Resetting criteria during ventricular overdrive pacing successfully differentiate orthodromic reentrant tachycardia from atrioventricular nodal reentrant tachycardia despite interobserver disagreement concerning QRS fusion Jonathan Z. Rosman, MD, Roy M. John, MD, PhD, FHRS, William G. Stevenson, MD, FHRS, Laurence M. Epstein, MD, Usha B. Tedrow, MD, MS, Bruce A. Koplan, MD, MPH, Christine M. Albert, MD, MPH, Gregory F. Michaud, MD, FHRS From the Department of Cardiovascular Medicine at Brigham and Women’s Hospital, and Harvard Medical School, Boston, Massachusetts. BACKGROUND The beginning of ventricular overdrive pacing (VOP) during supraventricular tachycardia (SVT) accurately distin- guishes orthodromic reentrant tachycardia (ORT) from atrioven- tricular nodal reentrant tachycardia (AVNRT) even when pacing terminates tachycardia. Tachycardia resetting most often occurs during this transition zone (TZ) of QRS fusion in ORT and after this TZ in AVNRT. The end of the TZ is marked by the first beat with a stable QRS morphology but is a subjective assessment. Disagree- ment concerning this beat may change tachycardia diagnosis. OBJECTIVE The purpose of this study was to assess interobserver agreement for identifying the TZ and whether disagreement af- fected diagnosis. METHODS Seventy-nine consecutive patients with inducible ORT and AVNRT were included. Resetting of tachycardia was evaluated by (1) atrial timing perturbation and (2) fixed stimulation–atrial activa- tion timing (SA). Two blinded observers identified the end of the TZ and used the two resetting criteria to establish a diagnosis. Diagnos- tic results were compared with standard criteria for SVT diagnosis. The diagnosis was considered correct if both electrophysiologists’ TZ as- sessment resulted in a correct diagnosis. RESULTS Agreement on the TZ occurred in 80% (148/186) of VOP trains. In ORT patients, tachycardia resetting occurred during the TZ and correctly diagnosed ORT based on atrial timing perturbation and fixed SA in 91% and 98% of VOP trains, respectively. In AVNRT patients, tachycardia resetting occurred after the TZ and correctly diagnosed AVNRT based on atrial timing perturbation and fixed SA in 93% and 94% of VOP trains, respectively. CONCLUSION Resetting criteria used during the VOP TZ accu- rately differentiate between ORT and AVNRT despite interobserver disagreement concerning identification of the TZ. KEYWORDS Accessory pathway; Atrioventricular nodal reentrant tachycardia; Entrainment; His-refractory premature ventricular com- plex; Orthodromic reentrant tachycardia; Supraventricular tachycardia ABBREVIATIONS AVNRT = atrioventricular nodal reentrant tachy- cardia; ORT = orthodromic reentrant tachycardia; PPI = postpacing interval; RV = right ventricular; SA = stimulation–atrial activation timing; SVT = supraventricular tachycardia; TCL = tachycardia cycle length; TZ = transition zone; VOP = ventricular overdrive pacing (Heart Rhythm 2011;8:2–7) © 2011 Heart Rhythm Society. All rights reserved. Introduction Resetting observed during the transition zone (TZ) of ventric- ular overdrive pacing (VOP) helps to distinguish orthodromic reentrant tachycardia (ORT) from atrioventricular nodal reen- trant tachycardia (AVNRT). 1–3 The TZ begins with fusion between the wavefronts from VOP and orthodromic tachycar- dia and ends with the first beat of stable QRS morphology assessed on the 12-lead ECG (Figure 1). 1 The first beat of stable QRS morphology can mark the beginning of entrain- ment with constant fusion as seen in patients with right-sided or septal accessory pathways (Figure 2). Alternatively it can represent a fully paced QRS morphology, seen in patients with AVNRT and accessory pathways more distant from the right ventricular (RV) pacing site, such as left free wall locations (Figure 3). Theoretically, accessory pathway-mediated tachy- cardia will be reset at or before the first beat of stable QRS morphology because ventricular activation is an obligatory part of the circuit. In contrast, patients with AVNRT demonstrate resetting of tachycardia after the TZ because ventricular acti- vation is not obligatory and the paced wavefront must first penetrate beyond the His bundle and lower common pathway before entering the AVNRT circuit (Figure 3). Examples of resetting are shown in Figure 4. Identification of the end of the TZ requires scrutinizing the 12-lead ECG for subtle changes in QRS morphology. Interob- Address reprint requests and correspondence: Dr. Gregory F. Michaud, Brigham and Women’s Hospital, 75 Francis Street, Boston, Massachusetts 02115. E-mail address: gfmichaud@partners.org. (Received August 10, 2010; accepted September 29, 2010.) 1547-5271/$ -see front matter © 2011 Heart Rhythm Society. All rights reserved. doi:10.1016/j.hrthm.2010.09.089