A narrow complex tachycardia with a short HV interval: What is the mechanism?

The case was a 15-year-old male with a history of paroxysmal supraventricular tachycardia refractory to medical therapy and prior catheter. A repeat electrophysiology study and catheter ablation were applied. Baseline AH and HV intervals were 100 and 55 ms during normal sinus rhythm (NSR), respectively. Programmed atrial stimulation induced a short RP narrow complex tachycardia (HV interval 22 ms) with an incomplete right bundle branch block configuration and right axis deviation. Tachycardia was terminated with ATP 5 mg injection. An atrial premature beat within the His refractory period advanced and reset tachycardia. Entrainment performed from the coronary sinus and left ventricle both showed a post-pacing interval minus tachycardia cycle length (TCL) of 90 ms. After confirming the diagnosis, left atrial mapping along mitral annulus was performed using trans-septal access and accessory pathway potentials were recorded during NSR and tachycardia at the superior mitral annulus. An irrigated ablation catheter guided by 3-D mapping was used to perform ablation during tachycardia. Tachycardia terminated immediately during the first RF application with ensuing automaticity exhibiting a warm-up during radiofrequency delivery and a cooling down upon the suspension of ablation. Therefore, all phenomena of Mahaim-fiber associated tachycardia were observed. In this case, we describe an antidromic atrio-ventricular reentry tachycardia using a left atrio-fascicular fiber inserting into the proximal left anterior fascicle.