Myocardial energy metabolism and functional recovery in coronary bypass surgery: A comparative study between continuous retrograde warm and mild hypothermic blood cardioplegia

Abstract

The optimal temperature for delivery of continuous retrograde blood cardioplegia is not known. Twenty-two patients admitted to the hospital for elective coronary artery bypass grafting were randomized into the warm (37°) and mild hypothermic (28°C) cardioplegia groups. Changes in myocardial energy metabolism during cardiopulmonary bypass, postoperative efflux of the creatine kinase MB isoform (CK-MB), functional hemodynamic recovery, and postoperative complications were followed. During aortic cross-clamp, the average oxygen consumption was higher in the warm than in the mild hypothermic group (3.4 ± 0.4 vs 2.4 ± 0.3 ml/minute, p = 0.06), as was the average transcardiac (outflow-inflow) pCO₂-difference (1.1 ± 0.1 vs 0.7 ± 0.1 kPa, p = 0.02). The average transcardiac pH-difference during aortic cross-clamp was lower in the warm group (−0.08 ± 0.01 vs −0.06 ± 0.01 U, p = 0.02), reflecting more profound tissue acidosis. In accordance with this, warm hearts produced more lactate, especially towards the end of the aortic cross-clamp period (p = 0.001). The net efflux of adenosine and its degradation products was more prominent during warm than during mild hypothermic cardioplegia (p = 0.016), indicating less degradation of high energy adenylates in the hypothermic group. The functional hemodynamic recovery during the first postoperative day was similar in both patient groups, although the myocardial injury estimated as postoperative leakage of CK-MB was less extensive in the hypothermic group (p = 0.011). There were no major differences in the postoperative complications between the study groups. We conclude that continuous retrograde mild hypothermic blood cardioplegia offers better myocardial protection during coronary bypass surgery than does continuous retrograde warm cardioplegia. The clinical significance of this should be tested in a larger series of patients, especially those with recent myocardial infarction, unstable angina, or severely depressed left ventricular function.