Locations of the components of a subcutaneous implantable cardioverter-defibrillator in situ. Image source: New England Journal of Medicine

An entirely subcutaneous implantable cardioverter-defibrillator (S-ICD) consistently detected and converted ventricular fibrillation induced during electrophysiological testing, and the device successfully detected and treated all 12 episodes of spontaneous, sustained ventricular tachyarrhythmia, according to small, nonrandomized studies published online May 12 in the New England Journal of Medicine.

The implantation of the S-ICD eliminates the need for lead placement in or on the heart and the need for imaging equipment. According to the authors, complications of ICD implantation have been associated mainly with transvenous lead insertion and have included pneumothorax, hemothorax and cardiac tamponade.

Video: HRS president Richard L. Page, MD responds to the NEJM study.

“Transvenous ICDs have proven to be an effective therapy for treating sudden cardiac arrest… As with any medical technology, however, there is room for improvement,” said lead author Gust H. Bardy, MD, of the Seattle Institute for Cardiac Research in Bellevue, Wash. Bardy is also co-founder of Cameron Health, based in San Clemente, Calif, which funded the studies.

First, the researchers conducted two short-term clinical trials to identify a suitable device configuration and assess energy requirements. They evaluated four S-ICD configurations in 78 patients who were candidates for ICD implantation and subsequently tested the best configuration in 49 additional patients to determine the subcutaneous defibrillation threshold in comparison with that of the standard transvenous ICD.

Subsequently, the investigators evaluated the long-term use of S-ICDs in a pilot study, involving six patients, which was followed by a trial involving 55 patients.

According to the authors, the “best device configuration” consisted of a parasternal electrode and a left lateral thoracic pulse generator. They wrote that this configuration was as effective as a transvenous ICD for terminating induced ventricular fibrillation, albeit with a significantly higher mean energy requirement (36.6 Joules vs. 11.1 Joules).

Among patients who received a permanent S-ICD, Bardy and colleagues reported that ventricular fibrillation was successfully detected in 100 percent of 137 induced episodes. Induced ventricular fibrillation was converted twice in 58 of 59 patients (98 percent) with the delivery of 65-Joule shocks in two consecutive tests.

Clinically significant adverse events included two pocket infections and four lead revisions, the researchers reported. “After a mean of 10 months, the device had successfully detected and treated all 12 episodes of spontaneous, sustained ventricular tachyarrhythmia,” they wrote.

Also, Bardy and colleagues acknowledged the “inherent limitations” of this device’s design. “Although transient post-shock pacing is available, the S-ICD cannot provide long-term pacing,” [and] therefore is not an alternative to transvenous ICDs when antibradycardia pacing is required,” they wrote. Also, the researchers noted that S-ICD is not designed to treat patients with ventricular tachycardia at rates slower than 170 beats per minute, which may be a limitation in patients with frequent, recurrent, monomorphic ventricular tachycardia.

“Ultimately, the relative benefit of subcutaneous ICDs, as compared with transvenous ICDs, will need to be shown in large, long-term, randomized, prospective, multicenter clinical trials,” the researchers concluded.

Therefore, to further the safety and efficacy of the new device, the first U.S. patient was enrolled on March 3, in the FDA pivotal trial, which is being conducted under an investigational device exemption. It is a prospective, multicenter, single-arm design involving up to 330 subjects at up to 35 sites in the U.S., Europe and New Zealand.

Heart Rhythm Society President Dr. Richard L. Page's response to the recent S-ICD research.