Slow atrioventricular nodal pathway affecting fast pathway conduction

A 15-year-old girl with a history of paroxysmal supraventricular tachycardia underwent an electrophysiology study (EPS) for diagnosis and ablation. Her baseline electrocardiogram and echocardiogram were normal. At EPS, she had dual atrioventricular nodal (AVN) conduction, but isoproterenol was needed to initiate the slow-fast form of AVN reentry. Before ablation without any isoproterenol, she began to have a spontaneous block in the fast pathway with continuous conduction over the slow pathway. After ablation of the slow pathway, all complexes conducted over the fast pathway during a 25-year follow-up. Possible electrotonic interaction between the slow and fast pathways is proposed as the mechanism for this phenomenon.