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\begin{document}

\title{High Defibrillation Threshold: Brace For Impact}



\author[1]{Karam Ayoub}%
\author[1]{Travis Richardson}%
\affil[1]{Vanderbilt University Medical Center}%


\vspace{-1em}



  
  \date{May 17, 2024}


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\textbf{High Defibrillation Threshold: Brace For Impact}

Karam Ayoub, MD \textsuperscript{1}; Travis Richardson ,
MD\textsuperscript{1}1. Division of Cardiac Electrophysiology,
Vanderbilt University Medical Center, Nashville, TN, USA.

\textbf{Disclosures} : Dr. Richardson has received research funding from
Medtronic Inc. and served as a consultant for Philips Inc. and
Biosense-Webster Inc.

\textbf{Conflict of interest} : none

\textbf{Funding} : none

\textbf{Key words} : Defibrillator , Mortality, Defibrillation
Threshold, Sudden Cardiac Death

\textbf{Total word count:} 1168

\textbf{Corresponding author:}

Travis D. Richardson, MD

Assistant Professor Cardiac Electrophysiology

Vanderbilt Heart and Vascular Institute

1215 21st Ave S. Nashville, TN

Medical Center East, South Tower, Suite 5209

travis.d.richardson@vanderbilt.edu

Sudden cardiac death (SCD) constitutes a major public health problem and
accounts for approximately 50\% of all cardiovascular deaths, including
230,000 to 350,000 deaths per year in the United States
alone\textsuperscript{1}. Affected by the death of his mentor, Michel
Mirowski was dedicated to find a solution for such a medical problem.
After years of work, he was able to build an implantable
cardioverter-defibrillator (ICD), introduced in humans in 1980, and
approved by the food and drug administration in 1985\textsuperscript{2}.
The ICD has transformed the treatment of patients at risk for sudden
cardiac death due to ventricular tachyarrhythmias. Initially, implanting
ICD leads required a thoracotomy, while the generator required an
abdominal surgery given that it was large and bulky. Over the last 35
years, tremendous development in capacitors resulted in significant
miniaturization of the ICD system and permitted subcutaneous pectoral
implantation in most patients. In 2012, the FDA approved the first
subcutaneous ICD, which opened the door to implant such life saving
devices in specific populations without the option of, or at high risk
for a transvenous approach. Not only are current ICDs smaller than early
generations, they also have the functionality of low and high-energy
shocks in multiple tachycardia zones along with antitachycardia pacing.

The transvenous ICD has been in clinical use for \textgreater{}3
decades, and robust data from high-quality randomized controlled trials
support its use in various patient populations including survivors of
cardiac arrest, patients with VT and structural heart disease, and
patients with significant LV dysfunction\textsuperscript{3}, leading to
the increased use of ICD in these populations at risk for ventricular
tachycardia. The role for defibrillation threshold (DFT) testing either
intraoperatively or postoperatively has changed significantly over time.
The definition of the DFT is a probabilistic value and is defined as the
minimum energy required at which two shocks will both successfully
terminate ventricular fibrillation. Such testing was routine at the time
of all ICD implantations in the past due to uncertainty surrounding the
device and a desire to better determine the probability of success in
treating ventricular arrhythmias. Generally, devices are programmed with
a safety margin of at least 10 J, although some trial data indicates
that a 5 J margin could provide equal efficacy\textsuperscript{4}.
However, when the safety margin is \textless{}10 J or when the device
fails to effectively terminate ventricular tachycardia, several
interventions can be performed, including medical therapy, device
reprogramming, or system revision/modification (table 1). However, there
is limited data to assess the long-term outcomes of patient undergoing
these modifications.

In this edition of the Journal of Cardiovascular Electrophysiology,
Najmul et al. present a retrospective cohort study of 6353 patients
undergoing ICD implantation and DFT testing; 191 of which had high DFT
(mean 32.1 +/- 3.7 J). High DFT was defined as \textgreater{} 25 J or
within 10 J of maximal device output. During more than 2000 days of
follow up, patients with high DFT had a higher mortality when compared
to patients with acceptable DFT (48\% vs 38\%, p =0.00046) with early
separation in Kaplan Meier (KM) analysis. Patients with high DFT were
more likely to be younger, taller, have non-ischemic cardiomyopathy, and
lower ejection fraction.

