Materials and methods
An experimental model to mimic the Fontan circuit was created. The crux
of the model was the creation of a junction between the branch pulmonary
arteries and the vena cavae. The approximate dimensions of relevant
blood vessels in an average adult Fontan patient such as superior vena
cava (SVC), inferior vena cava (IVC), right pulmonary artery (RPA) and
left pulmonary artery (LPA) were used as reference (see Table 1). A
4-arm setup to mimic these dimensions as far as practically possible was
designed and fabricated using acrylic pipes and a specially made
junction. Table 1 describes the dimensions of the vessels in the human
body and the dimensions provided in the simulated setup. There was some
limitation in mimicking the actual anatomical dimensions in the
simulated setup because we used standard available components in the
setup.
A mechanical arrangement was designed and fabricated to enable placement
of a ball inside the 4-arm setup along the RPA-LPA pathway. Furthermore,
a driving mechanism was also fabricated to mechanically oscillate the
ball along the LPA-RPA pathway. (Figure 1)(Video 1) The dimension of the
ball selected was 18 mm such that it could move easily along the RPA-LPA
pathway without completely occluding it and allow fluid to pass around
the ball. The reason for keeping some gap between the ball and the
pathway was to avoid total dependence of the movement of the blood and
hemodynamics only on the motion of the ball. In case the ball failed,
blood would be still able to flow around the ball thereby providing a
safety mechanism.
Two fluid inlet reservoirs, one each was attached to the top of the SVC
and IVC limbs (Figures 2 and 3). The reservoirs were made of sufficient
height so that a pressure head of fluid equivalent to the actual venous
pressure in the venous side of the Fontan circuit could be created by
filling fluid into the reservoirs. We used two different fluids in the
setup; one was water and another was a mixture of water and glycerine to
simulate the viscosity of blood. Typical pressures in the SVC and IVC
along with the corresponding height of fluid column in the fluid inlet
reservoirs are mentioned in Table 2. We conducted experiments at two
levels of venous pressure head equivalent to 20mmHg and 10mmHg pressure
to mimic a high and low venous pressure Fontan system.
Two fluid outlet reservoirs, one each was placed below the RPA and LPA
to collect the fluid coming out. Stopcocks were placed one each at the
end of RPA and LPA limbs to control the flow of the fluid out of RPA and
LPA. The purpose of the stopcocks was to simulate various levels of
resistance to flow as encountered in patients with failing Fontan
circuit. We conducted experiments with three levels of resistance; 30
degree opening of the stopcock indicating high resistance to flow, 60
degree indicating low resistance and 90 degree indicating no resistance
(free-flow) condition.
The oscillating ball was intended to act as a pump to increase the rate
of fluid output coming out from the RPA and LPA. We used three speeds of
oscillation; 40 strokes /min, 72 strokes/min and 100 strokes/min to test
the hypothesis. A condition without the ball was also tested as a
baseline. In order to simulate the viscosity of blood, a mixture
consisting of 52% Glycerin and 48% water by volume was
made.6 This blood analog was used as one of the fluids
in the setup.