Time domain analysis of the signal averaged electrocardiogram in patients with a conduction defect or a bundle branch block

Doubts have been expressed about the clinical usefulness of time domain analysis of the signal averaged electrocardiogram in patients with prolonged QRS complex duration. We studied 147 patients using a signal averaged ECG (40-250 Hz) whose QRS complex was longer than 100 ms. A baseline electrophysiology study was also performed in 128 of these patients. Seventy-seven patients had a minor (QRS < 120 and > 100 ms) conduction defect. Thirty-seven of these 77 had either induced or spontaneous sustained ventricular tachycardia (group I) and 40 had no sustained ventricular tachycardia (group II). Seventy patients had a major (QRS greater than or equal to 120 ms) conduction defect, 44 of whom had sustained ventricular tachycardia (group A). The remaining 26 without this condition formed Group B. Group I compared to group II patients had a longer filtered QRS duration (120.8 +/- 14 vs 104.5 +/- 9.5 ms, P < 0.001), a longer low amplitude signal duration (41 +/- 12.1 vs 31 +/- 12.6 ms, P < 0.0001) and a lower root mean square of the last 40 ms of the filtered QRS complex (27 +/- 29.8 vs 35 +/- 25.3 mu V, P = ns). Group A compared to group B had a longer filtered QRS duration (157.7 +/- 20.2 vs 140.7 +/- 15.7 ms, P < 0.001), a longer low amplitude signal duration (57.3 +/- 24.9 vs 37.8 +/- 20.3 ms P < 0.001) and a lower root mean square of the last 40 ms of the filtered QRS complex (14.3 +/- 11.2 vs 22.0 +/- 10.5 mu V, P < 0.01). Using conventional late potential criteria, the sensitivity and specificity of the signal averaged ECG for the detection of sustained ventricular tachycardia patients with a minor conduction defect were 89% and 75%, respectively. The same criteria applied to patients with a major conduction defect were sensitive (sensitivity: 87%) but non-specific (specificity: 50%). However, by using modified late potential criteria, such as the presence of two of any of the following three signal averaged parameters: filtered QRS duration greater than or equal to 145 ms, low amplitude signal duration greater than or equal to 50 ms, root mean square of the last 40 ms of the filtered QRS complex less than or equal to 17.5 mu V, we derived a non-optimal but still acceptable combination of sensitivity (68%) and specificity (73%). We conclude that traditional late potential criteria can be applied in patients with a minor conduction defect, but modification of these criteria is necessary to derive useful clinical information for risk stratification of patients with a QRS complex duration greater than or equal to 120 ms.