The prognostic significance of quantitative signal-averaged variables relative to clinical variables, site of myocardial infarction, ejection fraction and ventricular premature beats: A prospective study

A prospective study was undertaken of the prognostic significance of quantitative signal-averaged electrocardiographic (ECG) variables relative to clinical variables, site of myocardial infarction, left ventricular ejection fraction and characteristics of ventricular premature beats in 115 patients (mean age 62 ± 12 years) studied 10 ± 6 days after myocardial infarction. Signal-averaged variables included the root mean square voltage of the terminal 40 ms, the duration of the filtered signal-averaged QRS complex and low amplitude signals <40 μV determined at 25 and 40 Hz high pass filtering in all patients. Of the 115 patients, 51 (44%) had an abnormal signal-averaged ECG (one or more abnormal signal-averaged variables), 51 (44%) at 25 Hz and 48 (42%) at 40 Hz high pass filtering. A higher proportion of patients with an inferior wall infarction had an abnormal signal-averaged ECG as compared with patients with anterior wall infarction (58% versus 31%). Over a 14 ± 8 month follow-up period 16 patients (14%) had an arrhythmic event. An abnormal signal-averaged ECG at 40 Hz high pass filtering had a higher sensitivity (81 % versus 75%) and specificity (65% versus 61 %) than at 25 Hz high pass filtering. The predictive value of the signal-averaged ECG was superior to that of the ejection fraction (40% versus 20%) in anterior wall myocardial infarction, whereas in patients with inferior wall infarction, the predictive values of the two tests were equivalent. The prognostic power of 27 clinical and noninvasive variables was determined with the Cox proportional hazards regression model. The variables most significantly associated with an arrhythmic event were 1) signal-averaged QRS duration at 40 Hz (improvement χ² = 16.58 = 0.0001); 2) couplets (improvement χ² = 5.39 p = 0.02); and 3) ejection fraction (improvement χ² = 4.10, p = 0.04). In conclusion: 1) An abnormal signal-averaged ECG is seen in a high proportion of patients after an acute myocardial infarction and is associated with a significantly higher incidence of arrhythmic events. 2) High pass filtering provided a better sensitivity and specificity at 40 Hz than at 25 Hz. 3) The predictive value of the signal-averaged ECG in patients with anterior wall infarction is better than that of the ejection fraction, whereas the predictive value of these two tests is equivalent in inferior infarction. 4) The duration of the signal-averaged QRS complex is the most significant independent predictor of an arrhythmic event, followed by ventricular couplets and ejection fraction.