Failed Maximal Defibrillation Threshold Testing in the Subcutaneous Implantable Cardioverter Defibrillator

Successful defibrillation testing in patients undergoing elective subcutaneous implantable cardioverter-defibrillator generator replacement

AIMS

After implantation of a subcutaneous implantable cardioverter-defibrillator (S-ICD), a defibrillation test (DFT) is performed to ensure that the device can effectively detect and terminate the induced ventricular arrhythmia. Data on DFT efficacy at generator replacement are scarce with a limited number of patients and conflicting results. This study evaluates conversion efficacy during DFT at elective S-ICD generator replacement in a large cohort from our tertiary centre.

METHODS AND RESULTS

Retrospective data of patients who underwent an S-ICD generator replacement for battery depletion with subsequent DFT between February 2015 and June 2022 were collected. Defibrillation test data were collected from both implant and replacement procedures. PRAETORIAN scores at implant were calculated. Defibrillation test was defined unsuccessful when two conversions at 65 J failed. A total of 121 patients were included. The defibrillation test was successful in 95% after the first and 98% after two consecutive tests. This was comparable with success rates at implant, despite a significant rise in shock impedance (73 ± 23 vs. 83 ± 24 Ω, P < 0.001). Both patients with an unsuccessful DFT at 65 J successfully converted with 80 J.

CONCLUSION

This study shows a high DFT conversion rate at elective S-ICD generator replacement, which is comparable to conversion rates at implant, despite a rise in shock impedance. Evaluating device position before generator replacement may be recommended to optimize defibrillation success at generator replacement.

Pre-implant screening guided by cardiac silhouette fluoroscopy: a way to increase eligibility for the subcutaneous implantable cardioverter-defibrillator: a case report

BACKGROUND

Suitability for the subcutaneous implantable cardioverter-defibrillator (S-ICD) depends on a pre-implant electrocardiogram (ECG) screening to ensure appropriate sensing of electrical cardiac signals. Screening is performed positioning electrodes guided by chest surface anatomical landmarks.

CASE SUMMARY

We report a case of a patient with an initially negative conventional automatic screening for S-ICD, who underwent a modified screening guided by cardiac silhouette position, as seen under fluoroscopy, resulting in eligibility for the S-ICD.

DISCUSSION

The S-ICD reduces endovascular infection risk, providing therefore a potentially safer alternative in patients with prosthetic valves. It might be reasonable to perform a pre-implant ECG screening guided by fitting the cardiac silhouette in the shock vector, as this modified screening position could increase eligibility in patients who may benefit from S-ICD therapy such as the one presented.

Ventricular Fibrillation Refractory to Cutaneous Electrical Defibrillation in a Morbidly Obese Pediatric Patient With Hypertrophic Cardiomyopathy: A Case Report

We report a case of subcutaneous implantable cardioverter-defibrillator implantation in a morbidly obese pediatric patient with hypertrophic cardiomyopathy for the primary prevention of sudden cardiac death. During routine defibrillator threshold testing of the newly placed subcutaneous implantable cardioverter defibrillator, normal sinus rhythm could not be restored despite repeated attempts at defibrillation using the subcutaneous implantable cardioverter defibrillator and transcutaneous pads. Here, we describe the successful intraoperative resuscitation and management after failure to restore normal sinus rhythm using the newly placed subcutaneous implantable cardioverter defibrillator and repeated transcutaneous defibrillation attempts.

Defibrillation Threshold Testing: Current Status

When the transvenous ICD initially came into use for primary and secondary prevention of sudden cardiac death, defibrillation threshold (DFT) testing was universally performed. However, DFT testing is no longer routinely recommended for transvenous ICD implantation except in certain situations. Risk scores can help guide the decision to perform DFT testing. The subcutaneous ICD represents an area of uncertainty, with limited data available regarding the role of DFT testing in these devices. Current guidelines give a class I recommendation for performing DFT testing at the time of implant. Further studies are needed before this recommendation can be safely dismissed.

High defibrillation threshold with a subcutaneous implantable cardiac defibrillator due to the lead having been positioned in the fat layer

A 46-year-old female with a body mass index of 38.9 kg/m² and no organic heart disease underwent a subcutaneous implantable cardioverter-defibrillator implantation for secondary prevention of sudden cardiac death in the setting of idiopathic ventricular fibrillation. Defibrillation threshold (DFT) testing during implantation detected high shock impedance and high DFT. Fluoroscopy revealed subcoil fat between the lead and the sternum, which we suspected was the reason for the high shock impedance and high DFT. We repositioned the lead to a site just above the sternum and the shock impedance and DFT improved to within the respective normal ranges.

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.