Correlation Between Cerebral and Mixed Venous Oxygen Saturation During Moderate Versus Tepid Hypothermic Hemodiluted Cardiopulmonary Bypass

Objective: This study was undertaken to compare cerebral oxygen saturation (RsO2) and mixed venous oxygen saturation (SvO2) in patients undergoing moderate and tepid hypothermic hemodiluted cardiopulmonary bypass (CPB).Design: Prospective study.Settings: University hospital operating room.Participants: Fourteen patients undergoing elective coronary artery bypass graft surgery using hypothermic hemodiluted CPB.Interventions: During moderate (28°-30°C) and tepid hypothermic (33°-34°C) hemodiluted CPB, RsO2 and SvO2 were continuously monitored with a cerebral oximeter via a surface electrode placed on the patient’s forehead and with the mixed venous oximeter integrated in the CPB machine, respectively.Measurements and Main Results: Mean ± standard deviation of RsO2, SvO2, PaCO2, and hematocrit were determined prebypass and during moderate and tepid hypothermic phases of CPB while maintaining pump flow at 2.4 L/min/m2 and mean arterial pressure in the 60- to 70-mmHg range. Compared with a prebypass value of 76.0% ± 9.6%, RsO2 was significantly decreased during moderate hypothermia to 58.9% ± 6.4% and increased to 66.4% ± 6.7% after slow rewarming to tepid hypothermia. In contrast, compared with a prebypass value of 78.6% ± 3.3%, SvO2 significantly increased to 84.9% ± 3.6% during moderate hypothermia and decreased to 74.1% ± 5.6% during tepid hypothermia. During moderate hypothermia, there was poor agreement between RsO2 and SvO2 with a gradient of 26%; however, during tepid hypothermia, there was a strong agreement between RsO2 and SvO2 with a gradient of 6%. The temperature-uncorrected PaCO2 was maintained at the normocapnic level throughout the study, whereas the temperature-corrected PaCO2 was significantly lower during the moderate hypothermic phase (26.8 ± 3.1 mmHg) compared with the tepid hypothermic phase (38.9 ± 3.7 mmHg) of CPB. There was a significant and positive correlation between RsO2 and temperature-corrected PaCO2 during hypothermia.Conclusions: During moderate hypothermic hemodiluted CPB, there was a significant increase of SvO2 associated with a paradoxic decrease of RsO2 that was attributed to the low temperature-corrected PaCO2 values. During tepid CPB after slow rewarming, regional cerebral oxygen saturation was increased in association with an increase with the temperature-corrected PaCO2 values. The results show that during hypothermic hemodiluted CPB using the alpha-stat strategy for carbon dioxide homeostasis, cerebral oxygen saturation is significantly higher during tepid than moderate hypothermia.