Left atrial posterior wall isolation: the icing on the cake
J Interv Card Electrophysiol
Left atrial posterior wall isolation: the icing on the cake
Rong Bai 0
0 Department of Cardiology, Beijing Anzhen Hospital, Capital Medical University , Beijing 100029 , People's Republic of China
Catheter ablation for symptomatic, drug-resistant atrial fibrillation (AF) has emerged as a promising new therapeutic option over the past decade. Most paroxysmal AF (PAF) is initiated by premature beats from the pulmonary veins (PVs). Therefore, catheter ablation that electrically isolates PVs could abolish the arrhythmia. However, especially in more server cases like persistent or long-standing persistent AF (Per AF), PVs are not the only source and non-PV triggers may arise from any part of the heart but most commonly from the coronary sinus, left atrial posterior wall (LAPW), left atrial appendage (LAA), and superior vena cava (SVC) . These structures also serve as substrates to maintain AF episodes after they start. In view of this, it is recommended by the consensus statements that substrate modification should be considered on top of PVs isolation (PVI) in Per AF cases . There is no unique technique of atrial substrate modification while LAPW isolation has been adapted at many centers worldwide and shown to improve the success rate of AF ablation by many isolated studies [3-5]. In this issue of JICE, He et al. reported their results of a meta-analysis on the efficacy and safety profile of adding LAPW isolation to PVI during AF ablation procedures . Five studies with 594 AF patients were included, and the pooled data clearly showed that LAPW isolation reduces atrial tachyarrhythmia recurrence with comparable procedure-related complications and procedural time associated with PVI-alone strategy. Although sample size is small compared to other AF-related trials, this study included the largest series of AF patients treated with LAPW isolation + PVI strategy and provided solid evidence favoring this approach. Before data from a large-scale randomized controlled clinical trial become available, He et al.'s paper can potentially represent a reference for this topic.
While we are emphasizing that PVs play a crucial role in the
development of AF, we should remember that the LAPW and
the PVs are embryological Bsiblings.^ Actually, each PV we
see in adult forms by two parts that join together during the
human heart development. A primitive vein sprouts out of the
LA, which bifurcates twice to give four PVs which grow
towards the developing lungs. A plexus of veins is formed
in the mesoderm enveloping the bronchial buds; these veins
will meet with the developing PVs out of the left atrium to
establish a connection at the fifth week of gestation. As the left
atrium (LA) develops, it progressively incorporates the
common PV into the LA wall until all four PVs enter the LAPW
separately. The incorporated PVs form the smooth posterior
wall of the LA, while the trabeculated portion of the LA
comes to occupy a more ventral aspect [7, 8]. Anatomically,
there is an abrupt change in LA subendocardial fiber
orientation as this bundle traverses the posterior LA between the PVs,
which create a basis of reentry [9, 10]. Indeed, previous work
using noncontact mapping has demonstrated significant
conduction abnormalities in the posterior LA during sinus rhythm
in patients with PAF . Additionally, the LAPW myocytes
have a higher incidence of delayed afterdepolarizations, larger
late sodium currents, but smaller inward rectifier potassium
currents. The LAPW myocytes also have larger intracellular
Ca2+ transient and sarcoplasmic reticulum Ca2+ contents but a
less protein expression of Na-Ca exchanger . It is not
uncommon to see firings from the LAPW initiating AF
episodes. Hence, underlying tissue architecture and
electrophysiological characters in the LAPW may form the substrate
for onset and maintenance of AF. When AF becomes
sustained, atrial remodeling including fibrosis, fatty and
lymphomononuclear infiltration are more pronounced in the
LA septum and LAPW [12, 13]. Acknowledging the
aforementioned, we are confident that the LAPW should be
considered as a target following PVI when ablating AF. It is
expected that this approach will result in a more favorable
outcome as compared to PVI-alone. The question being left
unanswered, even by He et al.’s study, is whether we should
include LAPW isolation in all AF ablation procedures as data
has shown PVI-alone is sufficient to eliminate PAF. It seems
reasonable to add LAPW isolation on top of PVI if AF has
progressed to an advanced stage and the LA has undergone
significant remodeling, as in the cases we recently described
2 Technique to achieve LAPW isolation
To modify AF substrate in the LAPW, electrically isolating
this structure is the goal. Issues remaining controversial
include how to perform LAPW ablation and what is the
definition of LAPW isolation. The so-called box ablation is derived
from the Cox maze IV surgical procedure . It connects
bilateral PV-encircling lesions by placing two linear lesion
sets both superiorly on the roof and inferiorly at the bottom
of the LA. In this way, the entire LAPW is believed to be
isolated from the rest of the atria which can be confirmed by
an entrance block of the LAPW with or without exit block.
The technique developed by Natale’s group, however,
requires more extensive ablation targeting all near-field
potentials in the LAPW. Both entrance block and electrical silence
of the LAPW are used as the endpoint of Natale’s approach
. The study by Dr. He et al. failed to distinguish these
techniques from each other due to small sample size and might
introduce bias. Because of the complex architecture of the
atrial musculature, electrical isolation of the LAPW by a set
of linear lesions is always technically difficult. Gaps are
unavoidable on ablation lines and dormant conduction may
become manifested in the following months. Posterior interatrial
connections also allow firings in the LAPW to penetrate
epicardially and trigger AF episodes . Therefore,
elimination of all LAPW electrical activations is more likely
associated with long-term benefits.
3 Safety of LAPW ablation
He et al.’s study indicated that procedural time and rate of
procedure-related complications did not differ between
PVIalone and PVI + LAPW isolation strategies, regardless of the
technique utilized for LAPW isolation. One may question on
the LA mechanical function after LAPW isolation. However,
it is important to understand that the contractility of the LA
mainly rely on the anterior portion of LA wall while the
LAPW has limited contribution. Another unique, rare but
lethal complication associated with extensive LAPW ablation is
atrial-esophageal fistula. Caution should be taken when
pursuing LAPW isolation and new technologies including
contact force and esophageal temperature monitoring will help
minimize this complication.
Given the embryological homology of the PVs and the
LAPW, and the primary electrophysiological role of these
two structures in the development of AF, it is reasonable to
include LAPW isolation in AF ablation procedures in addition
to PVI, at least in cases with non-paroxysmal AF. Further
studies, especially randomized controlled clinical trials, are
warranted to identify the optimal strategy to achieve LAPW
Compliance with ethical standards
Funding This work was supported the BNational Natural Science
Foundation of China^ (NSFC-81370290), the BBeijing Natural Science
Foundation^ (7161003), and the BCapital Health Research and
Development of Special (2016-2-2062).^ Dr. Bai is supported by the
P r o g r a m o f B e i j i n g H i g h - C a l i b e r Ta l e n t f r o m O v e r s e a s
(BHTO201410007) and is an awardee of the Overseas High-Level
Talent of the Phoenix Plan of the Chaoyang District, Beijing.
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