Prognostic significance of the signal-averaged ECG depends on the time of recording in the postinfarction period

Abstract

Serial recordings of the signal-averaged ECG and the 24-hour ambulatory ECG were obtained from 156 patients with acute myocardial infarction up to 5 days (phase 1), 6 to 30 days (phase 2), and 31 to 60 days (phase 3) after the infarction. Left ventricular ejection fraction by radionuclide ventriculography was also determined in phase 2. The signal-averaged ECG was abnormal during one or more of the three phases in 51 patients (31%). In 35 of these patients (69%) the recording changed category between normal and abnormal with the highest prevalence of abnormal recording occurring during phase 2. Eight patients had ventricular tachycardia/ventricular fibrillation in the first 48 hours after myocardial infarction. The signal-averaged ECG was abnormal in only one of these patients. Twelve patients had late arrhythmic events during the first year of follow-up (four sudden deaths and eight instances of documented ventricular tachycardia or ventricular fibrillation). Nine of the 12 patients had an abnormal signal-averaged ECG in phase 2 and four of these nine had a normal recording in phase 1. Five patients had a transient abnormal signal-averaged ECG in phase 1, whereas six patients had an abnormal recording only in phase 3. None of these 11 patients had an arrhythmic event. Stepwice logistic regression showed that an abnormal signal-averaged ECG in phase 2 has the most significant relation to late arrhythmic events. Both an abnormal signal-averaged ECG and a left ventricular ejection fraction <40%, but not complex ventricular arrhythmias, were independent significant risk factors for late arrhythmic events. The combination of an abnormal signal-averaged ECG in phase 2, left ventricular ejection fraction <40%, and complex ventricular arrhythmias in phase 2 had an arrhythmic event rate of 54% compared to a rate of 2% when results of all three tests were normal. The following conclusions were drawn: (1) The signal-averaged ECG does not predict ventricular tachyarrhythmias during the acute phase of myocardial infarction. (2) Because of the dynamic nature of the signal-averaged ECG in the first 60 days after infarction, its prognostic significance for late arrhythmic events depends on the time of recording. An abnormal recording 6 to 30 days and possibly 6 to 14 days after infarction has the most significant relation to arrhythmic events in the first year. (3) A transient abnormal signal-averaged ECG in the first 5 days after infarction and late development of an abnormality in the second month after infarction are not associated with late arrhythmic events. (4) A battery of noninvasive tests including signal-averaged ECG, radionuclide ventriculography, and ambulatory ECG can help to stratify postinfarction patients into low- and high-risk groups for late arrhythmic events.