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\begin{document}

\title{High Density Pace-Mapping for Scar-related Ventricular Tachycardia
Ablation}



\author[1]{Travis Richardson}%
\author[1]{William Stevenson}%
\affil[1]{Vanderbilt University Medical Center}%


\vspace{-1em}



  
  \date{June 07, 2024}


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High Density Pace-Mapping for Scar-related Ventricular Tachycardia
Ablation

Travis D. Richardson MD and William G. Stevenson MD.

1 Division of Cardiovascular Medicine, Vanderbilt University Medical
Center, Nashville, TN, USA

Running Title: High Density Pace Mapping

Corresponding Author:

Travis D. Richardson, MD

Division of Cardiovascular Medicine

Vanderbilt University Medical Center

1211 Medical Center Dr

Nashville, TN 37232

USA

Email: travis.d.richardson@vumc.org

Word Count: 2,225

Conflicts of Interest :

Dr. Stevenson has received speaking Honoria from: Boston Scientific,
Medtronic, Abbott, Johnson and Johnson, and Biotronik; he is co-holder
of a patent for irrigated needle ablation that is consigned to Brigham
and Women's Hospital.

Dr. Richardson has received research funding from Medtronic Inc, Abbott
Inc and served as a consultant for Philips Inc and Johnson and Johnson.

This work did not receive any funding.

Despite advances in medical and interventional therapies, ventricular
tachycardia (VT) due to reentrant activity within complex regions of
myocardial scar remains a common late complication of myocardial
infarction.\textsuperscript{1} While implantable defibrillators (ICD)
may prevent sudden death, ICD shocks are painful and impact quality of
life\textsuperscript{2}. Catheter ablation reduces the likelihood of ICD
therapies and it's role early in the course of disease is
expanding\textsuperscript{3--5}. However, several factors limit the
success and safety of catheter ablation procedures. Scar-related reentry
circuits can be large with a critical isthmus shared by multiple loops.
Ablation of the isthmus is associated with a low risk of recurrence of
that VT\textsuperscript{6,7}. The critical isthmus can be identified
during VT by detailed activation mapping and entrainment. However,
prolonged mapping during VT is often not feasible or desired. Patients
undergoing VT ablation often have severe systolic heart failure as well
as other comorbid conditions. VT is often not hemodynamically tolerated
and even when tolerated, prolonged time in VT may lead to
decompensation. Strategies to limit initiation and mapping of VT may
improve procedural safety\textsuperscript{8}. Methods to guide ablation
based on characterization of the sinus rhythm substrate alone have
generally shown good results\textsuperscript{9}. A number of approaches
have been applied, including ablation over the entire low voltage area
(scar homogenization)\textsuperscript{10}. While this is often
successful, areas of scar can be quite extensive, and undoubtedly this
technique leads to ablation of more areas than absolutely necessary for
success. This approach is also more effective if epicardial ablation is
routinely included, which has the potential to increase procedural risk.
A strategy to focus on the critical regions, particularly when a
clinically relevant VT is known, remains a reasonable first step in the
procedure. A variety of electrogram markers of critical regions have
been described including late potentials, potentials that display
variable coupling to surrounding tissue during programmed
stimulation\textsuperscript{11} , and areas of slow conduction
identified by high density mapping \textsuperscript{12,13}. While these
are likely to increase the specificity of ablation targets compared to
electrogram voltage alone, they are also seen at bystander
areas\textsuperscript{14}.

Pace-mapping during sinus rhythm is useful to help identify the general
location of focal arrhythmia sources,\textsuperscript{15} and can also
be used in scar related reentry.\textsuperscript{16,17} At the reentry
circuit exit region the paced QRS morphology often resembles the VT QRS,
and this will also occur at sites proximal to the exit provided that the
stimulated wavefront follows the reentry path to the exit. A stimulus --
QRS \textgreater{} 40 ms is also consistent with slow conduction away
from the pacing site, that can be a marker for reentry
substrate\textsuperscript{17}.