In patients with high DFT, 63 \% (121/191) underwent system
modification(SM) with approximately 10J decrease in DFT from baseline
(33.3 +/- 3.4 J to 24.8 +/- 5.9 J, P= \textless{}0.001). A subcutaneous
coil (Medtronic 6996SQ-58) was most commonly used (66\%). Interestingly,
17 / 121 patients required further device reprogramming and/or lead
repositioning to attain a satisfactory DFT. Major complications of
cardiac arrest, pulmonary edema, and shock were observed in 12\% of
patients who underwent SM vs 11\% in patients with high DFT who did not
undergo SM. Further, when compared to patients with high DFT who did not
undergo SM to decrease DFT, patients who underwent SM had similar
mortality rate during follow up (48\% vs 47 \%, p =0.91). This held true
in patients with both primary and secondary prevention indications.
Notably, sudden/arrhythmic death could not be adjudicated in this
dataset.

DFT at the time of initial ICD placement was standard of care for many
years. With the advent of biphasic waveforms and high output shocks,
which result in more reliable defibrillation, the value of routine DFT
testing was revisited\textsuperscript{5}. This question was addressed in
the SAFE-ICD study where the primary end point (composite of severe
complications at ICD implant and sudden death or resuscitation at 2
years) was similar in patients who underwent DFT testing and those who
did not\textsuperscript{6}. Interestingly, in the SAFE-ICD study,
\textless{}7\% of patients had high DFT testing, which required an
intervention, similar to what was observed in the study by Najmul et al.
In light of these findings there has been a significant reduction in the
routine performance of DFT testing at the time of ICD
implantation\textsuperscript{7}. The findings of Najmul et al. further
support avoidance of routing DFT testing, especially if corrective
measures may not affect outcomes in general. This raises the question of
why, despite effectively reducing the DFT, SM did not affect outcomes.
It seems clear that patients with high DFT represent a high-risk
population independent of their risk of arrhythmic death. One
interesting observation was that the KM curves separate early during
follow up between patients with high DFT compared to acceptable DFT,
raising the question of whether intra-procedural or post-procedural
complications could have balanced out any reduction in sudden death risk
afforded by SM. Indeed, patients with high DFT had a 12\% complication
rate, likely related to prolonged procedure times, lead revisions, and
repeated inductions (median 3, with range between 1 to 12). Importantly,
these data cannot be generalized to patients in which ICD therapy has
failed to successfully treat a clinically observed ventricular
arrhythmia, as that population was not included and is likely to have a
much higher risk of recurrent sudden death.

The study by Najmul et al. further supports the avoidance of routine DFT
testing with current generation ICDs. These data raise doubt that system
modification affects outcomes, potentially because any decreased risk of
arrhythmic death may be balanced by increased procedural risk within
what is already a high-risk population. However, in a certain subset of
patients with high risk of both sudden death and high DFT, and certainly
in those with unsuccessful treatment of clinical events, system
modification to improve defibrillation efficacy should still be
considered.

Table 1 : Methods to decrease DFT, Least to Most Invasive\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
\begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
Intervention\strut
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
Description\strut
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
Reduction in DFT\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Reversal of Shock Polarity\textsuperscript{8}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Can to Coil vs. Coil to Can\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
15-30\%\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Changing Shock Waveform\textsuperscript{9}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Available with certain companies.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
3-5 J\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
Medical therapy \textsuperscript{10,11}\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
Sotalol and Dofetilide decrease DFT.\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
\textasciitilde{}4-5J with Sotalol \textasciitilde{}20\% with
Dofetilide\textsuperscript{*}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
SVC coil\textsuperscript{12,13}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
. Must be used at initial implant or requires lead replacement.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
2-3 J\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
RV ICD lead revision\textsuperscript{13}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Apical RV lead location may reduce DFT, RVOT with septal coil
alternative\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
NA\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Addition of other coils \textsuperscript{12,14,15}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Azygous, coronary sinus, subcutaneous array/coil\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Insufficient data\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

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