LAUSR

A 68-year-old man with a history of a structurally normal heart by echocardiography, an unsuccessful ablation of a narrow complex tachycardia and frequent symptoms of palpitations presents for an electrophysiology study and ablation. A baseline 12-lead EKG demonstrated sinus rhythm without ventricular pre-excitation. On isoproterenol at 1 μg/minute, a narrow complex tachycardia with concentric atrial activation, simultaneous with ventricular activation, is induced with single atrial extrastimuli. Entrainment of the tachycardia from the ventricle demonstrates a V-A-HV response with a long postpacing interval. After placing an ablation line from the tricuspid annulus to the coronary sinus os and ablating the roof of the proximal coronary sinus, the tachycardia remained largely unchanged. A transseptal puncture was then performed and in the left atrium APDs were inserted during tachycardia at several points along the mitral annulus. On the inferolateral mitral annulus an APD delivered during His refractoriness terminated the tachycardia (Fig. 1), and an APD delivered just after atrial depolarization advanced the next cycle of the tachycardia (Fig. 2). Sites just lateral or septal to this site did advance the tachycardia, but by a shorter interval. Based on these findings, what is the mechanism of the tachycardia and the effect of the pacing maneuvers on the tachycardia?