MiniImal Defibrillation Threshold in With a Subcutaneous Implantable-Defibrillator Patients Undergoing Elective Generator Replacement.

Implantable cardioverter defibrillators (ICDs) have proven to be effective in
treating life-threatening ventricular arrhythmias. For Transvenous ICDs
(TV-ICD) many studies have been performed on defibrillation treshold (DFT) and
programming a shock output with a safety margin of 10J above the DFT is
considered safe and effective. The relatively new subcutaneous ICD (S-ICD) is
implanted entirely extracardiac. Due to the extracardiac design the S-ICD
requires a higher energy output compared with the TV-ICD. The shock output of
the S-ICD is based on the results of defibrillation thresholds (DFT) tests in
two early acute studies with temporarily implanted S-ICDs following a step-down
protocol. The average DFT was 32.5J ± 17.0J determined by testing 61 patients
in the first study and 36.6J ± 19.8J determined by testing 49 patients in the
second study. Based on these results, the default shock output of the S-ICD
was set at 80J, which is higher than in TV-ICDs. New studies performed in S-ICD
patients report lower average DFT and may be the first step in reducing S-ICD
generator size by reducing the standard programmed shock output of 80J, which
is the main determinant for the device size. However it is important to note
that S-ICD DFT studies have only been performed directly post implant in de
novo S-ICD patients. Following guideline recommendations all patients implanted
with an S-ICD should be tested directly post-implant and all patients
undergoing a elective generator replacement should also be tested. Currently a
large randomized trial has begun enrolment with the goal of omitting DFT in de
novo S-ICD patients (NCT03495297). Patients undergoing S-ICD generator
replacement will not be included in this study. Considering these trends
surrounding DFT testing in S-ICD patients it is likely that DFT testing in
S-ICD patients will become obsolete in the near future. Whether this change in
practice can safely be extrapolated to S-ICD patients undergoing elective
generator replacement is unknown. During the years post-implant and prior to
generator replacement the pocket around the S-ICD generator is formed by
fibrotic tissue, whether this has a positive or negative effect on the actual
defibrillation threshold and consequently affects shock efficacy of the S-ICD
is unknown. There is no data available on DFT after generator replacement.

In this study, we assess the lowest energy which successfully converts the
induced ventricular arrhythmia following a predetermined step-down protocol, to
explore whether the average DFT in S-ICD patients undergoing elective generator
replacement is similar to patients directly post-implant. The aim of this study
is to explore whether results of DFT studies in de novo S-ICD patients can
safely be extrapolated to patients undergoing generator replacements.

To determine the lowest energy which successfully converts induced ventricular
arrhythmias in S-ICD patients who underwent elective generator replacement and
compare these data to average DFT results in de novo S-ICD patients from
previous studies.

This study is a prospective non-randomized single-arm study of 45 patients
undergoing a subcutaneous implantable cardioverter-defibrillator replacement.
We aim to enrol 20 patients with a BMI > 25 kg/m². If this is not achieved
within the first 40 consecutive enrolments of this study we will continue
enrolment of only patients with a BMI above 25 kg/m² with a maximum of 20
patients, to ensure enough variability in BMI in this explorative study.

In this study a standardized step-down DFT protocol will be used to obtain
accurate DFT data with the use of a minimal number of shocks. Performing a
step-down DFT protocol is a commonly used method during implant, which does not
expose patients to an increased risk. The DFT in a patient is determined by the
anatomy of the patient, the positioning of the device and the shock vector,
which is created by the positioning of the device in relation to the patients*
anatomy. These factors will not be altered in the setting of this study,
therefore there is no variability in DFT values within a patient that is tested
multiple times. The specific steps in the step-down protocol have been
pre-determined for this study (figure 3 in the protocol).

The participant will not benefit directly from this study, although a lower
defibrillation threshold could mean that the shock output will be programmed
lower than 80J in the future, which is currently the default setting of the
S-ICD. Battery longevity increases with lower shock output if S-ICD therapy is
given. There are no additional post-procedural limitations or visits required
for this study. The study does not increase the risks associated with ICD
generator replacement nor does it increase the radiation burden. ICD generator
replacement and subsequent defibrillation threshold testing are done as part of
routine clinical care in our hospital. In the current hospital protocol
generally 1 or 2 shocks of 65J are given (depending on shock success).The total
shock output of defibrillation testing in this study protocol will be lower
than the standard of 2 x 65J currently considered standard of practice.
Additional defibrillation tests will be performed following a step-down
protocol as shown in figure 3. Increasing the number of shocks during DFT
testing is not associated with an increased risk for heart failure, or death,
or future VT/VF episodes. For the majority of patients two shocks will be
required, with a lower total output than the current standard of practice,
which is 2 shocks of 65J, a total of 130J. In a small percentage of patients
three of four shocks will be required to determine the minimal defibrillation
threshold, however, the total amount of energy according to the study protocol
will not exceed 190J.

Risks discussed with the participants:
What will be discussed with the patient is that defibrillation testing is being
performed routinely during generator replacement of the S-ICD and with this
study protocol we will determine the actual defibrillation threshold using a
step-down protocol instead of performing two defibrillation tests at 65J. For
the majority of patients this will require two conversion tests at lower output
than 65J. A small proportion of patients will require three or four conversion
test to determine the actual defibrillation threshold. In these patients the
maximum amount of energy given will not exceed 190J. An increase in number of
shocks does not add any additional risk to the procedure.