Deterioration of left ventricular ejection fraction and contraction synchrony during right ventricular pacing in patients with left bundle branch block

Background: Electrocardiographic left bundle branch block (LBBB) may be intrinsic, due to ventricular conduction system disease, or induced by right ventricular pacing. Prior reports clearly delineate the derogatory impact of LBBB on left ventricular (LV) mechanical synchrony and global function, and suggest that the intrinsic and induced varieties are equivalent. This study sought to determine the difference in LV synchrony and global function between intrinsic LBBB and right ventricular apical pacing induced LBBB. Methods: Ten patients with heart failure, diminished ejection fraction (EF) (33 ± 11%), intrinsic LBBB and an implanted cardiac pacing device were studied. In each patient, separate gated SPECT acquisitions were performed during intrinsic ventricular activation (atrial pacing) and during induced LBBB (atrial and right ventricular pacing). During each condition, LVEF, contraction synchrony (phase standard deviation, PSD), and spatial pattern of activation were measured. Results: Compared to intrinsic, induced LBBB was associated with decreased EF (30 ± 11% vs 33 ± 11%, P =.007), contraction synchrony (PSD 49.7 ± 23.2 vs 41.6 ± 19.8, P =.02), and a disparate spatial pattern of activation. Conclusions: Induced LBBB is associated with significantly worse global and regional LV mechanical function than intrinsic LBBB.