Unmasking of left bundle branch potential in left bundle branch block during physiological pacing

Abstract

A 70-years-old lady had undergone left bundle branch pacing (LBBP) for left bundle branch block (LBBB) (Fig. 1A) with left ventricular ejection fraction of 35% using C315 sheath and 3830 SelectSecuretm lead [1]. Both non-selective to selective (Fig. 1B) and non-selective to septal capture transition could be demonstrated by decreasing the pacing output. Left bundle branch potential (LBB-Po) during sinus rhythm was unmasked without performing dual lead technique by gap phenomenon (Fig. 1D and E). Unipolar threshold measurement by rapid pacing resulted in loss of LBB capture at 0.2 V/0.5 ms followed by a blocked P-wave which peeled away the refractoriness of LBB transiently (phase 3 block) [2]. The next beat showed antegrade activation of LBB resulting in narrow QRS duration (Fig. 2A) and appearance of LB potential on the lead electrogram. Subsequent beats were conducted through right bundle (RB) with LBBB pattern. Programmed deep septal stimulation showed change in QRS morphology and R-wave peaking time at 500 ms (S1) and 300 ms (S2) confirming loss of LBB capture (Fig. 2B). Final paced QRS after optimizing AV delay was 98 ms (Fig. 1C). Burst pacing of the LBB to peel away the refractoriness is an alternative option to dual lead technique to unmask the LBB potential in patients with complete LBBB. Careful evaluation by electrophysiology study is required to document the phase 3 block.