This study describes electrophysiological breakthroughs from the left atrium to the pulmonary veins (PVs) in patients with atrial fibrillation (AF). The goal was to determine the extent of ostial ablation necessary to electrically disconnect PV myocardial extensions that initiate AF from the left atrium. Seventy patients underwent PV mapping with a circumferential 10-electrode catheter during sinus rhythm or left atrial pacing. PV potentials were present at 60% to 88% of their perimeter, but PV muscle activation was always sequential from a segment with earliest activation (breakthrough). Radiofrequency (RF) application at this breakthrough eliminated all PV potentials in 34 PVs, whereas a secondary breakthrough required RF applications at separate segments in 77; in others, >2 segments were ablated. A median of 5, 6, and 4 bipoles from the circular catheter were targeted in the right superior, left superior, and inferior PVs, respectively, to achieve PV disconnection. Early recurrence of arrhythmia was observed in 31 patients as a result of new venous or atrial foci or recovery of previously targeted PVs, most related to a single recovered breakthrough that was reablated with local RF application. The study concludes that although PV muscle covers a large extent of the PV perimeter, there are specific breakthroughs from the left atrium that allow ostial PV disconnection by use of partial perimetric ablation. The findings have practical implications, as circumferential mapping optimizes RF ablation at the PV ostia by directing energy at specific segments and avoiding unnecessary applications at others, thus minimizing the risk of PV stenosis. However, this technique may not be applicable to RF ablation outside the PV ostia, which may require complete circumferential lesions to produce distal disconnection. Other limitations include the continued high recurrence rate of AF due to unmasked foci from the ostial edge or atrial tissue characterized by difficulty in precise mapping and absence of a similar end point.This study describes electrophysiological breakthroughs from the left atrium to the pulmonary veins (PVs) in patients with atrial fibrillation (AF). The goal was to determine the extent of ostial ablation necessary to electrically disconnect PV myocardial extensions that initiate AF from the left atrium. Seventy patients underwent PV mapping with a circumferential 10-electrode catheter during sinus rhythm or left atrial pacing. PV potentials were present at 60% to 88% of their perimeter, but PV muscle activation was always sequential from a segment with earliest activation (breakthrough). Radiofrequency (RF) application at this breakthrough eliminated all PV potentials in 34 PVs, whereas a secondary breakthrough required RF applications at separate segments in 77; in others, >2 segments were ablated. A median of 5, 6, and 4 bipoles from the circular catheter were targeted in the right superior, left superior, and inferior PVs, respectively, to achieve PV disconnection. Early recurrence of arrhythmia was observed in 31 patients as a result of new venous or atrial foci or recovery of previously targeted PVs, most related to a single recovered breakthrough that was reablated with local RF application. The study concludes that although PV muscle covers a large extent of the PV perimeter, there are specific breakthroughs from the left atrium that allow ostial PV disconnection by use of partial perimetric ablation. The findings have practical implications, as circumferential mapping optimizes RF ablation at the PV ostia by directing energy at specific segments and avoiding unnecessary applications at others, thus minimizing the risk of PV stenosis. However, this technique may not be applicable to RF ablation outside the PV ostia, which may require complete circumferential lesions to produce distal disconnection. Other limitations include the continued high recurrence rate of AF due to unmasked foci from the ostial edge or atrial tissue characterized by difficulty in precise mapping and absence of a similar end point.