Hemodynamics of aortic valve stenoses

The aortic valve of a human heart is located between the left ventricle and the aorta. Its function is to open during the systole, allowing the blood to flow to the aorta in order to feed the organs and close during the diastole, preventing blood flow back into the left ventricle. Disease can lead to dysfunctionalities of the aortic valve and affect its performance. Calcific aortic valve disease (CAVD) is one of the most common valvular heart diseases which causes malfunctioning of the aortic valve leading to stroke, aortic aneurysm, heart attack and failure over time. Over the past few decades, researchers have investigated the effects of stenosis of the aortic valve on its hemodynamics and performance using in-vitro and in-vivo experiments, as well as numerical models. In-vitro experimentation and in-vivo measurements are broadly used for the investigation of the effect of aortic valve stenosis on the hemodynamics in the aortic root and the performance of the aortic valve. Numerical methods also play an important role in the modelling of the stenosis of the aortic valve and have obtained considerable attention in biomechanics application due to their very cost effective nature. Significant developments have been made in modelling the stenosis of the aortic valve and the effects on its hemodynamics and malfunctioning, however, the influence of vortex structures in the sinus on the aortic root and the coronary artery hemodynamics, as well as the performance of the valve and its correlation with the development of CAVD, are still unknown. The aim of this thesis is to develop an understanding of the flow behaviour inside the sinus cavity of the aortic valve in order to predict the shear stress distribution on the aortic valve leaflets and its correlation with CAVD. This aim has been achieved by answering the following research questions: (i) how stenosis of the aortic valve affects the aortic root and coronary artery hemodynamics; (ii) how the geometrical parameters of the aortic valve, such as the locations of the coronary artery ostia and the shape of the sinuses, influences the sinus hemodynamics, especially vortex structures within the sinuses; (iii) what are the effects of coronary artery stenosis on the shear stress distribution on the aortic valve leaflets; (iv) how vortex structures in the sinuses can change the wall shear stress distribution on the leaflets and its correlation with CAVD. In order to answer all of the above-mentioned questions and achieve the main objective of the project, the following tasks have been defined: • A brief overview of cardiovascular heart diseases with a focus on valvular heart diseases, and various techniques frequently used for diagnostics and treatment of stenosis of the aortic valve has been provided. • A comprehensive review of different techniques used for modelling aortic valve stenosis has been provided with a focus on the numerical Fluid-Structure Interaction (FSI) method and its advantages in modelling. • Two dimensional models of a healthy and a stenosed aortic valve have been extracted from the 2D echocardiography images available in the literature and developed in ANSYS- Fluent software. • Dynamic motion simulation of the aortic valve leaflets have been used to investigate the effects of stenosis of the aortic valve on the sinus vortex structures, coronary artery hemodynamics, wall shear stress on the leaflets and its correlation with CAVD. • A unique test rig has been designed, fabricated, and used for validation of the transvalvular pressure gradient and flow rate profile of the aortic valve. The test rig experimentally replicates the left ventricle of the heart and is capable of producing the heart beat flow conditions of different patients. The flow rate profile of the aortic valve and the pressure difference through the aortic valve are measured and compared with the simulated results. The developed model is capable of mimicking the dynamic motion of the aortic valve leaflets and predicting the wall shear stress distribution on the leaflets, which is associated with CAVD. The most important findings of the project showed that the wall shear stress distribution on the leaflets is highly dependent on the geometrical parameters of the aortic valve, such as the locations of the coronary artery ostia, as well as the shape of the sinuses. For example, an aortic valve with proximal coronary artery ostia experiences lower ranges of wall shear stress distribution on the leaflets. This means that a healthy valve with proximal coronary artery ostia are more prone to calcification over time in comparison with a healthy valve with distal and middle coronary artery ostia. Furthermore, the results demonstrated that a severely calcified aortic valve exhibits a lower range of wall shear stress distribution on the leaflets with higher probability of having smaller wall shear stress on the leaflets compared to a healthy valveThesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

