Dynamic changes in atrial activation sequence during supraventricular tachycardia

Abstract

A29?year?old woman presented with repeated paroxysms ofpalpitations resulting from a rapid narrow QRS complex tachycar-dia (NCT). Her family and personal history were free of anycardiovascular pathology. After the initiation of an NCT following a1:2 ventricular response to programmed atrial stimulation(Figure1), a premature ventricular complex (PVC) from the rightparahisian region was delivered at a time when the His bundleactivation was completed (Figure2); subsequently, a dynamicchange in the retrograde atrial activation sequence (AAS) andventriculoatrial (VA) intervals were developed (Figure3). Based onthe findings within the figures, what is the mechanism of tachycardia?