Fontan Junctional Rhythm-Innocent Bystander or Silent Killer?

This Viewpoint discusses the impact of sinus node dysfunction and escape junctional rhythm associated with the Fontan procedure on patient outcomes.

Sudden death in adults with repaired coarctation of the aorta: A case for sex-based risk factors.

Background: Sudden cardiac death (SCD) is an important risk for adults with repaired coarctation of the aorta (rCoA). We aimed determine if there are clinical risk factors for SCD in adults with rCoA. Methods and results: SCD events and clinical data from all adults with rCoA at a tertiary care center (2007-2017) were evaluated. In 167 adults with rCoA (39 ± 11 years old, 75 (45%) female) SCD occurred in 8 (5%) (vs. age-matched adults 0.9%). Those with SCD demonstrated significant QTc prolongation (QTc: 479 ± 16 vs. 434 ± 30 msec, p < 0.001). Overall, adults with rCoA and a prolonged sex-normative QTc interval had a 12-fold increased risk of SCD (x2 (1) = 12.3, p < 0.001), with men sustaining SCD at younger ages (42 ± 13 years vs. women 60 ± 10 years, p < 0.05). Multiple logistic regression modeling demonstrated that prolonged QTc selectively advanced risk for SCD in men only (x2 QTc prolongation 8.46, p < 0.005 and x2 age 0.29, p = 0.587), whereas in women, age was associated with SCD risk (x2 QTc prolongation 2.84, p = 0.092 and x2 age 7.81, p = 0.005). Non-sustained ventricular tachycardia, ventricular dysfunction, and myocardial fibrosis did not significantly impact SCD risk. Conclusions: There is an unanticipated high burden of SCD in adults with rCoA, occurring in men at younger age than women, suspicious for primary electrophysiologic dysfunction. Future investigation of sex-specific SCD risk in rCoA is important to better understand this disease and its late phenotype.

Multidetector Computed Tomography Assessment of Anatomical Ventricular Tachycardia Isthmuses in Repaired Tetralogy of Fallot.

BACKGROUND: Tetralogy of Fallot (TOF) is associated with risk for sustained monomorphic ventricular tachycardia (VT). Preemptive electrophysiology study before transcatheter pulmonary valve placement is increasing, but the value of MDCT for anatomical VT isthmus assessment is unknown. OBJECTIVES: The purpose of this study was to determine the impact of multidetector computed tomography (MDCT) in the evaluation of sustained monomorphic VT for repaired TOF. METHODS: Consecutive pre-transcatheter pulmonary valve MDCT studies were identified, and anatomical isthmus dimensions were measured. For a subset of patients with preemptive electrophysiology study, MDCT features were compared with electroanatomical maps. RESULTS: A total of 61 repaired TOFs with MDCT were identified (mean 35 ± 14 years, 58% men) with MDCT electroanatomical map pairs in 35 (57%). Calcification corresponding to patch material was present in 46 (75%) and was used to measure anatomical VT isthmuses. MDCT wall thickness correlated positively with number of ablation lesions and varied with functional isthmus properties (blocked isthmus 2.6 mm [Q1, Q3: 2.1, 4.0 mm], slow conduction 4.8 mm [Q1, Q3: 3.3, 6.0 mm], and normal conduction 5.6 mm [Q1, Q3: 3.9, 8.3 mm]; P < 0.001). A large conal branch was present in 6 (10%) and a major coronary anomaly was discovered in 3 (5%). Median ablation lesion distance was closer to the right vs the left coronary artery (10 mm vs 15 mm; P = 0.01) with lesion-to-coronary distance <5 mm in 3 patients. CONCLUSIONS: MDCT identifies anatomical structures relevant to catheter ablation for repaired TOF. Wall thickness at commonly targeted anatomical VT isthmuses is associated with functional isthmus properties and increased thermal energy delivery.

Long-Term Outcomes of Cardiac Resynchronization Therapy in Patients With Repaired Tetralogy of Fallot: A Multicenter Study.

