Discussion
The addition of posterior wall isolation (PWI) to pulmonary vein isolation (PVI) resulted in a considerably higher chance of freedom from AF over 12 months compared to PVI alone in patients with symptomatic persistent AF who were referred for CBA. Adding PWI to PVI also significantly reduced the recurrence risk of all atrial tachyarrhythmias. Furthermore, PWI was not associated with increased adverse events when compared to PVI, while total ablation time was significantly longer in the PWI + PVI group as compared to PVI group with a mean of 47±33 minutes and 24±19 minutes, respectively. This result is remarkable considering that a recently published RCT in patients with persistent AF using RF ablation did not show any significant difference in freedom from atrial arrhythmias with adjunct PWI as compared to PVI alone. (10)
To our knowledge, this is the first meta-analysis that has compared patients with atrial fibrillation undergoing PVI to PVI + PWI using CBA exclusively. Previous meta-analyses (17, 18, 19) also compared PVI with PVI+PWI, but they included studies that used RFA, whereas our meta-analysis explores the outcomes of only CBA in patients with persistent AF.
The front section of the LA wall yields the majority of left atrial (LA) contractility, while the posterior wall (PW) appears to play a rather minor role. Hence, LA posterior wall isolation does not result in a substantial loss in atrial mechanical function. (20)
The LA PW and the pulmonary veins (PV) share the same embryological origin in which the primitive mesodermal PV develops four different branches, and thus, the LA PW and PV exhibit similar histology. (21) The LA PW is a complicated structure made up of many layers of muscle bundles with varying wall thicknesses. (21, 22) In comparison to other sections of the LA, the LAPW frequently gives rise to rotors and spontaneous triggers. (25) (23) The LA PW and septum are the most common locations of structural atrial remodeling seen in AF. (24) Consequently, the LA PW similar to the PVs may be a relevant contributor to arrhythmogenesis. When compared to other LA sites, the LA PW has a greater rate of delayed afterdepolarizations, late sodium currents, but relatively small inward rectifier potassium currents, higher intracellular calcium transients, and sarcoplasmic reticulum calcium storage, and greater protein expression of the ryanodine receptor. (23)
Among individuals undergoing surgical ablation for AF, the routine inclusion of PWI with PVI has been shown to favorably impact the long-term procedural success. (11) Further, wide area circumferential PV isolation often leaves a narrow posterior wall channel that may be a viable reentry substrate. Fibrosis of the PW, may account for slow conduction and functional reentry. (27) ”Debulking” the left atrium by PWI may diminish critical mass required to sustain AF (28), and the PW’s epicardial fat pads containing ganglionic plex can be modified by ablation. (26) The DECAAF multicenter prospective study demonstrated that among patients with atrial fibrillation undergoing catheter ablation, atrial fibrosis estimated by delayed enhancement MRI was independently associated with the likelihood of recurrent arrhythmia. (29). However the DECAAF II trial did not demonstrate the incremental benefit of additional MRI-guided scar ablation when compared to conventional ablation in patients undergoing PVI (30).
Targeting LAPW following PVI may be a viable option for preventing AF recurrence in persistent AF. Depending on the operator’s choice and expertise, different techniques of PW can be utilized using either radiofrequency or cryo energy. Isolation can be accomplished via a ”box lesion set,” connecting the superior and inferior PV lesion sets with a roof and a low posterior line. A single ring has also been described that employs a single circle to include the PVs and the PW. Another technique is to eliminate all viable atrial potentials on the posterior wall with RF ablation. A potential explanation why PWI with RF ablation may not result in improved clinical outcomes may be that RF ablation in the area of the PW at altered energy settings may frequently result in incomplete lesion sets as a result of insufficient lesion debt and recovery of conduction in deeper epicardial layers of the posterior wall and left atrial roof.
Thorough PWI isolation is associated with longer procedure times. Another drawback of PWI is the possibility of complications, such as an atrio-esophageal (AE) fistula which appear to be more common using radiofrequency ablation (10). Operators have tried to decrease the risk of AE fistula by esophageal temperature monitoring, (31) employing high power short duration ablation, altered irrigation settings and rigorous contact monitoring, as well as (31) esophageal deflection. (32)
Advantages of CBA of the posterior wall may be a lower risk of esophageal injury and the creation of a more homogenous lesion set. Luminal esophageal temperature monitoring was done in 2 of the 3 included studies (14, 15). A trade of is the higher risk of phrenic nerve palsy. High output right atrial phrenic nerve stimulation (>10mA) from the superior vena cava was performed in all three studies to avoid phrenic nerve injury (13, 14, 15). Our meta-analysis found no evidence of a greater risk of complications with PWI than with PVI alone, particularly the development of an AE fistula.