Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle

Thrombogenesis and hemolysis have both been linked to the flow dynamics past heart valve prostheses. To learn more about the particular flow dynamics past mitral valve prostheses in the left ventricle under controlled experimental conditions, an in vitro study was performed. The experimental methods included velocity and turbulent shear stress measurements past caged-ball, tilting disc, bileaflet, and polyurethane trileaflet mitral valves in an acrylic rigid model of the left ventricle using laser Doppler anemometry. The results indicate that all four prosthetic heart valves studied create at least mildly disturbed flow fields. The effect of the left ventricular geometry on the flow development is to produce a stabilizing vortex which engulfs the entire left ventricular cavity, depending on the orientation of the valve. The measured turbulent shear stress magnitudes for all four valves did not exceed the reported value for hemolytic damage. However, the measured turbulent shear stresses were near or exceeded the critical shear stress reported in the literature for platelet lysis, a known precursor to thrombus formation.     

Two-component laser velocimeter measurements downstream of heart valve prostheses in pulsatile flow

Elevated turbulent shear stresses resulting from disturbed blood flow through prosthetic heart valves can cause damage to red blood cells and platelets. The purpose of this study was to measure the turbulent shear stresses occurring downstream of aortic prosthetic valves during in-vitro pulsatile flow. By matching the indices of refraction of the blood analog fluid and model aorta, correlated, simultaneous two-component laser velocimeter measurements of the axial and radial velocity components were made immediately downstream of two aortic prosthetic valves. Velocity data were ensemble averaged over 200 or more cycles for a 15-ms window opened at peak systolic flow. The systolic duration for cardiac flows of 8.4 L/min was 200 ms. Ensemble-averaged total shear stress levels of 2820 dynes/cm2 and 2070 dynes/cm2 were found downstream of a trileaflet valve and a tilting disk valve, respectively. These shear stress levels decreased with axial distance downstream much faster for the tilting disk valve than for the trileaflet valve.     

Turbulent shear stress measurements in the vicinity of aortic heart valve prostheses

A two dimensional laser Doppler anemometer system has been used to measure the turbulent shear fields in the immediate downstream vicinity of a variety of mechanical and bioprosthetic aortic heart valves. The measurements revealed that all the mechanical valves studied, created regions of elevated levels of turbulent shear stress during the major portion of systole. The tissue bioprostheses also created elevated levels of turbulence, but they were confined to narrow regions in the bulk of the flow field. The newer generation of bioprostheses create turbulent shear stresses which are considerably lower than those created by the older generation tissue valve designs. All the aortic valves studied (mechanical and tissue) create turbulent shear stress levels which are capable of causing sub-lethal and/or lethal damage to blood elements.     

Laser anemometry measurements of pulsatile flow past aortic valve prostheses

Experimental results are presented on physiological pulsatile flow past caged ball and tilting disc aortic valve prostheses mounted in an axisymmetric chamber incorporated in a mock circulatory system. The measurements of velocity profiles and turbulent normal stresses during several times in a cardiac cycle were obtained using laser-Doppler anemometry. Our results show that with increased angle of opening for the tilting disc valves, a large but locally confined vortex is observed along the wall in the minor flow region throughout most of the cardiac cycle. The turbulent normal stresses measured downstream to the tilting disc in the minor flow region parallel to the tilt axis were found to be larger than those measured downstream to the caged ball valves. Comparison of measurements with steady flow at flow rates comparable to peak pulsatile flow rate show that the turbulent normal stresses are larger by a factor of two in pulsatile flow with a frequency of 1.2 Hz.     

新型机械心脏瓣膜的研究

目前临床使用的各种机械心脏瓣膜的主要问题是血栓栓塞和与抗凝治疗有关的出血,其缺陷在于瓣膜开启时,碟片和支架将瓣膜的整个血流通道分隔成三至四个较小的血流通道。在这种受阻隔的血流通宫,形成容易诱发血栓的高剪应力区、紊流和滞流区。我们研制的两种机械心脏瓣膜在瓣膜开启时,没有任何支架和碟片分隔瓣膜的血流通道,使血流与天然心脏瓣膜中的相类似,可减少对血液的危害,从而可减少换瓣病人对抗凝治疗的依赖程度。     

