65 research outputs found

    Prediction of Turbulent Shear Stresses through Dysfunctional Bileaflet Mechanical Heart Valves using Computational Fluid Dynamics

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    There are more than 300,000 heart valves implanted annually worldwide with about 50% of them being mechanical valves. The heart valve replacement is often a common treatment for severe valvular disease. However, valves may dysfunction leading to adverse hemodynamic conditions. The current computational study investigated the flow around a bileaflet mechanical heart valve at different leaflet dysfunction levels of 0%, 50%, and 100%, and documented the relevant flow characteristics such as vortical structures and turbulent shear stresses. Studying the flow characteristics through these valves during their normal operation and dysfunction can lead to better understanding of their performance, possibly improved designs, and help identify conditions that may increase the potential risk of blood cell damage. Results suggested that maximum flow velocities increased with dysfunction from 2.05 to 4.49 ms-1 which were accompanied by growing eddies and velocity fluctuations. These fluctuations led to higher turbulent shear stresses from 90 to 800 N.m-2 as dysfunctionality increased. These stress values exceeded the thresholds corresponding to elevated risk of hemolysis and platelet activation. The regions of elevated stresses were concentrated around and downstream of the functional leaflet where high jet velocity and stronger helical structures existed

    Numerical comparison of the closing dynamics of a new trileaflet and a bileaflet mechanical aortic heart valve

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    [[abstract]]The closing velocity of the leaflets of mechanical heart valves is excessively rapid and can cause the cavitation phenomenon. Cavitation bubbles collapse and produce high pressure which then damages red blood cells and platelets. The closure mechanism of the trileaflet valve uses the vortices in the aortic sinus to help close the leaflets, which differs from that of the monoleaflet or bileaflet mechanical heart valves which mainly depends on the reverse flow. We used the commercial software program Fluent to run numerical simulations of the St. Jude Medical bileaflet valve and a new trileaflet mechanical heart valve. The results of these numerical simulations were validated with flow field experiments. The closing velocity of the trileaflet valve was clearly slower than that of the St. Jude Medical bileaflet valve, which would effectively reduce the occurrence of cavitation. The findings of this study are expected to advance the development of the trileaflet valve.[[incitationindex]]SCI[[booktype]]電子版[[booktype]]紙

    Fluid-structure Interaction Simulation of Bileaflet and Monoleaflet Mechanical Heart Valve Flow Dynamics

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    In this study, opening and closing behavior of monoleaflet and bileaflet prosthetic heart valves was simulated using 2D and 3D Fluid Structure Interaction (FSI) models. The FSI models were based on the arbitrary Lagrangian-Eulerian (ALE) method for moving boundaries. Leaflet and diaphragm motions were described by means of user-defined functions following the experimental setup in a previous study. The hemodynamic performance of monoleaflet valves at the opening angle of 45, 60, 75, 80, and 85 was compared and results from this study demonstrated that the optimal opening angle should fall between 75 and 80. As the opening angle further increased, even though the calculated flow parameters continued to improve, the large angle could prevent the valve to close properly, which might lead to the failure of the heart valve. Furthermore, the hemodynamic performance of bileaflet and monoleaflet heart valves following the design of St. Jude bileaflet valve with 85 of opening angle and Bjork-Shiley monoleaflet valve with 75 of opening angle was compared. Results demonstrated that the flow in the monoleaflet valve design had a lower maximum velocity compared to the bileaflet design during both opening and closing phases which resulted in lower chance for flow to transition to turbulence. The mean pressure gradients across the monoleaflet and bileaflet valves were similar and resulted in an analogous EOA for these valves. According to the results of this study, the bileaflet valve had higher chance of developing cavitation bubbles during the valve closure because of higher pressure drops across the valve.Mechanical & Aerospace Engineerin

    IN VITRO VISUALIZATION OF PEDIATRIC SIZED MECHANICAL HEART VALVE PERFORMANCE USING AORTIC ROOT MODEL IN MOCK CIRCULATORY LOOP

