BACKGROUND Beyond pulmonary vein (PV) isolation, anatomic isthmus transection is an adjunctive strategy for persistent atrial fibrillation. Data on the durability of multiple lines of block remain scarce.OBJECTIVE The purpose of this study was to evaluate the impact of gaps within such a lesion set.METHODS We followed 291 consecutive patients who underwent (1) vein of Marshall ethanol infusion, (2) PV isolation, and (3) mitral, cavotricuspid, and dome isthmus transection. Dome transec-tion relied on 2 distinct strategies over time: a single roof line with touch-ups applied in case of gap demonstrated by conventional ma-neuvers (first leg), and an alternative floor line if the roof line ex-hibited a gap during high-density mapping with careful electrogram reannotation (second leg).RESULTS Twelve-month sinus rhythm maintenance was 70% after 1 procedure and 94% after 1 or 2 procedures. Event-free survival af-ter the first procedure was lower in case of residual gaps within the lesion set (log-rank, P = .004). Delayed gaps were found in 94% of a second procedure performed in the 69 patients relapsing despite a complete lesion set with PV gaps increasing the risk of recurrence of atrial fibrillation (67% vs 34%; P = .02) and anatomic isthmus gaps supporting a majority of atrial tachycardias (60%). Between the first leg and the second leg, a significant decrease was found in roof lines considered blocked during the first procedure (99% vs 78%; P , .001) and in delayed dome gaps observed during a second procedure (68% vs 43%; P = .05).CONCLUSION Gaps are arrhythmogenic and can be reduced by optimized ablation and assessment of lines of block. Closing these gaps improves sinus rhythm maintenance.Read less <