The influence of different anatomical positions on the location of the S-ICD pulse generator

The subcutaneous implantable cardioverter defibrillator (S-ICD) is developed to
overcome complications seen in patients with a transvenous ICD (TV-ICD). The
S-ICD is therefore completely extravascular (figure 1.).

The position of the S-ICD generator and lead is related to the shock efficacy
(1). For a correct position, a good implantation technique and experience of
the implanter are important (2). Over the years new improved implantation
techniques are adopted (3). The shock efficacy is currently tested by
performing a defibrillation test (DFT) after implantation. The benefit of DFT
after implantation is debated and performing a DFT is not without a risk of
complications. The PRAETORIAN score is developed to estimate the shock efficacy
based on the implant position of the S-ICD using the standard of care taken
chest X-ray after implantation (4).

The PRAETORIAN score is calculated by 3 steps (figure 2). Step 1 and 3 are
related to the amount of fat between the coil and the sternum and between the
generator and the thoracic wall respectively. Step 1 and 3 will therefore not
be influenced by different anatomical positions. Step 2 however, can possibly
be influenced by a different anatomical position as this step is related to the
position of the generator towards the midline. A PRAETORIAN score * 90 is
described as an intermediate or high risk for shock failure and therefore
requires action, often a repositioning of the lead or generator.

Importance of arm position on the chest X-ray to avoid retakes
The PRAETORIAN score is based on a bidirectional chest X-ray after
implantation. Step 2 is based on the lateral chest X-ray with the arms in
standard upward position. However, lateral chest X-rays with the arms downwards
are frequently seen after S-ICD implantation. One explanation may be that this
arm position is recommended after TV-ICD implantation to avoid dislocation of
the TV-ICD lead. The influence of a different arm position on the S-ICD
generator position and therefore on the PRAETORIAN score is unknown. Knowledge
about this effect may avoid retakes of chest X-rays or may emphasize the
importance of the correct arm position regarding the PRAETORIAN score.

Future of a more efficient implantation process to avoid re-interventions
Currently, the standard of care chest X-ray is taken outside the
catheterization laboratory or operating room and after the patient wakes up. In
case of a high PRAETORIAN Score, re-intervention would be necessary. This can
be avoided if a supine chest X-ray immediately after implantation can be made.
On a standing chest X-ray gravity can influence spine angles and positioning
(5). Taking into account the fact that the S-ICD is implanted intermuscular
under the m. latissimus dorsi, which has his origin on the thoracic spine,
differences in spine angle may influence the generator position. The influence
of the supine and 90 degree angle of the arm versus the standard standing chest
X-ray on the generator position are unknown. This study will give information
on whether calculating the PRAETORIAN score on a supine chest X-ray is
feasible.

Primary Objective:
* The primary objective of this study is to determine the influence of
different anatomical positions on step 2 of the PRAETORIAN score.

Secondary Objective(s):
* The influence of the different anatomical positions on the total PRAETORIAN
score.
* The influence of the different anatomical positions on the PRAETORIAN score
categories.
* The possible displacement of the generator, measured in centimetres, in the
different anatomical positions.

This is an observational descriptive, single center, study in which the
influence of arm positon and supine position on the generator position is
studied in 30 patients. Patients who underwent an S-ICD implantation in the
Amsterdam UMC will be asked for participation. At least 9 patients will be
included with a step 2 PRAETORIAN score of x2 or x4 to study a possibly
stronger influence of generator displacement from a more anterior starting
point.

For all patients a standard standing chest X-ray will be performed following
standard protocols (PA arms downwards (A), lateral arms upwards (B) in figure
3.). Additionally, a lateral chest X-ray will be taken with the arms downwards
(C. in figure 3.) and a lateral supine chest X-ray will be taken with the left
arm at 90 degrees (D. in figure 3.). In total there will be 4 chest X-rays
executed.

The two standing lateral chest X-rays, with arms downwards and upwards (B. and
C. figure 3.), will be compared and step 2 of the PRAETORIAN score will be
determined. The lateral standing and lateral supine chest X-ray will also be
compared (B. and D. figure 3.) and step 2 of the PRAETORIAN score will be
determined. The PA chest X-ray (A. figure 3.) in combination with a lateral
chest X-ray (B., C. or D. figure 3.) will be used to determine the total
PRAETORIAN score. The distance in centimeters from the sternum (measured with a
90 degree angle) to the anterior side of the generator will be used to
calculate the exact possible dispalcement of the generator between two
different anatomical position. To avoid different scales of the X-ray to
influence the measurement, the distance will be calculated using the known
measurements of the S-ICD. Rating of the PRAETORIAN score will be performed by
2 independent raters and decision will be made on consensus.

Patients will not benefit directly from participating in this study. However,
this study will contribute to the understanding of the influence of different
anatomical positions on the generator position. This can contribute to the
avoidance of future retakes of chest X-rays. If there is agreement between the
different anatomical positions regarding the PRAETORIAN score this may benefit
the implantation process in the future. If patients need an S-ICD replacement
in the future they may benefit from this themselves.

There is no serious direct risk for patients participating in this study.
Patients who participate are exposed to radiation by 4 chest X-rays, see
radiation dose and risk estimates for further information. There is a
possibility that the chest X-rays show unknown findings. If so, the patient
will be informed about this by one of the investigators. Also the general
practitioner of the patient will be contacted to discuss the findings with the
patient if necessary and to take further steps if necessary.

Radiation dose and risk estimates
Patients participating in this study will receive 4 chest X-rays. One chest
X-ray can reach up to 0.1mSv. In total patients participating in this study
will therefore receive a maximum of 0.4 mSv. According to, Human Exposure to
Ionising Radiation for Clinical and Research Purposes: Radiation Dose & Risk
Estimates guidelines this translates to the risk category IIa. This category
represents an intermediate level of risk. The range of 0.1 to 1 mSv corresponds
with a maximum risk of five in hundred thousand and is less than the annual
background dose. To justify these risks a research proposal should at least
lead to potential health benefit for future patients. Our study has clear
health benefits for future patients and can even, depending on the outcome,
reduce the radiation exposure in future patients.