Native heart valves with limited functionality are commonly replaced by prosthetic heart valves in patients with valvular heart disease. Bileaflet mechanical heart valves (BMHVs) are currently the most widely implanted mechanical heart valve design owing to their long-term durability, with over 130,000 implants every year worldwide. However, despite the widespread clinical use of these valves and considerable improvement over the last two decades, the function of these devices remains imperfect. Recent studies have shown that BMHVs can still cause major complications, including promote hemolysis, platelet activation, and thromboembolic events. To avoid thromboembolic complications, patients with a BMHV must undergo lifelong anticoagulant therapy. However, side effects include significant risk of hemorrhage, infection, and autoimmune responses. Clinical reports and recent in vitro experiments suggest that the thrombogenic complications caused by BMHVs are mainly associated with the hemodynamic stresses imposed on blood elements by the complex non-physiologic flow through the valve and in particular through the hinge regions.
Therefore, full three dimensional characterization of the flow through the hinge region of prosthetic heart valves is essential to explore the dynamics of hinge flow structures and quantify their thrombogenic potentials based on shear stress history of blood elements combined with hinge residence times.
To date, flow phenomena occurring in the hinge region of BMHVs have largely been studied experimentally, but these studies provided only limited information. This study aims at numerically simulating the flow through the hinge region of a BMHV under physiologic pulsatile conditions so as to quantitatively and accurately predict the hinge flow features at a level that cannot be assessed by experiments alone. This research proposes to (1) develop an efficient and accurate computational fluid dynamics solver specifically tailored to simulate the pulsatile three-dimensional flow through the hinge region of BMHVs under physiologic conditions, (2) develop a Lagrangian framework to estimate the thromboembolic potential associated with the hinge region of BMHVs, and (3) apply the developed framework to assess the influence of hinge design on the blood damage potential associated with the hinge region of BMHV.
Accurate computational meshes of the hinge region of clinical valves are obtained from X-ray micro-computed tomography scans. The hinge flow solver is based on a hybrid Cartesian/Immersed Boundary approach specifically tailored to handle moving and colliding boundaries. The accuracy of the solver is assessed by comparing the simulated velocity field with earlier experimental data obtained using Hydrogen Bubble Flow visualization and two-component Laser Doppler Velocimetry techniques. Finally, a particle tracking method, coupled with existing blood damage models, is implemented to predict blood element trajectories through the hinge region and estimate the associated shear stress histories. This approach allows for the analysis of the computed hinge flow field as they are experienced by the blood cells flowing through the hinge and for an estimation of the potential for hemolysis and platelet activation.
Computational fluid dynamic simulations and Lagrangian particle tracking are performed on three different hinge configurations subjected to aortic conditions to provide new insights into the influence of hinge design on hemodynamics, hemolysis, platelet activation, and thrombus formation. The influence of the hinge gap width is studied by simulating the flow in the hinge region of two identical St Jude Medical (SJM) hinges with varying hinge gap width. The influence of wall curvature is investigated by comparing the performance of a SJM hinge and a CarboMedics (CM) valve.
Calculations reveal complex, unsteady and highly 3D flow fields, with flow patterns known to be detrimental to blood elements throughout the hinge and cardiac cycle. In particular zones of flow stagnation and recirculation, favorable to thrombosis, are identified. Elevated shear stresses, which may induce platelet activation, are seen in the hinge and near-hinge region. Hinge gap width and, more importantly, the shape of the hinge recess and leaflet ear are found to impact the levels of shear stresses experienced by the blood cells. In particular avoiding sharp corners or sudden shape transitions appears as a key geometrical design parameter to minimize flow disturbances and thromboembolic potential.
The implications of the present study are two fold. First, the computed flow fields underscore the need to perform full 3D pulsatile simulations throughout the cardiac cycle in order to fully capture the complexity and unsteadiness of the hinge flows. Then, though based only on three different hinge designs, this study provides general guidelines to optimize the hinge design based on hemodynamic performance and thromboembolic potential. The developed framework enables rapid and cost-efficient pre-clinical evaluation of prototype BMHV designs prior to valve manufacturing. Application to a wide range of hinges with varying design parameters will eventually help in determining the optimal hinge design.