The influence of device position on the flow within the Penn State 12 cc pediatric ventricular assist device

Abstract

Ventricular assist devices are a commonly used heart failure therapy for adult patients as bridge-to-transplant or bridge-to-recovery tools. The application of adult ventricular assist devices in pediatric patients has led to increased thrombotic events. Therefore, we have been developing a pediatric ventricular assist device (PVAD), the Penn State 12 cc PVAD. It is designed for patients with a body weight of 5-15 kg and has a stroke volume of 12 cc. Clot formation is the major concern. It is correlated to the coagulability of blood, the blood contacting materials and the fluid dynamics within the system. The intent is for the PVAD to be a long term therapy. Therefore, the system may be oriented in different positions according to the patient's behavior. This study evaluates for the first time the impact of position on the flow patterns within the Penn State 12 cc PVAD, which may help to improve the PVAD design concerning chamber and ports geometries. The fluid dynamics are visualized by particle image velocimetry. The evaluation is based on inlet jet behavior and calculated wall shear rates. Vertical and horizontal model orientations are compared, both with a beat rate of 75, outlet pressures of 90/60 mm Hg and a flow rate of 1.3 l/min. The results show a significant change of the inlet jet behavior and the development of a rotational flow pattern. Vertically, the inlet jet is strong along the wall. It initiates a rotational flow pattern with a wandering axis of rotation. In contrast, the horizontal model orientation results show a weaker inlet jet along the wall with a nearly constant center of rotation location, which can be correlated to a higher risk of thrombotic events. In addition, high speed videography illustrates differences in the diaphragm motion during diastole. Diaphragm opening trajectories measurements determine no significant impact of the density of the blood analog fluids. Hence, the results correlate to human blood.

Figure 1

(A) Schematic CAD drawing of the Penn State 12 cc PVAD acrylic model used for PIV. The dimensions correspond exactly to the clinical Penn State 12 cc PVAD. (B) Vertical and horizontal model orientations for PIV measurement and to high speed videography. C) Björk-Shiley Monostrut valve orientation for the inlet and outlet valves, which are rotated 15°.

Figure 2

(A) PIV parallel planes: At the zero position the first marginal flow is visible. The parallel planes with 7, 8.2 and 11 mm distance are chosen to be compared with previous studies. (B) PIV normal planes: The zero position for the inlet and outlet ports occurs at the left and right chamber edges, respectively. The 7.5 mm plane represents the normal middle of the inlet or outlet. The normal planes 3.75 and 11.25 mm on both sides are added to visualize the minor and major orifices valve flow patterns

Figure 3

The acrylic model of the Penn State 12 cc PVAD is connected via tubes to compliance chambers (CC) and a reservoir (R). Pressure probes (P1, P3) detect the diastolic/systolic pressure. The PVAD is driven by a pneumatic driver (PD), the pressure is measured at P2 with a pressure probe. A resistance plate (RP) regulates the pressure within the system. At F1/F2 position, the inflow/outflow is measured with flow probes.

Figure 4

The inflow/outflow were measured at the locations indicated in Figure 3. The average flow is 1.3 l/min in both device orientations.

Figure 5

The parallel plane coordinate system (x₀ to x₁), with the wall sections S1 to S4, are used for the wall shear rate calculations.

Figure 6

PIV diastolic flow fields (t: 150 ms to 350 ms) in the parallel planes for (A) the vertical orientation and (B) the horizontal orientation.

Figure 7

PIV diastolic flow fields (t: 150 ms to 350 ms) in the normal planes for (A) the vertical orientation and (B) the horizontal orientation.

Figure 8

Velocity profiles in cross sections (CS), shown in figure 5, at 200, 250, and 300 ms in mid-diastole for the (A) vertical and (B) horizontal orientations.

Figure 9

PIV systolic flow patterns at 550 ms and 600 ms in the (A) vertical and (B) horizontal orientations.

Figure 10

Wall shear rates (11 mm plane parallel plane) are plotted for each section (S1–S4) through the entire cardiac cycle (0 to 800 ms) for the (A) vertical and (B) horizontal orientations. The data is normalized by the 500 s⁻¹ threshold.

Figure 11

Wall shear rates at the apex wall location at the 11 mm parallel plane throughout the cardiac cycle with the vertical orientation denoted as squares and the horizontal orientation denoted as circles. In the vertical orientation, the 11 mm plane shows the highest apex shear rates peak at 1800 s⁻¹.

Figure 12

The collapsing diaphragm is described as a folding process with the vacuum engaged in the bottom of these images.

Figure 13

The diaphragm opening for each condition shows similar ascending slopes. After 250 ms, the diaphragm visualization is blocked by the sealing ring.