BACKGROUND: A growing number of patients with tetralogy of Fallot develop left ventricular systolic dysfunction and heart failure, in addition to right ventricular dysfunction. Although cardiac resynchronization therapy (CRT) is an established treatment option, the effect of CRT in this population is still not well defined. This study aimed to investigate the early and late efficacy, survival, and safety of CRT in patients with tetralogy of Fallot. METHODS: Data were analyzed from an observational, retrospective, multicenter cohort, initiated jointly by the Pediatric and Congenital Electrophysiology Society and the International Society of Adult Congenital Heart Disease. Twelve centers contributed baseline and longitudinal data, including vital status, left ventricular ejection fraction (LVEF), QRS duration, and NYHA functional class. Outcomes were analyzed at early (3 months), intermediate (1 year), and late follow-up (≥2 years) after CRT implantation. RESULTS: A total of 44 patients (40.3±19.2 years) with tetralogy of Fallot and CRT were enrolled. Twenty-nine (65.9%) patients had right ventricular pacing before CRT upgrade. The left ventricular ejection fraction improved from 32% [24%-44%] at baseline to 42% [32%-50%] at early follow-up (P<0.001) and remained improved from baseline thereafter (P≤0.002). The QRS duration decreased from 180 [160-205] ms at baseline to 152 [133-182] ms at early follow-up (P<0.001) and remained decreased at intermediate and late follow-up (P≤0.001). Patients with upgraded CRT had consistent improvement in left ventricular ejection fraction and QRS duration at each time point (P≤0.004). Patients had a significantly improved New York Heart Association functional class after CRT implantation at each time point compared with baseline (P≤0.002). The transplant-free survival rates at 3, 5, and 8 years after CRT implantation were 85%, 79%, and 73%. CONCLUSIONS: In patients with tetralogy of Fallot treated with CRT consistent improvement in QRS duration, left ventricular ejection fraction, New York Heart Association functional class, and reasonable long-term survival were observed. The findings from this multicenter study support the consideration of CRT in this unique population.

Intrahepatic Transcriptomics Differentiate Advanced Fibrosis and Clinical Outcomes in Adults With Fontan Circulation.

BACKGROUND: The molecular mechanisms underlying Fontan-associated liver disease (FALD) remain largely unknown. OBJECTIVES: This study aimed to assess intrahepatic transcriptomic differences among patients with FALD according to the degree of liver fibrosis and clinical outcomes. METHODS: This retrospective cohort study included adults with the Fontan circulation. Baseline clinical, laboratory, imaging, and hemodynamic data as well as a composite clinical outcome (CCO) were extracted from medical records. Patients were classified into early or advanced fibrosis. RNA was isolated from formalin-fixed paraffin-embedded liver biopsy samples; RNA libraries were constructed with the use of an rRNA depletion method and sequenced on an Illumina Novaseq 6000. Differential gene expression and gene ontology analyses were performed with the use of DESeq2 and Metascape. RESULTS: A total of 106 patients (48% male, median age 31 years [IQR: 11.3 years]) were included. Those with advanced fibrosis had higher B-type natriuretic peptide levels and Fontan, mean pulmonary artery, and capillary wedge pressures. The CCO was present in 23 patients (22%) and was not predicted by advanced liver fibrosis, right ventricular morphology, presence of aortopulmonary collaterals, or Fontan pressures on multivariable analysis. Samples with advanced fibrosis had 228 upregulated genes compared with early fibrosis. Samples with the CCO had 894 upregulated genes compared with those without the CCO. A total of 136 upregulated genes were identified in both comparisons and were enriched in cellular response to cytokine stimulus or oxidative stress, VEGFA-VEGFR2 signaling pathway, TGF-ß signaling pathway, and vasculature development. CONCLUSIONS: Patients with FALD and advanced fibrosis or the CCO exhibited upregulated genes related to inflammation, congestion, and angiogenesis.

Conduction System Pacing for Patients with Congenital Heart Disease.

For patients with congenital heart disease (CHD), chronic ventricular pacing may lead to progressive cardiomyopathy owing to electromechanical dyssynchrony. Cardiac conduction system pacing (CSP) has been proposed as a physiologic pacing strategy-directly engaging the His-Purkinje system and preserving electromechanical synchrony. CSP may be indicated for a wide variety of children and adults with CHD and has emerged as an important tool in the armamentarium for cardiac implantable electronic device operators. This review provides the rationale, background, and supportive evidence for CSP in patients with CHD and discusses implant strategies and outcomes in the context of dominant ventricular morphologic categories.

Ventricular Arrhythmias in Adults With Congenital Heart Disease, Part I: JACC State-of-the-Art Review.