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    Congenital heart valve disease is one of the most common abnormalities in children, with common valve defects being aortic stenosis, mitral stenosis, and valvular regurgitation. Although adult sized mechanical heart valve (MHV) replacements are widely studied and utilized, there are currently no FDA approved prosthetic heart valves available for the pediatric population. This is due to a variety of reasons such as a limited patient pool for clinical trials, limited valve sizes, and complex health histories in children. Much like adult sized mechanical heart valves, potential complications with pediatric heart valve replacements include thrombosis, blood damage due to high shear stresses, and cavitation. Due to pediatric sized MHVs being much smaller in size than adult MHVs, different fluid dynamic conditions and associated complications are expected. In order to accelerate the approval of pediatric sized heart valves for clinical use, it is important to first characterize and assess the fluid dynamics across pediatric sized heart valves. By understanding the hemodynamic performance of the valve, connections can be made concerning potential valve complications such as thrombosis and cavitation. The overall objective of this study is to better characterize and assess the flow field characteristics of a pediatric sized mechanical heart valve using flow visualization techniques in a mock circulatory loop. The mechanical heart valve chosen for this research was a size 17 mm Bjork-Shiley tilting disc valve, as this is a common size valve used for younger patients with smaller cardiovascular anatomy. The mock circulatory loop used in this research was designed to provide realistic pediatric physiological flow conditions, consisting of a Harvard Apparatus Pulsatile blood pump, venous reservoir, and a heart valve testing chamber. In order to expose the valve to realistic pediatric flow conditions, six unique pump operating conditions were tested that involved pre-determined heart rate and stroke volume combinations. In addition, a modified aortic root model was used to hold the mechanical heart valve in place within the loop and to provide more realistic aortic root geometry. This heart valve chamber was made from a transparent acrylic material, allowing for fluid flow visualization. A traditional Particle Image Velocimetry (PIV) experimental set up was used in order to illuminate the particles seeded within the fluid path, and thus allowing for the capture of sequential images using a high speed camera. The data collected throughout this study consisted of flow rate measurements using an ultrasonic flow meter, and the sequential PIV images obtained from the camera in order to analyze general flow characteristics across the pediatric valve. Such information regarding the flow profile across the valve allowed for conclusions to be made regarding the valve performance, such as average flow velocities and regions of regurgitant flow. By gaining a better understanding of the fluid dynamic profile across a pediatric sized heart valve, this may aid in the eventual approval of pediatric sized mechanical heart valves for future clinical use

    Time-Resolved Micro PIV in the Pivoting Area of the Triflo Mechanical Heart Valve

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    The Lapeyre-Triflo FURTIVA valve aims at combining the favorable hemodynamics of bioprosthetic heart valves with the durability of mechanical heart valves (MHVs). The pivoting region of MHVs is hemodynamically of special interest as it may be a region of high shear stresses, combined with areas of flow stagnation. Here, platelets can be activated and may form a thrombus which in the most severe case can compromise leaflet mobility. In this study we set up an experiment to replicate the pulsatile flow in the aortic root and to study the flow in the pivoting region under physiological hemodynamic conditions (CO = 4.5 L/min / CO = 3.0 L/min, f = 60 BPM). It was found that the flow velocity in the pivoting region could reach values close to that of the bulk flow during systole. At the onset of diastole the three valve leaflets closed in a very synchronous manner within an average closing time of 55 ms which is much slower than what has been measured for traditional bileaflet MHVs. Hot spots for elevated viscous shear stresses were found at the flanges of the housing and the tips of the leaflet ears. Systolic VSS was maximal during mid-systole and reached levels of up to 40 Pa

    Statistical Characteristics of Flow Field through Open and Semi-Closed Bileaflet Mechanical Heart Valve

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    The formation of thrombi on the streamlined surface of the bileaflet mechanical heart valves is one of the main disadvantages of such valves. Thrombi block the valve leaflets and disrupt the cardiovascular system. Diagnosis of thrombosis of the bileaflet mechanical heart valves is relevant and requires the creation of effective diagnostic tools. Hydroacoustic registration of the heart noise is one of the methods for diagnosing the operation of a mechanical heart valve. The purpose of the research is to determine the statistical characteristics of the vortex and jet flow through the open and semi-closed bileaflet mechanical heart valve, to identify hydroacoustic differences and diagnostic signs to determine the operating conditions of the valve. Experimental studies were conducted in laboratory conditions on a model of the left atrium and left ventricle of the heart between which there was the bileaflet mechanical heart valve. Hydrodynamic noise was recorded by miniature pressure sensors, which were located downstream of the valve. The vortex and jet flow behind the prosthetic heart valve were non-linear, random processes and were analyzed using the methods of mathematical statistics and probability theory. The integral and spectral characteristics of the pressure field were obtained and the differences in the noise levels and their spectral components near the central and side jets for the open and semi-closed mitral valve were established. It was shown that hydroacoustic measurements could be an effective basis for developing diagnostic equipment for monitoring the bileaflet mechanical heart valve operation. Doi: 10.28991/SciMedJ-2020-0204-1 Full Text: PD

