The relationship between mixed venous and regional venous oxygen saturation during cardiopulmonary bypass

The relationship between mixed venous and regional venous saturation during cardiopulmonary bypass (CPB), and whether this relationship is influenced by temperature, has been incompletely elucidated. Thirty patients undergoing valve and/or coronary surgery were included in a prospective, controlled and randomized study. The patients were allocated to two groups: a hypothermic group (28 degrees C) and a tepid group (34 degrees C). Blood gases were analysed in blood from the hepatic vein and the jugular vein and from mixed venous blood collected before surgery, during hypothermia, during rewarming, and 30 min after CPB was discontinued. Oxygen saturation in the hepatic vein was lower than in the mixed venous blood at all times of measurement (-24.0 +/- 3.0% during hypothermia, -36.5 +/- 2.9% during rewarming, and -30.5 +/- 3.0% postoperatively, p < 0.001 at all time points). In 23% of the measurements, the hepatic saturation was <25% in spite of normal (>60%) mixed venous saturation. There was a statistical correlation between mixed venous and hepatic vein oxygen saturation (r=0.76, p < 0.0001). Jugular vein oxygen saturation was lower than mixed venous saturation in all three measurements (-21.6 +/- 1.9% during hypothermia, p < 0.001; -16.7 +/- 1.9% during rewarming, p < 0.001; and -5.6 +/- 2.2% postoperatively, p = 0.037). No significant correlation in oxygen saturation could be detected between mixed venous and jugular vein blood (r = 0.06, p = 0.65). Systemic temperature did not influence the differences in oxygen saturation between mixed venous and regional venous blood at any time point. In conclusion, regional deoxygenation occurs during CPB, in spite of normal mixed venous saturation. Mixed venous oxygen saturation correlates with hepatic, but not with jugular, vein saturation. The level of hypothermia does not influence differences in oxygen saturation between mixed venous and regional venous blood.