Patients with congenital heart disease associated with a higher risk for ventricular arrhythmias (VA) and sudden cardiac death (SCD) can be divided conceptually into those with discrete mechanisms for reentrant monomorphic ventricular tachycardia (VT) (Group A) and those with more diffuse substrates (Group B). Part I of this review addresses Group A lesions, which predominantly consist of tetralogy of Fallot and related variants. Well-defined anatomic isthmuses for reentrant monomorphic VT are interposed between surgical scars and the pulmonary or tricuspid annulus. The most commonly implicated critical isthmus for VT is the conal septum that divides subpulmonary from subaortic outlets. Programmed ventricular stimulation can be helpful in risk stratification. Although catheter ablation is not generally considered an alternative to the implantable cardioverter-defibrillator (ICD) for prevention of SCD, emerging data suggest that there is a subset of carefully selected patients who may not require ICDs after successful monomorphic VT ablation.

Ventricular Arrhythmias in Adults With Congenital Heart Disease, Part II: JACC State-of-the-Art Review.

There are marked variations in the incidence of sudden cardiac death (SCD) and in the substrates for ventricular arrhythmias (VAs) across the gamut of congenital heart defects. In this 2-part review, patients with higher-risk forms of congenital heart disease (CHD) were conceptually categorized into those with discrete anatomic isthmuses for macro-reentrant ventricular tachycardia (VT) (Group A) and those with more diffuse or less well-defined substrates (Group B) that include patchy or extensive myocardial fibrosis. The latter category encompasses CHD lesions such as Ebstein anomaly, transposition of the great arteries with a systemic right ventricle (RV), and congenital aortic stenosis. For Group B patients, polymorphic VT and ventricular fibrillation account for a higher proportion of VA. The prognostic value of programmed ventricular stimulation is less well established, and catheter ablation plays a less prominent role. As cardiomyopathies evolve over time, pathophysiological mechanisms for VA among Groups A and B become increasingly blurred.

Patterns of Electrocardiographic Abnormalities in Children with Hypertrophic Cardiomyopathy.

Hypertrophic cardiomyopathy (HCM), a common cardiomyopathy in children, is an important cause of morbidity and mortality. Early recognition and appropriate management are important. An electrocardiogram (ECG) is often used as a screening tool in children to detect heart disease. The ECG patterns in children with HCM are not well described.ECGs collected from an international cohort of children, and adolescents (≤ 21 years) with HCM were reviewed. 482 ECGs met inclusion criteria. Age ranged from 1 day to 21 years, median 13 years. Of the 482 ECGs, 57 (12%) were normal. The most common abnormalities noted were left ventricular hypertrophy (LVH) in 108/482 (22%) and biventricular hypertrophy (BVH) in 116/482 (24%) Of the patients with LVH/BVH (n = 224), 135 (60%) also had a strain pattern (LVH in 83, BVH in 52). Isolated strain pattern (in the absence of criteria for hypertrophy) was seen in 43/482 (9%). Isolated pathologic Q waves were seen in 71/482 (15%). Pediatric HCM, 88% have an abnormal ECG. The most common ECG abnormalities were LVH or BVH with or without strain. Strain pattern without hypertrophy and a pathologic Q wave were present in a significant proportion (24%) of patients. Thus, a significant number of children with HCM have ECG abnormalities that are not typical for "hypertrophy". The presence of the ECG abnormalities described above in a child should prompt further examination with an echocardiogram to rule out HCM.

Sinus rhythm QRS morphology reflects right ventricular activation and anatomical ventricular tachycardia isthmus conduction in repaired tetralogy of Fallot.

BACKGROUND: Patients with repaired tetralogy of Fallot (TOF) are at risk for ventricular tachycardia (VT) related to well-described anatomical isthmuses. OBJECTIVE: The purpose of this study was to explore QRS morphology as an indicator of anatomical isthmus conduction. METHODS: Patients with repaired TOF and complete right bundle branch block referred for transcatheter pulmonary valve replacement (PVR) or presenting with sustained VT underwent comprehensive 3-dimensional mapping in sinus rhythm. Electrocardiographic characteristics were compared to right ventricular (RV) activation and anatomical isthmus conduction properties. RESULTS: Twenty-two patients (19 pre-pulmonary valve replacement and 3 clinical VT) underwent comprehensive 3-dimensional mapping (median 39 years; interquartile range [IQR] 27-48 years; 12 [55%] male). Septal RV activation (median 40 ms; IQR 34-46 ms) corresponded to the nadir in lead V1 and free wall activation (median 71 ms; IQR 64-81 ms) to the transition point in the upstroke of the R' wave. Patients with isthmus block between the pulmonary annulus and the ventricular septal defect patch and between the ventricular septal defect patch and the tricuspid annulus (when present), were more likely to demonstrate lower amplitude R' waves in lead V1 (5.8 mV vs 9.4 mV; P = .005), QRS fragmentation in lead V1 (15 [94%] vs 2 [13%]; P < .001), and terminal S waves in lead aVF (15 [94%] vs 6 [40%]; P < .001) than those with intact conduction. During catheter ablation, these QRS changes developed during isthmus block. CONCLUSION: For patients with repaired TOF, the status of septal isthmus conduction was evident from sinus rhythm QRS morphology. Low-amplitude, fragmented R' waves in lead V1 and terminal S waves in the inferior leads were related to septal isthmus conduction abnormalities, providing a mechanistic link between RV activation and common electrocardiographic findings.

