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Background Despite a rigorous screening process, including cardiac catheterization, a subset of patients with a single right ventricle (SRV) demonstrates suboptimal short‐term outcomes after the Fontan operation. The goal of this study was to perform a comprehensive assessment of diastolic function in pre‐Fontan patients with an SRV using invasive reference‐standard measures and determine their associations with post‐Fontan outcomes. Methods and Results Children aged 2 to 6 years with SRV physiology undergoing pre‐Fontan heart catheterization were recruited prospectively. Patients were divided into those who had an optimal or suboptimal outcome. A suboptimal outcome was defined as length of stay ≥14 days or heart transplant/cardiac death in first year after Fontan. Patients underwent pressure–volume loop analysis using reference‐standard methods. The measure of ventricular stiffness, β, was obtained via preload reduction. Cardiac magnetic resonance imaging for extracellular volume and serum draws for matrix metalloproteinase activity were performed. Of 19 patients with an SRV, 9 (47%) had a suboptimal outcome. Mean age was 4.2±0.7 years. Patients with suboptimal outcomes had lower ventricular stiffness (0.021 [0.009–0.049] versus 0.090 [0.031–0.118] mL−1; P=0.02), lower extracellular volume (25% [28%–32%] versus 31% [28%–33%]; P=0.02), and lower matrix metalloproteinase‐2 (90 [79–104] versus 108 [79–128] ng/mL; P=0.01) compared with patients with optimal outcomes. The only invasive measure that had an association with suboptimal outcome was β (P=0.038). Conclusions Patients with an SRV with suboptimal outcome after the Fontan operation had lower ventricular stiffness and evidence of maladaptive extracellular matrix metabolism compared with patients with optimal outcome. This appears to be a novel phenotype that may have important clinical implications and requires further study.

Background Heart failure phenotyping in single‐ventricle Fontan patients is challenging, particularly in patients with normal ejection fraction (EF). The objective of this study was to identify Fontan patients with abnormal diastolic function, who are high risk for heart failure with preserved ejection fraction (HFpEF), and characterize their cardiac mechanics, exercise function, and functional health status. Methods and Results Data were obtained from the Pediatric Heart Network Fontan Cross‐sectional Study database. EF was considered abnormal if 90th percentile (≥0.26 mL‐1). Patients were divided into: controls=normal EF and diastolic function; systolic dysfunction (SD) = abnormal EF with normal diastolic function; diastolic dysfunction (DD) = normal EF with abnormal diastolic pressure:volume quotient. Exercise function was quantified as percent predicted peak VO2. Physical Functioning Summary Score (FSS) was reported from the Child Health Questionnaire. A total of 239 patients were included, 177 (74%) control, 36 (15%) SD, and 26 (11%) DD. Median age was 12.2 (5.4) years. Arterial elastance, a measure of arterial stiffness, was higher in DD (3.6±1.1 mm Hg/mL) compared with controls (2.5±0.8 mm Hg/mL), P