Interference dissociation during a baseline complete left bundle-branch block with advanced right bundle-branch block.

A 64-year-old woman presented to the hospital with a complaint of dyspnea on effort. The 12‑lead electrocardiogram showed a baseline complete left bundle-branch block, in which the right bundle developed >2:1 block. The subsequent blocked sinus impulse resulted in a ventricular pause that caused the ventricular escape presenting as a QRS complex with right bundle-branch block pattern. However, some of them disclosed typical fusion beats and led to interference dissociation. Although other possibilities including Lenegre's disease or myocardial disease were not excluded, the effective refractory period of both bundle branches degenerated by myocardial ischemia might be attributed to this rhythm.