Subcutaneous implantable cardioverter-defibrillators (S-ICDs) have the potential to become a breakthrough therapy, according to Robert G. Hauser, MD. But until the technology proves its mettle against its transvenous cousins, it should be limited to a select group of patients, he wrote in the Jan. 8 issue of the Journal of the American College of Cardiology.

In September 2012, the FDA approved Boston Scientific’s S-ICD system, making it the first subcutaneous defibrillation therapy device to win market clearance in the U.S. The approval applied to patients with ventricular tachyarrhythmias who do not have symptomatic bradycardia, incessant ventricular tachycardia (VT) or spontaneous, frequently recurring VT that is reliably terminated with antitachycardia pacing (ATP). Approval was based on a prospective, nonrandomized, multicenter clinical study that enrolled 330 patients for implantation with the S-ICD system.

For efficacy, researchers did not test whether the device corrected spontaneous ventricular fibrillation. Rather, they induced heart arrhythmias and then tracked if the S-ICD system successfully converted abnormal heart rhythms to normal rhythms. "[B]y design, this study did not demonstrate the efficacy of S-ICD in ambulatory patients,” wrote Hauser, a cardiac electrophysiologist at the Minneapolis Heart Institute.

Transvenous devices can provide a tiered therapy of defibrillation, ATP and bradycardia pacing, but S-ICDs do not, Hauser explained. ATP often can painlessly terminate VT in many patients. Without ATP, patients may receive painful shocks. “But it is often difficult to determine which primary prevention patients will develop VT,” he pointed out. In the S-ICD trial, 64 percent of patients who experienced spontaneous VT or ventricular fibrillation during follow-up received shocks for monomorphic VT.

“Moreover, although controversial, current data suggest that all T-ICD shocks, both appropriate and inappropriate, might be associated with reduced longevity and quality of life,” he continued. “Thus, ATP might confer important advantages that are not available in shock-only devices.”

The larger issue, according to Hauser, is the scope of the evidence, including a narrow patient population and results that have yet to show S-ICDs are non-inferior to transvenous ICDs. Until there is more proof that S-ICDs are equivalent or better, he wrote, they should be limited to a subgroup of patients at high risk of sudden cardiac death: those lacking venous access; those who are not appropriate for thoracotomy; and those at risk of developing sepsis.

“The need for more data with this promising device should be paramount at this time,” Hauser recommended. “We should seize the opportunity to do the appropriate comparative effectiveness research in multiple patient cohorts.”

The FDA has ordered Cameron Health, the original manufacturer of S-ICDs that was acquired by Boston Scientific last year, to conduct a five-year 1,616-patient postmarket study to assess long-term safety and performance, and Boston Scientific said it planned to embark on a phased launch to train physicians on the use of the device.

Hauser described S-ICDs as a promising technology that could provide valuable therapy in countries that lack the resources for physicians to implant transvenous ICDs. “Indeed, if this new device that is based on simplicity and ease of implantation can be deployed at less cost and with non-inferior clinical outcomes, it could become a breakthrough therapy,” he proposed. But barring evidence from a comparative trial showing it is as effective as transvenous ICDs, he wrote, “a fully informed patient who is offered this device today should not want one.”