Abstract Body (Do not enter title and authors here): Introduction: Ablation index (AI) has been used as a predictor of forming durable lesions in pulmonary vein isolation (PVI) for atrial fibrillation (AF). Impedance drop (ID) had also been thought to be important for adequate lesion formation with radio frequency application (RF). However, relationship of ID and AI is unclear because AI does not take into account ID. In addition, there are some modes of ID, but it is unclear that what modes of dropping impedance is effective. Here, we investigated which modes of dropping impedance was effective for acute successful lesion formation. Association of ID and parameters of ablation lesion with RF including AI was also evaluated.

Methods: Ablation lesions made by RF using 35W power during PVI were studied. Only lesions created for the initial PVI line were analyzed. These points were classified into 5 groups according to the ways of ID. Points with gradual ID, 2-phased ID, and steep ID were respectively classified as group 1, 2, and 3, and these three groups were investigated. Points without dropping impedance, increase in impedance, or RF duration of less than 5 seconds were excluded. Acute success rate, decrease in impedance, duration of RF, average contact force (CF) and value of AI were investigated for each group.

Results: From 40 consecutive cases, 1921 lesions were obtained, of which 1843 were investigated. Group 1, 2, and 3 consisted of 511, 1184, and 148 points, respectively. Acute success rate for each group were 93.0, 98.6, and 93.2%, and was the highest in groups 2 (p<0.05). AI in group 2 was the highest among three groups (464±47 vs. 476±51 vs. 467±66 for each group, p<0.05). ID and CF were the largest in group 3 among three groups (7.6±3.5 vs 12.2±5.2 vs 14.6±5.9 ohm, p<0.05, for each group. 12.8±5.2 vs 15.0±6.0 vs 17.6±8.3 g, p<0.05, for each group). The duration of RF was longest in group 1 among three groups (24.5±4.7 vs 23.6±4.6 vs 21.0±6.2, p<0.05, for each group).

Conclusion: For optimal lesion formation, to obtain 2-phased ID in combination with moderate CF, ID, and the duration of RF was important. Evaluating modes of ID may make AI more reliable to create optimal lesions with RF at 35W of power.