Trends and In-Hospital Outcomes Associated With Adoption of the Subcutaneous Implantable Cardioverter Defibrillator in the United States

The Subcutaneous Implantable Cardioverter-Defibrillator: New Insights and Expanding Populations

Implantable cardioverter defibrillators (ICDs) have become a mainstay of treatment in patients at risk for sudden cardiac death. The majority of contemporary ICDs are implanted transvenously; however, this approach carries acute procedural and long-term risks. The subcutaneous ICD (S-ICD) was developed, in part, to circumvent some of these adverse events or as an alternative option in patients unable to undergo transvenous implantation. Early promising trials evaluating the S-ICD were small and focused on niche populations. More recently, larger trials included broader populations with worse heart failure and co-morbidities that may be more representative of typical ICD recipients. These studies have consistently demonstrated positive results. This review describes the S-ICD system, implantation, and the safety and efficacy of the device.

Safety, Efficacy and Evidence Base for Use of the Subcutaneous Implantable Cardioverter Defibrillator

The trans-venous implantable cardioverter defibrillator (TV-ICD) is effective in treating life-threatening ventricular arrhythmia and reduces mortality in high-risk patients. However, there are significant short- and long-term complications that are associated with intravascular leads. These shortcomings are mostly relevant in young patients with long life expectancy and low risk of death from non-arrhythmic causes. Drawbacks of trans-venous leads recently led to the development of the entirely subcutaneous implantable cardioverter defibrillator (S-ICD). The S-ICD does not require vascular access or permanent intravascular defibrillation leads. Therefore, it is expected to overcome many complications associated with conventional ICDs. This review highlights data on safety and efficacy of the S-ICD and is envisioned to help in identifying the role of this device in clinical practice.

Selection bias, interventions and outcomes for survivors of cardiac arrest

OBJECTIVE

Cardiac catheterisation and implantable cardioverter defibrillator (ICD) insertion are increasingly common following cardiac arrest survival. However, much of the evidence for the benefit is observational, leaving open the possibility that biased patient selection confounds the association between these invasive procedures and improved outcome. We evaluated the likelihood of selection bias in the association between cardiac catheterisation or ICD placement and outcome by measuring long-term outcomes overall and in a cause-specific approach that separated cardiac mortality from non-cardiac mortality.

METHODS

We performed a multivariable survival analysis of a clinical cohort between 2005 and 2013, with follow-up through 2015. We included patients who had out-of-hospital or inhospital cardiac arrest that survived to discharge, and evaluated the association between cardiac catheterisation or ICD insertion and all-cause, cardiovascular and non-cardiovascular mortality.

RESULTS

Among 678 patients who survived cardiac arrest, we observed lower all-cause mortality among patients who underwent cardiac catheterisation (adjusted HR (aHR) 0.40; P<0.01) or ICD insertion (aHR 0.55; P<0.01). However, cause-specific analysis showed that the benefits of cardiac catheterisation and ICD insertion resulted from reduced non-cardiac causes of death (cardiac catheterisation: aHR 0.24, P<0.01; ICD: aHR 0.58, P<0.01), while reduced cardiac cause of death was not associated with cardiac catheterisation (cardiac catheterisation: aHR 0.75, P=0.33).

CONCLUSIONS

There is evidence of selection bias in the secondary prevention survival benefit attributable to cardiac catheterisation for patients who survive cardiac arrest. Observational studies that consider its effects on all-cause mortality likely overestimate the potential benefit of this procedure.

Ventricular Fibrillation Conversion Testing After Implantation of a Subcutaneous Implantable Cardioverter Defibrillator: Report From the National Cardiovascular Data Registry

BACKGROUND

Compared with transvenous implantable cardioverter defibrillators (ICDs), subcutaneous (S)-ICDs require a higher energy for effective defibrillation. Although ventricular fibrillation conversion testing (CT) is recommended after S-ICD implantation to ensure an adequate margin between the defibrillation threshold and maximum device output (80J), prior work found that adherence to this recommendation is declining.

METHODS

We studied first-time recipients of S-ICDs (between September 28, 2012, and April 1, 2016) in the National Cardiovascular Database Registry ICD Registry to determine predictors of use of CT, predictors of an insufficient safety margin (ISM, defined as ventricular fibrillation conversion energy >65J) during testing, and inhospital outcomes associated with use of CT. Multivariable logistic regression analysis was used to predict use of CT and ISM. Inverse probability weighted logistic regression analysis was used to examine the association between use of CT and inhospital adverse events including death.

RESULTS

CT testing was performed in 70.7% (n=5624) of 7960 patients with S-ICDs. Although deferral of CT was associated with several patient characteristics (including increased body mass index, increased body surface area, severely reduced ejection fraction, dialysis dependence, warfarin use, anemia, and hypertrophic cardiomyopathy), the facility effect was comparatively more important (area under the curve for patient level versus generalized linear mixed model: 0.619 versus 0.877). An ISM occurred in 6.9% (n=336) of 4864 patients without a prior ICD and was more common among white patients and those with ventricular pacing on the preimplant ECG, higher preimplant blood pressure, larger body surface area, higher body mass index, and lower ejection fraction. A risk score was able to identify patients at low (<5%), medium (5% to 10%), and high risk (>10%) for ISM. CT testing was not associated with a composite of inhospital complications including death.

