By Michael H. Crawford, MD
Professor of Medicine, Lucy Stern Chair in Cardiology, University of California, San Francisco
SYNOPSIS: An FDA-mandated post-approval study of the subcutaneous cardioverter-defibrillator system in a real-world population revealed excellent safety and efficacy over five years.
SOURCE: Gold MR, El-Chami MF, Burke MC, et al. Postapproval study of a subcutaneous implantable cardioverter-defibrillator system. J Am Coll Cardiol 2023;82:383-397.
Real-world data have been lacking regarding the safety and efficacy of the subcutaneous implantable cardioverter-defibrillator (S-ICD). We know more now, thanks to the published results of the FDA-mandated S-ICD System Post Approval Study (PAS).
Clinicians at 86 enrolling centers followed patients up to five years. The primary endpoint was the overall shock efficacy in episodes of ventricular tachycardia (VT) or ventricular fibrillation (VF). The primary safety endpoint was freedom from complications directly related to the S-ICD system.
In six of 1,643 patients enrolled, device implantation was unsuccessful. Device implantation was successful among the remaining 1,637 patients (mean age was 53 years; 32% were women, 29% were Black). Ischemic heart disease was present in 41% of patients, 74% experienced clinical heart failure, and the average left ventricular ejection fraction (LVEF) was 32%. A total of 288 patients died; 686 exited the study before completing the five years of follow-up. That left 665 patients who completed the study (median follow-up = 4.2 years).
Overall shock efficacy for VT/VF was 98%, with 92% responding to the first shock, and did not vary over time. Also, shock efficacy was similar for polymorphic VT, monomorphic VT, and VT storm. The S-ICD complication-free rate was 93%. The most common complication was infection at the implantation site. For 6% of patients, surgeons removed the devices. No patient developed systemic infections caused by the device. Only 1.6% replaced their device with a transvenous (TV) ICD system because of a need for pacing.
The cumulative all-cause mortality rate was 22%. None of the deaths were attributed to the device. However, five patients died during the procedure for other reasons. A total of 7% of patients experienced inappropriate shocks at year 1 and 16% by year 5. Inappropriate sensing was the cause in 85% of the cases, with T-wave oversensing cited as the most frequent cause. The authors concluded S-ICD to be safe and effective.
COMMENTARY
The beauty of the S-ICD is that it is entirely subcutaneous. The cardioverter-defibrillator box is placed under a patient’s skin near the cardiac apex beneath the arm. The rhythm-sensing electrode attached to the box runs under the skin and up the left side of the sternum. One disadvantage of S-ICD is there is no way to pace the patient with this set up. In S-ICD PAS, the conversion rate to a TV-ICD system for this reason was less than 2%.
The average age of the study population was 53 years, which might explain the low rate of conversion because younger age often is a criterion for choosing a S-ICD. Complications of the S-ICD were almost exclusively related to issues associated with placing a foreign body under the skin, such as infection and skin erosion. These complications were more common in those who had received a previous ICD, women, and patients with an elevated body mass index.
Another critical issue with ICDs is inappropriate shocks. In S-ICD PAS, the rates were 7% at one year and 16% by year 5. Inappropriate shocks were more common in those with atrial fibrillation caused by rapid rate episodes with aberration, as well as patients on hemodialysis because of electrolyte disturbances, enhancing T wave oversensing.
The large size of the cohort, the diversity of this study population, and the fact the investigation was conducted at fewer high-volume academic centers all were strengths of this work. However, there also were weaknesses in S-ICD PAS. This was a non-randomized study that was observational in nature. There was a high exit rate, which the authors did not completely explain.
Additionally, there were no comparisons between high- and low-volume centers and operators. The authors evaluated shock efficacy as individual dysrhythmic episodes rather than as a clinical event. Finally, there was no systematic exploration of inappropriate shock events. Regardless, among a real-world population, the results of this work indicate the S-ICD system is safe and efficacious.
An FDA-mandated post-approval study of the subcutaneous cardioverter-defibrillator system in a real-world population revealed excellent safety and efficacy over five years.
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