Type A Wolff-Parkinson-White Syndrome Generating an Antidromic Atrioventricular (AV) Reentrant Tachycardia (AVRT) and an Orthodromic AVRT with a Long RP Interval Initiated only after Incomplete Impairment of an AV Accessory Pathway

We report on a case of a 23-year-old male with Wolff-Parkinson-White syndrome. At baseline, constant right atrial pacing induced antidromic atrioventricular reentrant tachycardia (AVRT), whereas constant right ventricular (RV) pacing only revealed a normal His-Purkinje system. Mapping below the mitral annulus during sinus rhythm revealed fusion of atrial and ventricular potentials at multiple lateral sites. After unsuccessful ablation at these sites, constant RV pacing induced a long RP interval, orthodromic AVRT with the earliest atrial site being located at an anterior aspect, where successful ablation was later achieved. These phenomena may indicate an unexpected arrhythmogenic effect of initial ablations.