Invited commentary

Abstract

The reappraisal of the off-pump technique in coronary artery bypass grafting (CABG) has renewed interest in looking into myocardial protection. There is now a growing body of evidence to suggest that avoidance of the use of cardiopulmonary bypass (CPB) during CABG could limit myocardial injury. Penttilä and colleagues should be congratulated for attempting to address this issue further by studying cellular metabolites and myocardial injury in patients undergoing CABG with or without CPB (and cardioplegia). The authors found that off-pump CABG is associated with better myocardial energy preservation and less myocardial damage compared with the conventional approach. In the light of this and other previous clinical observations, can we now conclude that the avoidance of CPB and cardioplegia means better myocardial protection? On close scrutiny, however, the answer to this question is not straightforward. Any clinical study of myocardial revascularization with or without CPB is in reality, a comparison of two entirely different surgical approaches, of which CPB is only a part. Besides CPB and cardioplegia, there are many other differences in the two groups of patients. These include different pharmacological management such as the heparin-protamine dosage; normothermia versus hypothermia; the use of intracoronary shunt or the blower during off-pump procedures (and their effects on endothelial function) and, most importantly, regional versus global ischemia. In fact, myocardial ischemia-reperfusion injury is now believed by many to be an acute inflammatory process in which all the above factors play a role. It would be difficult, therefore, if not impossible, to discern the relative contribution of each of these factors in a clinical study, even though it may be prospective, randomized in nature. Current myocardial protection techniques, using different variations of cardioplegia protocols, are based on the principle of decreasing myocardial energy demand by reducing electromechanical activity, cardiac wall tension, and temperature. None of these are significantly changed in off-pump CABG. Although it is now known that the off-pump approach is associated with a reduced inflammatory response, some important questions remain unanswered. For example, is global ischemia under CPB better or worse for the patient than regional ischemia in a beating, unsupported heart? The patients in the CPB group in the current study had a single period of aortic crossclamping. Whether the significantly higher intraoperative lactate levels detected simply reflect accumulation of the metabolite that cannot be continuously washed out is unclear. Also of note is that the biochemical differences observed between the two groups in this study were not translated into clinical benefits for the patients. The postoperative course in both groups was almost identical, which may again simply reflect the fact that all the patients in this study were relatively young with good left ventricular function. Hence, any potential subtle advantages conferred by one particular technique would not be clinically apparent. Scientific knowledge is built up by small additive increments. The observations made by Penttilä and colleagues in this study are provocative and stimulating. However, more basic scientific investigations would be required to further elucidate the exact mechanisms behind these observations.