IDF and AF substrate
The factors for the persistence of AF are regarded as trigger factors and arrhythmogenic substrate factors, which play a role as an AF perpetuator.17, 18 The triggers generally initiate AF and the substrate plays a critical role in AF persistence. Therefore, as the AF duration prolongs, LA remodeling proceeds, and the AF substrates increase. Thus, the success rate of AF cardioversion is inversely correlated to the LA size, because the fibrosis accompanying atrial dilatation increases the AF substrate.19 There are many reports about predictors associated with recurrence of AF after electrical cardioversion, such as the LA size, age, and BNP.20-22 However, there are few reports on cardioversion after AF ablation.
In the present study, RFCA significantly decreased the IDF threshold for restoring SR (Figure 1). The conditions for IDF were the same before and after the RFCA; we induced AF artificially with high rate burst pacing and ISP. After the AF sustained for more than two minutes, IDF was performed to detect any AF triggers. Although the main strategy for RFCA in our institute is an AF trigger ablation, an extensive encircling PVI or posterior wall isolation might decrease the AF substrate. That suggests that RFCA could eliminate or decrease both AF triggers and the substrate, and further, it could decrease the cardioversion threshold. Thus, the difference in the cardioversion threshold before and after RFCA may reflect the elimination of the AF substrate and triggers. There were some cases with LA enlargement but without a low voltage area in our study. In those cases, the IDF threshold could reflect the AF substrate, which could not be assessed by electrophysiology. Additionally, our study revealed that the lower output IDF (≤ 5 J) group after the RFCA was associated with a lower recurrence of AF. In fact, in patients with an output > 5 J, the LAD was larger, and there was a greater presence of an LVZ in the LA. Thus, the LA remodeling proceeded in those patients. An IDF threshold of > 5 J was a strong predictor of AF recurrence in a multivariate analysis after adjusting for the patient background, LAD, LVZ, duration of AF, and administration of AADs after the RFCA. Furthermore, the presence of an LVZ disappeared in the multivariate analysis model 1. Additionally, the ablation strategies (such as an SVC isolation, posterior wall isolation, and focal ablation) did not have any significant value for detecting AF recurrence or the IDF threshold (Tables 1 and 2 ). The IDF method was useful for assessing whether RFCA could decrease the AF substrate during the RFCA procedure.