Differential diagnosis of tachycardia with a typical left bundle branch block morphology

Figure 7 Pre-excitation via atriofascicular (“Mahaim”) bypass tracts. A: Antidromic AV reentry tachycardia. The tachycardia circuit conducts antegrade down the Mahaim fiber and inserts into the distal right bundle branch (RBB). It propagates retrograde back to the atrium via the more proximal portion of the RBB; B: Atrioventricular nodal reentrant tachycardia with a “bystander” Mahaim fiber present. The bypass tract is not part of the tachycardia circuit, but contributes to ventricular activation. A wide QRS complex will be present with a left bundle branch block configuration, but ablation of the bypass tract will not eliminate the tachycardia. Adapted from[18], with permission. LBB: Left bundle branch.