Effect of Alpha-Stat vs. pH-Stat Strategies on Cerebral Oximetry During Moderate Hypothermic Cardiopulmonary Bypass

Background: This study was undertaken to compare the effect of alpha-stat vs. pH-stat strategies for acid-base management on regional cerebral oxygen saturation (RsO2) in patients undergoing moderate hypothermic haemodilution cardiopulmonary bypass (CPB).

Methods: In 14 adult patients undergoing elective coronary artery bypass grafting, an awake RsO2 baseline value was monitored using a cerebral oximeter (INVOS 5100). Cerebral oximetry was then monitored continuously following anaesthesia and during the whole period of CPB. Mean +/- SD of RsO2, CO2, mean arterial pressure and haematocrit were determined before bypass and during the moderate hypothermic phase of the CPB using the alpha-stat followed by pH-stat strategies of acid-base management. Alpha-stat was then maintained throughout the whole period of CPB.

Results: The mean baseline RsO2 in the awake patient breathing room air was 59.6 +/- 5.3%. Following anaesthesia and ventilation with 100% oxygen, RsO2 increased up to 75.9 +/- 6.7%. Going on bypass, RsO2 significantly decreased from a pre-bypass value of 75.9 +/- 6.7% to 62.9 +/- 6.3% during the initial phase of alpha-stat strategy. Shifting to pH-stat strategy resulted in a significant increase of RsO2 from 62.9 +/- 6.3% to 72.1 +/- 6.6%. Resuming the alpha-stat strategy resulted in a significant decrease of RsO2 to 62.9 +/- 7.8% which was similar to the RsO2 value during the initial phase of alpha-stat.

Conclusion: During moderate hypothermic haemodilutional CPB, the RsO2 was significantly higher during the pH-stat than during the alpha-stat strategy. However, the RsO2 during pH-stat management was significantly higher than the baseline RsO2 value in the awake patient breathing room air, denoting luxury cerebral perfusion. In contrast, the RsO2 during alpha-stat was only slightly higher than the baseline RsO2, suggesting that the alpha-stat strategy avoids luxury perfusion, but can maintain adequate cerebral oxygen supply-demand balance during moderate hypothermic haemodilutional CPB.