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\begin{document}

\title{Tissue Coverage Matters}



\author[1]{Arvindh Kanagasundram}%
\author[1]{Travis Richardson}%
\author[2]{William Stevenson}%
\affil[1]{Vanderbilt University Medical Center}%
\affil[2]{Vanderbilt Heart and Vascular Institute}%


\vspace{-1em}



  
  \date{May 14, 2024}


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\textbf{Tissue Coverage Matters}

Arvindh Kanagasundram, Travis D Richardson, and William G Stevenson

\textbf{From the:} Division of Cardiovascular Medicine, Arrhythmia
Section,

Vanderbilt University Medical Center. Nashville, TN

\textbf{Total Word Count:}

\textbf{Address for Correspondence:}

William G. Stevenson, MD

Division of Cardiovascular Medicine

Vanderbilt University Medical Center

1215 21\textsuperscript{st} Ave South

Nashville, TN 37212

Phone: 1-615-322-2318

Fax: 1-615-936-2815

Email: william.g.stevenson@vumc.org

Although radiofrequency (RF) catheter ablation for ventricular
tachycardia has been shown to provide superior outcomes to escalating
anti-arrhythmic therapies, multi-center trials continue to report
arrhythmia recurrence rates in 20-50\% of patients. Failure of catheter
ablation is often due to deep intra-mural portions of reentry circuits
and anatomic barriers to ablation. Means of increasing lesion size are
of interest.

During ablation current passing through high resistance cell membranes
produces resistive heating. Heat produced by resistive heating is
transferred to the surrounding tissue by conduction and
convection.\textsuperscript{1} Tissue heating also heats the electrode
lying on the tissue. Energy delivery is limited by the denaturation of
blood proteins and formation of char on the electrode when temperatures
exceed 70 degrees C. Furthermore, if temperatures in the tissue reach
100 degrees C, steam formation can occur and explode through the tissue
with the potential for embolization and rupture.\textsuperscript{2} Thus
to achieve deep, durable RF lesions the goal is to deliver maximal
current into the tissue while maintaining tissue electrode temperatures
below that which generates coagulum and tissue temperatures below that
which creates steam pops. Irrigating the RF electrode reduces its
temperature and allows more tissue heating to occur before electrode
temperature reaches the level at which char formation occurs. Resulting
lesion size is due to a complex interplay of multiple factors, as was
evident early on when in experimental in-vivo studies RF lesions created
with the same RF ablation settings were variable in size and often
absent at some ablation sites, and optimizing RF parameters is still a
topic of great interest.

The greater the RF current, the greater the resistive heating in the
tissue. Current can be increased by increasing power, or lowering
resistance for a given power, as by moving the cutaneous disperisive
electrode closer to the heart or using two dispersive
electrodes.\textsuperscript{3} The RF electrode is in the blood pool and
only a fraction of the current passes through the myocardium, with the
remainder passing through the blood pool. Replacing the normal saline
irrigant with half normal (0.45\%) reduces shunting of current through
the blood pool, increasing the proportion of current passing through the
tissue\textsuperscript{4} \textsuperscript{5}.

The amount of current transferred into the tissue is also influenced by
electrode-tissue contact and the area of the electrode that is in
contact with the underlying tissue.\textsuperscript{6, 7} Electrode
contact force is an important determinant of lesion size with increased
force also resulting in a higher incidence of steam pops and thrombus
formation for a given energy setting\textsuperscript{8}. However, even
when all these parameters are measured, our ability to predict lesion
size and pops for a given set of parameters is inconsistent. In this
issue of the journal Bourier et al evaluate the relation between surface
area of the RF ablation electrode in contact, or covered by myocardium
with lesion size and steam pops while maintaining power, duration and
impedance the same. Porcine ventricular endocardium was ablated in a
tissue bath where the impedance was maintained at 120 Ohms by
alterations to the tonicity of the bath and RF lesions were with RF at
30W for 30 seconds. Three different ablation electrode tip
configurations were studied: only the tip in contact (I), tip electrode
pushed into the tissue such that half of the length of the electrode was
covered (II), and the entire electrode wedged into a trabeculum such
that it was entirely covered (III). Contact force was similar for each
preparation. Despite similar RF parameters greater electrode coverage
increased lesion size and steam pops, which occurred in 0\%, 10\% and
100\% of coverage I, II and III ablations respectively.

These observations are consistent with the effect of greater electrode
surface area contact to increase current flow into tissue rather than
the circulating fluid pool and increase lesion size, as has been
observed when comparing electrode that are oriented perpendicular versus
parallel to tissue. There are some caveats that should be considered in
interpretating their data. Consistent with greater heating, the
impedance of system decreased during ablation to a greater extent with
greater electrode coverage. With RF set to a fixed power, a decrease in
impedance results in an increase in current delivery to maintain the
same power, hence, this could play a role in increasing lesion size.
Secondly the impedance of the system would be expected to be greater
when the electrode is more covered by tissue, as opposed to when it is
in the blood pool.\textsuperscript{9, 10} They adjusted impedance to 120
Ohms in all situations by adjusting the tonicity of the tissue bath,
which could influence lesion size. Increasing tonicity to reduce
impedance could have the effect of decreasing current delivery into the
tissue; despite this covered lesions were larger.

