Operative myocardial protection: does an elevated level of creatine phosphokinase-MB isoenzyme always indicate myocardial necrosis?

Creatine phosphokinase-MB isoenzyme appears in the serum of virtually all patients who have undergone cardiac operations, but whether it represents myocardial cellular necrosis remains controversial. In 17 adult patients who underwent coronary artery bypass grafting or valve replacement, serial determinations of the serum levels of creatine phosphokinase-MB isoenzyme, ultrastructural studies of left ventricular myocardium before aortic cross-clamping and 10 minutes after reperfusion and postoperative electrocardiographic recordings were undertaken. The isoenzyme was detected in all patients; those having valve replacement had higher peak values than those who underwent coronary artery bypass grafting. No correlation was found between the duration of aortic cross-clamping and the peak level of isoenzyme after reperfusion. In spite of appreciable isoenzyme release from the myocardium, all the specimens obtained from this series of patients after reperfusion showed myocardial ultrastructural changes indicative of mild to moderate ischemic damage, considered to be reversible according to the criteria described by Jennings and Schaper. Except for one new Q wave, electrocardiographic changes in this series were nonspecific and transient. It is postulated that when sufficient myocardial mass is involved, such as in global ischemia during cardiac surgery, the MB isoenzyme of creatine phosphokinase leaked from reversibly damaged myocytes may become detectable and elevated in the serum. Thus, although the isoenzyme remains a sensitive indicator of myocardial protection, its presence does not necessarily indicate irreversible damage and myocardial necrosis.