Prosthetic heart valves are routinely used for replacing diseased heart valves; however, thromboembolism remains an obstacle to these devices.1,2 It is well known that flow-induced thrombogenicity by chronic platelet activation is the prominent aspect of this blood trauma.3–5 It has also been shown previously that patients with cardiac valve replacement, regardless of type of the valve, have increased level of plasma fibrinogen, plasma viscosity, and platelet aggregation compared with normal subjects.6 Although this problem is much more pronounced in patients with mechanical prostheses who need lifelong anticoagulant therapy, patients with bioprostheses are also at low risk of thrombogenesis in the short period after surgery so that anticoagulants are usually needed for a limited period.1,2,7
Fluid mechanical factors involved in platelet activation and aggregation include high shear rates, turbulence, and areas of flow stagnation or recirculation that are characterized by low shear and longer retention time.8,9 These factors should be considered in designing or modifying heart valves.
It has been postulated that decreasing the ventricular projection of the mitral bioprostheses has potential clinical benefits, such as facilitating the surgical procedure, but the effects on flow and circulation are unknown. In the present study, the effect of profile height change of Perimount mitral valves on flow field was examined at atrial and ventricular sides of the valve using digital particle image velocimetry (DPIV).10
Materials and Methods
Test valves consisted of one 27 mm clinical-quality CEP (Carpentier-Edwards Perimount pericardial) mitral as the control valve and three 27 mm mitral prototypes with approximate reduction in ventricular profile of 15%, 25%, and 33% with a corresponding increase in atrial projection with respect to the standard Perimount valve (Table 1, Figure 1). Valves were provided by the manufacturer (Edwards Lifesciences at Irvine, CA) for in vitro evaluation (Figure 1).
Heart Pulsed Flow Simulator
This study was accomplished using the Caltech’s left heart pulsed flow simulator located at the Cardiovascular and Biofluid Laboratory in Pasadena, California. It is composed of a ventricle, shaped according to molds in systolic state, and made of transparent silicone rubber. Test valves and a standard 23 mm aortic valve were mounted in mitral and aortic positions, respectively, in such a way that they could open and close freely with the flow. The ventricle was suspended at the upper part of a rigid, water-filled, cubic (to avoid optical distortion) Plexiglas container (Figure 2).
The container was connected to a pump system (Superpump system, VSI, SPS3891, Vivitro Systems Inc., Victoria, BC, Canada) and filled with particle-seeded water. The Superpump system consisted of a piston-in-cylinder pump head driven by a low-inertia electric motor. The oscillatory flows were generated by input of appropriate waveform to the power amplifier (VSI, SPA3891Z), which provided position and velocity feedback to the amplifier. The waveforms were representative of an instantaneous ventricular volume signed as FDA waveform by VSI. A cardiac output of 5 l/min, a mean arterial pressure of 100 mm Hg, and a heartbeat of 72 beats per minute were the investigated working conditions for the experiments.
The atrial part was a cylindrical tube with an orifice plate at its distal end for placing the valve. Atrial chamber geometrical specifications are shown in Figure 3. It was designed in a way that does not disturb the characteristics of flow that enter the ventricular sac from the reservoir through the mitral valve (Figure 3A). The design preserved the maximal visibility of the flow around the atrial side of the valve during the experiments. The ease of replacing the valve without a major change in the system setup was also an important consideration in designing the atrial part.
Digital Particle Image Velocimetry Measurements and Flow Visualization
The flow characteristic information (e.g., velocity field and circulation) was obtained with the DPIV technique. DPIV uses two digital images of a particle-seeded flow illuminated by a thin laser sheet to determine the displacement field of the particles in the field of view (sampling window) by cross-correlating pixels in a subsection of two images. Fluorescent particles as fluid markers, in conjunction with a laser light sheet, were used to make the flow visible. A high-resolution monochrome CCD digital camera (30 fps, 768 × 480; TM-9701, PULNiX America, Inc.) was positioned perpendicular to the ventricle container to capture the image sequences of particle field. The images were captured from illuminated sheet of fluorescent particles generated by a 25 mJ double-pulsed Nd:YAG laser.
