Resetting criteria during ventricular overdrive pacing successfully differentiate orthodromic reentrant tachycardia from atrioventricular nodal reentrant tachycardia despite interobserver disagreement concerning QRS fusion

Background The beginning of ventricular overdrive pacing (VOP) during supraventricular tachycardia (SVT) accurately distinguishes orthodromic reentrant tachycardia (ORT) from atrioventricular 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. Disagreement 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 affected 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 stimulationatrial activation timing (SA). Two blinded observers identified the end of the TZ and used the two resetting criteria to establish a diagnosis. Diagnostic results were compared with standard criteria for SVT diagnosis. The diagnosis was considered correct if both electrophysiologists\u27 TZ assessment 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 accurately differentiate between ORT and AVNRT despite interobserver disagreement concerning identification of the TZ. © 2011 Heart Rhythm Society