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.