Post myocardial infarction stratification and the signal-averaged electrocardiogram

Abstract

T HE CAUSE OF DEATH in patients who are discharged from the hospital following myocardial infarction (MI) is attributable to three factors: (1) reinfarction resulting in heart failure or a fatal arrhythmia, (2) cardiac rupture, and (3) primary ventricular tachyarrhythmia. Several studies’-” have demonstrated that the majority of patients who die suddenly have a sustained ventricular tachycardia that degenerates into ventricular fibrillation, and a minority have a bradyarrhythmia. Factors that precipitate the arrhythmic event remain poorly understood but likely include ischemia, drugs, premature ventricular depolarizations, and the presence of a substrate with slow conduction characteristics.” Mortality in patients with MI has decreased significantly in the last few years. This decrease has been shown for both early and late mortality. The significant decline in mortality is related to several factors.l’-‘” Among these are (1) the institution of coronary care units, (2) reduction in arrhythmic deaths resulting from ventricular fibrillation, (3) the use of intravenous and oral P-blocking agents, and (4) the use of thrombolytic agents. Whereas in 1966 the l-year mortality was 14% and the 6-year mortality was 42% for patients discharged from the hospital, today in the era of thrombolytic therapy the mortality is in the range of 3% to 5% for 1 year and likely 11% to 13% for 6 years” accounting for a 60% to 75% reduction in mortality. It is likely that mortality will remain high in patients who do not undergo thrombolysis, who are more than 70 years of age, who have multiple infarctions, and who have severe left ventricular dysfunction as well as ventricular arrhythmias. Thus, risk stratification assumes importance so that one can identify low-risk patients who require no intervention and highrisk patients who may require interventional therapy. A variety of tests have been used for risk stratification of patients post-MI. These tests have included exercise electrocardiogram (ECG), 24-hour Holter monitoring, radionuclear angiography, coronary angiography, and programmed ventricular stimulation.z”-3” In recent years the technique of signal averaging in association with high gain amplification has been used to detect late potentials,“‘-3y which are high frequency low amplitude signals that occur in the terminal part of the QRS complex and/or the ST segment. These late potentials have been postulated to be noninvasive markers of the arrhythmic substrate, which is characterized by slow inhomogenous propagation of conduction.l” The purpose of this article is to review the role of the signal-averaged ECG in risk stratification of patients post-MI..