Ablation of Left-Sided Accessory Pathways with Atrial Insertion Away from the Mitral Annulus Background A left-sided accessory pathway (AP) with atrial insertion away from the mitral annulus (MA) may result in difficulty or failed ablation along the MA. We report our initial experience of ablating this rare form of AP by a 3-dimensional electroanatomical mapping system (CARTO). Methods From January 2007 to August 2011, 29 patients with left-sided APs who failed previous ablations in other centers were enrolled in this study. Left atrium (LA) was reconstructed during orthodromic atrioventricuar reentry tachycardias (AVRTs) or ventricle pacing by using a 3-dimensional electroanatomical mapping system. The AP atrial insertion was defined as the earliest retrograde atrial activation and successful ablation of the AP at the site. Results Among the 29 patients who had failed previous ablation, 7 patients were found to have atrial insertions away from the MA. Out of the 7 patients, atrial insertions were at the base of the LA appendage in 5 patients and at the anterior roof of LA in 2 patients. Ablation at the atrial insertion successfully abolished AP conduction. The mean distance between the atrial insertion sites and the MA was 24.9 +/- 4.9 mm. No patients reported recovered AP conduction or recurrent tachycardias after at least 12-month follow-up. Conclusions Left-sided APs may have atrial insertions away from the MA. By using the CARTO system, atrial insertions can be reliably identified and ablated.