Defibrillation Threshold Testing in Patients with Hypertrophic Cardiomyopathy

Defibrillation threshold in elective subcutaneous implantable defibrillator generator replacements: Time to reduce the size of the pulse generator?

INTRODUCTION

The first step-down defibrillation studies in the subcutaneous implantable cardioverter-defibrillator (S-ICD) described a defibrillation threshold (DFT) of 32.5 ± 17.0 J and 36.6 ± 19.8 J. Therefore, the default shock output of the S-ICD was set at 80 J. In de novo implants, the DFT is lower in optimally positioned S-ICDs. However, a retrospective analysis raised concerns about a high DFT in S-ICD replacements, possibly related to fibrosis.

OBJECTIVE

We aimed to find the DFT in patients undergoing S-ICD generator replacement.

METHODS

This prospective study enrolled patients who underwent S-ICD generator replacement with subsequent defibrillation testing. A pre-specified defibrillation testing protocol was used to determine the DFT, defined as the lowest shock output that effectively terminated the induced ventricular arrhythmia.

RESULTS

A total of 45 patients were enrolled, 6.0 ± 2.1 years after initial implant. Mean DFT during replacement in the total cohort was 27.4 ± 14.3 J. In patients with a body mass index (BMI) 18.5-25 kg/m2 (N = 22, BMI 22.5 ± 1.6), median DFT was 20 J (IQR 17.5-30). In 18/22 patients, the DFT was ≤30 J and 5/22 patients were successfully defibrillated at 10 J. One patient with hypertrophic cardiomyopathy had a DFT of 65 J. In patients with a BMI >25 kg/m2 (N = 23, BMI 29.5 ± 4.2), median DFT was 30 J (IQR 20-40). In 15/23 patients, the DFT was ≤30 J and 4/23 patients had a successful defibrillation test at 10 J.

CONCLUSIONS

This study eases concerns about a high DFT after S-ICD generator replacement. The majority of patients had a DFT ≤30 J, regardless of BMI, suggesting that the shock output of the S-ICD could be safely reduced.

Patients with a high defibrillation threshold: Approaches to management

Implantable cardioverter-defibrillators (ICDs) have revolutionized the prognosis for patients at elevated risk of ventricular tachyarrhythmias. For safety, defibrillation should be effective with a minimum of 10 J below the device's maximum energy. While modern ICDs rarely deliver ineffective shocks in primary prevention, the surge in managing severe heart failure patients has led to an increased number of patients with high defibrillation thresholds (DFTs). This article elucidates the potential causes of high DFT, including clinical factors, lead and device placement, the presence of a Left Ventricular Assist Device (LVAD), prolonged ventricular arrhythmias, shock vectors, waveform tilt, medications, and manufacturer-specific options. We also detail management strategies, highlighting alternative shock coil placements, practical recommendations, and case studies from our institution. Our management algorithm suggests addressing preventable causes, re-evaluating coil positions, considering non-invasive system modifications, upgrading to a higher-capacity device, and adding extra coil(s).

Arrhythmias and Hypertrophic Cardiomyopathy: Unravelling the Connection

Hypertrophic cardiomyopathy (HCM) results from gene mutations affecting cardiac sarcomeres and is inherited in an autosomal dominant manner. With a prevalence of 1:200-1:500 in the general population, HCM is characterised by a hypertrophied and non-dilated left ventricle with predominant involvement of the interventricular septum. The myocardium's structural and intracellular factors, combined with triggers such as physical exertion, autonomic dysfunction, and ischemia, can lead to reentry events, and atrial and ventricular arrhythmias, including atrial fibrillation (AF) which is common among HCM patients. To manage the increased risk of mortality arising from congestive heart failure and thromboembolism, in patients with AF long-term anticoagulation and antiarrhythmic drugs are employed. HCM patients may also encounter supraventricular and ventricular arrhythmias, such as nonsustained ventricular tachycardia and ventricular premature beats, which can potentially lead to sudden cardiac death and necessitate treatment with implanted defibrillators. Physicians must comprehensively analyse clinical, anatomical, hemodynamic, rhythmic, functional, and genetic characteristics to identify HCM patients at high risk of sudden death. This article aims to discuss the pathophysiology of arrhythmia in HCM and clinical recommendations for various ventricular and atrial fibrillation including catheter ablation and implantable cardioverter-defibrillator (ICD).

