Comparative Outcomes of Subcutaneous and Transvenous Cardioverter-Defibrillators

Introduction

The subcutaneous implantable cardioverter-defibrillator (S-ICD) has emerged as a viable alternative to the conventional transvenous implantable cardioverter-defibrillator (T-ICD). The S-ICD is designed to be entirely subcutaneous, thereby avoiding the complications associated with intravascular leads. This makes it particularly attractive for patients who do not require pacing, are at increased risk for lead-related complications, or have limited venous access. The S-ICD is especially beneficial for young patients, those with a history of recurrent lead-related bloodstream infections, and those with congenital heart disease or cardiac channelopathies.

The efficacy and safety of the S-ICD have been demonstrated in several studies, including the Investigational Device Exemption (IDE) study and the Evaluation of Factors Impacting Clinical Outcomes and Cost Effectiveness of the S-ICD (EFFORTLESS) Registry. However, long-term comparative data between the S-ICD and T-ICD are limited. Known limitations of the S-ICD include its inability to provide bradycardia pacing and antitachycardia pacing, as well as concerns about inappropriate therapies primarily due to T-wave oversensing. This study aims to compare the efficacy and safety of the S-ICD with the single-chamber T-ICD in a predominantly hypertrophic cardiomyopathy (HCM) patient population.

Methods

Ethical Approval and Study Design

The study was approved by the Mayo Clinic Institutional Review Board, and all patients provided informed consent before the procedure. This observational study included patients who received an S-ICD at Mayo Clinic between January 1, 2012, and December 31, 2016. All patients had indications for ICD implantation according to the American College of Cardiology/American Heart Association/European Society of Cardiology guidelines. Patients were excluded if they had indications for pacing or a history of monomorphic ventricular tachycardia requiring antitachycardia pacing.

Patients were matched 1:1 with those who received a single-chamber T-ICD based on gender, age (±5 years), diagnosis of cardiomyopathy, primary vs. secondary prevention, and left ventricular ejection fraction (LVEF). Data on patient demographics, clinical indications, procedural details, device settings, procedure-related complications, and follow-up assessments were collected.

ICD Implantation

Patients were screened for S-ICD eligibility using the Boston Scientific electrocardiography screening tool. Eligible patients underwent standard S-ICD implantation using either the two-incision or three-incision technique. The two-incision technique involved creating a subcutaneous pocket in the left axillary area and a second incision at the xiphoid process. The lead was tunneled from the axillary pocket to the xiphoid incision and secured to the musculature. The distal end of the electrode was then tunneled subcutaneously and secured to the underlying muscular fascia. The three-incision technique added a third incision at the left parasternal area.

Defibrillation threshold (DFT) testing was performed in each patient after VF induction. A 65-J shock energy was delivered using a 50% tilt biphasic waveform. If the first shock was unsuccessful, external transthoracic defibrillation was used, and the next shock energy was increased to 70 or 80 J. DFT testing was not routinely performed in the T-ICD group.

ICD Programming

In the S-ICD group, devices were programmed with two zones: a VT zone ranging from 180 to 200 beats/min and a VF zone ranging from 200 to 220 beats/min. In the T-ICD group, devices were programmed with a VT monitor zone (detection rate 170 beats/min) and a VF zone (detection rate 200 beats/min).

Patient Follow-Up

Patients were followed up in person at the device clinic three months after implantation. Subsequent follow-ups were conducted via remote monitoring or outpatient visits every three months. All sustained ventricular arrhythmic events, including appropriate and inappropriate therapies, were reviewed by device-trained registered nurses and electrophysiologists. Device-related complications included any early or late complications deemed to be related to the device.

Statistical Analysis

Continuous variables were expressed as mean ± standard deviation (SD), and categorical variables were presented as actual numbers and frequencies. Categorical variables were compared using the Chi-squared test or Fisher’s exact test, and continuous variables were compared using the two-sample t-test or Wilcoxon rank-sum test. Kaplan-Meier analysis was used for survival and freedom from shock therapies after ICD implantation. A P-value <0.05 was considered statistically significant.

Results

Patient Characteristics

The study included 172 patients, with 86 in the S-ICD group and 86 in the T-ICD group. The mean age of the patients was 45 years, and 75% were male. The most common cardiac condition was hypertrophic cardiomyopathy (HCM, 37.8%), followed by ischemic cardiomyopathy (ICM, 22.7%) and dilated cardiomyopathy (DCM, 19.8%). The mean LVEF was 50%.