Pulsatile flow past aortic valve bioprostheses in a model human aorta

Pulsatile flow development past tissue valve prostheses in a model human aorta has been studied using qualitative flow visualization and quantitative laser-Doppler techniques. Experiments were conducted both in steady and physiological pulsatile flow situations and the measurements included the pressure drop across the valve, the instantaneous flow rate as well as the velocity profiles and turbulent stresses downstream to the valves. Our study shows that the velocity profiles with pericardial valves are closer to those measured past natural aortic valves. The porcine valves with a smaller valve opening area produce a narrower and stronger jet downstream from the valve with relatively larger turbulent axial stresses in the boundary of the jet. Our study suggests that the pericardial valves with turbulent stresses comparable to those of caged ball and tilting disc valves are preferable from a hemodynamic point of view.     

In vitro flow dynamics of four prosthetic aortic valves: a comparative analysis

The velocity fields downstream of four prosthetic heart valves were mapped in vitro over the entire cross-section of a model aortic root using laser Doppler anemometry. THe Bj?rk-Shiley 60 degrees convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.     

A comparative computational study of blood flow through prosthetic heart valves using the finite element method

The steady flow of blood through three common types of prosthetic heart valves was simulated numerically using the finite element method. The velocity, pressure and stress fields were obtained for the disk-type, tilting-disk and ball-type prosthetic heart valves in aortic position, for increasing Reynolds numbers up to 900, 1500 and 2000 respectively. Computer graphics of calculated velocities are presented, showing in detail the accelerated flow, recirculation and stagnation areas developed in the prosthesis. Maximum wall shear stresses were found at 0.5, 1.4, and 1.2 diameters from the sewing ring downstream for the disk, tilting-disk and ball valves being the values of 55, 18 and 33 dyn cm-2 respectively. In the vicinity of the occluder, maximum shear stresses of 38, 30 and 47 dyn cm-2 respectively were computed. The flow characteristics and performance for each valve are compared, the results are presented in terms of energy loss and maximum shear stress. The velocity and stress fields are compared with in vitro evaluations found in the literature.     

Hydrodynamic in vitro comparison of a new artificial heart valve with the Bj?rk-Shiley valve (standard)

Measurements performed to compare a newly developed tilting disc valve with the Bj?rk-Shiley valve included velocity profiles downstream of the heart valves, valve-induced flow turbulence and pressure drop across the opened valves. The velocity profiles measured with pulsed Doppler ultrasound are similar, although they do not permit a quantitative comparison of the valves. The interpretation of the 90 degrees-component of Doppler signals as a measure of the turbulence permits a quantitative comparison without the need for extensive measurements. However, only large vortices are recorded, so that our turbulent shear stresses are lower than these reported in the literature. The pressure drop across the opened valve is a measure of the energy loss, and important parameters for the valve can be derived from it. The pressure drop is dependent on the test conditions, and is therefore not a characteristic constant of the valve. The transformation of the power law Q = C delta P beta into a relation between Re- and Eu-number gives a nondimensional similarity number that is characteristic for tilting disc valves. Its verification requires more investigations, involving variation of valve size and the viscosity of the test fluid.     

Steady flow development past valve prostheses in a model human aorta—II. Tilting disc valves

The flow development in the model human aorta with uniform entry as well as with centrally occuluding valves mounted at the root of the aorta was described in Part I of this two-paper sequence. Part II deals with the flow development in the model aorta with tilting disc valves mounted at the root of the aorta. Bjork-Shiley and Hall-Kaster tilting disc valves were mounted in three different orientations with respect to the root of the aorta. The velocity profiles and turbulent stresses were measured with laser-Doppler anemometry. Our results under steady flow conditions in the model human aorta show quantitatively that the flow development in the ascending aorta as well as in the brachio-cephalic artery are strongly dependent on the orientation of the tilting disc valves. With the valves tilting towards the outer wall of curvature, our results suggest a tendency for flow separation at the flow divider region of the brachio-cephalic artery.     