Multicenter retrospective evaluation of magnetic resonance imaging in pediatric and congenital heart disease patients with cardiac implantable electronic devices.

BACKGROUND: Guidelines addressing magnetic resonance imaging (MRI) in patients with cardiac implantable electronic devices (CIEDs) provide algorithms for imaging pediatric and congenital heart disease (CHD) patients. Guideline acceptance varies by institution. Guidelines also do not support routine MRI scans in patients with epicardial or abandoned leads, common in pediatric and CHD patients. OBJECTIVE: The purpose of this study was to determine the incidence of MRI-related complications in pediatric and CHD patients with CIEDs, including epicardial and/or abandoned leads. METHODS: A multicenter retrospective review included patients with CIEDs who underwent any MRI between 2007 and 2022 at congenital cardiac centers. The primary outcome was any patient adverse event or clinically significant CIED change after MRI, defined as pacing lead capture threshold increase >0.5 V with output change, P- or R- wave amplitude decrease >50% with sensitivity change, or impedance change >50%. RESULTS: Across 14 institutions, 314 patients (median age 18.8 [1.3; 31.4] years) underwent 389 MRIs. There were 288 pacemakers (74%) and 87 implantable cardioverter-defibrillators (22%); 52% contained epicardial leads, and 14 (4%) were abandoned leads only. Symptoms or CIED changes occurred in 4.9% of MRI scans (6.1% of patients). On 9 occasions (2%), warmth or pain occurred. Pacing capture threshold or lead impedance changes occurred in 1.4% and 2.0% of CIEDs post-MRI and at follow-up. CONCLUSION: Our data provide evidence that MRIs can be performed in pediatric and CHD patients with CIEDs, including non-MRI-conditional CIEDs and epicardial and/or abandoned leads, with rare minor symptoms or CIED changes but no other complications.

Rationale and Design of the Multicenter Catheter Ablation of Ventricular Tachycardia Before Transcatheter Pulmonary Valve Replacement in Repaired Tetralogy of Fallot Study.

Patients with repaired tetralogy of Fallot are at elevated risk for ventricular arrhythmia and sudden cardiac death. Over the past decade, the pathogenesis and natural history of ventricular tachycardia has become increasingly understood, and catheter ablation has emerged as an effective treatment modality. Concurrently, there has been great progress in the development of a versatile array of transcatheter valves that can be placed in the native right ventricular outflow tract for the treatment of long-standing pulmonary regurgitation. Although such valve platforms may eliminate the need for repeat cardiac operations, they may also impede catheter access to the myocardial substrates responsible for sustained macro-reentrant ventricular tachycardia. This manuscript provides the rationale and design of a recently devised multicenter study that will examine the clinical outcomes of a uniform, preemptive strategy to eliminate ventricular tachycardia substrates before transcatheter pulmonary valve implantation in patients with tetralogy of Fallot.

Intrahepatic transcriptomics differentiate advanced fibrosis and clinical outcomes in adults with the Fontan circulation.

