CPB) is one of the major tools of cardiac surgery. However, no clear data are available for the ideal value of sweep gas flow to oxygenator during CPB. The aim of this study was to determine the best value for sweep gas flow during CPB. Thirty patients undergoing isolated CABG were randomly and equally allocated into three groups. Sweep gas flow to oxygenator was kept at 1.35 l/min/m(2) in group 1, 1.60 l/min/m(2) in group 2, and 2.0 l/min/m(2) in group 3. All patients were operated on under the same anaesthetic regime and surgical techniques. Samples for blood gas analysis were collected at T-1: before CPB; T-2: 5 min after the initiation of CPB; T-3: just before rewarming; and T-4: at the end of rewarming. Five minutes after the initiation of CPB (T-2), pCO(2) decreased significantly in groups 2 and 3 compared to group 1 (p<0.02). With the addition of hypothermia (T-3), the changes in the pH and pCO(2) became more profound and, in this period, the levels in group 3 patients outranged the physiologic limits, with pCO(2) and pH values being 28 +/- 3 mmHg and 7.50 +/- 0.04, respectively. At the end of the rewarming period (T-4), in spite of increased carbon dioxide production, pCO(2) values were below the physiologic limits in groups 2 and 3. We conclude that sweep gas flow to the oxygenator should be kept between 1.35 and 1.60 l/min/m(2) during CPB to avoid hypocapnia, which results in alkalosis and has hazardous effects on lung mechanics, cerebral blood flow, and the cardiovascular system.