RFCA and IDF
AF was induced artificially in all patients after the RFCA to provocate AF triggers. The IDF threshold to restore SR was significantly decreased after the RFCA (from 11.5 ± 8.6 J to 4.0 ± 3.8 J, p < 0.001). There was no difference in the IDF threshold between the recurrence and no-recurrence groups before the RFCA (Table 1 ), however, the threshold in the no-recurrence group was significantly lower than that in the recurrence group after the RFCA as shown inFigure 2 . A cut-off point analysis showed that as IDF threshold after the RFCA of > 5 J was the optimal point that discriminated those with AF recurrence from the rest of the participants as shown in Figure 3 (sensitivity 68.0% and specificity 65.2%). We divided the patients into two groups according to IDF outputs of ≤ 5 J or > 5 J. Of all the patients, the number of patients whose IDF output that restored SR was ≤ 5 J was 117. Table 2shows that the IDF threshold before the RFCA, LAD, and BNP in the patients with an output of ≤ 5 J were significantly lower than those in the patients with an output of > 5 J. On the other hand, there were no significant differences in the LVEF, duration of AF, and non-PV foci between the two groups. The presence of an LVZ in the LA significantly differed between the two groups (17 patients, 14.5% vs. 8 patients, 33.3%, p = 0.039).
The recurrence rate of AF was significantly higher in the patients with an output of > 5 J than in those with an output of ≤ 5 J (22 patients, 18.8% vs. 12 patients, 50%, p = 0.003). Further, the Kaplan-Meier model showed that an IDF threshold after RFCA of > 5 J was associated with a recurrence of AF (p< 0.001) as shown in Figure 4 .
A multivariate analysis using Cox proportional hazards models (models 1 and 2) after adjusting for the patient background, LAD, duration of AF, and use rate of AADs after the RFCA, revealed that an IDF threshold of > 5 J after the RFCA (HR, 3.99; 95% CI 1.93 - 8.22;p = 0.0001) was significantly associated with recurrence of AF (Table 3 ).