In the extracardiac Fontan operation, larger conduits are used when considering the
patients' growth rate. However, larger conduits may cause inefficient flow due to
turbulence or stagnation, resulting in late problems such as thrombosis or stenosis.
Our objective was to reveal the physiologic effects of respiration and exercise using
numerical models, based on the energy loss and flow stagnation, and to determine optimal
conduit size.
For the Fontan operation, a conduit from 14 to 22 mm was created based on angiographic
data from 17 Fontan patients (mean age, 36.0 months; mean body surface area, 0.53
m(2)). Respiratory-driven flow of the superior and inferior vena cava was determined
at rest and during exercise on two levels (0.5 and 1.0 W/kg) by magnetic resonance
imaging flow studies. Flow stagnation was defined as the volume of the region where
flow velocity was less than 0.01 m/second at both the expiratory and inspiratory phases.
In larger conduits, backward flow at the expiratory phase was prominent. Energy loss
was small even during exercise, but the change was slightly larger between 14 and
16 mm than other conduit sizes (14 mm, 5.759 mW; 16 mm, 4.881 mW; and 22 mm, 4.199
mW during 1.0 W/kg exercise). Stagnation volume at the expiratory phase increased
with an increase of conduit size (14 mm, 9.20% vs 22 mm, 33.9% conduit volume at rest).
Fontan circulation is a low-energy system even during exercise. Larger conduits were
proven to have redundant spaces, thus 16 and 18 mm conduits were optimal.