Abstract 11123: Intrinsic and Induced Left Bundle Branch Block are Not Interchangeable
Background: An extensive literature attests to the toxic impact of left bundle branch block (LBBB) on left ventricular (LV) mechanical function, whether the LBBB is naturally occurring (intrinsic) or is caused by right ventricular (RV) pacing (induced). In regard to impact on LV mechanical function, a preponderance of the literature would suggest that intrinsic and induced LBBB are equivalent.
Methods and Results: Twenty-one patients with intrinsic LBBB, intact atrioventricular conduction, and an implanted pacing device with right atrial and ventricular leads were studied. Each patient underwent echocardiographic imaging during two conditions: 1. atrial pacing (intrinsic); 2. atrial and ventricular pacing (AV delay 30 milliseconds; induced). Compared to intrinsic LBBB, induced LBBB was associated with diminished LV ejection fraction (22±7 vs 25±7 %, p<.001), longer total LV activation time (386±83 vs 331±104 milliseconds, p=.02), and greater intra-LV (standard deviation of time-to-peak strain 138±31 vs 117±39 milliseconds, p=.004) and inter-ventricular (interventricular mechanical delay 49±7 vs 44±8 milliseconds, p<.001) dyssynchrony. In addition, the region of latest LV mechanical activation during intrinsic LBBB and induced LBBB were disparate in 17 patients, with induced sites located more laterally and basally than intrinsic sites (p=.02, Figure).
Conclusions: Intrinsic and induced LBBB are dissimilar. Relative to intrinsic LBBB, RV pacing places a larger mechanical burden on the LV. The use of RV pacing in cardiac resycnhronization therapy should be reconsidered. If RV pacing is to be used, and LV lead location is to be targeted to the region of latest LV mechanical activation, this assay should be performed during induced LBBB and not intrinsic LBBB.
- © 2012 by American Heart Association, Inc.