New-onset intrinsic and paced QRS morphology of right bundle branch block pattern after atrioventricular nodal ablation: longitudinal dissociation or anatomical bifurcation?

We performed left bundle pacing combined with atrioventricular nodal (AVN) ablation in a patient with persistent atrial fibrillation and refractory symptomatic heart failure. The major findings were new-onset intrinsic and paced QRS morphology of right bundle branch block (RBBB) pattern after AVN ablation which was performed at a more atrial site compared with the pacing site and the paced RBBB pattern could not be corrected regardless of the pacing output. Longitudinal dissociation cannot explain this observation, while anatomical separation could. We also confirm this was proximal left bundle pacing rather than His bundle pacing.