176-29: Number of beats in the transition zone with fixed SA interval during right ventricular overdrive pacing determines accessory pathway location in orthodromic reentrant tachycardia

Abstract

Background: Ventricular overdrive pacing (VOP) produces reset during the transition zone (TZ) of QRS fusion in orthodromic reentrant tachycardia (ORT) and after the TZ in atrioventricular nodal reentrant tachycardia (AVNRT), and this represents a simple diagnostic maneuver to differentiate the 2 tachycardia mechanisms. Objective: The purpose of this study was to determine whether the number of beats with reset in the TZ predicts accessory pathway (AP) location in ORT. Methods: We retrospectively reviewed 57 patients with ORT (21 left-sided AP, 20 septal AP, and 16 right-sided AP) and 20 patients with AVNRT (19 typical AVNRT and 1 atypical AVNRT) who underwent VOP from the right ventricular apex. We analyzed the number of beats with reset during or after the TZ, demonstrated by fixed ventricular stimulus-atrial (SA) interval during VOP. Results: The overall mean tachycardia cycle length [CL] minus VOP CL was 22.6 ± 7.5 ms with no statistical difference between the groups (P = 0.480). The mean number of beats in the TZ with fixed SA interval was 2.5 ± 1.4 for the whole ORT group, 1.1 ± 0.4 for left-sided AP (range 1–2), 2.8 ± 0.9 for septal AP (range 1–5), and 4.0 ± 0.9 for right-sided AP (range 3–6) (P < 0.001). Using a cutoff >2 beats distinguished right- vs. left-sided AP in all cases.