Discordant Pacing Manoeuvers and a Narrow QRS Tachycardia – What is the Tachycardia Mechanism?

A 24-year-old female underwent an electrophysiological study because of recurrent episodes of drug-refractory, paroxysmal supraventricular tachycardia. During adrenergic stress, a narrow QRS tachycardia with eccentric atrial activation was reproducibly inducible. The response to premature ventricular extrastimulation during tachycardia suggested the presence of a slowly conducting accessory pathway. However, a comprehensive appraisal of the electrophysiological study delineated the tachycardia substrate as an atypical atrioventricular nodal reentrant tachycardia in the presence of a bystander nodofascicular pathway. Careful analysing of the basal pacing manoeuvers during sinus rhythm (para-Hisian and differential ventricular pacing) was crucial to establish the correct diagnosis and to avoid unnecessary left-side access to ablate this tachycardia.