In vivo evaluation of Fontan pathway flow dynamics by multidimensional phase-velocity magnetic resonance imaging

Hemodynamic efficiency of Fontan circulation is believed to be a major determinant of outcome. Prior research on flow dynamics in different modifications of Fontan circulation used in vitro models and computer-based simulation. This study was designed to compare in vivo flow dynamics in the systemic venous pathway between patients with atriopulmonary anastomosis (APA) and those with total cavopulmonary connection (TCPC).

Multidimensional phase-velocity magnetic resonance imaging (PV-MRI) studies were performed on 10 patients who had undergone a modified Fontan operation (5 with TCPC and 5 with APA) and were free of symptoms. The groups were comparable in terms of age and body surface area. The interval since surgery was longer for APA than for TCPC subjects. In each subject, the phase-velocity data sets were used to generate dynamic velocity-vector maps and to calculate quantitative flow indices describing the 3-dimensional blood-flow patterns throughout the cardiac cycle at the widest diameter of the Fontan pathway. Mean flow rate was comparable between groups. Velocity-vector maps showed areas of flow reversal, flow stagnation, and circular flow within APA but not TCPC pathways. Analysis of quantitative flow indices showed that compared with the APA group, flow velocities in the TCPC patients were significantly higher (mean velocity, 14+/-6 cm/s versus 5+/-3 cm/s; P=0.02), less variable (coefficient of variation, 19+/-2% versus 37+/-3.5%; P<0.0001), and more unidirectional (degree of unidirectionality, 89+/-7% versus 71+/-12%; P=0.03). APA pathways were significantly more dilated than were TCPC pathways (P<0.01) and showed a trend toward larger diameter with increased interval since surgery (R2=0.6, P=0.09). Fontan pathway dilatation correlated with flow velocity variability (R2=0.57, P=0.01) and inversely with flow unidirectionality (R2=0.75, P=0.001).

Blood flow patterns are more organized and uniform in TCPC than in APA pathways and are significantly influenced by pathway diameter. We speculate that TCPC may result in a more hemodynamically efficient circulation than APA because of differences in pathway dimension and uniformity.