The findings have important implications for ablation in the ventricles
and pectinated regions of the atrium where trabeculae can increase
coverage of the ablating electrode and provide an explanation of
unanticipated steam pops. They also suggest that electrode coverage can
not reliably predicted from contact force as recorded from ablation
catheters. Whether careful assessment of baseline impedance in the blood
pool versus at the target site, or analysis of the impedance curve
versus current during ablation might provide some insight in this regard
is not certain.

Despite extensive studies aimed at optimizing RF delivery, occasional
overheating and steam pops occur. Variations in electrode tissue
coverage in trabeculated regions of atrium and ventricles likely play a
role and are not easily appreciated during the procedure. Reducing power
in response to rapid falls in impedance and use of intra-cardiac
echocardiography for excessive bubble formation that likely indicates
overheating of the endocardial surface\textsuperscript{11} remain useful
means of avoiding steam pops.

Disclosures:

William G Stevenson has received speaking honoria from Medtronic,
Biotronik, Abbott, Johnson and Johnson and Boston Scientific, and holds
a patent for irrigated needle ablation that is consigned to Brigham
Hospital.

Arvindh Kanagasundram has received speaking honoraria from Janssen and
Johnson and Johnson

Travis D Richardson has no disclosures

Funding: None

References:

1. Haines D. Biophysics of ablation: application to technology. \emph{J
Cardiovasc Electrophysiol} . 2004;15:S2-S11.

2. Takami M, Lehmann HI, Parker KD, Welker KM, Johnson SB and Packer DL.
Effect of Left Atrial Ablation Process and Strategy on Microemboli
Formation During Irrigated Radiofrequency Catheter Ablation in an In
Vivo Model. \emph{Circulation Arrhythmia and electrophysiology} .
2016;9:e003226.

3. Shapira-Daniels A, Barkagan M, Rottmann M, Sroubek J, Tugal D,
Carlozzi MA, McConville JW, Buxton AE and Anter E. Modulating the
Baseline Impedance: An Adjunctive Technique for Maximizing
Radiofrequency Lesion Dimensions in Deep and Intramural Ventricular
Substrate: An Adjunctive Technique for Maximizing Radiofrequency Lesion
Dimensions in Deep and Intramural Ventricular
Substrate.\emph{Circulation Arrhythmia and electrophysiology} .
2019;12:e007336.

4. Nguyen DT, Gerstenfeld EP, Tzou WS, Jurgens PT, Zheng L, Schuller J,
Zipse M and Sauer WH. Radiofrequency Ablation Using an Open Irrigated
Electrode Cooled With Half-Normal Saline. \emph{JACC Clin
Electrophysiol} . 2017;3:1103-1110.

5. Nguyen DT, Tzou WS, Sandhu A, Gianni C, Anter E, Tung R, Valderrabano
M, Hranitzky P, Soeijma K, Saenz L, Garcia FC, Tedrow UB, Miller JM,
Gerstenfeld EP, Burkhardt JD, Natale A and Sauer WH. Prospective
Multicenter Experience With Cooled Radiofrequency Ablation Using High
Impedance Irrigant to Target Deep Myocardial Substrate Refractory to
Standard Ablation. \emph{JACC Clin Electrophysiol} . 2018;4:1176-1185.

6. Otomo K, Yamanashi WS, Tondo C, Antz M, Bussey J, Pitha JV, Arruda M,
Nakagawa H, Wittkampf FH, Lazzara R and Jackman WM. Why a large tip
electrode makes a deeper radiofrequency lesion: effects of increase in
electrode cooling and electrode-tissue interface area. \emph{J
Cardiovasc Electrophysiol} . 1998;9:47-54.

7. Haines DE and Verow AF. Observations on electrode-tissue interface
temperature and effect on electrical impedance during radiofrequency
ablation of ventricular myocardium. \emph{Circulation} . 1990;82:1034-8.

8. Yokoyama K, Nakagawa H, Shah DC, Lambert H, Leo G, Aeby N, Ikeda A,
Pitha JV, Sharma T, Lazzara R and Jackman WM. Novel contact force sensor
incorporated in irrigated radiofrequency ablation catheter predicts
lesion size and incidence of steam pop and thrombus. \emph{Circulation
Arrhythmia and electrophysiology} . 2008;1:354-62.

9. Strickberger SA, Vorperian VR, Man KC, Williamson BD, Kalbfleisch SJ,
Hasse C, Morady F and Langberg JJ. Relation between impedance and
endocardial contact during radiofrequency catheter
ablation.\emph{American heart journal} . 1994;128:226-9.

10. Zheng X, Walcott GP, Hall JA, Rollins DL, Smith WM, Kay GN and
Ideker RE. Electrode impedance: an indicator of electrode-tissue contact
and lesion dimensions during linear ablation. \emph{Journal of
interventional cardiac electrophysiology : an international journal of
arrhythmias and pacing} . 2000;4:645-54.

11. Okumura Y, Johnson SB, Bunch TJ, Henz BD, O'Brien CJ and Packer DL.
A systematical analysis of in vivo contact forces on virtual catheter
tip/tissue surface contact during cardiac mapping and
intervention.\emph{J Cardiovasc Electrophysiol} . 2008;19:632-40.

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