Measurements of steady flow through a bileaflet mechanical heart valve using stereoscopic PIV

Computational modeling of bileaflet mechanical heart valve (BiMHV) flow requires experimentally validated datasets and improved knowledge of BiMHV fluid mechanics. In this study, flow was studied downstream of a model BiMHV in an axisymmetric aortic sinus using stereoscopic particle image velocimetry. The inlet flow was steady and the Reynolds number based on the aortic diameter was 7600. Results showed the out-of-plane velocity was of similar magnitude as the transverse velocity. Although additional studies are needed for confirmation, analysis of the out-of-plane velocity showed the possible presence of a four-cell streamwise vortex structure in the mean velocity field. Spatial data for all six Reynolds stress components were obtained. Reynolds normal stress profiles revealed similarities between the central jet and free jets. These findings are important to BiMHV flow modeling, though clinical relevance is limited due to the idealized conditions chosen. To this end, the dataset is publicly available for CFD validation purposes.     

Transient, three-dimensional flow field simulation through a mechanical, trileaflet heart valve prosthesis

Thromboembolic complications are one of the major challenges faced by designers and researchers in development of artificial organs with blood-contacting devices such as heart valve prostheses, especially mechanical valves. Besides increasing the thrombogenic potential, these valves change the hydrodynamic performance of the heart. In this study, the flow through a trileaflet, mechanical heart valve prosthesis was modeled with transient computational fluid dynamics to analyze flow patterns causing thrombus formations on valves. The valve was simulated under conditions of a test rig (THIA II), which was specially designed to analyze different valves with respect to thrombosis. The main goal of this study was to mimic the exact conditions of the test rig to be able to compare numerical and experimental results. The boundary conditions were obtained from experimental data as leaflet kinematics and pressure profiles. One complete cycle of the valve was simulated. Numerical flow and pressure results were analyzed and compared with experimental results. Shear stress and shear rates were determined with respect to thrombogenic potential, especially in the pivot regions, which seem to be the main influence for activation and deposition of thrombocytes. Approximately 0.7% of the blood volume moving through the fluid domain of the valve was exposed to shear rates high enough to cause platelet activation. However, shear rates of up to 20,000 s?1 occurred in pivot regions. The pressure differences between the simulation and experimental data were approximately 2.5% during systole and increased up to 25% during diastole. The presented method, however, can be used to gain more information about the flow through different heart valve prostheses and, thus, improve the development process.     

Computer graphics analysis of stresses in blood flow through a prosthetic heart valve

Fluid dynamics principles and numerical analysis techniques are applied to the study of stress distribution in blood caused by the motion of the occluder in a prosthetic heart valve. An interactive computer graphics program is developed for the simulation of the flow process and the pictoral presentation of the solution for analysis. Resulting graphics displays show the stress distribution and other flow parameters which describe the movement of a disc occluder from fullclosed position to an almost full-open position. The possible contributions of this study to the understanding of hemolysis and thrombosis associated with prosthetic heart valves are discussed.     

CFD investigation of steady flow in bi-leaflet heart valve hinge

This paper focuses on local flow patterns inside the hinge socket of a bi-leaflet mechanical heart valve (MHV), where experimental measurements are difficult due to the extremely small flow region of about 40 microm. The overall objective of this study is to simulate the steady flow in this confined micro channel within the hinge region of a partially protruded ball hinge concept. A CFD simulation of flow through a bi-leaflet heart valve hinge was carried out. Steady flow with the valve leaflet in the fully open position during the valve systole phase was studied using FLUENT 4.4.7 running on Silicon Graphics. Body Fitted Coordinates (BFC) grid distribution was applied in the overall flow domain and great care was taken at the mesh distribution within the hinge local area. The flow study focused on local flow patterns inside the hinge socket of the valve where experimental measurements in the actual size valve are not practical. CFD results show evidence of flow in local area of hinge and no evidence of stagnation. Flow migrates across the clearance, and small vortices are formed after the hinge stoppers. The results indicate that flow in the hinge region is complex and critical for the valve to function effectively.     

