Subcutaneous ICD study details initial experiences

A study of entirely subcutaneous implantable cardioverter-defibrillators (S-ICD) found the devices effectively treat ventricular arrhythmias. The results, published Nov. 6 in the Journal of the American College of Cardiology, come on the heels of FDA approval of an S-ICD for the treatment of patients at risk for sudden cardiac arrest.

Implantable cardioverter-defibrillators (ICDs) are widely used in patients at risk of sudden cardiac death due to arrhythmic episodes associated with several cardiovascular diseases. In October, the FDA approved an S-ICD system (Boston Scientific, which bought its developer, Cameron Health, earlier this year) to provide defibrillation therapy for the treatment of ventricular tachyarrhythmias in patients who do not have symptomatic bradycardia, incessant ventricular tachycardia or spontaneous, frequently recurring ventricular tachycardia (VT) that is reliably terminated with antitachycardia pacing.

The traditional method of implanting ICDs requires gaining vascular access to the heart, which can lead to vascular complications, longer implantation times (which is associated with a higher rate of infection) and implantation failure. Intracardiac leads can migrate, fracture, fail, and/or lead to abrasions and infections can result in endocarditis. An entirely subcutaneous ICD eliminates the need for lead placement in or near the heart and therefore may alleviate some of the complications ICD patients experience.

Louise R.A. Olde Nordkamp, MD, of Academic Medical Center in Amsterdam, the Netherlands, and colleagues followed the first 118 patients who received S-ICDs at four high-volume centers in the Netherlands between December 2008 and April 2011. Patients had to have a Class I or IIa indication for ICD therapy for primary or secondary prevention of sudden cardiac death; researchers excluded patients with an indication for bradycardia or antitachycardia pacing (ATP) or resynchronization therapy.

Among patients receiving S-ICD, diagnoses included ischemic cardiomyopathy (38 percent), inherited cardiac disease (23 percent), dilated cardiomyopathy (19 percent), nonischemic/nondilated cardiomyopathy (6.8 percent) and other conditions. Eighty-nine percent of the patients were male and the mean age at implant was 50 years. Of the patients with inherited disease, 89 percent were receiving the S-ICD prophylactically.

The patients received S-ICD because they preferred it, or they had experienced complications with a transvenous ICD and reimplantation with another transvenous system was undesirable, or the patient was young and therefore S-ICD was a more appropriate choice.

Prior to implantation, patients received antibiotic prophylaxis of intravenous flucloxacillin (1,000mg or 3,000 mg) or caphazoline (1,000mg or 2,000mg). The implantation procedure used anatomical landmarks without fluoroscopy.

A postimplantation chest x-ray was performed to confirm correct placement of the device and subcutaneous lead. All patients were mobilized immediately, and most were discharged on the day of implantation or the following day.

After the first 15 patients, researchers noted a high incidence of inappropriate shocks and adjusted the software to decrease oversensitivity and allow for a slightly longer refractory period. Three of the first 20 patients experienced migration of the lead. The researchers began using an additional suture sleeve at the xiphoid position, and no additional lead migrations occurred.

After 18 months of follow-up, the researchers reported that the S-ICD successfully converted 98 percent of spontaneous VT/ventricular fibrillation (VF) events into sinus rhythm. "These results confirm the earlier reported very reliable shock efficacy of the S-ICD," they wrote.

The rate of inappropriate shocks was similar to that of transvenous ICDs. However, the researchers noted that when the software was upgraded early in the study to reduce T-wave oversensing, the rate of inappropriate shock decreased, although the cause of most inappropriate shocks remained T-wave oversensing. The authors explained that in some patients a different QRS to T-wave ratio developed during exercise, and that making a new template during exercise with these patients eliminated further inappropriate shocks. Therefore the researchers advised that automatic setup of the device occur during exercise when the patient is mobile.

The researchers also noted that one patient received an inappropriate shock due to double counting of a newly developed right bundle branch block. They suggested that "it would be useful if the device would be able to create an automatic template on a daily basis, to prevent shocks for newly developed intraventricular conduction delay. "

The S-ICD-related complication rate of was 14 percent (including three patients who experienced lead migration early in the trial), similar to the complication rate in transvenous ICD trials. Seven patients (5.9 percent) developed infections that required device removal. Among these patients, one was diabetic, one had an ongoing limb infection and one manipulated his wound. The authors pointed out that as this is a new device, there are no experienced implanters, and the novelty of the procedure led to increased implantation times, which is associated with higher risk of infection. They also reported that most of the infections were skin deep and easily managed, in contrast to transvenous ICD infections, which carry significant morbidity and mortality risk.

Two patients experienced significant skin erosion, but the researchers decided this was due to creating a pocket that was not sufficiently wide.  

The authors concluded that both inappropriate shocks and device-related complications were related to the learning curve for the device and the physician. Their findings suggest that the S-ICD is a viable alternative for an appropriate subset of patients, and they suggest randomized comparative trials to further refine the benefits and limitations of the device in selected patient populations.