Uncommon output-dependent paced QRS morphology transition during left bundle branch pacing.

We report a patient who underwent left bundle branch pacing (LBBP) because of intermittent complete heart block. During unipolar pacing at a deep septal site, a transition in QRS morphology from nonselective to selective pacing (presence of a discrete component on the intracardiac electrogram) was observed, but this was accompanied by concomitant prolongation of the stimulus-to-left ventricular activation time (Stim-LVAT) from 96 to 116 ms. However, with several additional turns of the lead, a transition from nonselective to selective LBBP with a short and constant Stim-LVAT of 78 ms was achieved. Our case suggests that a paced QRS morphology transition from nonselective to selective pacing does not always indicate LBBP. Assessment of high-output pacing is mandatory to achieve LBBP.