In vitro velocity measurements in the near vicinity of the Bjork-Shiley aortic prosthesis using a laser-Doppler anemometer

In this study,in vitro velocity measurements in the near vicinity of a Björk-Shiley aortic valve, one of the more commonly used aortic valve prostheses, were made using a laser-Doppler anemometer. The velocity measurements identified a zone of stagnation, about 20 mm wide, immediately downstream from the fully open disc. The measurements also showed that the flow through the valve was divided into two unequal regions, namely, the major and minor outflow regions. Because of the low flow in the minor outflow region, the shear stresses along the perimeter of the valve in that region were considerably lower than the shear stresses along the sewing ring of the major outflow region. Pathologic studies of nine recovered Björk-Shiley aortic valves indicated varying amounts of thrombus formation on the outflow face of the disc and excess growth of endothelial tissue along the perimeter of the minor outflow region. If the large stagnation zone and the relatively low shear in the minor outflow region which were observed in thein vitro measurements also existin vivo, they could lead to the clinically observed thrombus formation and tissue overgrowth, respectively.     

Biofluid mechanics--an interdisciplinary research area of the future.

Biofluid mechanics is a complex field that focuses on blood flow and the circulation. Clinical applications include bypass and anastomosis surgery, and the development of artificial heart valves and vessels, stents, vein and dialysis shunts. Biofluid mechanics is also involved in diagnostic and therapeutic measures, including CT and MRI, and ultrasound. The study of biofluid mechanics involves measuring blood flow, pressure, pulse wave, velocity distribution, the elasticity of the vessel wall, the flow behavior of blood to minimize complications in vessel,- neuro-, and heart surgery. Biofluid mechanics influence the lungs and circulatory system, the blood flow and micro-circulation; lymph flow, and artificial organs. Flow studies in arterial models can be done without invasive techniques on patients or animals. The results of fluid mechanic studies have shown that in the addition to basic biology, an understanding of the forces and movement on the cells is essential. Because biofluid mechanics allows for the detection of the smallest flow changes, it has an enormous potential for future cell research. Some of these will be discussed.     

Design considerations for the omniscience pivoting disc cardiac valve prosthesis

Parametric studies are conducted in an effort to determine the optimum curvature and eccentricity for concave-convex pivoting disc prosthetic heart valves. Steady-state pressure measurements with prototype valves and laser-Doppler anemometer measurements of the velocity field with a two-dimensional model are presented. These results suggest that an optimum curvature exists and that the minimum eccentricity that promotes full opening should be used. Estimates of the shear stress distribution in the wake of the valve are computed. The maximum shear stress for both the major and minor outflow areas appear to be: 1) fairly evenly distributed, 2) well below the level for damage to the formed elements of blood as they pass through the valve, and 3) above the level required for the formation of a stable thrombus.     

A Comparison of Flow Field Structures of Two Tri-Leaflet Polymeric Heart Valves

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this in vitro study, the velocity fields inside and downstream of two different prototype tri-lealfet polymeric heart valves were studied. Experiments were conducted on two 23 mm prototype polymeric valves, provided by AorTech Europe, having open or closed commissure designs and leaflet thickness of 120 and 80 m, respectively. A two-dimensional LDV system was used to measure the velocity fields in the vicinity of the two valves under simulated physiological conditions. Both commissural design and leaflet thickness were found to affect the flow characteristics. In particular, very high levels of Reynolds shear stress of 13,000 dynes/cm² were found in the leakage flow of the open commisure design. Maximum leakage velocities in the open and closed designs were 3.6 m/s and 0.5 m/s respectively; the peak forward flow velocities were 2.0 m/s and 2.6 m/s, respectively. In both valve designs, shear stress levels exceeding 4,000 dyne/cm² were observed at the trailing edge of the leaflets and in the leakage and central orifice jets during peak systole. Additionally, regions of low velocity flow conducive to thrombus formation were observed in diastole. The flow structures measured in these experiments are consistent with the location of thrombus formation observed in preliminary animal experiments.     

Echocardiographic characteristics of transprosthetic blood peak velocity in the Sorin Bicarbon bileaflet prosthetic heart valve

The Bicarbon prosthetic heart valve with two curved leaflets is designed so that the blood flows through the three orifices are parallel jets of equal size. This study was conducted to confirm that the Bicarbon valve functions clinically as designed. Forty-three patients underwent valve replacement with the Bicarbon valve. Forty-eight Bicarbon valves were implanted: 25 valves in the mitral position and 23 in the aortic position. Peak blood flow velocity through the three prosthetic orifices was measured postoperatively by Doppler echocardiography. The three flow jets through the prosthesis were parallel. The velocity through the lateral orifice was 2.33±0.38 m/min, and the velocity through the central orifice was 2.14±0.43 m/min at the aortic position (P>0.05). The velocity through the lateral orifice was 1.72±0.06 m/min at the mitral position, and that through the central orifice was 1.73±0.06 m/min (P>0.05). Serum lactic acid dehydrogenase values were also lower than those of patients or whom another bileaflet prosthesis had been implanted. The results confirm that the Bicarbon prosthetic heart valve performs clinically as designed, producing three parallel blood flow jets with equal flow velocity.     

