Functional abnormalities in isolated left bundle branch block. The effect of interventricular asynchrony

Abstract

Eighteen patients with isolated left bundle branch block (LBBB) were compared with 10 normal control subjects. Apexcardiograms, phonocardiograms, electrocardiograms, two-dimensional and dual M-mode echocardiograms, and radionuclide ventriculograms (RNV) were performed. There were no differences in the timing of right ventricular events between LBBB and normal subjects; however, striking delays in left ventricular systolic and diastolic events were apparent in the LBBB group. The delay was associated with shortening of left ventricular diastole and resultant increase in the ratio of right to left ventricular diastolic time in LBBB (1.2 ± 0.08) compared with normal (1.0 ± 0.06), p < 0.0001. First heart sound (S1) amplitude, expressed as the ratio S1/S2, was decreased in LBBB compared with normal (0.67 ± 0.2 compared with 1.34 ± 0.25, p < 0.01), in part due to wide separation of the valvular contributors to S1. The abnormal interventricular septal motion in LBBB corresponded to periods of asynchrony in contraction, ejection, end systole, and end diastole between right and left ventricles. Radionuclide ventriculograms revealed decreased regional ejection fraction of the septum in LBBB (40 ± 16%) compared with 67 ± 7% in normal subjects (p < 0.001), while the apical and lateral regional ejection fractions were similar in the two groups. This loss of septal contribution resulted in a reduction in global ejection fraction in LBBB compared to normals (54 ± 7% compared with 62 ± 5%, p < 0.005). The magnitude of systolic septal motion (echocardiography) and septal ejection fraction (RNV) were closely correlated to the ratio of right to left ventricular diastolic time (r = -0.86 and -0.85, respectively). Thus, isolated LBBB caused global ventricular abnormalities manifested by abnormalities in diastolic filling times, heart sounds, interventricular septal motion, and left ventricular ejection fraction.