Using CFD to Examine the Hemodynamics of Artifical Heart Valves

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Abstract

Background
Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bioprosthetic valves.Today, with over 200,000 implants worldwide each year, the most widely implanted design is the mechanical bileaflet prosthesis. Several other mechanical valves are currently available and many of them have good bulk forward flow hemodynamics. However, mechanical valve implants suffer from complications resulting from thrombus deposition and patients implanted with these valves need to be under long-term anti-coagulant therapy. In general blood thinners are not needed with bioprosthetic implants, but tissue valves suffer from structural failure with an average life-time of 10–12 years before replacement is needed.

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Flow-induced stresses on the formed elements in blood have been implicated in thrombus initiation within the mechanical valve prostheses. Regions of stress concentration on the leaflets during the complex motion of the leaflets have been implicated with structural failure of the leaflets with bioprosthetic valves. In vivo and in vitro experimental studies have yielded valuable information on the relationship between hemodynamic stresses and the problems associated with the implants. More recently, Computational Fluid Dynamics (CFD) has emerged as a promising tool, which, alongside experimentation, can yield insights of unprecedented detail into the hemodynamics of prosthetic heart valves. For CFD to realize its full potential, however, it must rely on numerical techniques that can handle the enormous geometrical complexities of prosthetic devices with spatial and temporal resolution sufficiently high to accurately capture all hemodynamically relevant scales of motion. Such algorithms do not exist today and their development is at the forefront of ongoing research. For CFD to further gain the confidence of valve designers and medical practitioners it must also undergo comprehensive validation with experimental data. Such validation requires the use of high-resolution flow measuring tools and techniques and the integration of experimental studies with CFD modeling.

The link between hemodynamics and clinical complications
Platelets can become hyper-activated due to shear forces and they present a risk for the development of a thrombotic event. As early as 1970, Harker and Slichter 13 showed that patients with first-generation mechanical valves such as the ball-and-cage and tiltingdisc had a shortened platelet half-life due to increased incidence of platelet destruction and activation. Direct mechanical trauma by impact with the valve suprastructure and shearing forces induced by turbulent flow are two possible mechanisms accounting for this destruction. Also, continuous sub lethal hemolysis can lead to alterations in red cell membrane morphology and reduced membrane flexibility.18,23 Such alterations can upset the balance of hemostasis as the endothelial lining appears to be very vulnerable to even low wall shear stresses generated in the forward flow fields of bileaflet valves in the aortic position.9,27 These findings have particular importance for prosthetic mitral valve recipients who may be in atrial fibrillation.20,25 In such patients, elevated plasma fibrinogen levels have been detected, indicating a heightened thrombotic tendency compared to population controls in normal sinus rhythm.20

The realization that blood flow induced stresses can trigger bio-chemical responses at the cellular level and cause clinical complications led to the need for a systematic characterization and quantification of the hemodynamical properties of prosthetic heart valves. Such undertaking, however, is far from trivial. Prosthetic valves are geometrically very complex and the shape and motion of their leaflets evolve dynamically in response to the instantaneous flow conditions. The pulsatile nature of blood flow and the ensuing fluid/structure interaction give rise to very complex and highly unsteady, borderline turbulent flow, characterized by regions of flow reversal, threedimensional separation and vortex formation and shedding. Furthermore, non-Newtonian effects can also become very important and need to be accounted for especially when analyzing the flow within valve components whose scale is comparable to that of formed blood elements, such as the hinge region of mechanical valves. Given these enormous complexities, sophisticated fluid dynamics testing of prosthetic heart valve flows requires a close synergy between advanced experimental and computational fluid dynamics techniques. />/>/>/>/>

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References
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