Influence of cardiac flow rate on turbulent shear stress from a prosthetic heart valve

Elevated turbulent shear stresses associated with sufficient exposure times are potentially damaging to blood constituents. Since these conditions can be induced by mechanical heart valves, the objectives of this study were to locate the maximum turbulent shear stress in both space and time and to determine how the maximum turbulent shear stress depends on the cardiac flow rate in a pulsatile flow downstream of a tilting disk valve. Two-component, simultaneous, correlated laser velocimeter measurements were recorded at four different axial locations and three different flow rates in a straight tube model of the aorta. All velocity data were ensemble averaged within a 15 ms time window located at approximately peak systolic flow over more than 300 cycles. Shear stresses as high as 992 dynes/cm2 were found 0.92 tube diameters downstream of the monostrut, disk valve. The maximum turbulent shear stress was found to scale with flow rate to the 0.72 power. A repeatable starting vortex was shed from the disk at the beginning of each cycle.     

PIV measurements of flow in a centrifugal blood pump: steady flow

Magnetically suspended left ventricular assist devices have only one moving part, the impeller. The impeller has absolutely no contact with any of the fixed parts, thus greatly reducing the regions of stagnant or high shear stress that surround a mechanical or fluid bearing. Measurements of the mean flow patterns as well as viscous and turbulent stresses were made in a shaft-driven prototype of a magnetically suspended centrifugal blood pump at several constant flow rates (3-9 L/min) using particle image velocimetry (PIV). The chosen range of flow rates is representative of the range over which the pump may operate while implanted. Measurements on a three-dimensional measurement grid within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser are reported. The measurements are used to identify regions of potential blood damage due to high shear stress and/or stagnation of the blood, both of which have been associated with blood damage within artificial heart valves and diaphragm-type pumps. Levels of turbulence intensity and Reynolds stresses that are comparable to those in artificial heart valves are reported. At the design flow rate (6 L/min), the flow is generally well behaved (no recirculation or stagnant flow) and stress levels are below levels that would be expected to contribute to hemolysis or thrombosis. The flow at both high (9 L/min) and low (3 L/min) flow rates introduces anomalies into the flow, such as recirculation, stagnation, and high stress regions. Levels of viscous and Reynolds shear stresses everywhere within the pump are below reported threshold values for damage to red cells over the entire range of flow rates investigated; however, at both high and low flow rate conditions, the flow field may promote activation of the clotting cascade due to regions of elevated shear stress adjacent to separated or stagnant flow.     

Numerical and experimental investigations of pulsatile blood flow pattern through a dysfunctional mechanical heart valve

Around 250,000 heart valve replacements are performed every year around the world. Due their higher durability, approximately 2/3 of these replacements use mechanical prosthetic heart valves (mainly bileaflet valves). Although very efficient, these valves can be subject to valve leaflet malfunctions. These malfunctions are usually the consequence of pannus ingrowth and/or thrombus formation and represent serious and potentially fatal complications. Hence, it is important to investigate the flow field downstream of a dysfunctional mechanical heart valve to better understand its impact on blood components (red blood cells, platelets and coagulation factors) and to improve the current diagnosis techniques. Therefore, the objective of this study will be to numerically and experimentally investigate the pulsatile turbulent flow downstream of a dysfunctional bileaflet mechanical heart valve in terms of velocity field, vortex formation and potential negative effect on blood components. The results show that the flow downstream of a dysfunctional valve was characterized by abnormally elevated velocities and shear stresses as well as large scale vortices. These characteristics can predispose to blood components damage. Furthermore, valve malfunction led to an underestimation of maximal transvalvular pressure gradient, using Doppler echocardiography, when compared to numerical results. This could be explained by the shifting of the maximal velocity towards the normally functioning leaflet. As a consequence, clinicians should try, when possible, to check the maximal velocity position not only at the central orifice but also through the lateral orifices. Finding the maximal velocity in the lateral orifice could be an indication of valve dysfunction.     

