CHANGES IN CEREBRAL OXYGENATION DURING CORONARY ARTERY BYPASS GRAFTING AND ITS DEPENDENCE ON HAEMATOCRIT, MEAN ARTERIAL PRESSURE AND PARTIAL PRESSURE OF OXYGEN IN ARTERIAL BLOOD
Authors: Sarvesh Pal Singh2,*, Minati Choudhury1, Ujjwal Kumar Chowdhury3, Sandeep Chauhan1
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Background: Cerebral protection has always been an important concern during cardiac surgery. Near infrared spectroscopy (NIRS) can continuously monitor cerebral oxygenation and is increasingly being used as a surrogate measure to ensure the wellbeing of brain. This prospective observational study was designed to observe the changes in cerebral oxygenation in patients undergoing coronary artery bypass surgery (CABG) with the aid of cardiopulmonary bypass (CPB) during hypothermia and normothermia, and to determine if there was any correlation between the cerebral oximetry values and variables like hematocrit (Hct), mean arterial pressure (MAP), partial pressure of oxygen in blood (PaO2), CPB flows and temperature. Methods: Forty patients scheduled to undergo elective CABG with the aid of CPB were enrolled in this study. The regional cerebral oxygen saturation (rso2), haematocrit, (MAP), PaO2, temperature and pump flows during CPB were measured at following time points during the surgery -T1:Baseline before induction of anaesthesia (on room air), T2:After induction of anaesthesia with a FiO2 of 100% , T3:After induction of anaesthesia with a FiO2 of 50% , T4:At the initiation of CPB (the lowest value of rso2 at the time of initiation of CPB), T5:On CPB at 35°C , T6:On CPB at 32°C, T7:On CPB after rewarming at 36°C , T8:After weaning from CPB with a FiO2 of 100% (after protamine administration) and T9:After weaning from CPB with a FiO2 of 50% (just before sternal closure). During CPB, pump flows were also recorded to find any deviation from the standard protocol. Results: The mean baseline rso2 values were 64.35 and 64.97 for right and left frontal lobes, respectively and there was a relative increase in rso2 values with increase in PaO2 levels in the preCPB period. There was a maximum relative decrease of 12% in rso2 values with the initiation of CPB and the values remained below baseline throughout the hypothermic CPB. An insignificant decrease in rso2 values occurred with hypothermia which reversed at rewarming. The rso2 values reached baseline values in the post-CPB period. Based on post hoc analysis we observed that rso2 values could be predicted as 0.329 X per unit change in haematocrit; 0.133 X per unit change in MAP and 0.005 X per unit change in PaO2. Conclusion: In patients undergoing cardiac surgery with CPB cerebral oximetry values were well maintained with maximal decrease of 12% at the time of initiation of CPB. Mild decrease in rso2 occurred with institution of CPB which reversed by the end of rewarming. The rso2 values differ insignificantly during hypothermic CPB. Cerebral oxygenation appears to be influenced by haematocrit, mean arterial pressure and partial pressure of oxygen in blood in pre, during and post CPB period.