Discussion
This proof of concept study suggest that a standardized approach for AF ablation using circumferential high power 50W setting guided by the ablation index, which was proofed in the CLOSE-protocol, was highly effective by reducing the total procedure time and RF application time, with low procedural complication rates, compared to the standard power settings. Especially in light of oesophageal lesions (EDEL), this study could not reveal increased rates of oesophageal lesions under ablation with 50W at the posterior left atrial wall, compared to the standard power settings, even without oesophageal temperature monitoring. Of note, the oesophageal lesions occurring with the 50W ablation were smaller, more superficial and had a faster healing tendency in comparison to the standard power settings. Further, fixed 50W AI-guided ablation did not result in any steam pops, increased incidence of pericardial tamponade or early AF recurrence rate compared to the standard power settings. The absence of steam-pops is remarkable, since Chen et al reported up to 8% steam-pop rates in comparable ablation settings. Close monitoring of impedance drop and stability during lesion formation, as well as avoiding excessive contact force and operator experience in a single-centre setting might have contributed.
Sustainable RF lesions are the most important requirement for a durable PVI. Lesion formation is influenced by RF power, RF application time, catheter contact force and catheter stability 6. Duytschaever and colleagues evaluated the ablation index in the CLOSE and close-to-cure study 3,4. The AI combines the different above mentioned factors in a nonlinear weighted formula, in which the RF power has the greatest contribution. The resulting index seems to be very predictive for the lesion quality and quantity. Nonetheless, there is an ongoing discussion about the most important contributor of this index - the ideal RF power. Whereas most ablation centres use conventional power settings of 25-35W, some previous trials like the power-AF study showed that a 45W RF-power CLOSE-protocol in paroxysmal AF patients significantly increases the global procedural efficiency with similar mid-term efficacy 12. Other studies compared ablation protocols with very high power settings and short RF application time not relying on the ablation index: Deisenhofer et al. could show that a high power short-duration ablation using 70W for 5–7 s leads to significantly less arrhythmia recurrences after 1 year and to significantly shortened RF- and procedural time13. In addition, the QDot-fast trial was able to show that an irrigated very high power short duration ablation (vHPSD: 90W/4s) is feasible and safe to achieve durable PVI in the most cases in a very short procedural- and fluoroscopy time 14. Of note, only in 78.8% of cases, PVI was achieved using the vHPSD mode only. Early PV reconnection occurred predominantly in the area of the posterior wall, requiring re-ablation with standard settings in some cases 14. The biophysical background of the different RF power settings is the way in which the energy is delivered into the tissue. Using higher RF power (HPSD), the tissue heating is caused resistive as direct catheter-myocardium interaction and within the myocardium itself. This direct heating begins immediately and stops with the end of RF energy application 6. Histological data shows that the resulting HPSD lesions differ compared to the standard RF power. In contrast, ablation with standard RF settings and a longer RF application time causes a conductive, i.e. indirect heating which transfers the energy as passive process to more distant tissue. This conductive heating continues, even after the RF power application has stopped and results in deeper lesions compared to HPSD6. Therefore, from a biophysical point of view, it is quite conceivable that HPSD ablation may cause less damage on neighbouring structures such as the oesophagus.
Here, procedural safety was high both in the standard and HPSD group, underlining the advances in atrial fibrillation procedures15. Chest pain was reported by almost ever second patient treated in the HPSD group, however, most patients reported merely minor discomfort and did not require pain medication or prolonged hospitalisation. The pain was mostly described as being breath dependant, i.e. pleuritic. Typically, the development of Dressler’s syndrome is a rare complication after catheter ablation of AF16 - while the occurrence of Dressler’s syndrome in one patient receiving HPSD PVI with extensive left atrial substrate modification might be coincidence, it will require further alert attention, since the mechanism of tissue damage differ between normal and HPSD ablation.
Recently, Sommer and colleagues could show in the OPERA trial that the peak oesophageal temperature during AF ablation was not predictive for the development of thermal oesophageal lesions 17. In the present study no more EDEL could be detected compared to previously reported ablation studies, even without oesophageal temperature monitoring and HPSD ablation at the posterior wall 17. While the overall incidence of EDEL was identical in the standard and HPSD ablation approach, lesion characteristics differed significantly, with most of the HPSD lesions consisting of small ulcerations or only superficial mucosal detachments that are poorly characterized in the currently used classification of EDEL 11. These findings support the above mentioned biophysical considerations and are in line with previous publications of HPSD lesions 18. In the present trial, all patients received post-procedural esophagoscopy and were followed up until the lesions resolved, or showed healing tendency. In further clinical practice, one might consider performing index esophagoscopy to characterise lesions. Smaller ulcerations or mucosal detachments are likely to resolve within 1-2 weeks, without the need for extensive follow-up, whereas patients with larger lesions might be monitored more intensively. However, there is little to no experience and evidence in the development of atrial-oesophageal fistula after HPSD ablation, and time-line characteristics might differ from fistula formation previously described for normal power ablation 19,20. To avoid the development of EDEL while using HPSD at the posterior wall, the recommendations published in the power-AF study should be followed: avoiding high contact force and adapted stability criteria to avoid delayed appearance of the Visitag above the target AI value12. In our study, post-hoc analysis revealed that the largest EDEL occurred in a patient where some posterior points where achieved with a mean contact force of 24 and above grams, with maximum lesion AI of 430 on the posterior wall.
There are some limitations of the current study. First, the study was not designed as a mid-term or long-term outcome trial including re-ablation of patients suffering from AF recurrence. Thus, the long term durability of the ablation lesions, created with the AI guided 50W setting, as well as the resulting clinical outcome remains speculative in lack of robust follow-up data. Further, like in the previous trials the number of the studied subjects was too small to make a valid statement about the real incidence of atrial-oesophageal fistulas using the fixed 50W power setting.
However, while most previous studies have used higher-power ablation with less than 50W, or with reduced power on the posterior wall to <35W, or without relying on ablation index guidance, our study is the one of the first to perform circumferential 50W HPSD PVI with AI guidance and the CLOSE protocol, confirming the data of the FAFA AI High Power Study by Chen et al 21 - but in contrast, without performing esophageal temperature monitoring, and including more complex left atrial procedures with an additional substrate based approach, even including 50W HPSD inferior wall isolation.