Tomographic PIV in a model of the left ventricle: 3D flow past biological and mechanical heart valves

Left ventricular flow is intrinsically complex, three-dimensional and unsteady. Its features are susceptible to cardiovascular pathology and treatment, in particular to surgical interventions involving the valves (mitral valve replacement). To improve our understanding of intraventricular fluid mechanics and the impact of various types of prosthetic valves thereon, we have developed a custom-designed versatile left ventricular phantom with anatomically realistic moving left ventricular membrane. A biological, a tilting disc and a bileaflet valve (in two different orientations) were mounted in the mitral position and tested under the same settings. To investigate 3D flow within the phantom, a four-view tomographic particle image velocimetry setup has been implemented. The results compare side-by-side the evolution of the 3D flow topology, vortical structures and kinetic energy in the left ventricle domain during the cardiac cycle. Except for the tilting disc valve, all tested prosthetic valves induced a crossed flow path, where the outflow crosses the inflow path, passing under the mitral valve. The biological valve shows a strong jet with a peak velocity about twice as high compared to all mechanical heart valves, which makes it easier to penetrate deeply into the cavity. Accordingly, the peak kinetic energy in the left ventricle in case of the biological valve is about four times higher than the mechanical heart valves. We conclude that the tomographic particle imaging velocimetry setup provides a useful ground truth measurement of flow features and allows a comparison of the effects of different valve types on left ventricular flow patterns.     

Numerical simulation of steady flow in a two-dimensional total artificial heart model.

In this paper, a numerical simulation of steady laminar and turbulent flow in a two-dimensional model for the total artificial heart is presented. A trileaflet polyurethane valve was simulated at the outflow orifice while the inflow orifice had a trileaflet or a flap valve. The finite analytic numerical method was employed to obtain solutions to the governing equations in the Cartesian coordinates. The closure for turbulence model was achieved by employing the k-epsilon-E model. The SIMPLER algorithm was used to solve the problem in primitive variables. The numerical solutions of the simulated model show that regions of relative stasis and trapped vortices were smaller within the ventricular chamber with the flap valve at the inflow orifice than that with the trileaflet valve. The predicted Reynolds stresses distal to the inflow valve within the ventricular chamber were also found to be smaller with the flap valve than with the trileaflet valve. These results also suggest a correlation between high turbulent stresses and the presence of thrombus in the vicinity of the valves in the total artificial hearts. The computed velocity vectors and turbulent stresses were comparable with previously reported in vitro measurements in artificial heart chambers. Analysis of the numerical solutions suggests that geometries similar to the flap valve (or a tilting disk valve) results in a better flow dynamics within the total artificial heart chamber compared to a trileaflet valve.     

Model studies at mechanical aortic heart valve prostheses--Part I: Steady-state flow fields and pressure loss coefficients

In a 3:1 scaled model of the human aorta models of the Omniscience, Bj?rk-Shiley Convexo-Concave, Bj?rk-Shiley Monostrut, Medtronic-Hall, Duromedics (Hemex) and the Saint Jude Medical heart valve prostheses are studied in steady flow representing the systolic peak flow phase. Detailed flow visualization experiments show flow separations at all inner ring surfaces as well as at most of the occluders. The resulting stagnation areas increase the risk of thrombus accumulation. Flow separations also stimulate vortex formation and turbulent mixing at the downstream jet boundaries and thus may intensify blood damage by turbulent shear stresses. The different influences of struts and occluder guides on the flow around the occluders are discussed. The effects of the individual valve components on the flow fields are analyzed and correlated with the resulting pressure losses.     

Two-dimensional color-mapping of turbulent shear stress distribution downstream of two aortic bioprosthetic valves in vitro.

Since artificial heart valve related complications such as thrombus formation, hemolysis and calcification are considered related to flow disturbances caused by the inserted valve, a thorough hemodynamic characterization of heart valve prostheses is essential. In a pulsatile flow model, fluid velocities were measured one diameter downstream of a Hancock Porcine (HAPO) and a Ionescu-Shiley Pericardial Standard (ISPS) aortic valve. Hot-film anemometry (HFA) was used for velocity measurements at 41 points in the cross-sectional area of the ascending aorta. Three-dimensional visualization of the velocity profiles, at 100 different instants during one mean pump cycle, was performed. Turbulence analysis was performed as a function of time by calculating the axial turbulence energy within 50 ms overlapping time windows during the systole. The turbulent shear stresses were estimated by using the correlation equation between Reynolds normal stress and turbulent (Reynolds) shear stress. The turbulent shear stress distribution was visualized by two-dimensional color-mapping at different instants during one mean pump cycle. Based on the velocity profiles and the turbulent shear stress distribution, a relative blood damage index (RBDI) was calculated. It has the feature of combining the magnitude and exposure time of the estimated shear stresses in one index, covering the entire cross-sectional area. The HAPO valve showed a skewed jet-type velocity profile with the highest velocities towards the left posterior aortic wall. The ISPS valve revealed a more parabolic-shaped velocity profile during systole. The turbulent shear stresses were highest in areas of high or rapidly changing velocity gradients. For the HAPO valve the maximum estimated turbulent shear stress was 194 N m-2 and for the ISPS valve 154 Nm-2. The RBDI was the same for the two valves. The turbulent shear stresses had magnitudes and exposure times that might cause endothelial damage and sublethal or lethal damage to blood corpuscules. The RBDI makes comparison between different heart valves easier and may prove important when making correlation with clinical observations.     

