A numerical investigation of blood damage in the hinge area of aortic bileaflet mechanical heart valves during the leakage phase.

Previous experimental and numerical blood studies have shown that high shear stress levels, long exposure times to these shear stresses, and flow recirculation promote thromboembolism. Artificial heart valves, in particular bileaflet mechanical heart valves (BMHVs), are prone to developing thromboembolic complications. These complications often form at the hinge regions of BMHVs and the associated geometry has been shown to affect the local flow dynamics and the associated thrombus formation. However, to date no study has focused on simulating the motion of realistically modeled blood elements within the hinge region to numerically estimate the hinge-related blood damage. Consequently, this study aims at (a) simulating the motion of realistically modeled platelets during the leakage (mid-diastole) phase in different BMHV hinge designs placed in the aortic position and (b) quantitatively comparing the blood damage associated with different designs. Three designs are investigated to assess the effects of hinge geometry and dimensions: a 23 mm St. Jude Medical Regent™ valve hinge with two different gap distances between the leaflet ear and hinge recess; and a 23 mm CarboMedics (CM) aortic valve hinge. The recently developed lattice-Boltzmann method with external boundary force method is used to simulate the hinge flow and capture the dynamics and surface shear stresses of individual platelets. A blood damage index (BDI) value is then estimated based on a linear shear stress-exposure time BDI model. The velocity boundary conditions are obtained from previous 3D large-scale simulations of the hinge flow fields. The trajectories of the blood elements in the hinge region are found to be qualitatively similar for all three hinges, but the shear stresses experienced by individual platelets are higher for the CM hinge design, leading to a higher BDI. The results of this study are also shown to be in good agreement with previous studies, thus validating the numerical method for future research in BMHV flows. This study provides a general numerical tool to optimize the hinge design based on both hemodynamic and thromboembolic performance.

Numerical investigation of the performance of three hinge designs of bileaflet mechanical heart valves.

Thromboembolic complications (TECs) of bileaflet mechanical heart valves (BMHVs) are believed to be due to the nonphysiologic mechanical stresses imposed on blood elements by the hinge flows. Relating hinge flow features to design features is, therefore, essential to ultimately design BMHVs with lower TEC rates. This study aims at simulating the pulsatile three-dimensional hinge flows of three BMHVs and estimating the TEC potential associated with each hinge design. Hinge geometries are constructed from micro-computed tomography scans of BMHVs. Simulations are conducted using a Cartesian sharp-interface immersed-boundary methodology combined with a second-order accurate fractional-step method. Leaflet motion and flow boundary conditions are extracted from fluid-structure-interaction simulations of BMHV bulk flow. The numerical results are analyzed using a particle-tracking approach coupled with existing blood damage models. The gap width and, more importantly, the shape of the recess and leaflet are found to impact the flow distribution and TEC potential. Smooth, streamlined surfaces appear to be more favorable than sharp corners or sudden shape transitions. The developed framework will enable pragmatic and cost-efficient preclinical evaluation of BMHV prototypes prior to valve manufacturing. Application to a wide range of hinges with varying design parameters will eventually help in determining the optimal hinge design.

Simulation of the three-dimensional hinge flow fields of a bileaflet mechanical heart valve under aortic conditions.

Thromboembolic complications of bileaflet mechanical heart valves (BMHV) are believed to be due to detrimental stresses imposed on blood elements by the hinge flows. Characterization of these flows is thus crucial to identify the underlying causes for complications. In this study, we conduct three-dimensional pulsatile flow simulations through the hinge of a BMHV under aortic conditions. Hinge and leaflet geometries are reconstructed from the Micro-Computed Tomography scans of a BMHV. Simulations are conducted using a Cartesian sharp-interface immersed-boundary methodology combined with a second-order accurate fractional-step method. Physiologic flow boundary conditions and leaflet motion are extracted from the Fluid-Structure Interaction simulations of the bulk of the flow through a BMHV. Calculations reveal the presence, throughout the cardiac cycle, of flow patterns known to be detrimental to blood elements. Flow fields are characterized by: (1) complex systolic flows, with rotating structures and slow reverse flow pattern, and (2) two strong diastolic leakage jets accompanied by fast reverse flow at the hinge bottom. Elevated shear stresses, up to 1920 dyn/cm2 during systole and 6115 dyn/cm2 during diastole, are reported. This study underscores the need to conduct three-dimensional simulations throughout the cardiac cycle to fully characterize the complexity and thromboembolic potential of the hinge flows.

Fluid mechanics of artificial heart valves.

1. Artificial heart valves have been in use for over five decades to replace diseased heart valves. Since the first heart valve replacement performed with a caged-ball valve, more than 50 valve designs have been developed, differing principally in valve geometry, number of leaflets and material. To date, all artificial heart valves are plagued with complications associated with haemolysis, coagulation for mechanical heart valves and leaflet tearing for tissue-based valve prosthesis. For mechanical heart valves, these complications are believed to be associated with non-physiological blood flow patterns. 2. In the present review, we provide a bird's-eye view of fluid mechanics for the major artificial heart valve types and highlight how the engineering approach has shaped this rapidly diversifying area of research. 3. Mechanical heart valve designs have evolved significantly, with the most recent designs providing relatively superior haemodynamics with very low aerodynamic resistance. However, high shearing of blood cells and platelets still pose significant design challenges and patients must undergo life-long anticoagulation therapy. Bioprosthetic or tissue valves do not require anticoagulants due to their distinct similarity to the native valve geometry and haemodynamics, but many of these valves fail structurally within the first 10-15 years of implantation. 4. These shortcomings have directed present and future research in three main directions in attempts to design superior artificial valves: (i) engineering living tissue heart valves; (ii) development of advanced computational tools; and (iii) blood experiments to establish the link between flow and blood damage.