In this issue of the \emph{Journal of Cardiovascular
Electrophysiology,}Guenancia et al. review their technique of using high
density pace mapping to guide VT ablation\textsuperscript{18}. Their
method takes advantage of software available in electroanatomic mapping
systems that assigns a measure of correlation between two different QRS
morphologies; in this case the VT and the paced QRS
morphology.\textsuperscript{19} A pacing correlation map is generated by
pacing multiple sites within the ventricle and color coding the
algorithmically derived score for display at each point on the anatomic
map. Sites near the exit from the reentry circuit isthmus, typically
along the border of a scar, will display good correlation with induced
VT. As one moves along the isthmus deeper into the low voltage scar the
S-QRS prolongs due to the conduction time between the pacing site and
the exit region. If the isthmus is anatomically defined, such that it is
present during VT and sinus rhythm, the QRS morphology remains similar
to the VT as long as the paced wavefront follows the isthmus out to the
exit. Moving to the entrance or adjacent sites outside the isthmus can
produce an abrupt transition to a markedly different paced QRS because
the wavefront can propagate away without following the path of the
isthmus.\textsuperscript{20} Thus, the pace-map correlation maps can
outline the location of a reentry circuit isthmus during sinus rhythm,
as they illustrate.

Their method can also help identify cases in which the critical isthmus
is not located on the surface being mapped. When the VT circuit is
epicardial or intramural, the earliest endocardial activation may appear
focal. Similarly, the pace-map correlation maps may reveal a concentric
or focal pattern of matching, potentially allowing recognition of this
situation without the need for activation mapping during VT.

We agree with the fundamental principles described, and feel this
technique can be a helpful substrate mapping approach. There are several
caveats. Evaluation to clarify its specificity and sensitivity is
limited. The authors report that in their unpublished experience an
abrupt transition is seen in the majority of post-infarct cases, they
have also published a series of 10 post-infarct patients undergoing VT
ablation during which the pacing correlation maps visually matched VT
activation maps.\textsuperscript{21}

This technique is likely to be effective in cases where the VT isthmus
is confined to the ventricular surface being mapped. Pacing can capture
deep to the endocardium depending on current
strength.\textsuperscript{22} Whether this technique can detect
intramural isthmuses and whether deep tissue that can be captured with
pacing can also be ablated from the pacing site is not clear.

It is important to point out that very good correlations with VT can be
observed pacing in an outer loop immediately adjacent to the exit where
one would not anticipate RF ablation delivery would be effective. If a
focal pattern is seen on both the endocardial and epicardial surfaces
very little can be inferred about the VT circuit; the site with better
correlation would be expected to be closer to the exit. In this setting
entrainment during a brief episode of induced VT with assessment of the
post-pacing interval can potentially clarify the proximity to the
reentry circuit.

During VT, areas of functional conduction block may be present that are
absent during sinus rhythm. Functional block can also occur remote from
the reentry isthmus and alter activation wavefronts during VT changing
the QRS morphology. Theoretically it is then possible to have poor
correlation between the VT and paced QRS at its exit. In animal models
of post-infarction VT exit regions have been shown to harbor very slow
areas of conduction which could be prone to altering total ventricular
activation during VT.\textsuperscript{23}.

We would caution against generalizing these techniques to patients with
dilated cardiomyopathies where confluent regions of low voltage scar are
absent. Diffuse interstitial fibrosis may play a greater role in some of
these VT circuit and anatomically fixed isthmus sites are less likely to
be present.

Further study is needed before utilizing this technique when anatomical
structures within the ventricle are involved in the VT circuit.
Structures such as the moderator band may by definition have multiple
exits and varied QRS morphologies\textsuperscript{24}, and papillary
muscles may display large areas of similar paced
morphology\textsuperscript{25}, potentially distorting pacing
correlation maps.

This technique is unlikely to correctly characterize VT circuits that
involve a portion of the cardiac conduction system as occurs in some
scar-related VTs and in bundle-branch reentry.\textsuperscript{26} These
circuits may demonstrate a focal pattern at the left or right
ventricular apical septum on pacing correlation maps due to the long,
insulated nature of the reentrant circuit itself, and ablation at the
exit site is very unlikely to be effective.

This strategy of high density pace mapping adds to the available
substrate mapping methods for guiding VT ablation while limiting VT
induction. This strategy does not rely on electrogram interpretation,
making it of particular interest in regions of very low voltage. Indeed,
when utilizing larger recording electrodes, such as an ablation
catheter, pacing will often reveal the presence of excitable tissue
where a local electrogram is not always apparent. In post-infarct
ventricular tachycardia circuits with a well-defined scar and a short
anatomically bounded isthmus, pacing correlation maps are likely to be
revealing. More study is warranted to further assess this method in
relation to other substrate mapping methods, in complex substrate with
intramural components, and in other disease substrates. It is useful to
have multiple tools in the tool box. More studies are needed to further
define which tools work best for which substrate.

References:

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4. Cronin EM, Bogun FM, Maury P, et al.: 2019 HRS/EHRA/APHRS/LAHRS
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