Estimation of infarct size from serum MB creatine phosphokinase activity: Applications and limitations

Results of enzymatic estimates of infarct size have been verified under defined experimental conditions, and close correlations have been obtained between enzymatically and morphologically estimated infarct size in patients. Nevertheless, to provide a basis for improved enzymatic estimates we explored several aspects of the original model. The first order disappearance rate of creatine phosphokinase (CPK) was verified by observed high correlation coefficients of the logarithm of CPK versus time after myocardial infarction in patients or intravenous injection of purified myocardial CPK in dogs. Selected hemodynamic interventions simulating derangements accompanying myocardial infarction including acceleration of heart rate, diminution of cardiac output and reduction of renal or hepatic perfusion in conscious dogs did not markedly alter CPK disappearance. To exclude contributions from noncardiac CPK to enzymatic estimates we performed studies with the MB CPK isoenzyme. Under standard assay conditions, MB CPK was found virtually exclusively in myocardium. Serial serum MB CPK curves paralleled those of total CPK from patients with uncomplicated infarction. Similar MB curves were obtained even in patients whose noncardiac CPK values distorted the total CPK curve after intramuscular injections. The correlation coefficient between infarct size estimated from total CPK and MB CPK was 0.97 in 12 patients with hemodynamically uncomplicated infarction. Thus, hemodynamic perturbations associated with infarction are unlikely to affect CPK disappearance and hence should not lead to spurious enzymatic estimates of infarct size. Furthermore, improved enzymatic estimates can be obtained by quantitative assay of MB CPK, a more specific myocardial marker, avoiding spurious estimates due to contributions from noncardiac enzyme.