Myocardial oxygen balance and cardiopulmonary bypass in patients undergoing coronary artery bypass grafting

The frequency of anaerobic myocardial metabolism was studied in 14 patients undergoing coronary artery bypass surgery during enflurane-supplemented high-dose fentanyl anesthesia and compared with other clinical monitors of myocardial ischemia including the configuration of the pulmonary capillary wedge pressure (PCWP) and electrocardiographic findings. Hemodynamic parameters, coronary sinus blood flow, myocardial oxygen and lactate extractions, and a seven-lead ECG were recorded before and after cannulation of the aorta and vena cava, during total cardiopulmonary bypass (CPB) in a vented heart, during rewarming after global myocardial ischemia and cold cardioplegia, and 15 minutes after coming off bypass. The cannulation for CPB induced no changes in the central or coronary hemodynamics, but four patients had abnormal lactate metabolism. Two of these also had ST segment depression, and two had prominent AC waves on the PCWP tracing. Coronary sinus blood flow and myocardial oxygen extraction were maintained at the beginning of CPB, but lactate extraction decreased markedly or turned to lactate production, and ECG changes indicating myocardial ischemia were seen in five patients. During rewarming and after CPB, all patients had abnormal lactate metabolism despite decreased myocardial oxygen extraction, adequate coronary perfusion pressure, and adequate coronary sinus blood flow. During these periods most patients also had cardiac conduction disturbances that made the interpretation of the ST segment impossible. Only one patient had clearly abnormal AC and V waves on the PCWP tracing after CPB. Two patients had ECG evidence of a perioperative myocardial infarction, but they had no significant clinical consequences. Four patients had a fascicular block at discharge. These results indicate that anaerobic myocardial metabolism is common during and after CPB, and that associated myocardial ischemia cannot always be reliably detected by changes in the ECG or the PCWP tracings.