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

Abstract 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.