Abstract Body (Do not enter title and authors here): Background
The Fontan operation has been an effective treatment plan for patients with single ventricle anomalies, greatly improving survival into adulthood. Yet, Fontan patients are at increased risk of numerous cardiac and noncardiac complications, and premature mortality.

Methods
A cohort of 735 patients with a prior Fontan surgery (median age 7.6yrs at first encounter, 57.4% male, 41.0% Black), with at least one healthcare encounter from a pediatric or adult healthcare system between 2010-2019, was linked to encounter data and death certificates. Heart failure (HF) was defined by ICD-9-CM and ICD-10-CM codes determined by clinicians. The primary composite outcome included heart transplant, hepatocellular carcinoma (HCC) and death. Hospitalizations were defined as an inpatient encounter spanning at least two consecutive days. Time to event was anchored at initial encounter.

Results
Characteristics of those with and without HF are seen in Table 1. Median age was 17.1yrs [3.0-70.5] and 5.5yrs [3.0-56.0] for those with and without HF, respectively. The composite outcome occurred in 85 Fontan patients (11.6%) at median 3.26yrs after initial encounter. Of those with HF, 12.5% experienced a heart transplant compared to 0.9% without HF (p<0.001), 3.0% with HF had HCC vs 0.5% without HF (p<0.05), and 9.4% with HF died at mean age 29.3yrs compared to 2.7% without HF at mean age 16.3yrs (p<0.001). Kaplan-Meier event-free survival curves appear in Figure 1. Of those with HF, 69.0% were hospitalized at least once compared to 53.0% of those without HF (p<0.001). Fontan patients without HF were 95.9% likely to be event-free at 5yrs follow-up compared to those with HF who were 84.7% likely to be event-free at 5yrs.

Conclusion
Fontan patients more commonly experience HF at older ages. HF is associated with heart transplant, HCC, and death during follow-up compared with Fontan patients without HF. Hospitalizations were common in both groups, but occurred more commonly with HF.