This study investigates the extent of ostial ablation necessary to electrically disconnect pulmonary vein (PV) myocardial extensions that initiate atrial fibrillation from the left atrium. Seventy patients underwent PV mapping with a circumferential 10-electrode catheter during sinus rhythm or left atrial pacing. After assessing the perimetric distribution and activation sequence of PV potentials, ostial ablation was performed at segments showing earliest activation, aiming for PV disconnection. A total of 162 PVs were ablated, and PV potentials were present at 60% to 88% of their perimeter. Radiofrequency (RF) application at the breakthrough segment eliminated all PV potentials in 34 PVs, while secondary breakthroughs required additional RF applications in 77 PVs. The study concludes that while PV muscle covers a large extent of the PV perimeter, there are specific breakthroughs from the left atrium that allow ostial PV disconnection with minimal ablation. This finding optimizes RF ablation at the PV ostia by directing energy at specific segments, minimizing the risk of PV stenosis. However, the high recurrence rate of AF due to unmasked foci or atrial tissue remains a challenge.This study investigates the extent of ostial ablation necessary to electrically disconnect pulmonary vein (PV) myocardial extensions that initiate atrial fibrillation from the left atrium. Seventy patients underwent PV mapping with a circumferential 10-electrode catheter during sinus rhythm or left atrial pacing. After assessing the perimetric distribution and activation sequence of PV potentials, ostial ablation was performed at segments showing earliest activation, aiming for PV disconnection. A total of 162 PVs were ablated, and PV potentials were present at 60% to 88% of their perimeter. Radiofrequency (RF) application at the breakthrough segment eliminated all PV potentials in 34 PVs, while secondary breakthroughs required additional RF applications in 77 PVs. The study concludes that while PV muscle covers a large extent of the PV perimeter, there are specific breakthroughs from the left atrium that allow ostial PV disconnection with minimal ablation. This finding optimizes RF ablation at the PV ostia by directing energy at specific segments, minimizing the risk of PV stenosis. However, the high recurrence rate of AF due to unmasked foci or atrial tissue remains a challenge.