Nonreentrant supraventricular tachycardia misdiagnosed as inappropriate sinus tachycardia.

We report a case of a woman with incessant palpitations initially misdiagnosed as inappropriate sinus tachycardia that proved refractory to β-blockers. At the time of electrophysiologic testing, a sustained narrow-complex tachycardia with a 1:2 atrioventricular relationship was repeatedly initiated by a posterior fascicle depolarization induced by means of a timed ventricular extrastimulus. The tachycardia was repeatedly terminated with a timed atrial extrastimulus, which excluded junctional bigeminy and confirmed the diagnosis of nonreentrant supraventricular tachycardia. Catheter ablation of the slow pathway eliminated dual-pathway conduction and tachycardia.