4.2. The Success Rate with Different Application Settings
The success rate of AF ablation has risen over the past two decades with the evolution of three-dimensional electroanatomic mapping, contact force sensing catheters, and catheter irrigation [
17]. Cryoablation is a promising alternative technique to RFA for treating PAF with encouraging results. Chen YH et al. [
18] demonstrated that cryoablation presented comparable long-term AF/atrial tachycardial-free survival and procedure-related adverse events compared with RFA. Meanwhile, cryoablation markedly shortens the procedure time with negligible impact on the fluoroscopy time. However, the HPSD ablation strategies are only applicable when using single point-by-point ablation devices. AI and lesion size index (LSI) are novel ablation quality markers to predict lesion quality that incorporates CF, time, and power in a weighted formula [
19]. However, the exact power settings and AI values that result in a high success rate with AF are unclear. Currently, the setting of foreign and domestic mainstream AI is 400–600 for the anterior wall and 400–450 for the posterior wall. However, the results vary from study to study, with similar or slightly higher success rates in the HPSD group with the guidance of AI values than in the conventional group [
20,
21,
22,
23]. With the targeted AI values of 450–500 for the anterior wall and 350–400 for the posterior wall in two groups, Liu Z et al. demonstrated the HPSD-AI group (≥45 W) had lower recurrence of atrial arrhythmia at 12 months (6.8% vs. 28.3%,
p = 0.011), higher PV first-pass isolation, shorter ablation times, and fewer patients with anatomical leakages and sites of unreached AI compared with the low power-AI group (<35 W) [
24]. O’Brien J et al. [
23] illustrated that with a power setting of 50 W and target AI values of 550–600 in the anterior LA region and 400–450 in the posterior LA region, there was no significant difference in the success rate at 12-month follow-up compared with the AI-guided conventional group with a power setting of 35–40 W (80.2% vs. 82.8%,
p = 0.918). Similarly, for PAF patients, there was no significant difference in the success rate between the HPSD group (40 W) and the conventional group (30 W) at 12 months of follow-up under the same AI-guided ablation (400 for posterior and 500 for anterior wall) in both groups (92% vs. 84%, respectively,
p = 0.22) [
25]. In addition, the measurement of the local impedance might predict optimal lesion formation. A local impedance drop more than 21.80 ohms on the anterior wall and more than 18.30 ohms on the posterior wall significantly increased the probability of creating a successful lesion [
26]. Boussoussou M et al. [
27] found that LA wall thickness did not influence the first-pass isolation rate during PVI guided by the modified CLOSE protocol (AI 400 on the posterior wall and 500 on the anterior wall, CF 10–40 g). Only the diameter of the right superior PV was associated with the success rate of right-sided first pass isolation, as a wider right superior PV diameter led to an easier first-pass isolation.
However, fewer studies have compared the effectiveness of a HPSD group with a conventional group guided by relatively low AI values. Solimene F et al. [
28] reported on 156 AF patients (124 PAF patients and 32 PeAF patients) undergoing AI-guided PVI with target AI values of 400–450 for anterior and 330–350 for posterior LA regions, with 89.2% of patients (91% PAF vs. 78% PeAF) being in the sinus rhythm at 14 ± 6 months. Okamatsu H et al. [
29] investigated 60 AF patients undergoing AI-guided PVI (AI values of 400 at the anterior, 360 at the posterior LA wall, and 260 on the esophagus) randomly assigned to 3 groups (LP group: 30 W at the anterior and 20 W at the posterior wall; MP group: 40 W at the anterior and 30 W at the posterior wall; HP group: 50 W at the anterior, 40 W at the posterior wall and 30 W on the esophagus) and found no significant difference among the groups (100%, 95%, and 95% in LP, MP, and HP groups, respectively,
p = 0.44).
Additionally, a very high power ablation strategy applied clinically improved procedural efficiency with comparable safety compared with the conventional strategy. Kottmaier M et al. [
30] demonstrated that very high power, short-duration (vHPSD) applications with a high power of 70 W for 5–7 s had significantly less arrhythmia recurrence during the one-year follow-up (26.9% vs. 34.9%,
p < 0.013) with no major complications. Additionally, vHPSD ablation performed by applying 90 W for 4 s during follow-up had more than a 90% success rate with comparable safety [
31,
32]. A prospective, observational cohort study showed that both HPSD (50 W, AI 500 on the anterior and 400 on the posterior LA wall) and vHPSD RFA settings (90 W/4 s) shorten procedure and RF time and result in a higher rate of first-pass isolation at 9-month AF recurrence rate (10%, 8% and 36%,
p < 0.01) compared to LPLD RFA settings (30 W, AI 500 on the anterior and 400 on the posterior LA wall). Moreover, the presence of first-pass isolation was associated with a lower AF recurrence rate at 9 months (OR = 0.09, 95% CI 0.04–0.24,
p < 0.01) [
33].