Fluid Dynamics Modeling and Sound Analysis of a Bileaflet Mechanical Heart Valve

Cardiovascular disease (CVD) is one of the main causes of death in the world. Some CVD involve severe heart valve disease that require valve replacement. There are more than 300,000 heart valves implanted worldwide, and about 85,000 heart valve replacements in the US. Approximately half of these valves are mechanical. Artificial valves may dysfunction leading to adverse hemodynamic conditions. Understanding the normal and abnormal valve function is important as it help improve valve designs. Modeling of heart valve hemodynamics using computational fluid dynamics (CFD) provides a comprehensive analysis of flow, which can potentially help explain clinical observations and support therapeutic decision-making. This detailed information might not be accessible with in-vivo measurements. On the other hand, finite element analysis (FEA), is an efficient way to analyze the interactions of blood flow with blood vessel and tissue layers. In this project both CFD and FEA simulations were performed to investigate the flow-induced sound generation and propagation of sound waves through a tissue-like material. This method is based on mapping the transient pressure (force) fluctuations on the vessel wall and solving for the structural vibrations in the frequency domain. These vibrations would then be detected as sound on the epidermal surface. Advantages of the methods used in the current study include: (a) capability of providing accurate solution with a faster solution time; (b) inclusion of the fluid–structure interaction between blood flow and the arterial wall; and (c) accurately capturing some of the spectral features of the velocity fluctuation measured over the epidermal surface

Characterization of disturbed hemodynamics due to stenosed aortic jets with a Lagrangian Coherent structures technique

Isfahan University of Technology. The aortic valve is located at left ventricular outlet and is exposed to the highest pressure in the cardiovascular system. Problems associated with the valve leaflet movement can cause complications for the heart. Specifically, aortic stenosis (AS) arises when aortic leaflets do not efficiently open. In the present study, Lagrangian Coherent Structures (LCSs) were utilized by processing a variety of Computational Fluid Dynamics (CFD) models velocity vector data further to identify the characteristics of AS jets. Particularly, effective orifice areas (EOA) for different cases were accurately identified from unstable manifolds of finite time Lyapunov exponent (FTLE) fields. Calcified leaflets were modeled by setting the leaflet's Young modulus to 10 MPa and 20 MPa for moderately and severely calcified leaflets respectively while a healthy leaflet's Young modulus was assigned to be 2 MPa. Increase in calcification degree of the leaflet caused destruction of the vortex structures near the fibrosa layer of the leaflet indicating a malfunctioning for the movement mechanism of the leaflet. Furthermore, when we analyzed stable manifolds, we identified a blockage region at the flow upstream due to the stagnant blood here. Compared to a healthy case, for the calcified valve, this blockage region was enlarged, implying an increase in AS jet velocity and wall shear stress on leaflets. As a conclusion, results from the present study indicate that aortic leaflet malfunctioning could be accurately evaluated when LCS technique was employed by post processing velocity vector data from CFD. Such precise analysis is not possible using the Eulerian CFD approach or a Doppler echocardiography since these methods are based on only analyzing instantaneous flow quantities and they overlook fluid flow characteristics of highly unsteady flows

Energetic and Hemodynamic Characteristics of Paravalvular Leak Following Transcatheter Aortic Valve Replacement

Transcatheter aortic valve replacement (TAVR) has emerged as an alternative treatment for inoperable and high risk patients with severe symptomatic aortic stenosis. TAVR short and medium term results are very promising, however paravalvular leak (PVL) post-TAVR still represents a significant complication. PVL post-TAVR is shown to be an independent predictor of short-term and long-term mortality. Despite, its importance and prevalence, with a wide range of reported incidences, only few studies addressed the PVL after TAVR. In the present study, first, the mathematical lumped parameter model is used to model the simplified circulatory system in presence of PVL and to evaluate the performance of TAVR by computing the variation of the left ventricle stroke work (LVSW) under several pre-TAVR and post-TAVR conditions. Results show that in a large majority of cases, TAVR significantly reduced LVSW. However, in cases with pre-existing aortic stenosis conditions with trace/mild aortic regurgitation, it did not significantly reduce LVSW or even led to an increase. Second, a three-dimensional (3D) computational fluid dynamics (CFD) simulation is performed in order to investigate the effect of PVL on the diastolic flow-field characteristics post-TAVR. Results show that PVL leads to significant disturbances in blood flow, which characterized by high speed jets, coherent structures and markedly elevated shear stress on both sides of the implanted aortic leaflets, which could promote a more rapid degeneration of the valve leaflets. Results could be useful in understating the hemodynamics of PVL post-TAVR and estimating some important parameters, which could not be obtained during the medical assessment (e.g. wall shear stress). Also, they could be a help in the process of choosing the appropriate valve for TAVR procedure, based on comparing the pre and post TAVR different scenarios

