S-ICD – Subcutaneous Implantable Cardioverter Defibrillator – Boston Scientific
|‘Regular’ Pacemaker/ICD with Leads and a ‘Can’
hen we think of Pacemakers
we naturally think of a ‘Can’ and Leads that track down into the heart. Whilst these devices work fantastically well and will continue to do so. Unfortunately the ‘lead’ part of the device opens the door for a few complications to possibly arise. Those who have a Pacemaker
will probably be familiar with concerns over;
- Systemic Infection – Infections travelling down the Leads into the Heart
- Lead Displacement – The Lead moving away from the heart tissue and thus becoming pretty useless.
- Vascular/Organ Injury – Damage to the blood vessels being used for access or perforation of heart wall.
- Pneumothorax (damage to the lining around the Lung), Haemothorax (build up of blood in the chest cavity), and air embolism (air bubble trapped in a blood vessel).
These complications are one of the key motivations behind developing ‘leadless’ devices the first of which the St Jude Nanostim
, a small VVI Pacemaker
that fits directly into the heart.
Another device to address these issues is the Boston Scientific S-ICD
What is the Boston Scientific S-ICD?
The S-ICD is what is sometimes referred to as a ‘shock box’ it does not have the pacemaker functionality that many other ICD’s
do have. It is ONLY there to terminate dangerous Arrhythmias
*It does not have the pacing functionality of traditional ICD
‘s because it DOES NOT HAVE A LEAD THAT ENTERS THE HEART.*
Without the lead(s) ENTERING the heart via a blood vessel there is a reduction in the risks mentioned previously that are associated traditional device. Another of the benefits is that the S-ICD is positioned and implanted using anatomical landmarks (visible parts of your body) and not Fluoroscopy (video X-Ray) which reduces radiation exposure to the patient.
Positioning of the S-ICD.
‘ (metal box that contains all the circuitry and battery), is buried under the skin on the outside of the ribs. Put your arms down by your sides, the device would go where your ribs meet the middle of your bicep. A lead is then run under the skin to the centre of your chest where its is anchored and then north, under the skin again until the tip of the lead is roughly at the top of the sternum.
For you physicians out there the ‘can’ is positioned at the mid-axillary line between the 5th and 6th intercostal spaces, the lead is then tunnelled to a small Xiphoid incision and then tunnelled north to a superior incision.
How is an S-ICD Implanted?
Having spoken to Boston Scientific it is becoming more apparent that the superior incision (cut at the top of the chest) may actually be removed from the procedure guidance as simply tunnelling the lead and ‘wedging’ the tip at that point is satisfactory – THIS IS NOT CONFIRMED AT THE MOMENT AND IS THEREFORE NOT PROCEDURE ADVICE.
|Image Courtesy of
How does the S-ICD Work?
A ‘Shock Box’ basically needs to do 2 things. Firstly
be able to SENSE
if the heart has entered a Dangerous Arrhythmia
, be able to treat it.
The treatment part of the functionality is the easy bit – it delivers an electric shock
across a ‘circuit’ that involves a large amount of the tissue in the heart. The lead has two ‘electrodes’ and the ‘Can’ is a third electrode allowing you different shocking ‘vectors’. By vectors we mean directions and area through which the electricity travels during a shock. This gives us extra options when implanting a device as some vectors will work better than others for the treatment of dangerous arrhythmias.
This is a concept you are familiar with without even thinking about it… when you are watching ER or another TV program and they Defibrillate the patient using the metal paddles, where do they position them? One either side of the heart? Precisely!! this is creating a ‘vector’ across the heart to involve the cardiac tissue. The paddles would be a lot less effective if you put one on the knee and one on the foot!
Now because the ‘Vectors’ used by the S-ICD are over a larger area than those with a traditional device – more energy has to be delivered to have the same desired affect. The upshot of this is that a larger battery is required to deliver the 80J! Bigger Battery = Bigger Box. This image shows a demo device but this is the exact size compared to a One Pound Coin! Now yes it is big but because of the extra room where they place the device it is pretty discrete and hidden in even slender patients.
The S-ICD System delivers up to 5 shocks per episode at 80 J with up to 128 seconds of ECG storage per episode and storage of up to 45 episodes.
The heart rate that the S-ICD is told to deliver therapy is programable between 170 and 250 bpm. Quite cleverly the device is able to also deliver a small amount of ‘pacing’ after a shock, when the heart can often run slowly. This is external pacing and will be felt!! It can run for 30s.
The S-ICD uses its electrodes to produce an ECG similar to a surface ECG.
Now the Sensing functionality
is the devices ability to determine what Rhythm the heart is in! Without a lead in the heart to give us really accurate information the device is using a large area of heart, ribs and muscle. This means there is more potential for ‘artefact’. Artefact is the electrical interference and confusion – that could potentially lead to a patient being shocked
when they do not require it – or not being shocked when they do…
Boston Scientific have come up with a very clever software/algorithm called ‘Insight’. Insight uses 3 separate methods to determine the nature of a heart rhythm.
- Normal Sinus Rhythm Template (Do your heart beats look as they should)
- Dynamic Morphology Analysis (A live comparison of heart beat to previous heart beat, do they all look the same or do they keep changing?)
- QRS Width analysis (Are the tall ‘peaks’ on your ECG, the QRS’, wider than they normally are?)
These questions (with some very complex maths) and the rate of a rhythm are used to decide whether to ‘shock’ or not.
|Image Courtesy of http://www.bostonscientific.com/
How does Insight and the S-ICD compare to other ICD Devices?
The statistics for treatment success and inappropriate shocks (an electrocuted patient that did not need to be) actually compare very similarly if not favourably compared to other devices on the market – these two studies are well worth a read if you have the time 🙂
1. Burke M, et al. Safety and Efficacy of a Subcutaneous Implantable-Debrillator (S-ICD System US IDE Study). Late-Breaking Abstract Session. HRS 2012.
2. Lambiase PD, et al. International Experience with a Subcutaneous ICD; Preliminary Results of the EFFORTLESS S-ICD Registry. Cardiostim 2012.
3. Gold MR, et al. Head-to-head comparison of arrhythmia discrimination performance of subcutaneous and transvenous ICD arrhythmia detection algorithms: the START study. J Cardiovasc Electrophysiol. 2012;23;4:359-366.
|Template used to assess eligibility!
Image Courtesy of
Well essentially anyone who qualifies for a normal ‘shock box’ ICD
but with one other requirement. The Insight Software requires that a person has certain characteristics on their ECG. This is essentially showing that they have tall enough and narrow enough complexes to allow the algorithm to perform effectively. A simple 12 lead ECG Laying and Standing will be obtained and then a ‘Stencil’ is passed over the Print out – If the complexes fit within the boundaries marked on the ‘stencil’ then you potentially qualify. If your ECG does not meet requirements then it will not be recommended for you to have the S-ICD.
There you have it a quick overview of the Boston Scientific S-ICD.
Time for some Sales Shopping!
Thanks for Reading