In our study, after using propensity-score matching, compared with the conventional RFA strategy (without AI, 35 W at anterior wall, 25–35 W at the posterior wall, and 35–40 W was applied in the other segments), the HPSD RFA strategy with relatively lower AI values (40–50 W, AI values of 350–400 for anterior, 350–380 for superior and inferior, and 320–350 for posterior LA wall) had higher early recurrence, similar freedom from atrial arrhythmia, and comparable safety during 12 months of follow-up. Additionally, we are a high-volume electrophysiology center with operators who have over 10 years of AF ablation experience, and there may be no significant difference between the guidance with AI and non-AI ablation. AI is a quantitative metric that may help shorten the learning curve. We used a relatively low AI-guided ablation, which may reflect better long-term outcomes if ablation guided with a relatively high AI is currently available at other centers.
The internal aspect of the LA is relatively smooth, but its wall thickness is not uniform, with an average thickness of 4.5 ± 0.6 mm (range, 3.5–6.5 mm) at the roof, 3.9 ± 0.7 mm (range, 2.5–4.9 mm) at the left lateral wall, and 3.3 ± 1.2 mm (range, 1.5–4.8 mm) at the anterior wall, 2 mm near the vestibule of the mitral annulus, and 4.1 ± 0.7 mm (range, 2.5–5.3 mm) at the posterior wall. The wall thickness of PVs varied from heart to heart. At 0.5 cm away from the junction, the thickness of the left superior PV (LSPV), left inferior PV (LIPV), right superior PV (RSPV), and right inferior PV (RIPV) was 2.8 ± 0.5 mm (range, 1.9–3.5 mm), 1.5 ± 0.4 mm (range, 0.9–2.1 mm), 2.5 ± 0.5 mm (range, 1.8–3.3 mm) and 2.0 ± 0.4 mm (range, 1.5–2.5 mm), respectively. At 0.5 cm away from the junction, the thickness of the LSPV, LIPV, RSPV, and RIPV was 2.3 ± 0.4 mm (range, 1.8–2.8 mm), 1.2 ± 0.3 mm (range, 0.5–1.7 mm), 2.0 ± 0.3 mm (range, 1.5–2.5 mm), and 1.5 ± 0.2 mm (range, 0.9–2.2 mm), respectively. Additionally, the LA posterior wall thickness increased from the most superior to the most inferior measured level, whereas the wall was thinner in the middle and between the inferior venous orifices in those with AF (for AF patients: 2.1 ± 0.9 mm between the superior PVs, 2.2 ± 1.0 mm in the center of the posterior LA wall, and 2.5 ± 1.3 mm between the inferior PVs; for non-AF patients: 2.3 ± 1.0 mm between the superior PVs, 2.6 ± 1.0 mm in the center of the posterior LA wall, and 2.9 ± 1.3 mm between the inferior PVs) [
34,
35,
36]. The myocardium of the left lateral ridge at its superior level was thicker than at its inferior level (2.8 ± 1.1 (range 1.5–4.2 mm) vs. 1.7 ± 0.8 mm (range 0.5–3.5 mm), respectively,
p < 0.001) [
37]. The minimal distance between the right PV antrum, left PV antrum, LA posterior wall, and esophageal wall was 6.3 ± 2.8 mm (range, 3.4–11.5 mm), 5.6 ± 2.2 mm (range, 3.3–10.5 mm), and 6.2 ± 2.5 mm (range, 3.6–13.5 mm), respectively [
38]. The HPSD RFA strategy with a power of 50 W for 7 s (LSI 4.8 ± 0.52) creates wider but shallower lesions that had a diameter of 4.98 ± 0.91 mm and a depth of 2.2 ± 0.76 mm, whereas the conventional RFA strategy with a power of 25 W for 30 s (LSI 4.73 ± 0.59) had a diameter of 4.45 ± 0.74 mm and a depth of 2.8 ± 1.56 mm [
39]. Additionally, the HPSD settings for 90 W/4 s in the beating heart of swine produced wider lesions (6.02 ± 0.2 mm vs. 4.43 ± 1.0 mm) and similar depth (3.58 ± 0.3 mm vs. 3.53 ± 0.6 mm) compared with the conventional RFA strategy for 25 W/20 s (CF of two groups range from 5–40 g) [
16]. From the anatomy of the LA, the thickness of the posterior wall ranged from 2.5 mm to 5.3 mm [
36]. Therefore, using relatively conservative AI values in patients is more consistent with safety principles, while AI values of 320–350 for posterior walls are able to balance effectiveness and safety.
