BACKGROUND AND OBJECTIVES: Radiofrequency (RF) catheter ablation of the pulmonary vein (PV) can treat drug refractory focal atrial fibrillation (AF). However, high RF energy (RFE) can cause severe PV damage, and the lower limits of effective RFE in PV have not yet been elucidated. This study attempts to evaluate the changes of PV structure after various modes of RFE delivery. MATERIALS AND METHODS: Right heart and transseptal catheterization were performed in 5 anesthetized mongrel dogs. RFE was delivered at the right superior pulmonary vein (RSPV) with 50 watts and 70degreesC and at the left superior pulmonary vein (LSPV) with 20 watts and 50degreesC. After the procedures, the endocardiums of the left atrium and both superior PVs were examined. RESULTS: Total applied RFE in both PVs was 14.2+/-.2 vs. 13.5+/-.6 watts, 46.6+/-.1 vs. 64.5+/-.2degreesC (p<0.05), in LSPV vs. RSPV, respectively. Follow up pulmonary venograms showed that total occlusions of PV branches and severe stenosis of proximal PV (>70% luminal narrowing) developed in 4 dogs with high RFE. Mild stenosis (<50% luminal narrowing) of PV developed in 1 dog with low RFE. Histological examination of the 5 dogs revealed coagulation necrosis over the whole PV layer, including the adventitia and some portion of the myocardial sleeve, and severe hemorrhage and destruction of PVs from high energy treatment, in contrast to intimal damage and swelling of subintimal PV layers in low energy treatment. CONCLUSION: High RF current may result in severe damage of pulmonary veins and sub-structures whereas low RF current may cause suboptimal pulmonary vein damage such as intimal only damage.