A supraventricular tachycardia and the response to parahisian pacing: What is the mechanism?

A 21-year-old man underwent an electrophysiological study because of a documented supraventricular tachycardia (SVT). Baseline atrial-His and His-ventricular intervals were normal, and programmed ventricular extrastimulation (VES) demonstrated decremental ventriculoatrial (VA) conduction with a concentric atrial activation sequence (Figure 1A ). An SVT was induced during programmed atrial stimulation with an identical atrial activation sequence as that during VES, but attempts at right ventricular (RV) entrainment consistently terminated the tachycardia (Figure 1B). The His-RV catheter was removed, and a mapping catheter was inserted instead to perform parahisian pacing. The response is shown in Figure 2A . Based on these electrophysiological findings, what is the mechanism of this tachycardia and the most appropriate interpretation of the parahisian pacing response? Figure 2A: Response to parahisian pacing. B: Recovery from the mechanical bump of the bypass tract (BT). From the third beat, the BT potential emerges, with shortening of the ventriculoatrial (VA) interval. Arrows indicate BT potentials. HA = His-atrial; SA = stimulus-atrial; other abbreviations as in Figure 1. View Large Image Figure Viewer Download Hi-res image