CONCLUSIONS

Use of CT testing after S-ICD implantation was driven by facility preference to a greater extent than patient factors and was not associated with a composite of inhospital complications or death. ISM was relatively uncommon and is associated with several widely available patient characteristics. These data may inform ICD system selection and a targeted approach to CT.

Subcutaneous Implantable Cardioverter Defibrillators Implantation Without Defibrillation Threshold Testing: A Single Center Experience

Background

Subcutaneous implantable cardioverter defibrillator (S-ICD) system has been proven to be an effective therapy for prevention of sudden cardiac death (SCD) in selected patients. Although the Shockless IMPLant Evaluation (SIMPLE) trial has shown that defibrillation threshold (DFT) testing is not necessary for transvenous ICD (TV-ICD) systems, it is still recommended for S-ICD systems. We aimed to study the efficacy and safety of S-ICD implantation without DFT in our Heart Center with the comparison of S-ICD patients’ outcome to those with a single chamber TV-ICD without DFT in the same period.

Methods

A retrospective analysis of patients underwent S-ICD without DFT from December 2014 to May 2016 with the comparison to single chamber TV-ICD patients implanted during the same period.

Results

Thirty consecutive patients (23 males (76.7%); mean age 41 ± 13 years; mean left ventricular ejection fraction 30±12%) received a S-ICD for primary (25 patients, 83.3%) or secondary prevention (five patients, 16.7%) of SCD. During a mean follow-up of 710.6 ± 190 days, three patients received 38 appropriate ICD shocks (90.5%), and two patients received four inappropriate shocks (9.5%). There were two mortalities (6.7%): one cardiac and one non-cardiac. When compared to 30 consecutive who received a single chamber TV-ICD during the same period, there was no significant difference in mortality.

Conclusions

Implantation of S-ICD using intermuscular approach without DFT seems to be safe and effective. Data from large S-ICD registries with long-term follow-up, and preferably randomized controlled studies, are needed to confirm this finding.

Predictors of Elevated Defibrillation Threshold with the Subcutaneous Implantable Cardioverter-defibrillator

There are limited data regarding defibrillation thresholds (DFTs) for the subcutaneous implantable cardioverter-defibrillator (S-ICD), and factors associated with elevated DFTs remain incompletely understood. The objective of this study was to determine the factors associated with elevated DFTs in patients undergoing S-ICD implantation. A retrospective cross-sectional analysis of all patients undergoing S-ICD implantation at our institution between 2013 and 2016 who underwent step-down DFT testing was performed. Factors associated with a higher DFT were analyzed. In total, 56 patients (mean age: 49.3 ± 13.1 years, mean left ventricular ejection rate: 31.1% ± 13.7%) underwent S-ICD implantation in the study period. Full DFT testing was performed in 31 of the 56 patients (55%), with an average DFT of 46.4 joules (J) ± 25.9 J found among this cohort. The DFT was > 65 J in five of the 31 patients (16%). A high DFT was associated with increased body mass index (BMI) (37.7 kg/m² versus 29.4 kg/m²; p = 0.02) and either increased septal or posterior wall thickness (1.5 cm versus 1.0 cm; p = 0.0003 and 1.4 cm versus 1.1 cm; p= 0.003, respectively). Patients with high DFTs also had higher failed shock impedance values (138 Ω versus 71 Ω; p = 0.005). Renal failure did not appear to affect DFT (51.4 J versus 51.7 J; p = 0.99). BMI, body surface area (BSA), and septal and posterior left ventricular wall thickness predicted elevated DFT on univariate analysis, although findings were not significant with multivariate analysis due to the small sample size. Thus, elevated S-ICD DFT appears to be associated with increased BMI, BSA, and septal or posterior wall thickness. In contrast, dialysis-dependent renal failure is not associated with elevated DFT. Further investigation is necessary in order to better characterize and predict which patients are at-risk for high DFTs.

The Subcutaneous Defibrillator

The transvenous implantable cardioverter-defibrillator (ICD) has been shown in multiple studies to be effective in the prevention of sudden cardiac death in select populations. The Achilles heel of traditional ICD technology has been the transvenous lead. The subcutaneous ICD provides effective sudden death protection while avoiding lead-related complications of traditional transvenous systems. The subcutaneous ICD is a reasonable option for patients with an ICD indication who do not need bradycardia pacing or cardiac resynchronization therapy.

The Effects of Catheter Ablation on Permanent Pacemakers and Implantable Cardiac Defibrillators

Catheter ablation is a procedure that is frequently performed in patients with cardiac implantable electronic devices. Here, we review all of the potential interactions that can occur among patients undergoing catheter ablation while having implantable cardiac electronic devices, and discuss the precautionary measures to minimize such interactions.