An experimental study of steady flow patterns of a new trileaflet mechanical aortic valve

Hemodynamic research shows that thrombosis formation is closely tied to flow field turbulent stress. Design limitations cause flow separation at leaflet edges and the annular valve base, vortex mixing downstream, and high turbulent shear stress. The trileaflet design opens like a physiologic valve with central flow. Leaflet curvature approximates a completely circular orifice, maximizing effective flow area of the open valve. Semicircular aortic sinuses downstream of the valve allow vortex formation to help leaflet closure. The new trileaflet design was hemodynamically evaluated via digital particle image velocimetry and laser-Doppler anemometry. Measurements were made during peak flow of the fully open valve, immediately downstream of the valve, and compared with the 27-mm St. Jude Medical (SJM) bileaflet valve. The trileaflet valve central flow produces sufficient pressure to inhibit separation shear layers. Absence of downstream turbulent wake eddies indicates smooth, physiologic blood flow. In contrast, SJM produces strong turbulence because of unsteady separated shear layers where the jet flow meets the aortic sinus wall, resulting in higher turbulent shear stresses detrimental to blood cells. The trileaflet valve simulates the physiologic valve better than previous designs, produces smoother flow, and allows large scale recirculation in the aortic sinuses to help valve closure.     

Development of squeeze flow in mechanical heart valve: a particle image velocimetry investigation

Fluid between the reducing flow channel of the valve occluder and the orifice wall tends to be squeezed out of the flow channel, causing a high-speed flow. The squeeze flow is accompanied by a sharp local pressure drop, which may result in potential cavitation phenomenon in a mechanical heart valve (MHV). Limited experimental investigation has been conducted into the flow physics of this squeeze flow phenomenon, which is likely to be the origin of MHV cavitation. We used a pulsatile test loop simulating physiologic flow conditions and an actual-size transparent MHV model for flow visualization. A digital particle image velocimetry (DPIV) system incorporated with a microscope was applied to observe flow within a narrowing channel. A triggering mechanism was designed so that the DPIV system could be timed to capture images when the valve occluder was near its closing position. A series of images within the channel from 1.4 to 0.1 mm were captured. As the gap between the tip of the valve occluder and orifice wall becomes narrower, evidence of high-speed jet flow becomes more apparent. When the flow channel is reduced to around 0.1 mm, flow velocity of up to 2 m/s was noted. A sudden increase in high-speed jet flow causes a corresponding reduction in local pressure, and is a likely source for potential cavitation.     

In vitro comparison of steady and pulsatile flow characteristics of jellyfish heart valve

In examining the hydrodynamic performance of artificial heart valves in vitro, experiments are carried out under either steady or pulsatile flow conditions. Steady flow experiments are simple to set up and analysis of the data is also simple; however, their validity and accuracy have been questioned. In this study, the flow characteristics of jellyfish valves are evaluated and analyzed for steady and pulsatile flow conditions. The analysis is given in terms of velocity and shear stress distributions for a cardiac flow rate of 4.5l/min, and the corresponding steady flow rate is measured at two locations, 0.5D and 1D downstream of the valve face (D being the diameter of the pipe). At the 0.5D location, the velocity profile results obtained for both flow conditions indicated that jetting flow occurred close to the wall, and flow reversal as well as stagnation zones occurred in the core of the valve chamber. These phenomena were also evident in the shear stress profiles for both pulsatile and steady flow conditions. At this location, the maximum difference between the steady and pulsatile values of peak velocity is about 18%. However, the maximum difference between the peak shear stresses was in the range of 5%–7%. At the 1D location, the flow characteristics observed under both the pulsatile and steady flow conditions were almost identical, with a maximum difference between the peak values of less than 4%. From the data presented here, it can be stated that, at least in the initial optimization of the valve hemodynamic performance, the steady hydrodynamic evaluation of the valve could be an effective tool for analyzing the flow characteristics.     

Mathematical mdoel for turbulent blood flow through a disk-type prosthetic heart valve

Computational fluid dynamic techniques are used to construct a mathematical model for turbulent blood flow through a disk-type prosthetic heart valve in the aortic position. The TEACH computer code is used to solve the k-6 turbulence model numerically over the axisymmetric flow field of the valve during systole. Stream function, mean axial velocity profiles, turbulent shear stresses and wall shear stress distributions are computed for Reynolds numbers between Re_D=600 and 10 000 (corresponding to steady flow rates of 2·63 lmin⁻¹ and 43·89lmin⁻¹, respectively). The location, length and maximum reverse flow velocities of separated, flow regions are presented and compared with experimental observations. The largest computed mean axial velocities are 4·4 to 4·8 times the inflow velocity and occur near the downstream corner of the sewing ring. The maximum wall shear stress computed is 229·7 Nm⁻² at the upstream corner of the disk occluder for Re_D=10000. The location of maximum walls shear stress occurs at the downstream corner of the sewing ring for Re_D≤2000. Turbulent shear stresses of up to 380·7 Nm⁻² are computed in the region between the sewing ring and the disk occluder for the physiological Reynolds number Re_D=6054. The numerical solutions are shown to compare favourably with available experimental measurements.     