Immersed boundary method for cavitating and biological flows

The aim of the present work is the development of a computational tool to ease the numerical simulation of cavitating flows in domains of complex topology or with arbitrary moving boundaries. Within the framework of Computational Fluid Dynamics(CFD), an Immersed Boundary (IB) Method has been developed. According to the IB methodology, the grid that discretises the computational domain does not need to conform to the geometry and the solid boundaries are modelled on a fixed canonical grid by alternations of the governing equations in their vicinity. This modelling strategy is beneficial in terms of both computational cost and numerical solution. The grid generation, which is a complex and time consuming process, is simplified as a regular canonical grid, non-conformal to the boundaries, can be used. In addition, when moving boundaries are present, a conformal grid would need to adapt or deform following the motion of boundaries, which would increase the computational cost of the simulations in the first case and affect the solution in the latter case; the use of IB method alleviates these issues. The developed method follows the direct-forcing approach, which simply adds to the governing equations a source term to account for the body force acting on the fluid. The simplicity of the method makes it suitable for complex flow regimes, including phase change, strong shocks and compressibility effects, as well as Fluid Structure Interaction (FSI). Since cavitation dynamics regard a wide range of applications of engineering interest, from hydraulic machines to novel therapeutic techniques, the method is designed to be applicable in a wide range of flow regimes. Turbulent modelling and flow induced motion has been taken into account. The method has been successfully applied to cavitating and incompressible cases where conventional techniques are not easily or at all applicable. The shock-wave interaction with material interfaces is studied via the high-speed impact of a solid projectile on a water jet, which has been studied only experimentally before and only qualitative observations existed. The numerical investigation with the proposed methodology unveiled rich information regarding the physics of the impact, the resulting shock formation, cavitation development and interface instabilities initiation. Moreover, the methodology was applied on the thoroughly studied pulsatile flow through a bi leaflet Mechanical Heart Valve, to provide additional information regarding shear stress development. The methodology aids an experimental campaign employing novel shear stress measuring techniques, carried out by our collaborators. The research work and the developed method described in the present Thesis, intend to set the foundations for more elaborate numerical investigations of highly complex problems of Fluid Dynamics

Application of passive flow control to mitigate the thromboembolic potential of bileaflet mechanical heart valves: an in-vitro study

