Abstract Body (Do not enter title and authors here): Background: The physiology of the Fontan circulation is consistent with chronic heart failure, characterized by elevated central venous pressure and reduced cardiac output. In adults with conventional heart failure, iron deficiency is common and is associated with increased morbidity and mortality. However, data is lacking on the prevalence of iron deficiency and its relationship with cardiovascular outcomes in pediatric patients with Fontan circulation.

Aim: To characterize the relationship between iron profiles and measures of cardiovascular health in pediatric patients with Fontan circulation.

Hypothesis: Iron deficiency is associated with worse measures of cardiovascular health.

Methods: Cross-sectional study comprising patients with Fontan circulation who had iron labs measured from 2010 - 2024 at a single urban center. Exclusion criteria were hematologic disease other than iron deficiency, acute bleeding, and acute illness other than heart failure. Cardiovascular health was evaluated with NYHA or Ross class, echocardiogram, and cardiac MRI. Kruskal-Wallis tests and linear regression were used to analyze associations between iron labs and measures of cardiovascular health. Indexed oxygen delivery (iDO2) was calculated from cardiac index measured by cardiac MRI, pulse oximetry, and hemoglobin.

Results: 100 patients (mean 12.5±4.7 years, 46% female) were included. Iron deficiency was present in 45% or 51% based on conventional criteria of transferrin saturation (TSAT) <20% or ferritin <30 ng/mL, and 94% based on suggested criteria for heart failure (ferritin <100 ng/mL or ferritin 100-299 ng/mL and TSAT <20%). TSAT was significantly lower in patients with NYHA or Ross class III/IV vs. I/II symptoms (median 14% [IQR 10-19%] vs. 20% [15-25.5%], p=0.014) and in those with history of ≥ moderate vs. < moderate atrioventricular valve regurgitation (median 12.5% [10-21.8%] vs. 20.5% [15-24.5%], p=0.049). Ferritin trended toward being significantly lower in those with ≥ moderate vs. < moderate ventricular dysfunction (median 19.5 [11-34] vs. 30.5 [18-56], p=0.095). Among patients with hemoglobin <15 g/dL, TSAT strongly correlated with iDO2 (R²=0.495, p=0.034).

Conclusion: Iron deficiency is common and is associated with both worse heart failure symptoms and objective measures of cardiovascular health in pediatric patients with Fontan circulation. Interventions to improve iron stores offer promising avenues for mitigating heart failure risk and improving outcomes.