Numerical dye washout method as a tool for characterizing the heart valve flow: study of three standard mechanical heart valves

To date, no ideal heart valve prosthesis for the replacement of a diseased natural valve or for use in ventricular assist devices exists. Valves still cause thromboembolic complications originating from thrombus formations in the valve's stagnant and recirculation zones. Optimization of valve design requires detailed flow field investigations. Usually, the regions that are more prone to thrombus formation can be estimated using a dye washout experiment. This successful experimental method was simulated using numerical methods. The proposed method was applied to three standard mechanical heart valves--Bj?rk-Shiley, St-Jude, and Starr-Edwards valve. The dye washout was characterized by a time course of the gray value averaged over a defined region of interest. Finally, these curves were quantified by a half dye time (HDT), which characterizes the blood residence time. The HDT in the best valve, the Starr-Edwards valve, was 0.0747 s. The HDT in the worst valve, the Bj?rk-Shiley, was 0.0942 s. The analysis of the hemodynamic valve parameters (pressure drop, velocity magnitudes and turbulence) revealed that the best valve is the St-Jude valve. The Starr-Edwards valve displayed the worst hemodynamic parameters. This study shows that the proposed numerical method of dye washout visualization can be used as an additional tool for the flow characterization.     

Bileaflet mechanical heart valve closing sounds: in vitro classification by phonocardiographic analysis

Bileaflet mechanical heart valves, which exhibit hemodynamic performance fairly similar to that of native valves, can be investigated by the analysis of their closing sounds. Signal spectra calculated from the closing sounds are characterized by specific features that are suitable for the functional evaluation of the valves. Five commercial bileaflet mechanical heart valves were studied under different conditions that were simulated in vitro using a Sheffield pulse duplicator for the aortic position. The closing sounds were acquired by means of a phonocardiographic apparatus, analyzed by a specifically implemented algorithm, and were statistically compared. This article was aimed at classifying the investigated valves on the basis of their signal spectra: different profiles were identified, depending on the working conditions; moreover, closing sound reproducibility and intensity allowed the ranking of valve performances with respect to the “noise” produced by valve closure. In particular, results demonstrated which valves were characterized by the lowest noise (i.e., the Medtronic Advantage and St. Jude Regent valves) and which were characterized by the highest reproducibility (OnX, Medtronic Advantage, and St. Jude Regent valves) under the examined experimental conditions.     

Observation of cavitation in a mechanical heart valve in a total artificial heart

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures occurring in implanted mechanical heart valves. The cause of cavitation in mechanical heart valves was investigated using the 25 mm Medtronic Hall valve and the 23 mm Omnicarbon valve. Closing of these valves in the mitral position was simulated in an electrohydraulic totally artificial heart. Tests were conducted under physiologic pressures at heart rates from 60 to 100 beats per minute with cardiac outputs from 4.8 to 7.7 L/min. The disk closing motion was measured by a laser displacement sensor. A high-speed video camera was used to observe the cavitation bubbles in the mechanical heart valves. The maximum closing velocity of the Omnicarbon valve was faster than that of the Medtronic Hall valve. In both valves, the closing velocity of the leaflet, used as the cavitation threshold, was approximately 1.3-1.5 m/s. In the case of the Medtronic Hall valve, cavitation bubbles were generated by the squeeze flow and by the effects of the venturi and the water hammer. With the Omnicarbon valve, the cavitation bubbles were generated by the squeeze flow and the water hammer. The mechanism leading to the development of cavitation bubbles depended on the valve closing velocity and the valve stop geometry. Most of the cavitation bubbles were observed around the valve stop and were generated by the squeeze flow.     

