Abstract Body (Do not enter title and authors here): Background: The Fontan procedure has significantly extended the lifespan of patients with single-ventricle congenital heart disease. Atrial tachyarrhythmias (AT) are known to be associated with an increased risk of morbidity and mortality in this population. Understanding the anatomic, surgical, and clinical elements associated with AT is essential for risk stratification and guiding quality care.

Research Question: What is the prevalence of AT in the Fontan population? What are the clinical, imaging, and surgical risk factors for AT development?

Methods: The Fontan Outcomes Registry Using CMR Examinations (FORCE) was utilized for the analysis, which contains retrospective clinical and imaging data of Fontan patients from 42 centers. Cardiac MRI (CMR) data spanned 3/1999-1/2025. Patients were included if they had at least one CMR prior to AT diagnosis. The latest CMR assessment served as the starting point for modeling. AT was defined as atrial fibrillation, atrial flutter, or atrial tachycardia. Differences in characteristics between AT groups were assessed using Wilcoxon-Mann-Whitney or Chi-square/Fisher’s exact tests, as appropriate. Cox proportional hazards regression models were used to estimate hazard ratios for the relationship between predictors and risk of AT. Kaplan-Meier survival curves were used to compare freedom from AT across time between differing risk factor groups.

Results: The study included 2,534 patients (42% female) with an average age at CMR of 15.7 years old. At a median follow-up of 2 years (IQR: 0.8-4.1 years) post-CMR, 266 (9.3%) developed AT with an overall incidence of 32.0 per 1000 person-years. Patients were 93.8%, 88.4%, and 78.3% free from AT at 2, 4, and 8 years, respectively. In the multivariable model, older age at CMR, atrial-pulmonary or lateral tunnel Fontan, >3 cardiac surgeries, higher BMI, and higher single-ventricle end-diastolic volume index (EDVi) were significantly associated with AT (Image 1 & 2). Kaplan-Meier analysis demonstrated that increased number of risk factors led to progressively worsened risk of AT (Image 3).

Conclusion: Fontan type, number of surgeries, age, and BMI increase the risk of AT development in the Fontan population. Elevated single-ventricle EDVi was identified as a novel imaging risk marker for AT that underscores the utility of CMR in this population. Incorporating CMR into clinical risk stratification provides the opportunity to identify patients at risk for AT development post-Fontan.