Spatio-temporal flow analysis in bileaflet heart valve hinge regions: potential analysis for blood element damage.

Point-wise velocity measurements have been traditionally acquired to estimate blood damage potential induced by prosthetic heart valves with emphasis on peak values of velocity magnitude and Reynolds stresses. However, the inherently Lagrangian nature of platelet activation and hemolysis makes such measurements of limited predictive value. This study provides a refined fluid mechanical analysis, including blood element paths and stress exposure times, of the hinge flows of a CarboMedics bileaflet mechanical heart valve placed under both mitral and aortic conditions and a St Jude Medical bileaflet valve placed under aortic conditions. The hinge area was partitioned into characteristic regions based on dominant flow structures and spatio-temporal averaging was performed on the measured velocities and Reynolds shear stresses to estimate the average bulk stresses acting on blood elements transiting through the hinge. A first-order estimate of viscous stress levels and exposure times were computed. Both forward and leakage flow phases were characterized in each partition by dynamic flows dependent on subtle leaflet movements and transvalvular pressure fluctuations. Blood elements trapped in recirculation regions may experience exposure times as long as the entire forward flow phase duration. Most calculated stresses were below the accepted blood damage threshold. Estimates of the stress levels indicate that the flow conditions within the boundary layers near the hinge and leaflet walls may be more detrimental to blood cells than bulk flow conditions, while recirculation regions may promote thrombus buildup.

Effect of hinge gap width on the microflow structures in 27-mm bileaflet mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Most bileaflet mechanical heart valves (BMHVs) incorporate some retrograde flow through their hinge mechanism to prevent flow stasis and inhibit microthrombus formation. This reverse flow is characterized by high velocities and shear stresses, thereby promoting platelet activation and hemolysis inside the hinge region. In the present study, the thromboembolic potential of three 27-mm BMHVs with varying hinge gap widths was assessed via in-vitro characterization of the hinge microflow structures. METHODS: Three 27-mm BMHV prototypes with different hinge gap widths (50, 100, and 200 microm) were provided by St. Jude Medical Inc. The valves were mounted in the mitral position of a left heart flow simulator, and two-dimensional laser Doppler velocimetry was used to measure the hinge velocity fields. RESULTS: All three valve prototypes revealed Reynolds shear stress (RSS) levels above 2000 dynes/cm2, which exceeded the threshold for platelet activation and hemolysis. The hinge flow fields were characterized by leakage jets during systole, and a strong vortical flow during diastole. The leakage jet size and corresponding RSS levels were found to increase with the hinge gap width. All three gap widths had RSS >4000 dynes/cm2 (range: 5640 to 13,315 dynes/cm2). The hinge with the smallest gap width registered the highest jet velocity magnitude (2.08 m/s) during systole. CONCLUSION: The study results showed that the hinge gap width influences washout and RSS levels inside the hinge recess. The 100-microm hinge gap width provided optimum fluid dynamic performance. In contrast, the two valves with large and small hinge gap widths may have higher thromboembolic potential.

Fluid dynamic assessment of three polymeric heart valves using particle image velocimetry.

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this paper we characterize the in vitro velocity and Reynolds Shear Stress (RSS) fields inside and downstream of three different prototype trileaflet polymeric heart valves. The fluid dynamic differences are then correlated with variations in valve design parameters. The three valves differ in leaflet thickness, ranging from 80 to 120 mum, and commisural design, either closed, opened, or semi-opened. The valves were subjected to aortic flow conditions and the velocity measured using three-dimensional stereo Particle Image Velocimetry. The peak forward flow phase in the three valves was characterized by a strong central orifice jet of approximately 2 m/s with a flat profile along the trailing edge of the leaflets. Leakage jets, with principle RSS magnitudes exceeding 4,500 dyn/cm(2), were observed in all valves with larger leaflet thicknesses and also corresponded to larger leakage volumes. Additional leakage jets were observed at the commissural region of valves with the open and the semi-open commissural designs. The results of the present study indicate that commissural design and leaflet thickness influence valve fluid dynamics and thus the thrombogenic potential of trileaflet polymeric valves.

Comparison of the hinge flow fields of two bileaflet mechanical heart valves under aortic and mitral conditions.

BACKGROUND: Animal and clinical studies have shown that bileaflet mechanical heart valve designs are plagued by thromboembolic complications, with higher rates in the mitral than in the aortic position. This study evaluated the hinge flow dynamic of the 23 mm St. Jude Medical (SJM) Regent and the 23 mm CarboMedics (CM) valves under aortic conditions and compared these results with previous findings under mitral conditions. METHOD: Velocity and Reynolds shear stress fields were captured using two-component laser Doppler velocimetry. RESULTS: Under aortic conditions, both the SJM and CM hinge flow fields exhibited a strong forward flow pattern during systole (maximum velocities of 2.31 and 1.75 m/s, respectively) and two main leakage jets during diastole (maximum velocities of 3.08 and 2.27 m/s, respectively). CONCLUSIONS: Aortic and mitral flow patterns within the two hinges were similar, but with a more dynamic flow during the forward flow phase under aortic conditions. Velocity magnitudes and shear stresses measured under mitral conditions were generally higher than those obtained in the aortic position, which may explain the higher rates of thromboembolism in the mitral implants when compared with the aortic implants.