A redo MI ablation procedure was performed in 15 patients (11 patients with previously successful MI block, 4 patients with failed MI block) during the follow-up. Reconnection of MI was observed in 9 (81.8%) patients with previously successful MI block. Redo ablation of the MI was tried, and MI block was achieved in 7 (77.8%) patients. Both endocardial MI and distal CS ablations were required to achieve BDB across the MI in 5 patients, and the remaining 2 patients eventually needed new MI line ablation to achieve MI block. On the other hand, redo MI ablations were also tried in 4 patients with previously failed MI block. However, only 1 (25%) patient achieved MI block with endocardia MI and distal CS ablation.
Cardiac tamponade occurred in 10 (4.2%) patients during the entire procedure. All patients were stabilized with immediate pericardiocentesis and supportive care without surgery. Stroke occurred in 1 (0.4%) patient who had an embolic stroke in the cerebellum 1 day after the procedure but experienced a full neurological recovery without sequelae.
We described our experience using different ablation strategies to achieve MI block or PMB and presented the outcomes. The main findings of our study were as follows:
Some anatomical obstacles for MI block have been previously suggested. Anatomical features, including myocardial thickness, presence of recesses or fold, cooling effect of adjacent vessels (left circumflex artery or CS), and presence of epicardial CS connection over MI, might contribute to the failure of MI conduction block.[9,16–18] Making a new separate MI line or anterior line may be a good option if the previous ablation line was located on the recesses or fold. Ablation targeting the distal CS or VOM may be useful for elimination of epicardial conduction over the MI. And these methods may provide incremental benefits by providing successful PMB. A prospective study evaluating the long-term efficacy of these additional lesions is needed.
Extensive ablation to achieve MI block can cause serious complications, including cardiac tamponade, circumflex artery occlusion, and atrio-esophageal fistula.[4,19,20] Increasing the RF power and ablation duration should be avoided because of the increased risk of complications in difficult cases of achieving MI block. Fortunately, left circumflex artery occlusion or atrio-esophageal fistula was not observed in our study population. Cardiac tamponade occurred in 4.2% of the patients. Previous studies regarding MI ablation have reported the incidence of cardiac tamponade as 0.8% to 8%.[4,21,22] Although different ablation catheters and RF power were used, we believe our results are consistent with previous results.
Both MI and anterior line ablation are effective in treating macro-reentrant PM AFL. Superiority of the anterior line has been suggested, but the advantage of MI ablation over the anterior line has also been discussed. First, the MI is the area of the latest atrial activation during sinus rhythm. Thus, linear ablation in this area does not alter the LA activation pattern. However, anterior line ablation, by transecting the transatrial conduction through the LA, can change the propagation pattern with the late activation of LAA during sinus rhythm. This change may be related to hemodynamic alteration (simultaneous contraction of the LAA and left ventricle) and increased risk of stroke. Especially, careful considerations are required when failed MI ablation is switched to anterior line ablation because the combination of these 2 procedures can cause LAA isolation and thrombus formation despite oral anticoagulation medications. Although LAA isolation was not observed in the present study, substantial delays of LAA activation even after inscriptions of the QRS complex were observed in 13.0% (3/23) of the patients who underwent anterior line ablation after failed MI block. They all subsequently showed no measurable late diastolic or mitral A wave during sinus rhythm in follow-up echocardiography (Fig. 6). Second, MI is anatomically shorter (2–4 cm). Thus, theoretically, MI ablation carries less chance of leaving a conduction gap than does anterior line ablation. This conduction gap is well known to be proarrhythmogenic of re-entrant arrhythmia.
In the present study, successful PMB was not associated with a lower recurrence rate of atrial tachyarrhythmia. In fact, the benefits of additional substrate modification, including MI ablation after PVI in patients with persistent AF, is controversial. A recent randomized control trial failed to demonstrate the benefits of additional linear lesions or complex fractionated atrial electrogram ablation after PVI. This finding is in contrast to previous reports that have suggested sinus rhythm maintenance benefits of MI ablation, particularly in patients with persistent AF.[4,5,28] Our results appeared to be consistent with the results of a recent multi-center trial, but cautious interpretation is required because our study subjects were not randomized. A future large-scale study is required to clarify this issue.
This study had several limitations. First, this was a retrospective observational study performed at a single center, so selection bias may have influenced our results. Second, selection of the additional ablation strategy was left to the discretion of the operators, and the relative efficacies of different ablation strategies (double MI line, additional anterior line, or VOM ablation) could not be assessed. Third, extensive monitoring of the recurrence of atrial tachyarrhythmia, such as 7-day Holter monitoring or implantation of a loop recorder, was not performed in our patient cohort. Thus, the recurrence rate of atrial tachyarrhythmia might have been underestimated. Lastly, all procedures including PVI were performed using RF ablation. Thus the results of present study may not be applicable to the patients, treated with cryoballoon that is an another excellent treatment option for paroxysmal and persistent AF.[29,30]
Single endocardial MI line with or without distal CS ablation alone was insufficient to achieve MI block. Additional RF applications including double MI line ablation, anterior line ablation, or ablation targeting the VOM improved the MI block and PM block success rate. However, the benefits of PMB for prevention of atrial tachyarrhythmias were not clear in this study. A further large-scale study is required to resolve this issue.
The authors would like to thank JY Jung, SH Moon, JP Yun, DY Han, JH Kwon, JH Kim, SH Kim, H Kim, HJ Ahn, KH Lee, and SH Lee for their excellent technical supports during the electrophysiologic procedures and data collection.
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