The present study introduced and evaluated a novel method of catheter ablation of MI involving EI-VOM, "V-shape" linear ablation, and endocardial and epicardial ablation of key ablation targets (KATs), abbreviated as "MVK". Our main findings were as follows: 1) The implementation of the novel "V-shape" linear ablation technique demonstrated a positive impact on achieving MI block; 2) precise ablation of KATs reduced unnecessary lesions, thereby minimizing the risk of complications; 3)The stepwise method contributed to a high bidirectional conduction block rate across the MI region; 4) the integration of established techniques in the stepwise method increases its reproducibility by different electrophysiologists across different medical centers.
Challenge of MI block
The mitral isthmus is an anatomic area between the mitral annulus and antrum of LPV, playing an important role in maintaining perimitral atrial arrhythmia(11, 12). The linear ablation of MI has been recommended as an initial or repeat ablation strategy for PeAF(1). However, achieving a bidirectional conduction block is challenging due to its complex anatomical structure. Detailed anatomy reveals that MI was surrounded by various structures, including LAA, LPV, CS, ligament of Marshall (LOM), and ramus circumflexus, as well as being inserted by some fat pads(13). This anatomical complexity introduces several considerations when performing MI ablation. Firstly, there is a significant variation in myocardium thickness along the MI. Secondly, the ramus circumflexus and CS, two epicardial blood veins, serve as an epicardial "heat-sink" that prevents the formation of transmural lesions by lowering the conductive heating. Thirdly, autopsy studies revealed various numbers and morphologies of myocardium around the CS and/or LOM connecting with LA and serving as an endocardial-epicardial bridge across the MI line(5, 13, 14). These difficulties pose challenges in creating transmural and durable lesions solely through endocardial ablation.
Efficacy of EI-VOM
The VOM follows a posterior and superior course over the epicardial surface of the LA before draining into the CS. It forms an epicardial bridge consisting of VOM myofibers that can traverse the MI region from the CS to the ridge between the LPVs and LAA (15). The presence of epicardial connections, particularly those involving the VOM, can bypass the ablation lesions created in the MI area. This can result in an incomplete conduction block in the MI despite multiple radiofrequency applications(16, 17). The selective ablation of Ethanol infusion of the vein of Marshall (EI-VOM) targets the atrial myocardium supplied by the VOM. By eliminating ectopic triggers originating from the VOM and blocking residual conduction mediated by epicardial connections, EI-VOM not only addresses the specific anatomic characteristics of the VOM but also extends its impact to the anterior wall of the LPV and the ridge between the LAA and LPV. This enhances the durability of pulmonary vein isolation and modifies the substrate in the MI region (18, 19). In a recent study, Valderrabano et al. found that EI-VOM combined with RF catheter ablation superior to RF ablation alone improved the success rate of AF freedom and resulted in MI conduction block(7). EI-VOM has now become widely accepted as a step in MI ablation procedures. Therefore, we chose EI-VOM as our first step of MI ablation. Nevertheless, despite previous demonstrations of its effectiveness, several pitfalls are associated with EI-VOM. Firstly, the chemical ablation of EI-VOM may not be capable of creating transmural lesions that cover the entire MI region. Secondly, the precision of the injury caused by EI-VOM can be challenging, and visible VOM is only present in a percentage of cases, ranging from 84–92% (7–9). Thirdly, there were other epicardial connections except for VOM, which served as important residual MI gaps. These pitfalls associated with EI-VOM accounted for the 0% block rate observed in the present study.
"V-shape" endocardial linear ablation of MI
Endocardial linear ablation of MI is the most important step in achieving a conduction block. The conventional method defined the narrow region between LIPV and MA as the ablation line, generally originating from the root of LIPV to 2–3 o'clock of MA(11). However, this method often resulted in a low acute block rate and high reconnection rate in the later stages of ablation due to the heterogeneous and complex structure of MI. In our study, a "V-shape" two-line design of endocardial ablation was selected. This method was inspired by previous ablation techniques for AFL related to the cavotricuspid isthmus(CTI) in the early years(20) and an ablation description of the epicardial connection of CTI in one study(21). Multiple ablation lines were created to encompass all potential arrhythmogenic sites that were difficult to eliminate directly. In the present study, the upper line of "V- shape" ablation lines extended along the posterior root of LAA from the 1–2 o'clock point of the MA to LIPV while the lower was from the LIPV to the ostium of VOM in the present study. There were several advantages to this approach follows. Firstly, the ablation line was positioned away from the thick myocardium and the complex structure of the MI to some extent. Secondly, the "V-shape" block lines encompassed the residual epicardial connections after the initial EI-VOM ablation. Therefore, a higher acute block rate of MI (57.9%) was observed in our study compared to previous studies.
Endocardial and epicardial ablation of KATs
Due to the complex structure of the MI and variations in myocardium thickness, different RF deliveries are needed to achieve durable lesions. In our study, we described ELs as lesions where the activation sequence changed or the conduction time prolonged during the ablation process, indicating that myocardial conduction passed through those sites. Enhanced ablation of these ELs not only improved the acute success rate of MI conduction block but also reduced the likelihood of late reconnection. Mapping and ablation of EASs were performed when the initial linear ablation did not achieve conduction block, which helped avoid the need for extensive additional ablation (22, 23). Chen et al. reported that conduction breakthroughs were observed in 78.4% of patients after the initial linear ablation(22). In our study, the residual connections were identified in 50 patients(42.0%), which may be attributed to the early EI-VOM, which eliminated some epicardial connections. Endocardial ablation of EASs could facilitate conduction block in 22 cases, primarily at the root of LAA(36.4%) and the ostium of VOM(45.5%), possibly because of thicker myocardium or elimination of some epicardial connections at the two positions. Ablation of VOM ostium was performed due to the incomplete alcoholization caused by balloon occlusion(24). The remaining 28 patients needed epicardial ablation to achieve a conduction block, attributed to the residual epicardial connections. Precise endocardial and epicardial ablation of these connections reduced the need for excessive ablation and decreased the recurrence of complications, such as cardiac tamponade.
In our study, three techniques were involved: EI-VOM, endocardial linear RF ablation, and endocardial and epicardial ablation of KATs. Those techniques have been used maturely for years, and their maneuverability and efficacy have been described and demonstrated in previous studies. Therefore, no technical problem limits the feasibility of this stepwise method, and different cardiac electrophysiologists could reproduce it. Moreover, advances in technologies, including contact force (CF)-sensing catheters, high-power ablation strategy, and visualized steerable sheaths, facilitated the creation of contiguous lesions by resistive heating, which was beneficial in reducing the occurrence of late reconnections(25–27).
Limitation
Firstly, despite the encouraging results of our study, it is important to acknowledge its limitations, including a small sample size from a single center without a control group. Secondly, long-term outcomes have not been specifically evaluated in the present work. All patients in our study were followed by our research group, and the follow-up results will be published, providing insights into the short and long-term prognosis.