Abstract Body (Do not enter title and authors here): Introduction: More patients with Fontan circulation are surviving into adulthood and transitioning to adult congenital heart disease (ACHD) care. There are limited long-term outcome data in these patients.

Research Questions: We sought to describe a cohort of adults with Fontan circulation and determine patient characteristics associated with long-term survival.

Methods: A retrospective review of adults with Fontan circulation followed in our ACHD clinic between 2009 and 2023 was performed. The outcome of interest was transplant-free survival. Survival analysis was performed to explore association between patient factors and time from Fontan completion to patient death or transplant. Patients without events were censored at last follow-up date.

Results: In our cohort of 429 patients, median age at first ACHD visit was 24 (IQR 22-29) years with median follow-up time of 3.9 (IQR 0.6-6.4) years. The most common underlying anatomies were tricuspid atresia (26.5%) and hypoplastic left heart syndrome (25.4%). Fontan completion, most commonly lateral tunnel (59.1%), was performed at a median age of 29.7 months. Extracardiac comorbidities in our cohort included: cirrhosis (42.9%), cyanosis (25.9%), and protein losing enteropathy (PLE) (7.0%). Forty-one patients (9.6%) died, 17 (4.0%) had a heart-liver transplant, and 2 (0.5%) had a heart transplant. Transplant-free survival at 20, 30, and 40 years after Fontan completion was 99%, 87%, and 63%. In total, 10 patients died from end-stage heart failure, 8 from sudden cardiac arrest, and 5 from post-transplant complications (Figure). Morphologic right ventricle, cirrhosis, cyanosis, and PLE were associated with worse transplant-free survival (Table).

Conclusions: Factors associated with decreased transplant-free survival in a cohort of adult patients with Fontan circulation were identified. These data highlight the importance of early identification and potential intervention for these comorbidities to improve long-term outcomes.