In 2012, the Food and Drug Administration approved the first subcutaneous implantable cardioverter-defibrillator (S-ICD) system for use in the United States. The approval was granted after extensive review of data obtained from pilot studies and the European (EFFORTLESS) S-ICD Registry.
Traditional ICD systems comprise the generator and one or more transvenous leads and have sensing, anti-bradycardia and anti-tachycardia pacing, and shocking capabilities. The need in some patients for a system that avoids the use of transvenous leads has been long recognized. Patients with underlying congenital or structural cardiac abnormalities or with limited or difficult vascular access that precludes placement of transvenous leads require epicardial leads and patches. Individuals with channelopathies that confer a risk of sudden cardiac death often have no need for routine pacing, and transvenous leads rarely last the life of the (usually) young individuals receiving a device for this indication.
Since intravascular leads become fibrosed in place over time, lead revision and extraction procedures are challenging and not without risk. A system that does not require intracardiac leads may be appealing in other primary prevention settings. The S-ICD is a system without transvenous leads that provides defibrillation for patients at risk of sudden cardiac death due to ventricular tachyarrhythmias.
Clinical trials involving S-ICDs have demonstrated the efficacy and optimal configuration of the device. The pilot study, which was published in The New England Journal of Medicine in 2010, established concept viability and identified the ideal device configuration: the pulse generator along the left lateral chest wall and the subcutaneous electrode in a left parasternal position.
The device was as effective as standard transvenous devices in terminating induced ventricular fibrillation, although with higher energy requirements (36.6 J±19.8 J for S-ICD vs. 11.1± 8.5 J for transvenous ICD); the higher impedance and greater distance from the heart inherent in subcutaneous systems increases the energy requirements approximately threefold for successful defibrillation. Induced ventricular fibrillation was detected by the device in all 137 episodes.
A subsequent investigational device exemption trial involving 330 patients and the ongoing European EFFORTLESS registry have confirmed these initial positive results. The incidence of inappropriate shocks in EFFORTLESS is about 7 percent, and most occurred in individuals who received early implants and who did not have recommended preprocedural ECG screening to ensure proper QRS and T-wave sensing. Current devices have both a "shock zone," which commits to shock therapy based strictly on heart rate, and a "conditional shock zone," which employs additional discriminators to determine whether shock therapy is warranted.
The system has some limitations. S-ICD is not indicated in patients who require anti-bradycardia pacing or in those with heart failure for whom cardiac resynchronization is indicated. The device can deliver post-shock pacing therapy, but in doing so, it also paces the muscle wall, which can be uncomfortable in conscious patients. It cannot provide anti-tachycardia pacing, which can painlessly terminate ventricular tachycardia, and is not designed to treat ventricular arrhythmias at rates lower than 170 bpm. The incidence of device infection was 2.5 percent in the EFFORTLESS registry. The use of lead anchoring sleeves mitigated the risk of subcutaneous lead migration.
Mayo Clinic campuses in Minnesota, Arizona and Florida offer S-ICD to patients for whom standard ICD placement is precluded or not preferred. Candidates include patients with structural heart disease, patients who lack venous access for transvenous lead placement, patients with channelopathies that confer risk of sudden cardiac death and who do not need anti-bradycardia pacing, some patients awaiting cardiac transplantation, and those primary prevention patients who are best treated without a transvenous lead due to tricuspid valve concerns or a previously infected transvenous system.