Implantable cardioverter-defibrillators for hypertrophic cardiomyopathy: The Times They Are a-Changin'

The implantable cardioverter-defibrillator (ICD) is a life-saving therapy in patients with hypertrophic cardiomyopathy (HCM) at high risk of sudden cardiac death. The heterogeneity of clinical scenarios in HCM and the availability of ICDs with distinct characteristics emphasizes the need for selecting the right device for the right patient. There is growing awareness that unnecessarily complex devices can lead to short- and long-term complications without adding significant clinical benefits. Young patients have the greatest potential years of life gained from the ICD but are also most exposed to device-related complications. This increases the complexity of decision-making of ICD prescription in these often otherwise well patients in whom device selection should be tailored to preserve survival benefit without introducing morbidity. In the light of the multiple clinical phenotypes characterizing HCM, the present article offers evidence-based perspectives helpful in predicting the individual impact of the ICD and choosing the most appropriate device.

The Role of Subcutaneous ICDs in the Prevention of Sudden Cardiac Death

The ICD is an important therapy in the prevention of sudden cardiac death. The transvenous-ICD (TV-ICD) has been the primary device used for this purpose. However, mechanical and infectious complications occur with traditional TV-ICDs increasing morbidity and mortality. The subcutaneous-ICD (S-ICD) system was developed to circumvent some of these complications, but S-ICDs have their inherent set of limitations as well. These include inappropriate shock delivery, lack of bradycardia, antitachycardia or CRT pacing therapy and shorter device longevity. The S-ICD is now included in guidelines as an acceptable alternative to TV-ICDs among patients without pacing indications. This review discusses the rationale for S-ICDs by reviewing studies including the PRAETORIAN, PAS and UNTOUCHED trials.

Azygos Vein ICD Lead Implantation Lowers Defibrillation Threshold in a Patient with Hypertrophic Cardiomyopathy

A 14-year-old boy with hypertrophic cardiomyopathy (HCM) diagnosed at the age of 1 year and with massive left ventricular hypertrophy suffered an episode of ventricular fibrillation during mild effort. He underwent a dual-chamber implantable cardioverter defibrillator (ICD) implantation. The defibrillation threshold testing (DFT) was ineffective. Subcutaneous multi-coli arrays tunneled into the left postero-lateral position and connected to the superior vena cava (SVC) port of the dual-chamber ICD were added to increase the myocardial mass involved in the defibrillation shock pathway. A new DFT was unsuccessful. The patient was transferred to our hospital for myectomy. An epicardial defibrillation patch was placed on the left ventricular lateral wall, but again, DFT testing was ineffective using the right ventricular (RV) coil to lateral patch as shock pathway. Another epicardial defibrillation patch was then placed on the inferior wall. In this case, DFT testing was effective with a defibrillation pathway between the two patches and the can. In November 2015, a high shock impedance alarm was recorded through remote monitoring, thus compromising the safety of the ICD shock pathway. The patient underwent the implant of a new trans-venous defibrillation coil lead in the azygos vein. After few months, the patient developed symptomatic severe aortic regurgitation and underwent an aortic valve replacement. During the operation, DFT testing was performed and was successful. Our case illustrates that azygous vein ICD lead implantation is efficacious in HCM with massive hypertrophy and high DFT, and prompts further studies to systematically investigate its efficacy in this particular subgroup of the HCM population.

