Abstract Body (Do not enter title and authors here): Total cavo-pulmonary connection (Fontan) is the final stage of single ventricle palliation. In patients with an extracardiac (EC) Fontan, the synthetic conduit connecting the inferior vena cava to the pulmonary artery does not grow and may become smaller over time.

We hypothesize that smaller measured Fontan conduit diameter will be associated with lower functional capacity as measured by percent predicted peak VO2 (ppVO2).

A cross-sectional analysis of the FORCE registry was performed. Cardiac magnetic resonance imaging (CMR), exercise stress test (EST), and clinical data were extracted. Patients with EC-Fontan who had a CMR and EST within 1 year of each other from Jan 2003-June 2024 were included. Fontan conduits were manually segmented in 3D-Slicer. Center-line based assessments of conduit diameter were performed using circle-equivalent (CE) and maximum inscribed sphere (MIS) techniques (Figure 1). The relationships between minimum and length-averaged CE and MIS diameters indexed to body surface area (BSA) and percent predicted VO2 (ppVO2) were examined in bivariate and multivariable analyses, controlling for known predictors of exercise performance in Fontan patients: ventricular morphology, ventricular ejection fraction, atrioventricular valve regurgitation, time since Fontan, and presence of a patent fenestration.

Adequate imaging was present in 393 patients, of these 311 patients from 27 contributing centers had complete data for EST and CMR analysis. The median age at the time of CMR was 15.5 years and the median time between CMR and EST was 61 days. The median ppVO2 was 59% (IQR 50-68%). The median minimum Fontan conduit diameters by MIS and CE respectively were 14.6mm (IQR 13.1-16.0mm) and 17.1mm (IQR 15.8-18.7mm), while the median length-averaged Fontan conduit diameters by MIS and CE were 16.1mm (IQR 14.7-17.3mm) and 18.5 (IQR 17.0-20.0mm) (Figure 2). After indexing to BSA all four measures were associated with ppVO2, but average MIS was slightly more predictive, so was used in the multivariable model (Figure 3). After controlling for the known predictors listed above, each mm/m2 increase in measured conduit diameter was associated with a 1.6% increase in ppVO2 (p<0.001).

Fontan diameter is extremely variable across individuals. Smaller indexed Fontan conduit diameter is associated with lower functional capacity as measured by ppVO2. Further data is needed to determine if strategies to increase Fontan conduit size improve functional capacity.