Circulation and Particle Residence Time
With the intention of studying the wash out around each valve, circulation (Γ) was calculated based on the vorticity field of each captured frame. Vorticity (ω) is defined as the curl of the velocity vector:
Circulation is defined as the line integral of the velocity. Based on the Stoke’s theorem, the circulation around a reducible curve (c) is equal to the flux of vorticity through an open surface (A) with unit normal vector n bounded by the curve, that is:
Where u is the velocity vector. Particle residence time calculates as:
Where x and v are the displacement and velocity vectors for each particle, respectively. Tp represents particle residence time, i denotes the number of frames that the tracking particle is still in the area of interest, and X is the dimensions of the area. This formula calculates the time that it takes a designated particle to leave the area of interest. In the present study, the area of interest for atrial and ventricular sides was considered the area around the valve below the tip of the prototype (Figure 4).
Successful sets of experiment for each valve profile were conducted to get the DPIV data from the flow in atrial and ventricular sides of the system. DPIV measurements were completed for different laser pulse separation ranging between 0.5 and 5 milliseconds to obtain the best cross-correlation peak velocities between the frames. Appropriate seeding of the water inside the system with orange fluorescent particles, using a 32 × 32-pixel sampling window and a 16 × 16-pixel step size, resulted in an excellent cross-correlation peak velocity.
Figures 5A and 5B show the typical velocity vector fields of the inflow and outflow to the mitral valve during the diastole when the fluid jet is passing across the mitral valve. The color bar represents the range of velocity, and the length of each vector represents the magnitude at any given point. The measurement units are millimeters and the velocity measurements are based on millimeters per second.
Residence time was measured by tracing 1,000 random particles distributed uniformly at the adjacent valve area in atrial and ventricular sides separately (Figure 5). The results for the residence time are summarized in Tables 1 and 2. Circulation was computed between the base and the tip of the valves at ventricular and atrial sides. In Figure 6, the magnitude of circulation (atrial and ventricular) averaged over 72 cycles is compared for all the examined valves.
Results from DPIV experiments show that on the atrium, by increasing the atrial projection of the mitral valve, the extent of circulation around the valve decreases. Mean particle residence time adjacent to the valve increases from 0.86 ± 0.74 seconds in the control CEP valve to 5.76 ± 1.90 seconds in the prototype with 33% atrial elevation (Table 2).
Considering the results in the ventricular sac, extreme reduction of profile height increased the amount of circulation around the valve compared with the other three profiles (Figure 6). This leads to a minimum particle residence time among the other three prototypes (Table 3).
This study confirms the relation between the valve’s height in atrial and ventricular sides and the degree of circulation around the valve. It has also been shown that, by increasing the amount of circulation around the valve, particle residence time decreases. An optimal design should therefore try to decrease the ventricular projection as much as possible, to both ease implantation and increase the circulation, but without increasing the atrial projection to avoid potential blood stagnation.
The magnitude of circulation was not consistently decreased in the 15% or 25% reduction valve profiles, which is also coherent with the particle residence time. This variation in magnitude of circulation can be attributed to size/shape of the confined area between the valve’s cusp and the sac. Another possible reason for this variation is the limitations in visibility around the valve from the commissure tips to the cusp resulted from different designs of the sewing ring. Because portions of the cusp were recessed into the sewing ring, the same region of interest was not fully visible for all four bioprostheses. This can potentially result in different evaluations of flow around the valve. Additional limitations of this study include different flow fields attributed to the valve replacement and limited visibility around the valve due to the unavoidable reflections of the laser at the interface of the silicone sac.
In the present study, we examined the flow around four different Perimount valve profiles. It has been observed that by changing the height of the bioprosthesis, the circulation around the valve changes, and this can influence the particle residence time adjacent to the bioprosthesis.
Because fluid dynamic factors involved in platelet activation and aggregation also include high rates of shear and deformation in addition to areas of flow stagnation or recirculation, shear stress analysis and platelet activation modeling for each bioprosthesis should also be performed as an additional study to evaluate the bioprosthesis performance.
The conclusion of this study is limited by the fact that DPIV captures two-dimensional characteristics of the flow. However, in real life, three dimensionality can complicate the flow around the valve. Further in vivo studies are needed to evaluate the potential hemodynamic benefits of low-profile designs.
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