The significance of persistent myocardial perfusion during aortic cross-clamping in myocardial protection

Abstract

Persistent residual myocardial blood flow during aortic cross-clamping can wash out the cardioplegic solution and metabolic end products, continue to deliver substrates, and rewarm the heart. Since microsphere tracers previously used to study “noncoronary” flow could be trapped in the lung before the bronchial circulation reaches the heart, we used xenon 133 washout from the myocardium to determine the residual flow during normothermic cardiopulmonary bypass and aortic cross-clamping. In 16 dogs, with either normal or hypertrophic left ventricles, the persistent myocardial flows during aortic cross-clamping were found to be significant and variable, ranging between 10% and 50% of pre-cross-clamp flows, and were consistently higher in nonvented hearts. The normal hearts with left ventricular venting had residual flow of 13.3 ± 4.0 ml/100 gm/min; those without venting had 37.4 ± 13.0 ml/100 gm/min. The hypertrophic hearts had 9.7 ± 0.7 ml/100 gm/min with venting and 14.1 ± 2.0 ml/100 gm/min without venting. Myocardial biopsy specimens were obtained before and at the end of a 30 minute aortic cross-clamp period, and the myocardial tissue lactate levels were determined. There was an inverse relationship between the magnitude of myocardial residual flow and the tissue lactate accumulation, indicating the metabolic significance of such residual flow. Certain controversies on myocardial protection, which arose due to discrepancies in results obtained from isolated in vitro preparations as compared with those from the in vivo hearts (e.g., the roles of glucose and calcium), may be explained by considering the effects of such residual myocardial flow. Furthermore, the magnitude of myocardial perfusion during aortic cross-clamping can be modulated by a number of maneuvers in order to achieve better myocardial protection during cardiac operations.