Single Coil Implantable Cardioverter Defibrillator Leads in Patients With Hypertrophic Cardiomyopathy

Patients with hypertrophic cardiomyopathy (HC) may require higher energies to terminate ventricular fibrillation (VF); thus, dual coil defibrillation leads are often implanted. However, single coil leads may be preferred in young patients. All patients with HCM implanted with a transvenous ICD from years 2000 to 2014 were included. Of 249 patients, 223 underwent VF testing including 150 with a dual coil lead and 73 a single coil. Patients tested with dual coil compared with single coil had lower successful VF energies (15.7 ± 6.1 joule to 20.2 ± 7.9 joule (p <0.0001)). Adequate safety margin for defibrillation was noted in 97.3% of patients. Notably, 6 (4 with single coil leads) had inadequate safety margins (defined as ≥10 joule). Three of these 6 patients required replacement of a single coil lead with a dual coil lead. The remaining 3 underwent waveform tilt alteration, higher energy ICD, or removal of the can from the shock vector. There were no clinical or implant predictors of inadequate safety margins. In follow-up of 16 ± 30 months (range 0 to 170), there were 24 arrhythmias including 13 VF, all successfully terminated. In conclusion, in HC patients undergoing ICD implantation, single coil leads can provide adequate safety margins. In conclusion, defibrillation testing should be considered in all HC patients undergoing ICD implantation, and should be performed in those undergoing implantation with a single coil lead.

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.

Routine DFT testing in patients undergoing ICD implantation does not improve mortality: A systematic review and meta-analysis

Defibrillation threshold (DFT) testing has been an integral part of implantable cardioverter-defibrillator (ICD) implantation to confirm appropriate sensing of ventricular fibrillation and to establish an adequate safety margin for defibrillation. However, there is a lack of evidence regarding benefits of routine DFT testing. Therefore, we performed a meta-analysis to assess its mortality benefit. We searched MEDLINE for studies comparing mortality outcomes in ICD recipients who underwent DFT testing to those who did not. For the second analysis, studies comparing outcomes in patients with high- vs low-energy DFT were included. Odds ratio and standard errors were calculated, and inverse variance method in a random-effect model was used to combine effect sizes. Fifteen studies with 10,975 subjects comparing outcomes in patients who underwent routine DFT testing during ICD implantation and those who did not were included. There was no difference in the group that did not undergo DFT testing with regards to all-cause mortality (OR 0.935; CI 0.725-1.207; P = 0.606), cardiac mortality (OR 0.709; CI 0.385-1.307; P = 0.271), noncardiac mortality (OR 0.921; CI 0.701-1.210; P = 0.554), and arrhythmic mortality (OR 1.152; CI 0.831-1.596; P = 0.396). Percentage of successful appropriate first shocks among the two groups showed no difference. Five studies with 2278 subjects were included in the second analysis comparing patients with low DFT vs high DFT. Patients with high DFT had no significant increase in all-cause mortality compared to patients with low DFT (OR 0.527; CI 0.034-8.107; P = 0.646). Patients requiring higher DFT had no increased all-cause mortality compared to patients with lower DFT. Routine DFT testing during ICD implantation does not confer any significant benefit.

Implantable cardioverter-defibrillator in hypertrophic cardiomyopathy

Sudden cardiac death (SCD) is the most devastating complication in hypertrophic cardiomyopathy (HCM). The implantable cardioverter–defibrillator (ICD) has proven to be effective in SCD prevention in several clinical scenarios. In HCM population, it has demonstrated to successfully abort life-threatening ventricular arrhythmias despite the extreme morphology characteristic of HCM, often with massive degrees of left ventricular hypertrophy and/or LV outflow tract obstruction. Studies showed a high rate of appropriate intervention in secondary prevention and in primary prevention of patients considered at high risk. This appropriate intervention rate is even more significant considering the young and otherwise healthy patients that compose HCM population. Since SCD incidence in HCM is relatively low, optimal identification of patients at high risk is crucial. Classical strategy of risk stratification based on clinical risk factors has several limitations and has proven to overestimate risk. A new risk prediction model that provides individual 5-year estimated risk appears to be superior to traditional models based on bivariate risk factors. Perioperative complications seem to be similar to those related to the implant of other cardiac devices, while long-term complications have been traditionally in the spotlight. HCM patients are considered more vulnerable to ICD-related complications and inappropriate ICD therapy because of their young age at implant and increased prevalence of atrial fibrillation, but long-term follow-up data on ICD-related complications in general practice is limited. The subcutaneous implantable cardioverter defibrillator seems to be a safe and effective alternative in HCM, although long-term data are scarce.