Closing behavior of the mechanical heart valve in a total artificial heart

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures occurring in implanted mechanical heart valves. The cause of cavitation in mechanical heart valve was investigated in both 25-mm Björk–Shiley and 25-mm Medtronic Hall valves. The closing events of these valves in the mitral position were simulated in an electrohydraulic total artificial heart with a stroke volume of 85?ml. The tests were conducted under physiologic pressures at heart rates of 60, 70, 80, and 90 beats/min with cardiac outputs of 4.5, 5.5, 6.4, and 7.5?l/min, respectively. The disk closing behavior was measured by a laser displacement sensor. The closing behaviors were investigated under various atrial and aortic pressures. In both valves, the duration of closing decreased with an increase in the cardiac output. The greater the amount of atrial pressure, the shorter the closing duration of both valves. The maximum closing velocity of the Medtronic Hall monostrut valve ranged from 0.8 to 0.9?m/s, and that of the Björk–Shiley monostrut valve ranged from 0.73 to 0.78?m/s. In both valves, the maximum closing velocities were less than the reported cavitation thresholds. This suggests that there should be no possibility of occurrence of cavitation in an electrohydraulic total artificial heart with mechanical heart valve.     

Characteristics of pulsatile blood flow through the curved bileaflet mechanical heart valve installed in two different types of blood vessels: velocity and pressure of blood flow

The aim of this study was to investigate the flow fields of blood flowing through the curved bileaflet mechanical heart valve. A numerical analysis was carried out with the fluid-structure interaction between the blood flow and the motion of leaflets in two different types of blood vessels (type A, with sinus blood vessel, and type B, without sinus blood vessel). When the leaflet was fully opened, a fluttering phenomenon was detected in association with the blood flow, and recirculation flows were observed in the sinus region of the blood vessel for type A. During the closing phase, regurgitation was formed between the ring and the edge of the each leaflet for both types. When the leaflet came into contact with the valve ring at the end of the closing phase, rebound of the leaflet occurred. In consideration of the entire domain, the pressure drop occurs mainly in the valve region. The present results showed tendencies similar to those obtained by previous experiments for blood flow and contribute to the development of the curved bileaflet mechanical heart valve prostheses.     

Numerical study of turbulent blood flow through a caged-ball prosthetic heart valve using a boundary-fitted co-ordinate system

A numerical model is developed for steady turbulent flow through a fully open Starr-Edwards caged-ball prosthetic heart valve in the aortic position. An orthogonal boundary-fitted co-ordinate system is generated for the axisymmetric flow domain in the vicinity of the valve. The boundary lines follow the left ventricular wall, an idealised sinus, the aortic wall, and the ball occluder. The governing partial differentiation equations, written in a stream function-vorticity formulation, are recast into their curvilinear equivalents before being discretised into finite-difference equations. The equations are then solved iteratively. Regions of separated flow and elevated fluid stress are identified at several flow rates. Analysis of the numerical solutions reveals a simple power-law relationship between the computed turbulent shear stress and the steady flow rate at important flow field locations. The maximum turbulent shear stress occurs consistently near the sewing-ring tip. However, the peak turbulent shear stress in the sinus separation zone is observed to increase significantly with higher flow rates, exceeding values in many other regions. The numerical solutions compare satisfactorily with experimental measurements.     

脉动流下机械心脏瓣膜瓣阀开启状态的评价

背景：体外机械型人工心脏瓣膜(机械瓣)性能的评价涉及心输出量、反流量、有效瓣口面积、跨瓣压差,以及应力场、流场和成穴现象等。 目的：对3种机械瓣的瓣阀开启状态进行可视性观察和评价。 方法：用脉动流模拟循环装置系统,维持系统整个状态不变,在模拟心搏出量4 L/min、模拟心率75次/min和收缩时间占其循环周期46.2%的条件下,分别将久灵双叶瓣、Carbomedics双叶瓣和C-L侧倾碟瓣置于主动脉瓣位,将高速摄像机置于模拟循环装置动脉腔的正上方,观察10个连续模拟心动周期中瓣阀开启状态。利用自编图像处理软件包,捕获瓣阀开启角度最大的1幅图像,作为计算该只瓣膜在1个心动周期中最大开放面积和开启角度的基准。 结果与结论：脉动流下,25 mm CarboMedics瓣、25 mm和23 mm久灵双叶瓣在开放到最大位时,可见瓣阀抖动现象,27 mm C-L侧倾碟瓣未见瓣阀抖动。用不同的计算方法测量上述瓣膜的瓣口面积显示,由厂家提供的瓣口实际面积最大,用Green公式计算的瓣膜开放面积次之,用Gorin公式计算的有效瓣口面积最小。根据三角形定理计算的瓣阀开放角度,久灵双叶瓣和CarboMedics瓣的两个瓣阀的开放角度不一致,并均小于瓣膜固有的开放角度;C-L侧倾碟瓣的开放角度也未达其固有的开放角度。提示机械型人工心脏瓣膜双叶瓣的瓣阀开放不同步,瓣阀有抖动现象;瓣阀在脉动周期中呈不完全性开启。     

