4.1 Main findings
The main findings of our study were as follows: (1) A gap following attempted LAPWI was often located either at the center or right aspect of the line; (2) HVZs ≥ 2.64 mV and an LA wall ≥ 2.04 mm in thickness at the ablation line, and a long roof ablation line ≥ 43.4 mm were predictive of a gap in the ablation line; and (3) the best cutoff modified AI value for predicting acute durable LAPWI was ≥ 199 AU/mV.
Whether LAPWI provides additional benefits over PVI alone is controversial.2–4,15 Achievement of complete LAPWI without additional ablation within the lines is only modest (45%–72%),15 which might hinder the efficacy of the procedure. Incomplete LAPWI may result from failure to create transmural lesions in the roof line or floor line. To overcome this problem, several new technologies such as AI and high-power short duration ablation have been developed. These modalities are aimed at improving the “operator factors” by generalizing and standardizing the physicians’ skills and experience as applied to the ablation procedure. However, ablation lesions can also be influenced by “patient factors,” such as the wall thickness and tissue characteristics, e.g., diseased vs. healthy tissue. Results of our previous studies have shown that a high bipolar voltage area ≥ 2.69 mV and a thick LA wall were obstacles to achievement of complete PVI, and the modified AI, which takes into account both operator and patient factors, of ≥ 189 AU/mV was found to predict acute durable PVI.8 In the study reported herein, we obtained similar results, with our data indicating that the modified AI can also be applied to LAPWI.
Among our study patients, average LAPW thickness varied, ranging from 0.6 mm to 5.5 mm. The center of the roof line lies above the thickest part, measuring up to 6.5 mm. This is because the septopulmonary bundle runs behind Bachmann’s bundle and passes over the roof.16,17 Other investigators have reported that gaps were frequently located at the center and right of the roof line because of lower stability and thicker tissue with interatrial muscular bridges.16,18 Results of our study are consistent with these reported findings. Furthermore, bipolar voltage is reported to be a surrogate marker of LA wall thickness.9Although wall thickness is also a good predictor of acute PV reconnections,11 it is not informative with respect to tissue characteristics. Real-time assessment and accurate measurement of the thickness of the wall just underneath the ablation tags are also limited due to the technical difficulties of incorporating such information into a 3D mapping system. Bipolar voltage is a simple real-time variable obtained at sites accurately mapped in 3D.
In contrast, another major obstacle to creating a continuous floor line lesion would be its proximity to the esophagus, frequently forcing a limit on the ablation power or duration to avoid esophageal temperature rises.15 However, we found no association between gaps and sites where the esophageal temperature rose. A gap in the floor line might be due simply to catheter instability.
One major concern is that patient factors cannot be adjusted by the operators. Therefore, an AI that accounts for both the operator and patient factors is needed for predicting durable LAPWI. With the goal of producing durable but non-excessive ablation, we developed a modified AI, which accounts for both operator and patient factors. We intended to deliver the RF energy with same output, duration, and contact force resulting in the same AI to all the lesion for the calculation of the modified AI. We found the modified AI to be of good prognostic performance for predicting gaps in the ablation lines when attempting LAPWI, and we found the best cutoff value to be ≥ 199 AU/mV. For example, this cutoff value indicated that an AI of ≥ 398 AU will be needed if bipolar voltage at the target ablation site is 2 mV and an AI of ≥ 597 AU for 3 mV. The modified AI-guided ablation strategy considering bipolar voltage might be a potential ablation method to increase the acute lesion durability and reduce the procedure time and complications. Repeated ablation sessions for acute PV reconnections are time-consuming and may produce tissue edema that can cause failure of transmural lesions.19 Further prospective studies are needed to clarify the effectiveness of modified AI-guided LAPWI.