2014 Summer.Implantation of a bileaflet mechanical heart valve (BMHV) continues to be associated with risk of thromboembolic complications despite anti-coagulation therapy. Mechanical heart valves have been the gold standard in valve heart replacement since the 1950s with BMHVs currently still being the valve of choice for younger patients. Given that a large body of literature points to thromboembolic complications due to poor hemodynamics, improvements to the hemodynamic performance of BMHVs are needed. In this study, we explore the concept of passive flow controls that have been widely used in aerospace industry as a novel approach towards improving BMHV design. Passive flow control elements are small features on solid surfaces, such as vortex generators (VGs), that alter flow to achieve desired performance. The specific aims of this study are (1) develop a methodology to evaluate thromboembolic potential (TEP) of BMHVs using in-vitro particle image velocimetry technique, (2) quantify the efficacy of rectangular VGs distributed on BMHV leaflets to reduce TEP, and (3) quantify the hemodynamic performance impact of rectangular VGs. An in-vitro pulsatile flow loop along with Particle Image Velocimetry (PIV) flow visualization technique was developed, validated, and utilized to acquire time-resolved velocity fields and shear stress loading: Lagrangian particle tracking analysis of the upstream and downstream flow during diastole and systole enabled the calculation of predicted shear stress history and exposure times corresponding to platelets. This information was then used in numerical models of blood damage to predict the TEP of test heart valves using established platelet activation and platelet lysis parameters. BMHV leaflets were constructed using 3D printing technology with VGs based on micro-CT scans of a model BMHV leaflet. Two configurations were constructed: co-rotating VGs and counter-rotating VGs. Co-rotating VGs consist of single features 1mm tall and 2.8mm long spaced equally apart (5mm) at an angle of attack of 23 degrees. Counter-rotating VGs consist of mirrored feature pairs 1mm from each other with the same dimensions as the co-rotating VGs. The leaflets were tested using the methodology described above to elucidate their effect on the TEP of the BMHV compared to the control leaflets. For systolic flow downstream of the valve, we report a decrease in the average platelet activation and average platelet lysis TEP (both normalized by the average exposure time) largely in the central jet, with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. However, for diastolic flow upstream of the valve, we report an increase in the average platelet lysis TEP and average platelet activation TEP (both normalized by the average exposure time) largely in the regurgitant jet zone with the vortex generator equipped leaflets compared to the control leaflets at a p-value of 0.05. Also, steady and pulsatile flow experiments were conducted to calculate the transvalvular pressure drop across the model BMHV with control leaflets (no VGs) and leaflets containing VGs to calculate effective orifice area (EOA), which is an index of valve performance and is related to the degree to which the valve obstructs blood flow. We report a significant increase in EOA values for valves with leaflets containing passive flow control elements in both steady and pulsatile flow experiments compared to the control leaflets. Under steady flow, the co-rotating VGs configuration had the best EOA value compared to the control leaflet and counter-rotating vortex generator configuration. However, under pulsatile conditions, the counter-rotating VGs configuration had the best EOA value compared to the control leaflet and co-rotating vortex generator configuration. PIV measurements highlight the delay in flow separation caused by the VGs and corroborate the increased pulsatile flow EOA values. This study shows that the TEP of BMHVs can be accurately evaluated using in-vitro PIV techniques and that there is room for improvement in BMHV design using passive flow control elements. With optimization of passive flow control configuration and design, it is possible to further decrease the TEP of BMHVs while increasing their hemodynamic performance; thus creating a safer, more efficient BMHV

Development of a prosthetic heart valve with inbuilt sensing technology, to aid in continuous monitoring of function under various stenotic conditions