Measurements of steady turbulent flow through a rigid simulated collapsed tube

Axial and transverse components of liquid velocity are measured by laser Doppler anemometer in a perspex tube that has been deformed at one point to resemble the shape of the throat of a partially collapsed flexible tube, conveying fluid while being compressed externally. The Reynolds number is 5900. The flow downstream of the throat consists of two side-jets with reverse flow extending all across the cross-section between them. The jets spread out around the central retrograde-flow zone, initially forming crescents of high-speed forward flow and then, at three diameters downstream, an almost complete annulus of forward flow around a central zone of lower-speed but now forward flow. Comparison is made between the features of this turbulent flow and those of a previously investigated laminar flow through the same geometry. In both, retrograde flow ceases between two and three diameters downstream of the centre of the throat. However, the laminar flow is annular at three diameters downstream, whereas here the jets remain influential at that station. The maximum normalised turbulence intensity exceeds 1.35.     

Axial flow pump support in a Marfan patient with an aortic mechanical heart valve: case report and literature review

The presence of a mechanical heart valve in the aortic position is usually considered a contraindication for the use of cardiac assist devices. Only a few cases with the combination of mechanical circulatory support and valve prostheses have been reported in the literature to date, and the experience is even more limited in the new generation of miniaturized axial flow pumps. We present a case report of a patient with a mechanical aortic heart valve who was successfully supported with a continuous flow pump and discuss the literature available on this problem. Further on, the patient was weaned from his ventricular assist device after 456 days of support.     

Flow analysis of the bileaflet mechanical prosthetic heart valves using laser Doppler anemometer: Effect of the valve designs and installed orientations to the flow inside the simulated left ventricle

Ever since the first introduction of the ball-type valve by Hufnagel in 1952, which was installed in the descending aorta to correct aortic valve insufficiency, great efforts have been aimed to produce a hemodynamically and structurally superior prosthetic heart valve. Bileaflet valves, commercially initiated by the St. Jude medical (SJM) valve, perform satisfactorily, and now the majority of the mechanical-type prosthetic heart valves used clinically are of this type. The recent trend in bileaflet valve design seems to be concentrated on the hinge mechanism and leaflet design to improve performance against thromboembolic complications and hemolysis. This paper studied the effects of hinge location, leaflet configuration, valve opening angle, and valve installed orientation to the flow field inside the simulated ventricle using laser Doppler anemometry. As a model prosthetic valve, the SJM valve was selected as a reference, and newer bileaflet valves, including the ATS, the Carbomedics (CM), and the Jyros (JR) valves, were selected for comparison. The test program also utilized a flow visualization technique to map the velocity field inside the simulated ventricle to complement the information obtained using the LDA system. Comparison of the velocity profiles at corresponding flow phases revealed the effects of the differences in valve design and orientation. Based on precise examination of the data, the following general conclusions can be made: all valves (SJM, ATS, CM, and JR) show distinct circulatory flow patterns when the valve is installed in the antianatomical orientation. The small differences in hinge location and leaflet configuration can generate noticeable differences, particularly during the accelerating flow phase of the valve. The ATS and the CM valves open less during the forward flow phase, and this results in generally diverse and less distinct flow patterns and slower velocity. This is particularly noticeable for the flow through the central orifice. The SJM valve maintains a relatively higher velocity through the central orifice. The curved leaflet JR valve generates higher but divergent flow during the accelerating and peak flow phases.     

Turbulence characteristics downstream of a new trileaflet mechanical heart valve

Design limitations of current mechanical heart valves cause blood flow to separate at the leaflet edges and annular valve base, forming downstream vortex mixing and high turbulent shear stresses. The closing behavior of a bileaflet valve is associated with reverse flow and may lead to cavitation phenomenon. The new trileaflet (TRI) design opens similar to a physiologic valve with central flow and closes primarily due to the vortices in the aortic sinus. In this study, we measured the St. Jude Medical 27 mm and the TRI 27 mm valves in the aortic position of a pulsatile circulatory mock loop under physiologic conditions with digital particle image velocimetry (DPIV). Our results showed the major principal Reynolds shear stresses were <100 N/m2 for both valves, and turbulent viscous shear stresses were smaller than 15 N/m2. The TRI valve closed more slowly than the St. Jude Medical valve. As the magnitudes of the shear stresses were similar, the closing velocity of the valves should be considered as an important factor and might reduce the risks of thrombosis and thromboembolism.