Mechanisms of Recurrence After Surgical Treatment for Atrial Fibrillation. Background: Video-assisted thoracoscopic bilateral pulmonary vein (PV) isolation with left atrial appendage (LAA) excision is a novel surgical treatment for patients who have atrial fibrillation (AF) but no indication for open heart surgery. However, the electrophysiological mechanisms of the recurrent atrial tachyarrhythmias after this procedure are unknown. Methods: Eight consecutive patients with highly symptomatic atrial tachyarrhythmias after failed video-assisted thoracoscopic surgery were included in this study. A predetermined stepwise ablation protocol, aimed at termination of the arrhythmia and isolation of all PVs, was conducted. The conduction across the remnant of the LAA was also evaluated in 4 patients. Results: Three patients had AF, which was converted into AT by complex fractionated atrial electrogram ablation in 2. Eleven sustained ATs in 7 patients were mapped during the procedure. A majority of ATs (10 of 11) were terminated by ablation before PV isolation. In total, 10 PV gaps in 7 patients were identified. All residual PV gaps were distributed exclusively in the roof or the bottom of the PV antrum. The conduction time across the remnant of the LAA was 90.7 ± 11.5 ms. One patient underwent a repeat successful ablation procedure. After a mean follow-up of 10.1 ± 5.0 months after the last ablation procedure, 7 of 8 patients were free of clinical atrial tachyarrhythmias recurrence. Conclusion: PV gaps are present, with a characteristic distribution, in the majority of patients who fail this surgical procedure, but these gaps are not responsible for the arrhythmias identified. Instead, most are macro-reentrant, isthmus-dependent arrhythmias related to clamp-associated or LAA excision-associated scars. (J Cardiovasc Electrophysiol, Vol. 20, pp. 1313-1320, December 2009) © 2009 Wiley Periodicals, Inc.