An unusual atrioventricular accessory pathway with an oblique course

Case report An 18-year-old male patient experienced frequent episodes of palpitations for 6 months. He was referred to our hospital for catheter ablation. This patient had no prior history of structural cardiovascular disease. His chest radiograph and transthoracic echocardiogram showed no abnormalities. The electrocardiogram (ECG) showed Wolff–Parkinson–White syndrome at baseline (Figure 1A) and supraventricular tachycardia (SVT) when symptomatic. A manifest left anterolateral AP was suspected, according to the Arruda algorithm. During electrophysiological study, a 6F decapolar catheter (Biosense Webster, Diamond Bar, CA) was positioned within the coronary sinus (CS) with its proximal electrode pair 9-10 (CS9-10) at CS ostium and distal electrode pair (CS1-2) at lateral mitral annulus (MA), which showed the earliest ventricular activation at electrode pair 5-6 (CS5-6) in sinus rhythm (Figure 2A). Programmed incremental right ventricular apex (RVa) pacing at approximately 500–340 milliseconds revealed eccentric retrograde atrial activation, with the earliest atrial activation at CS1-2 and the second earliest at CS9-10. RVa pacing at 300 milliseconds could repeatedly induce the first orthodromic atrioventricular reentrant tachycardia with the same atrial activation sequence as that observed during RVa pacing (SVT1, cycle