CardioExchange Archive

Earlier today, at the annual meeting of the Heart Rhythm Society in Denver, Dr. Gust Bardy presented the initial experience with a promising new device, an entirely subcutaneous ICD (S-ICD). The paper was published simultaneously online in the New England Journal of Medicine. Dr. Bardy graciously agreed to answer questions about the device from the editors of CardioExchange. We invite our readers to ask their own questions in the Comments section at the bottom of the page.

Background: Transvenous leads are the main limitation of ICDs. In the NEJM paper, Bardy and colleagues reported that they tested 4 different S-ICD configurations in 78 patients, ultimately settling on a parasternal electrode and a left lateral thoracic pulse generator configuration. They then tested that configuration in 49 patients and established that the device was as capable as a traditional transvenous device of achieving defibrillation, though the subcutaneous device required a greater energy expenditure.

Subsequently, the S-ICD successfully detected VF in 61 patients who received a permanent device. Some 98% of induced VF episodes were successfully converted with 65-J shocks. After 10 months of treatment, the device had successfully detected and treated 12 episodes of VT/VF. The investigators reported 2 pocket infections and 4 lead revisions.

You mention that younger patients with ICD indications may be the most ideal candidates for this type of therapy.

Certainly, the patients who will live the longest are likeliest to garner maximum benefit.  Patients who require ICD therapy for many years are the ones most likely to suffer lead failures.  Those with transvenous lead failures would then require lead removals and replacements via a procedure that carries significant risk.  Use of an S-ICD preserves the subclavian veins for when they actually are required.  Younger patients, by virtue of the type of diseases they have (Long QT, Brugada’s, CPVT, HCM, RV Dysplasia, etc.) are also those more likely to only need the life-saving benefit for protection against death from ventricular fibrillation.  I should also point out that there is no age limit on avoiding complications.  Elderly people without the requirement for anti-bradycardia pacing, for example, who are at risk of ventricular fibrillation by virtue of their ischemic heart disease or dilated cardiomyopathy, would also benefit.  Avoiding pneumothorax, hemothorax, cardiac perforation, tricuspid valve injury, and tamponade benefits anyone.

Who do you think would not be eligible for this new device? 

Any patient with known VT at rates less than 170 bpm is not eligible.  The device only treats “rapid” ventricular tachycardia.  Patients that require anti-bradycardia pacing also are not eligible as the device cannot provide permanent pacing.  Finally, patients who have well-documented recurrent, monomorphic VT known to terminate with anti-tachycardia pacing are also less-than-ideal candidates.  Essentially, S-ICD candidates potentially may be any primary prevention patient (SCD-HeFT or MADIT II indication) or those with a history of cardiac arrest and no known history of documented recurrent, monomorphic VT amenable to anti-tachycardia pacing.

Will this device expand the pool of patients who could benefit from ICDs?

Perhaps, by virtue of decreasing complications, but not by expanding beyond known patient groups indicated for the therapy.  This therapy makes implantation simpler and provides an alternative approach to physicians and patients who are already identified as needing protection from sudden cardiac arrest.

What about inappropriate shocks, which have been a great concern with current ICDs? Is there any information on how uncomfortable shocks are from a completely subcutaneous versus conventional transvenous device?

Shocks are known to be uncomfortable even at very low energies, down to <1 J.  Consequently, whether a shock is 1 J or 35 J or 80 J, patients would be very unlikely to differentiate one shock energy from another because all of these energies make every muscle in the chest contract nearly instantaneously, thereby causing the uncomfortable sensation that a patient feels.  There is no reason to believe than an inappropriate shock with an S-ICD will have any different effect on the patient than one from a transvenous ICD. The goal is to decrease the number of inappropriate shocks altogether, as we anticipate this therapy will do. A few patients who have received S-ICDs have had previous transvenous systems.  At least one of them has had shocks from both systems and could not differentiate between the two.

This device may attract a lot of interest in the cardiology community. What are the next steps in its development?

The device is approved for use in Europe.  The next step in the United States is to complete the FDA trial currently underway.  Physicians interested in having their patients treated with the S-ICD should contact one of the many U.S. sites participating in the trial (see http://clinicaltrials.gov/ct2/show/NCT01064076?term=cameron+health&rank=4 ).

Do you have any sense when it might become available?

It is likely to be available in the U.S. (outside of research trials) sometime in 2012, but that decision will be up to the FDA.