BACKGROUND: Atrial fibrillation and atrial flutter are relatively rare in young people and the incidence of thromboembolic complications is unknown. These issues contribute to the limited utility of present guidelines regarding anticoagulation in this population. OBJECTIVE: To report the number of thromboembolic complications in a cohort of pediatric and young adult patients presenting with atrial fibrillation (AFib) or atrial flutter (AFl) while also assessing anticoagulation practice in a multicenter cohort of young patients with these arrhythmias. METHODS: Multicenter, retrospective cohort of patients aged <
25 years old who presented with atrial flutter (AFl) or atrial fibrillation (AFib) between 2000 and 2019 to several large, quaternary pediatric centers, excluding episodes occurring within 30 days of an invasive cardiac procedure. RESULTS: There were 311 episodes of AFib/AFl among 210 patients with a median age of 17 (IQR 14, 20) years. Structural heart disease (SHD) was present in 120 patients (57%) and 20 patients (10%) had a primary cardiomyopathy. Twelve AFib/AFl episodes (8 with AFL, 4 with AFib) were associated with a thrombus, of which 8 (67%) patients had SHD, 2 (17%) had cardiomyopathy and 5 (42%) ventricular dysfunction. The CHADSVASc score was 0-3 for all patients with a thrombus. Of patients with no thrombus at presentation who were cardioverted, 102 (34%) patients were discharged on aspirin and 80 (27%) on anticoagulation. Two patients had symptoms concerning for a thromboembolic event on follow-up, but none had a newly documented thrombus. CONCLUSIONS: Thromboembolic complications occur in children and young adults presenting with AFib/AFl. The majority are associated with SHD, cardiomyopathy, and/or ventricular dysfunction. CHADSVASc was calculated for each patient and was of limited utility. Thromboembolic complications at follow up after cardioversion are rare, and anticoagulation strategies at discharge varied.