A 20-year-old man with recurrent supraventricular tachycardia (Figure 1) was referred for catheter ablation. The baseline ECG (Figure 2) showed sinus rhythm with a PR interval of 0.20 seconds and QRS duration of 0.085 seconds. Application of irrigated radiofrequency current to a site 8 mm below the apex of Koch’s triangle was terminated after 18 seconds because of an accelerated junctional rhythm. After radiofrequency delivery, there was sinus rhythm with a markedly widened QRS complex (Figure 3). What is the cause of the wide QRS complex after ablation? Figure 1. A narrow QRS complex tachycardia with cycle length of 320 ms and an RP interval of 145 ms was induced by rapid atrial pacing. The QRS morphology is identical to that recorded during sinus rhythm at baseline. Figure 2. The baseline 12-lead ECG showed sinus rhythm with a cycle length of 685 ms with no …