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:
- Ablation lines are created either as a single ring encircling the PVs
and PW or as a box lesion set, where a roof line connecting the
superior PVs and an inferior line between the inferior PVs are done.
However, the durability of these lines has remained a challenge due to
the formation of conduction gaps.
- Extensive substrate-based ablation, or “debulking of the atrium”
where the endpoint is complete electrical silence (60). Nonetheless,
although initial results were promising, the small patient population
in this study has limited its widespread use.
- A “hybrid” approach, that combines a minimally invasive
pericardioscopic epicardial with endocardial ablation, allowing for
the formation of durable transmural lesions, while minimizing the risk
of injury to adjacent structures(61). Regardless, this strategy is
based on a small study, and further trials are required on this novel
multi-disciplinary procedure.
- Use of cryoballoon energy to isolate the PW has shown to be promising
in two recent RCTs, however complete isolation with cryoablation alone
was not achieved in a significant proportion of subjects(62,63).
Although no difference in acute complications were reported in these
studies, esophageal injury remains a major concern, especially for
larger cryoablation balloons. The PIVoTAL study (Comparison of
Pulmonary Vein Ablation with or without Left Atrial Posterior Wall
Ablation for Persistent AF; NCT03057548) is an ongoing RCT to evaluate
the incremental benefit of cryoablation of PWI in addition to PVI.
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).