Antidromic His capture during ventricular entrainment of an orthodromic AV re-entrant tachycardia. What is the mechanism?

Abstract

A 32-year-old lady was evaluated for recurrent episodes of palpitation. During one of the palpitation episodes a regular narrow QRS tachycardia was documented, and it got terminated with the administration of IV adenosine. The baseline 12 lead electrocardiogram (ECG) did not show any manifest preexcitation. There was no evidence of structural heart disease by echocardiogram. Patient underwent an electrophysiology [EP] study after informed consent. Quadripolar catheters were placed at the His region and right ventricular (RV) apex. A decapolar catheter was placed in the coronary sinus (CS) with CS 9,10 dipoles at CS OS region and CS 1,2 dipoles at CS distal region (at 3 O clock of mitral anulus). A mapping & ablation catheter was positioned at right atrial (RA) appendage. Baseline atrial and ventricular pacing protocols could not be performed as both atrial and ventricular pacing were easily inducing a regular narrow QRS tachycardia. Ventricular pacing manoeuvres including delivery of His refractory premature ventricular extra (PVE) stimulus [Figure 1] & ventricular overdrive pacing (VOD) [Figure 2 & 3] were performed during the tachycardia from the right ventricular (RV) apex. Antidromic His capture during VOD of orthodromic AV re-entry had occurred, which was unusual and might be expected to terminate AV re-entrant tachycardia rather than entrain it. Why did this not happen? This article is protected by copyright. All rights reserved.