In spite of technological advances in the design of prosthetic heart valves, they are still often subject to complications after implantation. One of the common complications is valve stenosis, which involves the obstruction of the valve orifice caused by biological processes. The greatest challenge in diagnosing the development of valve failure and complications is related to the fact that the valve is implanted and isolated. To continuously monitor the state of the valve and its performance would be of great benefit but practically can only be achieved by instrumenting the implanted valve. In this thesis, we explore the development of a prosthetic valve with inbuilt sensing technology to aid in continuous monitoring of valve function under various stenotic conditions. 22mm polyurethane valves were designed via dipcoating. A custom made mock circulatory system was designed and hydrodynamic testing of the polyurethane valves under different flow rates were performed with Effective orifice area (EOA) and Transvalvular Pressure Gradient (TVPG) being the parameters of interest. Valves were subjected to varying levels of obstruction to investigate the effect obstruction has on the pressure gradient across the valves. Similar tests were performed on a Carpentier Edwards SAV 2650 model bioprosthetic valve for comparison. Polyurethane valves were then instrumented with strain gauges to measure peak to peak strain difference, in response to varying levels of obstructions. All the polyurethane valves exhibited good hydrodynamic performance with EOA (>1cm2) under baseline physiological conditions. It was also discovered that pressure difference across the valves was directly proportional to the flow rate. The pressure difference also demonstrated a slow increase during the initial stages of simulated stenosis and a sudden increase as the obstruction became severe. This provides further evidence to support the ideal that stenosis is a slow progressive disease which may not present symptoms until severe. The peak to peak strain differences also tend to decrease as the severity of the obstruction was increased. The peak to peak strain difference is indicative of the pressures within the valve (intravalvular pressure). The results suggest that directly monitoring the pressures within the valve could be a useful diagnostic tool for detecting valve stenosis. Future works involves miniaturisation of the sensors and also the incorporation of telemetry into the sensor design.In spite of technological advances in the design of prosthetic heart valves, they are still often subject to complications after implantation. One of the common complications is valve stenosis, which involves the obstruction of the valve orifice caused by biological processes. The greatest challenge in diagnosing the development of valve failure and complications is related to the fact that the valve is implanted and isolated. To continuously monitor the state of the valve and its performance would be of great benefit but practically can only be achieved by instrumenting the implanted valve. In this thesis, we explore the development of a prosthetic valve with inbuilt sensing technology to aid in continuous monitoring of valve function under various stenotic conditions. 22mm polyurethane valves were designed via dipcoating. A custom made mock circulatory system was designed and hydrodynamic testing of the polyurethane valves under different flow rates were performed with Effective orifice area (EOA) and Transvalvular Pressure Gradient (TVPG) being the parameters of interest. Valves were subjected to varying levels of obstruction to investigate the effect obstruction has on the pressure gradient across the valves. Similar tests were performed on a Carpentier Edwards SAV 2650 model bioprosthetic valve for comparison. Polyurethane valves were then instrumented with strain gauges to measure peak to peak strain difference, in response to varying levels of obstructions. All the polyurethane valves exhibited good hydrodynamic performance with EOA (>1cm2) under baseline physiological conditions. It was also discovered that pressure difference across the valves was directly proportional to the flow rate. The pressure difference also demonstrated a slow increase during the initial stages of simulated stenosis and a sudden increase as the obstruction became severe. This provides further evidence to support the ideal that stenosis is a slow progressive disease which may not present symptoms until severe. The peak to peak strain differences also tend to decrease as the severity of the obstruction was increased. The peak to peak strain difference is indicative of the pressures within the valve (intravalvular pressure). The results suggest that directly monitoring the pressures within the valve could be a useful diagnostic tool for detecting valve stenosis. Future works involves miniaturisation of the sensors and also the incorporation of telemetry into the sensor design

Hemodynamics of Native and Bioprosthetic Aortic Valves: Insights from a Reduced Degree-of-Freedom Model

Heart disease is the leading cause of deaths in the US with aortic valve (AV) diseases being major contributors. Valve replacement is the primary therapeutic indication for AV diseases and transcatheter aortic valve replacement (TAVR) provides a safe and minimally invasive option. However, post-TAVR patient outcomes show considerable variability with deployment parameters. TAVR valves are also susceptible to failure mechanisms like leaflet thrombosis which increase the risk for serious thromboembolic events. Early detection and intervention can avert such outcomes, but symptoms often manifest at advanced stages of valve failure. Continuous monitoring can facilitate early detection, but regulatory and technological challenges may hinder developing such technology through experimental or clinical means. Computer simulations enable unprecedented predictive capabilities which can help gain insights into the pathophysiology of valvular diseases, conduct in silico trials to design novel monitoring technologies and even guide surgeries for optimal valve deployment. However, accurate, yet efficient numerical models are required. This study describes the implementation of a versatile, efficient AV dynamics model in a previously developed fluid-structure interaction solver, and its application to each of these tasks. The model accelerates simulations by simplifying the constitutive parameter space and equations governing leaflet motion without compromising accuracy. It can simulate native and prosthetic valve dynamics exhibiting physiological and pathological function in idealized and personalized aorta anatomies. This computational framework is used to generate canonical and patient-specific simulation datasets describing hemodynamic differences secondary to healthy and pathological AVs. These differences help identify biomarkers which reliably predict the risk of valvular and vascular diseases. Changes in these biomarkers are used to assess whether TAVR can deter aortic disease progression. Next, statistical differences in such biomarkers recorded by virtual wearable or embedded sensor systems, between normal and abnormal AV function, are analyzed using data-driven methods to infer valve health. This lays the groundwork for inexpensive, at-home diagnostic technologies, based on digital auscultation and in situ embedded-sensor platforms. Finally, a simulation describing the deployment of a commercially available TAVR valve in a patient-specific aorta anatomy and the associated hemodynamics is presented. Such simulations empower clinicians to optimize TAVR deployment and, consequently, patient outcomes