Study of the influence of left bundle branch block on the signal-averaged electrocardiogram: a qualitative and quantitative analysis. To study the influence of left bundle branch block (LBBB) on the signal-averaged electrocardiogram (SAECG), quantitative and qualitative analyses of SAECG parameters were undertaken in 48 patients with electrocardiographic evidence of intrinsic LBBB and in 39 patients with a "normal" surface QRS duration (less than 120 msec) who underwent right ventricular pacing-induced LBBB. We assumed pacing of the right ventricular apex to be a suitable model of this conduction defect. Sustained monomorphic ventricular tachycardia (SMVT) was inducible in 16 of 48 patients with intrinsic LBBB and in 23 of 39 patients with pacing-induced LBBB. Utilizing a filter setting of 25 to 250 Hz, late potentials were defined as a total filtered QRS duration greater than or equal to 120 msec, a root mean square voltage in the terminal 40 msec (RMS 40) of less than or equal to 25 microV, and the duration of signals less than 40 microV (LAS 40) of greater than or equal to 38 msec. Only RMS 40 and LAS 40 criteria were used in patients with LBBB. Prolongation of LAS 40 and fragmentation of signals in the terminal portion of the filtered QRS were characteristic of all patients with LBBB aberration. Of those patients with intrinsic LBBB, the mean total filtered QRS duration, RMS 40, and LAS 40 for inducible and noninducible patients were significantly different (170 +/- 28, 16 +/- 10, 55 +/- 24, and 153 +/- 18 msec, 25 +/- 10 microV, 33 +/- 16.9 msec; p = 0.04, 0.009, and 0.007, respectively). Noninducible patients with a normal QRS duration demonstrated a 60% decrement in the mean RMS 40 value during pacing-induced LBBB. These changes resulted in a 59% false positive incidence of late potentials during pacing-induced LBBB. This correlated with a similarly low mean RMS 40 value in patients with intrinsic LBBB and no inducible SMVT, hence giving rise to a false positive incidence of late potentials of 63%. Since "standard" RMS 40 and LAS 40 criteria resulted in low specificity and positive predictive value, new parameters were selected and analyzed. The combination of RMS 40 less than or equal to 17 microV plus LAS 40 greater than or equal to 55 msec yielded the best overall statistical result, with a sensitivity, specificity, and total predictive accuracy of 69%, 81%, and 77%, respectively. In conclusion: (1) A reduction of RMS 40, prolongation of LAS 40, and fragmentation of signals in the terminal portion of the filtered QRS are characteristics of LBBB.(ABSTRACT TRUNCATED AT 400 WORDS). 