Comprehensive hemodynamic assessment of the Fontan pathway using four-dimensional flow magnetic resonance imaging

OBJECTIVES: Conventional hemodynamic assessment of Fontan circulation relies on limited quantitative measurements. We evaluated the feasibility and clinical value of comprehensive flow visualization using four-dimensional flow magnetic resonance imaging to characterize patient-specific hemodynamic patterns in extracardiac total cavopulmonary connection. METHODS: Eight patients with extracardiac total cavopulmonary connection underwent four-dimensional flow magnetic resonance imaging at a median of 13.7 years postoperatively (interquartile range: 9.0-16.4 years). The Fontan pathway was segmented into five regions: extracardiac conduit, superior vena cava, central confluence, left pulmonary artery, and right pulmonary artery. Energy loss, maximum velocity, and kinetic energy were quantified for each segment using iTFlow software. Clinical outcomes were assessed during follow-up. RESULTS: Median age at total cavopulmonary connection was 1.8 years (interquartile range: 1.5-3.5), with 18 mm conduits in seven patients and 22 mm conduit in one patient. During a median follow-up of 17.3 years, two patients underwent re-operation after magnetic resonance imaging evaluation (tricuspid valve replacement and ascending aortic reconstruction); all remained alive without Fontan failure. Energy loss differed significantly across segments (p = 0.011), being highest in the conduit and central confluence, with remarkable inter-patient variability superimposed (conduit values varying 35-fold from 0.025 to 0.887 mW). Maximum velocity strongly correlated with energy loss (r = 0.92, p = 0.001), suggesting velocity may serve as an accessible surrogate. Atrioventricular valve regurgitation severity did not consistently correlate with energy loss. CONCLUSIONS: Four-dimensional flow magnetic resonance imaging with iTFlow software enables comprehensive hemodynamic assessment throughout the Fontan pathway, revealing substantial patient-specific heterogeneity in energy efficiency. With larger cohorts and correlation with clinical outcomes, this technique may become a valuable screening tool for post-Fontan evaluation and long-term surveillance.