Background: The molecular mechanisms underlying Fontan associated liver disease (FALD) remain largely unknown. We aimed to assess intrahepatic transcriptomic differences among patients with FALD according to the degree of liver fibrosis and clinical outcomes. Methods: This retrospective cohort study included adults with the Fontan circulation at the Ahmanson/UCLA Adult Congenital Heart Disease Center. Clinical, laboratory, imaging and hemodynamic data prior to the liver biopsy were extracted from medical records. Patients were classified into early (F1-F2) or advanced fibrosis (F3-F4). RNA was isolated from formalin-fixed paraffin embedded liver biopsy samples; RNA libraries were constructed using rRNA depletion method and sequencing was performed on Illumina Novaseq 6000. Differential gene expression and gene ontology analyses were carried out using DESeq2 and Metascape. Medical records were comprehensively reviewed for a composite clinical outcome which included decompensated cirrhosis, hepatocellular carcinoma, liver transplantation, protein-losing enteropathy, chronic kidney disease stage 4 or higher, or death. Results: Patients with advanced fibrosis had higher serum BNP levels and Fontan, mean pulmonary artery and capillary wedge pressures. The composite clinical outcome was present in 23 patients (22%) and was predicted by age at Fontan, right ventricular morphology and presence of aortopulmonary collaterals on multivariable analysis. Samples with advanced fibrosis had 228 up-regulated genes compared to early fibrosis. Samples with the composite clinical outcome had 894 up-regulated genes compared to those without it. A total of 136 up-regulated genes were identified in both comparisons and these genes were enriched in cellular response to cytokine stimulus, response to oxidative stress, VEGFA-VEGFR2 signaling pathway, TGF-beta signaling pathway, and vasculature development. Conclusions: Patients with FALD and advanced liver fibrosis or the composite clinical outcome exhibit up-regulated genes including pathways related to inflammation, congestion, and angiogenesis. This adds further insight into FALD pathophysiology.

Predictors of arrhythmia during pregnancy in adults with congenital heart disease.

BACKGROUND: Risk prediction of arrhythmia during pregnancy in adult congenital heart disease (ACHD) patients is currently lacking, and the impact of preconception catheter ablation on future antepartum arrhythmia has not been studied. METHODS: We conducted a single-center, retrospective cohort study of pregnancies in ACHD patients. Clinically significant arrhythmia events during pregnancy were described, predictors of arrhythmia were analyzed, and a risk score devised. The impact of preconception catheter ablation on antepartum arrhythmia was assessed. RESULTS: The study included 172 pregnancies in 137 patients. Arrhythmia events occurred in 25 (15%) of pregnancies, with 64% of events occurring in the second trimester and sustained supraventricular tachycardia being the most common rhythm. Univariate predictors of arrhythmia were history of tachyarrhythmia (OR 20.33, 95% CI 6.95-59.47, p < 0.001, Fontan circulation (OR 11.90, 95% CI 2.60-53.70, p < 0.001), baseline physiologic class C/D (OR 3.72, 95% CI 1.54-9.01, p = 0.002) and history of multiple valve interventions (OR 3.10, 95% CI 1.20-8.20, p = 0.017). Three risk factors (excluding multiple valve interventions) were used to formulate a risk score, with a cutoff of ≥2 points predicting antepartum arrhythmia with sensitivity and specificity of 84%. While recurrence of the index arrhythmia was not observed following successful catheter ablation, preconception ablation did not impact odds of antepartum arrhythmia. CONCLUSIONS: We provide a novel risk stratification scheme for predicting antepartum arrhythmia in ACHD patients. The role of contemporary preconception catheter ablation in risk reduction needs further refinement with multicenter investigation.

Transcatheter Leadless Pacing in Children: A PACES Collaborative Study in the Real-World Setting.

BACKGROUND: Transcatheter Leadless Pacemakers (TLP) are a safe and effective option for adults with pacing indications. These devices may be an alternative in pediatric patients and patients with congenital heart disease for whom repeated sternotomies, thoracotomies, or transvenous systems are unfavorable. However, exemption of children from clinical trials has created uncertainty over the indications, efficacy, and safety of TLP in the pediatric population. The objectives of this study are to evaluate clinical indications, procedural characteristics, electrical performance, and outcomes of TLP implantation in children. METHODS: Retrospective data were collected from patients enrolled in the Pediatric and Congenital Electrophysiology Society TLP registry involving 15 centers. Patients ≤21 years of age who underwent Micra (Medtronic Inc, Minneapolis, MN) TLP implantation and had follow-up of ≥1 week were included in the study. RESULTS: The device was successfully implanted in 62 of 63 registry patients (98%) at a mean age of 15±4.1 years and included 20 (32%) patients with congenital heart disease. The mean body weight at TLP implantation was 55±19 kg and included 8 patients ≤8 years of age and ≤30 kg in weight. TLP was implanted by femoral (n=55, 87%) and internal jugular (n=8, 12.6%) venous approaches. During a mean follow-up period of 9.5±5.3 months, there were 10 (16%) complications including one cardiac perforation/pericardial effusion, one nonocclusive femoral venous thrombus, and one retrieval and replacement of TLP due to high thresholds. There were no deaths, TLP infections, or device embolizations. Electrical parameters, including capture thresholds, R wave sensing, and pacing impedances, remained stable. CONCLUSIONS: Initial results from the Pediatric and Congenital Electrophysiology Society TLP registry demonstrated a high level of successful Micra device implants via femoral and internal venous jugular approaches with stable electrical parameters and infrequent major complications. Long-term prospective data are needed to confirm the reproducibility of these initial findings.

