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.