4.3. Characteristics of Recurrent Atrial Arrhythmia
In a recent publication by Vassallo F et al. [
40], the HPSD RFA strategy increased the incidence of early recurrence and reduced late recurrence compared with the conventional strategy, while AT or AFL occurring during the blanking period is more common in the HPSD group. However, Bunch et al. [
41] reported a similar recurrence of AF at one year and three years between two groups, and a higher rate of AFL at one year and three years was observed in patients treated with the HPSD RFA. In our study, the propensity-score matching analysis showed that a similar recurrence of AF was observed during the blanking period in both groups, while a higher rate of atrial arrhythmia was observed after one year’s follow-up in patients treated with the HPSD RFA strategy.
The early recurrence for AF patients was associated with an inflammatory process caused by the 40–50 W power setting used in this ablation protocol [
42]. After the blanking period, the spontaneous resolution of the arrhythmias with sinus rhythm conversion may be explained by the decrease in inflammatory responses or the use of AADs. Because the HPSD RFA strategy had relatively low AI values, shallower lesions, and less total energy, transmural damage may not be achieved, resulting in no difference in the late recurrence rate between the two groups. Therefore, further studies are required to explore the most optimal power setting and AI values for RFA to yield greater clinical value.
PVI is the cornerstone of AF ablation, whereas PVR is attributed to catheter instability, tissue edema, and reversible non-transmural injury [
43]. Therefore, the continuous and transmural lines are key to the success of ablation. In animal studies, HPSD applications resulted in 100% contiguous lines with all transmural and improved lesions showing lesion-to-lesion consistency compared with conventional applications [
15,
16]. Recent studies [
44,
45,
46,
47] have shown different consequences in terms of chronic PVR during the second procedure between two groups. Yavin HD et al. [
47] demonstrated that incidence of chronic PVR during the redo procedure was lower in a HPSD group than in a conventional group (16.66% vs. 52.2%,
p = 0.03), and reconnection sites occurred in the septal aspect of the right PV or the anterior left PV in the HPSD group and in the anterior left PV, septal right PV, and posterior wall in the conventional group. However, Hansom SP et al. [
45] reported that the right PV carinal segments had a higher rate of reconnection in HPSD applications compared with conventional applications, but there was no difference in chronic PVR between the two groups.
In our study, there was no difference in the proportion of chronic PVR during repeat procedure between the two groups. Some patients who did not undergo redo procedures had recurrence during the follow-up period. Additionally, areas of chronic PVR were co-located at sites of decreased catheter stability in both groups. Reconnection sites occurred more in right PVs and left carina in both groups. In terms of anatomy, it is more difficult for a catheter to reach right PVs than left PVs. Furthermore, the myocardial thickness of the carina is thicker, and the catheter attachment of the carina is more difficult, and thus the HPSD strategy with conservative AI values and lower energy may lead to non-transmural lesions. Therefore, for all the reasons listed above, we may underestimate the rate of chronic PVR, contributing to chronic PVR in both groups. Non-PV triggers were frequently found in the superior vena cava, LA anterior wall, LA posterior wall, coronary sinus, and vein of Marshall [
48]. In our study, since 15% of patients in both groups underwent superior vena cava isolation in the first ablation procedure, the number of patients requiring superior vena cava isolation in the repeat ablation procedure was minimal. Additionally, both groups had similar rates of PV triggers. The proportion of non-PV triggers was relatively high in the HPSD group compared with the conventional group, but there was no significant difference in non-PV triggers between the two groups.
This implies that the relatively low AI values might not have produced transmural injury lesions, while the thicker carina and the difficulty of catheter apposition during the procedure contributed to the chronic PVR in both groups. Furthermore, a limited number of recurrent patients had repeat procedures, and additional lesions were performed during the initial procedure in both groups, which might have reduced the incidence of non-PV triggers. More well-designed and large-scale RCTs are required to confirm these findings.