Therapy for cardiac resynchronization: When left ventricular–only “fusion” pacing is not enough

In patients with left bundle branch block (LBBB), the right bundle branch (RBB) is responsible for left ventricular (LV) activation, following slow transseptal conduction (Figure, left). Once initiated, propagation across the LV free wall is usually rapid.1 Cardiac resynchronization therapy aims to restore electrical synchronization. Conventionally, this is delivered by LV pacing to the site of latest LV activation. It is appealing to time this with intrinsic RBB conduction (“LV fusion” pacing) to avoid deleterious right ventricular (RV) effects of RV pacing.