Masked right bundle branch conduction delay pattern during left bundle branch pacing

The specific criteria for left bundle branch pacing (LBBP) include right bundle branch delay (RBBD) pattern (qR/Qr/rSRʹ) in lead V1 and transition from nonselective to selective left bundle branch (LBB)– or left ventricular septal (LVS)–only capture at near-threshold outputs.1 An absent RBBD pattern during deep septal pacing would generally indicate that the pacing lead has not reached the LBB/LVS and requires additional rotations. Rarely the qR/Qr/rSRʹ pattern in lead V1 may be masked (QS pattern) despite LBBP (Figures 1A and 1B , blue circles). The aim of our study was to describe the incidence and mechanisms of the masked RBBD pattern during nonselective LBB capture.