Catheter Ablation of a Right Atrial Free Wall Diverticulum-Related Accessory Pathway Masquerading as a Septal One.

Epicardial right-sided accessory pathway (AP) ablation is challenging. In rare cases, the atrial insertion of the AP is related to unconventional sites and associated with repeated and complex ablation procedures. In this study, we report a case of right free wall diverticulum-related AP with a distinct surface electrocardiogram (ECG).A 45-year-old male patient with repetitive palpitation for 2 years was referred for an electrophysiological (EP) study. His resting surface ECG showed manifest ventricular preexcitation with a negative delta wave and a "QS" wave in precordial lead V1, which is most consistent with right mid-septal AP.In the EP study, orthodromic atrioventricular reentrant tachycardia could be easily induced with the earliest atrial activation at the right atrium (RA) free wall, but the AP failed to be blocked by ablating the earliest activation on the tricuspid annulus edge. An epicardial free wall AP was then suspected.Inadvertent catheter manipulation into a narrow and long chamber was noted on the RA geometry. Angiography via contrast injection from the ablation tip revealed a diverticulum extending from the RA to the right ventricle side. The epicardial AP was suspected to be related to this diverticulum. The earliest atrial activation, as shown through a detailed activation mapping, was located at the entrance of the diverticulum. Subsequent ablation at the atrial insertion site successfully abolished the antegrade and retrograde AP conduction without any complication. A postprocedural computed tomography scan proved the presence of a free wall diverticulum associated with the right atrial appendage.A diverticulum-related AP at RA free wall might exhibit surface ECGs mimicking that of an AP at the RA septum. The approach targeting the atrial insertion of the epicardial AP is effective and might be facilitated by clarification of structural malformations prior to the ablation procedure.