Conduction System Pacing Versus Conventional Cardiac Resynchronization Therapy in Congenital Heart Disease.

BACKGROUND: Dyssynchrony-associated left ventricular systolic dysfunction is a major contributor to heart failure in congenital heart disease (CHD). Although conventional cardiac resynchronization therapy (CRT) has shown benefit, the comparative efficacy of cardiac conduction system pacing (CSP) is unknown. OBJECTIVES: The purpose of this study was compare the clinical outcomes of CSP vs conventional CRT in CHD with biventricular, systemic left ventricular anatomy. METHODS: Retrospective CSP data from 7 centers were compared with propensity score-matched conventional CRT control subjects. Outcomes were lead performance, change in left ventricular ejection fraction (LVEF), and QRS duration at 12 months. RESULTS: A total of 65 CSP cases were identified (mean age 37 ± 21 years, 46% men). The most common CHDs were tetralogy of Fallot (n = 12 [19%]) and ventricular septal defect (n = 12 [19%]). CSP was achieved after a mean of 2.5 ± 1.6 attempts per procedure (38 patients with left bundle branch pacing, 17 with HBP, 10 with left ventricular septal myocardial). Left bundle branch area pacing [LBBAP] vs HBP was associated with a smaller increase in pacing threshold (Δ pacing threshold 0.2 V vs 0.8 V; P = 0.05) and similar sensing parameters at follow-up. For 25 CSP cases and control subjects with baseline left ventricular systolic dysfunction, improvement in LVEF was non-inferior (Δ LVEF 9.0% vs 6.0%; P = 0.30; 95% confidence limits: -2.9% to 10.0%) and narrowing of QRS duration was more pronounced for CSP (Δ QRS duration 35 ms vs 14 ms; P = 0.04). Complications were similar (3 [12%] CSP, 4 [16%] conventional CRT; P = 1.00). CONCLUSIONS: CSP can be reliably achieved in biventricular, systemic left ventricular CHD patients with similar improvement in LVEF and greater QRS narrowing for CSP vs conventional CRT at 1 year. Among CSP patients, pacing electrical parameters were superior for LBBAP vs HBP.

Atrial Standstill in the Pediatric Population: A Multi-Institution Collaboration.

BACKGROUND: Atrial standstill (AS) is a rare condition characterized by absence of electrical activity within the atria. Studies to date have been limited. OBJECTIVES: The authors sought to describe the clinical characteristics, genetics, and outcomes of patients with AS. METHODS: This was a retrospective multicenter study of patients <18 years at AS diagnosis, defined as absence of atrial activity documented during an electrophysiology study, device placement, or noninvasive rhythm tracings and confirmed by echocardiogram. Patients with acquired disorders were excluded. Clinical details and genetic variants were recorded and analyzed. RESULTS: Twenty patients were diagnosed at a median age of 6.6 years (IQR: 2.9-10.8 years). Arrhythmias included 16 (80%) with atrial/supraventricular arrhythmias and 8 (40%) with ventricular tachycardia, including 4 with cardiac arrests. A type 1 Brugada pattern was documented in 4. Pacemakers were implanted in 18 (90%). Although atrial leads were attempted in 15, only 4 achieved pacing at implantation. During a median follow-up of 6.9 years (IQR: 1.2-13.3 years), 7 (35%) had thromboembolic events. Of these, none had atrial pacing, 6 were not on anticoagulation, and 1 was on aspirin. Genetic testing identified SCN5A variants in 13 patients (65%). Analyses suggest SCN5A loss-of-function may be one mechanism driving AS. Ventricular arrhythmias and cardiac arrest were more commonly seen in patients with biallelic SCN5A variants. CONCLUSIONS: AS may be associated with loss-of-function SCN5A variants. Patients demonstrate atrial and ventricular arrhythmias, and may present challenges during device placement. Patients without the capacity for atrial pacing are at risk for thromboembolic events and warrant anticoagulation.