Corresponding Author
Jason S. Chinitz, MD
Northwell Health Physician Partners, Cardiology at Brightwaters
402 Potter Boulevard
Brightwaters, NY 11718
jchinitz@northwell.edu
In the ongoing quest to optimize outcomes for atrial fibrillation
ablation, efforts continue to balance the reliable creation of durable
transmural ablation lesions, while minimizing risk to neighboring
sensitive structures. Various effective strategies have been developed
to guide titration of radiofrequency (RF) energy delivery, leveraging
our understanding of ablation biophysics and titratable factors such as
contact force, power and duration of ablation. The Ablation Index (AI,
Biosense Webster) utilizes this information to guide more predictable
lesion creation, and has led to improved procedural success and
efficiency.1 However, despite important advancements
such as this, the ideal amount of ablation in a given location, with
variable tissue characteristics and surrounding structures, remains
unknown. Strategies designed to improve lesion efficacy must be weighed
against the risks of complications from over-ablation, including
esophageal injury during ablation on the posterior wall of the left
atrium.
The CLOSE protocol combined AI targets and structured intertag distance
limits to standardize and optimize ablation delivery during pulmonary
vein isolation (PVI), and has shown impressive short-term results
including 1 year freedom from atrial tachy-arrhythmia recurrence
>90% and low rates of pulmonary vein reconnection at redo
procedures.2 The original CLOSE protocol involved
standard power ablation at 25-35W, but derivations to the CLOSE protocol
have adapted this strategy with high power ablation (45-50W), also
producing impressive clinical outcomes.3High-power-short-duration (HSPD) ablation has been shown to produce a
more homogenous, broad based lesion, and may further improve efficacy of
PVI as well as procedure efficiency; theoretically, HSPD may also
improve safety of ablation in the left atrium adjacent to the esophagus
by limiting lesion depth and conductive heating.4However, as most esophageal lesions resulting from ablation are
asymptomatic, smaller clinical studies evaluating novel ablation
strategies may under-appreciate the risk of esophageal injury, and favor
strategies that promote efficacy at the expense of safety.
This manuscript by Francke et. al.5 focuses on the
safety of the combined high-power CLOSE-guided ablation approach, using
endoscopy in 300 patients to identify esophageal lesions following AF
ablation. In this study, PVI was performed with contiguous 50W RF
applications, delivered on the posterior wall of the left atrium until
an Ablation Index target of 400 was reached. Whereas traditional HPSD
ablation limits ablation to around 5 seconds when adjacent to the
esophagus, using pre-specified AI targets to guide ablation duration in
this study protocol resulted in RF duration of 11-12 seconds on average
per lesion on the posterior wall. The ablation strategy employed in this
study did not use esophageal temperature monitoring or any other defined
strategy to avoid collateral esophageal damage. Accordingly, the effects
to the esophageal tissue from uninterrupted high power ablation guided
by AI targets were more directly examined.
The findings reported 35/300 (11.7%) of patients with endoscopy
detected esophageal lesions (EDEL), including 10/300 patients (3.3%)
with type 2 esophageal lesions based on the Kansas City Classification
(and four patients with type 2b lesions, indicating deep
ulceration). Notably these findings are overall consistent or better
than results of prior investigations with standard power ablation which
have shown EDEL in up to 15% of cases.6 In one recent
multi-center study evaluating high-power AI-guided ablation with target
AI of 350 on the posterior wall, pooled analysis in 953 patients showed
EDEL in 6%, half of which were type 2 lesions and one that resulted in
esophageal perforation.7 Though the overall risk of
clinically relevant esophageal injury remains very low, the residual
incidence of esophageal injury demonstrable by endoscopy with the
potential for catastrophic progression remains concerning, and warrant
even better efforts for risk mitigation.
One clinically relevant finding in this study was that high contact
force during posterior wall ablation was a primary risk factor for
esophageal injury, independent of other baseline or ablation parameters.