Subcutaneous ICD implantation in a patient with hypertrophic cardiomyopathy after transvenous ICD failure: A case report

We describe the case of a patient with hypertrophic cardiomyopathy who experienced the failure of a transvenous implantable cardioverter defibrillator (T‐ICD) lead and the following inability of a second T‐ICD to convert a ventricular fibrillation. A subcutaneous ICD (S‐ICD) was finally implanted and was effective at defibrillation test.

Risk Factors for Inadequate Defibrillation Safety Margins Vary With the Underlying Cardiac Disease: Implications for Selective Testing Strategies

INTRODUCTION

In view of the shift from routine toward no or selective defibrillation testing, optimization of the current risk stratification for inadequate defibrillation safety margins (DSMs) could improve individualized testing decisions. Given the pathophysiological differences in myocardial substrate between ischemic and nonischemic heart disease (IHD/non-IHD) and the accompanying differences in clinical characteristics, we studied inadequate DSMs and their predictors in relation to the underlying etiology.

METHODS AND RESULTS

Cohort of routine defibrillation tests (n = 785) after first implantable cardioverter defibrillator (ICD)-implantations at the Radboud UMC (2005-2014). A defibrillation threshold >25 J was regarded as an inadequate DSM. In total, 4.3% of patients had an inadequate DSM; in IHD 2.5% versus 7.3% in non-IHD (P = 0.002). We identified a group of non-IHD patients at high risk (13-42% inadequate DSM); the remainder of the cohort (>70%) had a risk of only 2% (C-statistic entire cohort 0.74; C-statistic non-IHD 0.82). This was based upon two identified interaction terms: (1) non-IHD and age (aOR 0.94 [95% CI 0.91-0.97]); (2) non-IHD and the indexed left ventricular (LV) internal diastolic diameter (aOR 3.50 [95% CI 2.10-5.82]).

CONCLUSION

The present study on risk stratification for an inadequate DSM not only confirms the importance of making a distinction between IHD and non-IHD, but also shows that risk factors in an entire cohort (LV dilatation, age) may only apply to a subgroup (non-IHD). Appreciation of this concept could favorably affect current risk stratification. If confirmed, our approach may be used to optimize individualized testing decisions in an upcoming era of non-routine testing.

Subcutaneous Implantable Cardioverter Defibrillator in Patients With Hypertrophic Cardiomyopathy: An Initial Experience

Background

The subcutaneous implantable cardioverter defibrillator (S‐ICD) has been developed to avert risks associated with transvenous defibrillator leads. The technology is attractive for younger patients, such as those with hypertrophic cardiomyopathy (HCM). However, there are limited data on S‐ICD use in HCM.

Methods and Results

HCM patients identified at risk for sudden death were considered for S‐ICD implantation. Patients were screened for potential oversensing by surface electrocardiography (ECG). At implant, defibrillation threshold (DFT) testing was performed at 65, 50, and 35 joules (J). Twenty‐seven patients were considered for S‐ICD implantation, and after screening, 23 (85%) remained eligible. The presence of a bundle branch block was associated with screening failure, whereas elevated body mass index (BMI) showed a trend toward association. One patient passed screening at rest, but failed with an ECG obtained after exercise. At implant, the S‐ICD terminated ventricular fibrillation (VF) with a 65J shock in all 15 implanted patients and a 50J shock was successful in 12 of 15. A 35J shock terminated VF in 10 of 12 patients. DFT failure at 50 J was associated with a higher BMI. There were no appropriate shocks after a median follow‐up of 17.5 (3–35) months, and 1 patient received an inappropriate shock attributable to a temporary reduction in QRS amplitude while bending forward, resulting in oversensing, despite successful screening.

