B-011-23 QRS CHARACTERISTICS DURING LEFT BUNDLE BRANCH PACING WITH AND WITHOUT BIDIRECTIONAL LEFT INTRAHISIAN BLOCK

Abstract

LBB (left bundle branch) pacing has been proposed as a novel modality to achieve cardiac resynchronization. However, selective capture of the left bundle is physiologically expected to produce a wide QRS, with variability based on the integrity of retrograde right bundle conduction. To study QRS characteristics during left bundle branch pacing with and without bidirectional left intrahisian block. 27 patients underwent multielectrode mapping of the left conduction system with pacing from the region of the left bundle branch. QRS duration was compared between selective and nonselective LBB pacing, in addition to the S-His interval to assess for the presence of retrograde LBB block at the left in the left His fibers. Amongst 27 patients with antegrade LBB, pacing from the selective LBB resulted in wider QRS compared to nonselective pacing (147± 21ms vs 130±17 ms, p=0.02). Left intrahisian block was bidirectional in 100% of cases of LBBB with complete conduction block with long stimulus to His intervals (148±42 in nonselective LBB pacing vs 176±42ms in selective LBB pacing, p<0.001). In patients with intact Purkinje activation, pacing more distally in the left conduction system resulted in progressively wider QRS due to smaller contribution from retrograde right bundle resynchronization. The stimulus to left ventricular activation time became shorter as pacing more distal LBB. In patients with right bundle branch block, same QRS width with pacing different site of LBB due to the failure of RBB resynchronization. Due to the presence of bidirectional intrahisian left bundle branch block, wide QRS paced morphologies were uniformly observed during selective LBB capture. Nonselective LBB capture results in shorter QRS duration due to myocardial fusion with more rapid engagement of the right bundle. The physiology of LBB pacing for CRT is predicated upon fusion with intrinsic QRS and nonselective myocardial capture.