Three-dimensional visualization of stationary flow behind artificial heart valves]

The visualization and quantitative analysis of flow offers a possibility for the hydrodynamic characterization of artificial heart valves. Different types of valves can be compared if velocity profile and the turbulent shear stress caused by the prosthesis are known. The tracer technique was selected, since it permits visualization also of turbulent flow through the valve. With the aid of a simple optical device the three-dimensional flow pattern behind the valve is determinable. The main features of the method are: The regions of interest can easily be identified. Velocity profiles can be determined and shear stress and turbulence intensities estimated. The experimental setup is simple, calibration is not necessary, and it can be used for turbulent flows. The method can be used only with transparent fluids and vessels; measurements in blood are not possible. Because of the large number of measuring points required the method is very time-consuming. The use of an automatic picture analyzing system would make it possible to increase the number of pictures processed, and thus increase resolution. The velocity profile of a three-finger-valve, the TAD 29, was established at a distance of 20 mm from the ring, and compared with known profiles from the literature. The valve has an opening angle of 70 degrees. All typical regions for the flow of an artificial heart valve, such as jet, stagnation gone, backflow and turbulence were demonstrated.     

Comparison of hinge microflow fields of bileaflet mechanical heart valves implanted in different sinus shape and downstream geometry

The characterization of the bileaflet mechanical heart valves (BMHVs) hinge microflow fields is a crucial step in heart valve engineering. Earlier in vitro studies of BMHV hinge flow at the aorta position in idealized straight pipes have shown that the aortic sinus shapes and sizes may have a direct impact on hinge microflow fields. In this paper, we used a numerical study to look at how different aortic sinus shapes, the downstream aortic arch geometry, and the location of the hinge recess can influence the flow fields in the hinge regions. Two geometric models for sinus were investigated: a simplified axisymmetric sinus and an idealized three-sinus aortic root model, with two different downstream geometries: a straight pipe and a simplified curved aortic arch. The flow fields of a 29-mm St Jude Medical BMHV with its four hinges were investigated. The simulations were performed throughout the entire cardiac cycle. At peak systole, recirculating flows were observed in curved downsteam aortic arch unlike in straight downstream pipe. Highly complex three-dimensional leakage flow through the hinge gap was observed in the simulation results during early diastole with the highest velocity at 4.7 m/s, whose intensity decreased toward late diastole. Also, elevated wall shear stresses were observed in the ventricular regions of the hinge recess with the highest recorded at 1.65 kPa. Different flow patterns were observed between the hinge regions in straight pipe and curved aortic arch models. We compared the four hinge regions at peak systole in an aortic arch downstream model and found that each individual hinge did not vary much in terms of the leakage flow rate through the valves.     

Wall shear stress distribution along the cusp of a tri-leaflet prosthetic valve

High levels of wall shear stress on the surface of valvular cusps can cause mechanical damage to the blood cells and the cusp surfaces. The shear stresses are also responsible for mechanical failure of prosthetic heart valves. Quatitative measurements of wall shear stress in the vicinity of the leaflets are thus essential for diagnosis of suspected complications and provide important information for the design and fabrication of bioprosthetic heart valves. For this purpose we measured the velocity distribution along the inside wall of the cusps of a tri-leaflet heart valve with a two colour laser Doppler anemometer system. The wall shear stresses on the cusp surface were computed and found to range from 80 to 120 N/m² during the ejection phase. Wall shear stresses of up to 180 N/m² were measured in loci of cusp flexure and the accelerated boundary layer. The results of this study show a correlation between the high shear stress loci and the clinically (animal) observed regions of cusp calcification.