The Subcutaneous Implantable Defibrillator (S-ICD)

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S-ICD. Image provided courtesy of Boston Scientific.Copyright 2014 Boston Scientific Corporation or its affiliates. All rights reserved

Implantable defibrillators - also called implantable cardioverter defibrillators (ICDs) - are surgically implanted medical devices that automatically deliver treatment for life threatening cardiac arrhythmias, specifically, for ventricular tachycardia and ventricular fibrillation. ICDs are recommended for people who have a high risk of sudden death from cardiac arrest. These devices can significantly increase survival in such people.

ICDs consist of a “generator” (a titanium container which houses batteries, capacitors, a computer and other sophisticated electronics), and “leads” (wires) which transmit the heart’s electrical signals back to the generator, where they are continuously analyzed. If a dangerous arrhythmia is detected, the ICD immediately treats it by either pacing or shocking the heart through the leads.

Standard ICD generators are inserted beneath the skin just below the collarbone. The ICD leads (usually two or three of them) are then passed through nearby blood vessels to specific locations within the heart.

The Subcutaneous Implantable Defibrillator

In 2012 the FDA approved a new type of ICD - the subcutaneous implantable defibrillator, or S-ICD.* (Boston Scientific)

The feature that makes the S-ICD unique and attractive is that the entire S-ICD system, including the lead, is placed under the skin, and not inside the blood vessels or the heart.

Because the heart and blood vessels remain untouched, the most dreaded complications seen with standard, transvenous (that is, “through the veins”) ICD systems are eliminated. These complications include damage to the heart or blood vessels, and infections of the cardiovascular system.

The picture (above) shows how an implanted S-ICD system is situated.

The S-ICD generator is placed beneath the skin along the far left side of the chest, over the 5th and 6th ribs. The S-ICD’s single lead is also inserted just beneath the skin, and from the generator it extends along the 6th or 7th rib to the left edge of the breastbone, and then bends upward, following the edge of the breastbone to the 2nd rib. The entire S-ICD system remains outside of the ribcage.

The implantation surgery is minimally invasive, and requires only three small incisions (one at the site of the generator, a tiny incision at the junction of the 6th rib and the breastbone, and another tiny incision at the 2nd rib and the breastbone).

Effectiveness of the S-ICD

While the S-ICD is a relatively new device, and therefore long-term experience with it is lacking, clinical studies with the S-ICD indicate that this system is as effective as the transvenous ICD in detecting and terminating lethal cardiac arrhythmias.

Advantages of the S-ICD

The chief advantage of the S-ICD is that it does not touch the heart or blood vessels.

With transvenous ICDs, the leads are the “weak link.” Intravascular leads can cause obstruction of blood vessels, blood clots, and perforation of the blood vessels or the heart. Intravascular leads move with each heartbeat, and the resultant lead “fatigue” produces lead failure in approximately 20% of leads within 10 years. Infections of any ICD system usually involve the leads, and since the leads with standard ICDs extend into the heart itself, infections often become very dangerous, producing endocarditis and spreading throughout the vascular system. Other kinds of lead malfunctions can occur (some of which have recently led to well-publicized lead recalls), and these malfunctions can be life-threatening when leads are located inside the heart.

Any of these lead problems usually require extraction of the leads themselves. Transvenous ICD leads tend to form adhesions within the blood vessels and the heart, making lead extraction a difficult and occasionally hazardous procedure. Even in expert hands there is a substantial risk of major complications with lead extraction, and some risk of death.

Lead problems are substantially less consequential with the S-ICD - and this is the main advantage of the S-ICD system. S-ICD leads, of course, can also fail or become involved in infection. However, because these leads are easily accessible, and because they are located outside the vascular system, the consequences of lead problems are much less significant, and S-ICD leads can be removed and replaced relatively easily.

Clinical studies confirm that infections involving the S-ICD (which occur about as commonly as they do with transvenous ICDs), remain localized, tend to be easily managed, and tend not to develop into life-threatening, system-wide infections, as they commonly do with transvenous ICDs.

There are other advantages to the S-ICD system. The implantation procedure for the S-ICD system is substantially simpler than for transvenous ICDs, and does not require fluoroscopy. Programming the S-ICD (that is, setting up the device to function as desired) is also far less complicated than for most transvenous ICDs.

Despite the fact that the S-ICD generator is larger than a standard ICD generator, patients receiving the S-ICD (especially women) tend to find this system cosmetically more appealing than a transvenous ICD system. In particular, women who have received S-ICDs commonly report that a bra or the top of a two-piece swimsuit hides the evidence that they have received one of these devices.

Further, the S-ICD delivers its shock to the chest wall, instead of directly to the heart (as is the case with transvenous ICDs). At least some evidence suggests that avoiding a direct shock to the heart lessens any cardiac damage caused by a shock.