This finding is consistent with prior studies which have shown reduction
in esophageal lesions by limiting contact force to <20
grams.8 As applied contact force is now controllable
during ablation, avoiding high contact force on the posterior wall may
be an important strategy to prevent direct esophageal injury,
particularly when high power is used. However, with reduced contact
force, longer duration of ablation is needed to reach AI targets. The
target AI of 400 consistently takes >10 seconds of
continuous ablation at 50W to reach, and the algorithm will not even
begin to display the dynamic AI until after at least 6 seconds of
ablation, thereby precluding the use of AI during traditional
time-limited HPSD ablation. In the POWER-AF study, using 45W in the
posterior wall required an average of 13 seconds (range 11-14 seconds)
to achieve a target AI of 400.9 Similar ablation
duration was required to reach AI of 400 using 50W in this study, with
maximum RF duration on the posterior wall reported up to 19.9 +- 3.5
seconds.5 As acknowledged by the authors in the
Discussion, there is a more narrow safety margin with high power
ablation, and therefore prolonged high power applications may negate the
safety benefits of HPSD biophysics.3,9 Steam pops have
been shown to occur before target AI of >450 are reached
when using high power ablation, particularly at higher starting
impedances.10 In addition, the limits in time
associated with HPSD ablation not only reduce the risk of over-ablation,
but may allow for more consistent catheter-tissue contact during RF
application. It is therefore questionable whether the same AI targets
are appropriate when using high power, and whether lower AI targets,
time-limited ablation, or use of other ablation strategies may achieve
similar efficacy with less risk.
A uniform approach to ablation over the posterior left atrial wall may
not be ideal in all patients. Given variations in atrial myocardial
thickness, tissue characteristics, esophageal location and proximity to
sites of ablation, the optimal duration and pattern of high-power
ablation is variable in clinical practice. In addition, “heat
stacking” has been demonstrated with multiple HPSD lesions delivered in
close spatial and temporal proximity, arguing against consecutive and
contiguous ablation applications as described in this study
protocol.11 Patient specific characteristics including
baseline tissue impedance, and extent and slope of impedance decrease
during ablation gives important feedback regarding tissue heating, and
can further guide effective and safe lesion delivery.
Esophageal temperature monitoring was not utilized in this study, and it
is unknown whether addition of this information would have changed the
results. No incremental clinical benefit has been demonstrated in recent
studies using luminal esophageal temperature monitoring with either
standard or high-power ablation.12 Skepticism
regarding the benefit of esophageal temperature monitoring is rooted in
the limited sensitivity of the luminal temperature probe, which is
dependent on the location of the probe relative to the site of ablation,
and can underestimate or completely miss heating changes at other
regions of the esophagus-left atrial interface. Furthermore, the latency
of temperature changes may permit esophageal damage even before
temperature rise is detected, particularly with HPSD
ablation.11 Regardless, esophageal temperature
monitoring when used appropriately can alert the operator when
esophageal heating occurs, and should prompt a change in approach to
avoid unnecessary additional ablation in at-risk regions until
temperature decreases. Other strategies such as esophageal deviation and
active esophageal cooling also have shown limited clinical benefits and
carry additional costs and potential adverse effects that have limited
their adaptation. Despite the limitations, until a better understanding
of lesion titration and risk mitigation on the posterior wall exists, it
seems prudent to encourage concomitant use of compatible strategies to
avoid this potentially life-threatening complication.
With major progress in technology and understanding of ablation
biophysics, the risk of esophageal injury may have been tempered. While
the approach and findings described in this study provide a step in the
right direction in our understanding of ablation to achieve effective
and safe PVI, even better strategies to minimize the collateral risk are
still needed. Using high-power and pre-specified AI targets may increase
the likelihood of lesion transmurality, but with risk of over-ablation
at sensitive locations, where less aggressive ablation may be
sufficient. Optimal AI targets with HPSD require further study, and
further evaluation is needed to determine whether AI provides clinical
benefit over time-limited HPSD alone. A combination of approaches is
needed, including tailoring of ablation delivery to take into account
tissue thickness and proximity of ablation to sensitive structures. As
AF ablation now more frequently includes posterior wall isolation in
many institutions, rates of clinically relevant esophageal injury will
have to be driven even closer to zero. Otherwise, Pulsed Field Ablation
is set to challenge RF as a primary approach to AF ablation in the near
future.