Posterior Wall Isolation (PWI)
The PW of the LA shares its embryologic origin with the PVs (53), which explains its similar electrophysiologic properties and higher arrhythmogenic potential when compared to anterior wall myocytes (54). Furthermore, the PW is exposed to higher stress and tension (which is associated with LVA and electrical scar (55), and is in close proximity to epicardial fat pads and ganglionic plexi (GP), structures known to enhance excitability of the myocardium. The role of GP ablation has been supported in small, randomized studies and a meta-analysis especially for PAF; however, data on long term outcomes is lacking (56-58) (Figure 7). The significant number of AF driver regions located in the PW was further substantiated in a prospective study using non-invasive body surface mapping (59). As such, ablation of the PW has now emerged as an appealing adjunctive strategy to PVI (Figure 8). Several strategies have been developed to achieve PWI, and each approach has theoretical advantages and shortcomings:
Despite the evidence behind PW being a vital focus of AF triggers, data regarding its outcomes have been largely conflicting (64-66), perhaps secondary to failure to achieve complete bidirectional block. The RCT PEACEFUL (Electrical Posterior Box Isolation in Persistent Atrial Fibrillation Changed to Paroxysmal Atrial Fibrillation) was unable to find any added benefit of PWI. Reconnection of the PW was observed in all patients who received a second procedure during follow-up (67).
In a meta-analysis of ten studies including patients with PAF and non-PAF, patients who underwent concomitant PWI experienced less recurrence of all-atrial arrhythmias with significant reduction noted in non-PAF subjects(68).
PWI is associated with certain limitations, including technical difficulties in achieving lesions with durable isolation, thermal esophageal injury, and the concern of atrial mechanical dysfunction. The generation of gaps along the ablation lines or the presence of epicardial and sub-epicardial connections are thought to be the triggers for PW reconnection, even after extensive ablations (69,70). These reconnections have been described in 40-70 % of patients (most frequently in the roof line), despite confirmed bidirectional block and the use of adenosine or isoproterenol during the initial ablation procedure (65,71).
Esophageal injury occurs in 47 % of cases of PWI and can range from erythema or ulceration, to the most life-threatening complication, atrio-esophageal fistula (AEF) formation (72). A large global database recorded the incidence of esophageal perforation and AEF to be 0.016 % and 0.011% respectively (73). Continuous luminal esophageal temperature monitoring could theoretically reduce the risk of esophageal lesions. Termination of ablation is advised when esophageal temperature reaches 38°C, as it can continue to rise above 39°C in at least half of patients, even after stopping ablation (74,75). Other techniques, such as adjusting to low irrigation parameters and mechanical displacement of the esophagus, can be considered. The use of contact-force sensing catheters and restricting contact force to < 20 g has been shown to minimize esophageal injury in a single-center randomized study (76).
The concern for deterioration of LA contractile function has been raised with extensive PW ablation. However, LA contractility is primarily a function of the muscular cells at the anterior, septal, inferior, and lateral walls with negligible contribution from the PW. As such, no decline in LA function has been reported after extensive PW ablation (77,78).