Implantation Success

All patients in the S-ICD group had successful device and lead implantation. DFT testing at 65 J was successful in 77 patients, with polarity reversed in nine patients who initially failed. Nine patients did not undergo DFT testing due to various reasons, including Eisenmenger syndrome, spontaneous coronary artery dissection, and left ventricular thrombus.

Procedure Complications

At a mean follow-up of 23 months, there were no significant differences in device-related infections between the two groups. Two patients in each group developed pocket hematoma. Infective endocarditis occurred in two patients in the T-ICD group but none in the S-ICD group. Lead malfunction was found in three patients (3.5%) in the T-ICD group but none in the S-ICD group. Three patients (3.5%) in the S-ICD group underwent device removal or revision due to pocket infection and T-wave oversensing. Five patients (5.8%) in the T-ICD group underwent lead removal due to infective endocarditis and lead malfunction.

ICD Therapies

There was no significant difference in the appropriate ICD therapy rate between the two groups (1.2% vs. 4.7%, P=0.211). In the S-ICD group, one patient received an appropriate shock for monomorphic VT. In the T-ICD group, four patients received appropriate shocks after antitachycardia pacing failed. The inappropriate ICD therapy rate was also not significantly different between the two groups (9.3% vs. 3.5%, P=0.211). However, inappropriate shocks due to T-wave oversensing occurred in eight patients (9.3%) in the S-ICD group and none in the T-ICD group (P=0.007). Six of these eight patients underwent successful device reprogramming, while two required device removal.

Survival

Survival was not significantly different between the two groups (P=1.000). In the S-ICD group, five patients (5.8%) died during follow-up: one from respiratory failure and four from end-stage heart failure. In the T-ICD group, five patients (5.8%) died of heart failure. None of the study patients required biventricular pacing during follow-up.

Patients with HCM

Among the 65 patients with HCM, 32 were in the S-ICD group and 33 in the T-ICD group. At a mean follow-up of 21 months, no patient in the S-ICD group underwent lead revision, while two patients in the T-ICD group had device removal for infection and lead malfunction (P=0.492). In the S-ICD group, one patient received inappropriate ICD therapy, while neither appropriate nor inappropriate therapy occurred in the T-ICD group.

Discussion

S-ICD Implant Success

All patients in the S-ICD group had successful implantation, with no lead dislocations in the two-incision technique group. The two-incision technique is simpler, requires less procedure time, and causes less patient discomfort. DFT testing at 65 J was successful in all patients, confirming the sufficient energy delivery capacity of the S-ICD.

Device-Related Complications

The infection rate in the S-ICD group was similar to that in the T-ICD group and lower than in previous studies. The avoidance of transvenous leads in the S-ICD group minimized the risk of bloodstream infection and endocarditis. Lead malfunction was not observed in the S-ICD group, while lead fractures were the main cause of lead malfunction in the T-ICD group.

Appropriate and Inappropriate ICD Shocks

The appropriate ICD therapy rate was low in both groups, with no significant difference. However, the S-ICD group had a higher incidence of inappropriate shocks due to T-wave oversensing. Reprogramming the device to a different sensing vector was successful in most cases, but two patients required device removal. Pre-implant screening and programming a high detection rate may mitigate this drawback.

Survival

Mortality was not significantly different between the two groups, with most deaths due to heart failure. This finding is consistent with previous studies, indicating that the S-ICD is a safe and effective alternative to the T-ICD.

Benefit of ICD in Patients with HCM

Patients with HCM did not have increased rates of appropriate and inappropriate ICD shocks. The S-ICD is a good alternative to the T-ICD for patients with HCM, especially those at high risk for transvenous lead-related complications. Pre-implant screening is critical to identify appropriate candidates and minimize the risk of T-wave oversensing.

Conclusion

The S-ICD is an effective and safe therapy for treating ventricular arrhythmias compared with the T-ICD. It is associated with fewer major complications, particularly in patients at high risk for transvenous lead-related infections. T-wave oversensing remains a challenge, but pre-implant screening and appropriate programming can mitigate this issue. The S-ICD is a viable alternative to the T-ICD, especially in young patients and those with HCM.

doi.org/10.1097/CM9.0000000000000133

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