Wide to narrow complex tachycardia: What is the diagnosis?

Wide to narrow complex tachycardia: What is the diagnosis? 1CASEA 19‐year‐old male with Ebstein's anomaly of the tricuspid valve underwent electrophysiology study for recurrent palpitations. Baseline 12‐lead electrocardiogram showed preexcitation consistent with a posteroseptal pathway. During incremental ventricular pacing, atrial activation was central. On cessation of ventricular pacing, a wide QRS tachycardia of left bundle branch (LBBB) morphology was seen, which changed to a narrow QRS tachycardia after a few beats (Figure ). What is the mechanism of the tachycardia and what is the mechanism of the change in QRS morphology?Tracing showing change in tachycardia. After 3 beats of wide QRS tachycardia, the QRS complex becomes narrow. Marked are the cycle length (CL), His‐ventricular (HV), ventriculo‐atrial (VA), and His‐atrial (HA) intervals for the wide and narrow complex tachycardias2DISCUSSIONThe tracing shows a wide QRS tachycardia of LBBB morphology for the first 3 beats, a narrow QRS for the last 5 beats, and 1 fusion beat in between. The LBBB morphology, a fixed and short VH interval, and very early RV activation are consistent with antegrade activation proceeding by an atriofascicular pathway. Shortening of the AH interval precedes the fusion beat and subsequent change to a narrow QRS suggesting shortening of AV nodal conduction time. The shortening is due to accommodation http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Cardiovascular Electrophysiology Wiley

Wide to narrow complex tachycardia: What is the diagnosis?