    Cavitation in Biological and Bioengineering Contexts

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    There are an increasing number of biological and bioengineering contexts in which cavitation is either utilized to create some desired effect or occurs as a byproduct of some other process. In this review an attempt will be made to describe a cross-section of these cavitation phenomena. In the byproduct category we describe some of the cavitation generated by head injuries and in artificial heart valves. In the utilization category we review the cavitation produced during lithotripsy and phacoemulsification. As an additional example we describe the nucleation suppression phenomena encountered in supersaturated oxygen solution injection. Virtually all of these cavitation and nucleation phenomena are critically dependent on the existence of nucleation sites. In most conventional engineering contexts, the prediction and control of nucleation sites is very uncertain even when dealing with a simple liquid like water. In complex biological fluids, there is a much greater dearth of information. Moreover, all these biological contexts seem to involve transient, unsteady cavitation. Consequently they involve the difficult issue of the statistical coincidence of nucleation sites and transient low pressures. The unsteady, transient nature of the phenomena means that one must be aware of the role of system dynamics in vivo and in vitro. For example, the artificial heart valve problem clearly demonstrates the importance of structural flexibility in determining cavitation occurrence and cavitation damage. Other system issues are very important in the design of in vitro systems for the study of cavitation consequences. Another common feature of these phenomena is that often the cavitation occurs in the form of a cloud of bubbles and thus involves bubble interactions and bubble cloud phenomena. In this review we summarize these issues and some of the other characteristics of biological cavitation phenomena

    Glycosaminoglycan Stabilization Reduces Tissue Buckling in Bioprosthetic Heart Valves

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    Currently, bioprosthetic heart valves are crosslinked with glutaraldehyde to prevent tissue degradation and to reduce tissue antigenicity. Glutaraldehyde forms stable crosslinks with collagen via a Schiff base reaction of the aldehyde with an amine group of the hydroxylysine/lysine in collagen. However, within a decade of implantation, 20-30% of these bioprostheses will become dysfunctional and over 50% will fail due to degeneration within 12-15 years post-operatively. Gylcosaminoglycans, a major constituent of valvular tissue, play an important role in maintaining a hydrated environment necessary for absorbing compressive loads, modulating shear stresses, and resisting tissue buckling. One of the disadvantages of glutaraldehyde crosslinking is its incomplete stabilization of GAGs, which lack the amine functionalities necessary for fixation by aldehyde addition. Previous studies have reported a greater depth of buckling in glutaraldehyde crosslinked aortic valves, one of the major causes of failure in these bioprostheses. Buckling occurs at sites of sharp bending, producing large stresses that can eventually lead to mechanical fatigue and consequent valvular degeneration. Local structural collapse occurs at these areas of tissue buckling to minimize compressive stresses, which subsequently causes a reduction in tissue length. Previous studies have reported the loss of GAGs in glutaraldehyde crosslinked porcine cusps during fixation, storage, in vitro fatigue experimentation, and in vivo subdermal implantation due to enzyme-mediated GAG degradation. Additionally, GAG loss has been observed in failed porcine bioprosthetic heart valves following clinical use. Therefore, to evaluate the potential role of GAGs in reduction of buckling in bioprosthetic heart valves, we used two 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) based crosslinking chemistries that link GAG carboxyl groups to the amine groups of proteins. Neomycin trisulfate, a hyaluronidase inhibitor, was employed to effectively stabilize the GAGs and subsequently prevent its enzymatic degradation. Previously, stabilization of valvular GAGs using neomycin trisulfate, a GAG-enzyme inhibitor, coupled with carbodiimide fixation chemistry was found to resist in vitro and in vivo enzymatic degradation of GAGs. Thus, using the above-mentioned GAG-targeted fixation strategies, we demonstrate that the retention of valvular GAGs reduces the extent of buckling in bioprosthetic heart valves, which may subsequently improve the durability of these bioprostheses
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