Limitations of the S-ICD

The chief limitation of the S-ICD is that the only therapy it can use to terminate a life-threatening arrhythmia is a shock, whereas the transvenous ICD can also attempt to terminate arrhythmias with pacing techniques, which (in contrast to a shock), are painless. So the S-ICD usually should not be the first choice for a person who has recurrent ventricular tachycardia - an arrhythmia that can often be treated with pacing techniques.

Furthermore, because the S-ICD cannot provide chronic pacing therapy, it is not suitable for patients who need a pacemaker to maintain a normal heart rate. In contrast, transvenous ICDs offer full pacemaker therapy.

Similarly, while transvenous ICDs are available that also provide cardiac resynchronization therapy (CRT, an effective treatment for some patients with heart failure), the S-ICD cannot provide such therapy.

The S-ICD delivers a bigger shock (80 joules) than transvenous ICDs (30 or 40 joules), and some believe the shock may be more painful as a result. However, others point out that once a threshold of shock energy is administered (probably around 5 joules), the shock engages the pain receptors in the chest wall - and it is not at all clear that higher energy shocks cause substantially more pain.

The currently available S-ICD is predicted to last about 5 years before a new generator is needed. In contrast, transvenous ICDs are predicted to last 7 - 10 years. Also the current generation of S-ICD does not provide remote monitoring (that is, the ability to transmit information from the device over the Internet), as do most transvenous ICDs.

The incidence of inappropriate shocks (a shock delivered because the ICD mistakenly believes a life-threatening arrhythmia is occurring) appears to be about the same for transvenous ICDs and S-ICDs.

The Bottom Line

As with any new paradigm-shifting therapy, the S-ICD has aroused strong feelings among many cardiac arrhythmia specialists (electrophysiologists), either positive or negative. The negative feelings generally are expressed as relating to the S-ICD’s lack of sophisticated pacing features. But these pacing features, while very useful in some patients, are very often merely “nice-to-have” instead of essential, and the fact that they are lacking usually ought to be considered a trade-off instead of a deal killer.

One can be forgiven for suspecting that perhaps what’s really bothering some electrophysiologists is that the S-ICD represents an existential threat to their turf. It is a very common kind of apprehension, one which medical specialists will always encounter when one of their belovedly complex processes or procedures is greatly simplified by advancing technology.

Trying to look at the S-ICD as objectively as possible, there are circumstances where the S-ICD clearly ought to be strongly considered as preferred therapy, other circumstances where it should not be strongly considered, and yet others where it is at least a reasonable option.

The S-ICD ought to be strongly considered in patients who have difficult vascular access (such as congenital malformations of the blood vessels, or blood vessel obstruction from previous surgery or previous ICDs), in patients who have had previous vascular infections from an ICD, in patients with certain types of valvular heart disease or artificial heart valves, in patients who have an increased risk of infection (such as patients with compromised immune systems, or patients on dialysis), in patients who are awaiting cardiac transplantation, or in patients who are relatively young (and therefore who can expect to require ICD therapy for a couple of decades or more - so long-term lead complications would become a serious threat with a transvenous ICD).

An S-ICD generally should not be used in patients who need chronic pacing therapy (including CRT) where a single transvenous device could do the job, or in patients whose ventricular arrhythmias are judged to be treatable with pacing therapy.

The S-ICD is a reasonable option in anyone else who needs an implantable defibrillator, and patients in this last category are owed at least a discussion of the S-ICD (including potential advantages and disadvantages) by their electrophysiologist.

*Boston Scientific has cleverly trademarked the term “S-ICD.” This likely means that when other companies begin selling their own versions of the subcutaneous ICD, some new shorthand nomenclature for this technology will have to be invented. This simple example illustrates just one of the ways in which medical jargon can quickly become unnecessarily complicated.

Disclosures: The author is a member of the Boston Scientific Patient Safety Advisory Board.

Sources:

Bardy GH, Smith WM, Hood MA, et al. An entirely subcutaneous implantable cardioverter-defibrillator. N Engl J Med 2010; 363:36.

Maisel WH. Transvenous implantable cardioverter-defibrillator leads: the weakest link. Circulation. 2007 May 15;115(19):2461–3.

Weiss R, Knight BP, Gold MR, et al. Safety and efficacy of a totally subcutaneous implantable-cardioverter defibrillator. Circulation 2013; 128:944.

Lambiase PD, Theuns DAMJ, Neuzil P, et al. International experience with a subcutaneous ICD; preliminary results of the EFFORTLESS S-ICD registry. Europace 2012; 14:i36.

Killingsworth CR, Melnick SB, Litovsky SH, et al. Evaluation of acute cardiac and chest wall damage after shocks with a subcutaneous implantable cardioverter defibrillator in Swine. Pacing Clin Electrophysiol. 2013 Oct;36(10):1265–72.

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