Conclusions

In a high‐risk HCM cohort without a pacing indication referred for consideration of an ICD, the majority were eligible for S‐ICD. The S‐ICD is effective at recognizing and terminating VF at implant with a wide safety margin.

Risk Markers and Appropriate Implantable Defibrillator Therapy in Hypertrophic Cardiomyopathy

BACKGROUND

Risk stratification of sudden cardiac death (SCD) in hypertrophic cardiomyopathy (HCM) is mainly based on evaluations from patients at highly specialized centers.

AIM

To evaluate risk markers for appropriate implantable cardioverter defibrillator (ICD) therapy in an unselected, nationwide cohort of HCM.

METHODS

Patients with an ICD due to HCM were identified from the Swedish ICD Registry since its start in 1995, merged with Patient Register data, and medical records were retrieved. Risk markers for ventricular arrhythmias leading to appropriate ICD therapy were analyzed using Cox proportional hazard ratio (HR).

RESULTS

Of 321 patients (70.1% males), at least one appropriate therapy occurred in 77 (24.0%) during a mean follow-up of 5.4 years (5.3% per year; primary prevention 4.5%, secondary prevention 7.0%). Cumulative incidences at 1 year, 3 years, and 5 years were 8.1%, 15.3%, and 21.3%, respectively. Cardioversion effectively restored rhythm in 52% of the first episode and antitachycardia pacing was sufficient in the remaining. For the whole cohort, ejection fraction (EF) <50% (HR 2.63; P < 0.001) was associated with appropriate ICD therapy. In primary prevention, patients with established risk markers experienced appropriate therapy; atrial fibrillation (AF; HR 2.54; P = 0.010), EF < 50% (HR 2.78; P = 0.004), and nonsustained ventricular tachycardia (HR 1.80; P = 0.109) had the highest HR, and wall thickness ≥ 30 mm, syncope, exercise blood pressure response, or family history of SCD had weaker associations.

CONCLUSION

ICD therapy successfully terminates ventricular arrhythmias in HCM. In addition to conventional risk markers, a history of AF or EF < 50% may be considered in risk stratification.

Predictors of high defibrillation threshold in patients with implantable cardioverter-defibillator using a transvenous dual-coil lead

BACKGROUND

Defibrillation testing (DT) is considered a standard procedure during implantable cardioverter-defibrillator (ICD) implantation. However, little is known about the factors that are significantly related to patients with high defibrillation threshold (DFT) using the present triad system.

METHODS AND RESULTS

We examined 286 consecutive patients who underwent ICD implantation with a transvenous dual-coil lead and DT from December 2000 to December 2011. We defined patients who required 25 J or more by the implanted device as the high DFT group, and those who required less than 25 J as the normal DFT group. For each patient, assessment parameters included underlying disease, comorbidities, NYHA functional class, drugs, and echocardiographic measures. The high DFT group consisted of 12 patients (4.2%). Multivariate analysis identified 3 independent predictors for high DFT: atrial fibrillation (odds ratio (OR) 4.85, 95% confidence interval (CI) 1.24-22.33, P=0.023), hypertension (OR 4.01, 95% CI 1.08-15.96, P=0.039), thickness of interventricular septum (IVS) >12 mm (OR 4.82, 95% CI 1.17-20.31, P=0.030).

CONCLUSIONS

Atrial fibrillation, hypertension and IVS hypertrophy were significantly associated with high DFT. Identification of such patients could help to lower the risk of complications with DT.