The Influence of Valve Leaflet Stiffness Variability on Aortic Wall Shear Stress

Aortic stenosis is a common cardiac condition that impacts the aorta’s hemodynamics downstream of the affected valve. We sought to better understand how non-uniform stiffening of a stenotic aortic valve would affect the wall shear stress (WSS) experienced by the walls of the aorta and the residence time near the valve. Several experimental configurations were created by individually stiffening leaflets of a polymer aortic valve. These configurations were mounted inside an in vitro experimental setup. Digital particle image velocimetry (DPIV) was used to measure velocity profiles inside a model aorta. The DPIV results were used to estimate the WSS and residence time. Our analysis suggests that leaflet asymmetry greatly affects the amount of WSS by vectoring the systolic jet and stiffened leaflets have an increased residence time. This study indicates that valve leaflets with different stiffness conditions can have a more significant impact on wall shear stress than stenosis caused by the uniform increase in all three leaflets (and the subsequent increased systolic velocity) alone. This finding is promising for creating customizable (patient-specific) prosthetic heart valves tailored to individual patients.

     

Flow in Prosthetic Heart Valves: State-of-the-Art and Future Directions

Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bioprosthetic valves. Today, the most widely implanted design is the mechanical bileaflet, with over 170,000 implants worldwide each year. Several different mechanical valves are currently available and many of them have good bulk forward flow hemodynamics, with lower transvalvular pressure drops, larger effective orifice areas, and fewer regions of forward flow stasis than their earlier-generation counterparts such as the ball-and-cage and tilting-disc valves. However, mechanical valve implants suffer from complications resulting from thrombus deposition and patients implanted with these valves need to be under long-term anti-coagulant therapy. In general, blood thinners are not needed with bioprosthetic implants, but tissue valves suffer from structural failure with, an average life-time of 10–12 years, before replacement is needed. Flow-induced stresses on the formed elements in blood have been implicated in thrombus initiation within the mechanical valve prostheses. Regions of stress concentration on the leaflets during the complex motion of the leaflets have been implicated with structural failure of the leaflets with bioprosthetic valves. In vivo and in vitro experimental studies have yielded valuable information on the relationship between hemodynamic stresses and the problems associated with the implants. More recently, Computational Fluid Dynamics (CFD) has emerged as a promising tool, which, alongside experimentation, can yield insights of unprecedented detail into the hemodynamics of prosthetic heart valves. For CFD to realize its full potential, however, it must rely on numerical techniques that can handle the enormous geometrical complexities of prosthetic devices with spatial and temporal resolution sufficiently high to accurately capture all hemodynamically relevant scales of motion. Such algorithms do not exist today and their development should be a major research priority. For CFD to further gain the confidence of valve designers and medical practitioners it must also undergo comprehensive validation with experimental data. Such validation requires the use of high-resolution flow measuring tools and techniques and the integration of experimental studies with CFD modeling.     

Fluid Dynamic Assessment of Three Polymeric Heart Valves Using Particle Image Velocimetry

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this paper we characterize the in vitro velocity and Reynolds Shear Stress (RSS) fields inside and downstream of three different prototype trileaflet polymeric heart valves. The fluid dynamic differences are then correlated with variations in valve design parameters. The three valves differ in leaflet thickness, ranging from 80 to 120 μm, and commisural design, either closed, opened, or semi-opened. The valves were subjected to aortic flow conditions and the velocity measured using three-dimensional stereo Particle Image Velocimetry. The peak forward flow phase in the three valves was characterized by a strong central orifice jet of approximately 2 m/s with a flat profile along the trailing edge of the leaflets. Leakage jets, with principle RSS magnitudes exceeding 4,500 dyn/cm², were observed in all valves with larger leaflet thicknesses and also corresponded to larger leakage volumes. Additional leakage jets were observed at the commissural region of valves with the open and the semi-open commissural designs. The results of the present study indicate that commissural design and leaflet thickness influence valve fluid dynamics and thus the thrombogenic potential of trileaflet polymeric valves.     

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.     

人造机械瓣心音的分析研究

在一些致命性心脏病的诊断中,心音听诊是最有效也是应用得最成功的手段之一.鉴于目前机械瓣的使用非常普遍,研究简单有效的机械瓣病变判别方法对于临床诊断来讲具有重要意义.运用希尔波特-黄变换(HHT),针对不同的机械瓣心音进行分析,并设计一种基于Hilbert边界谱特征的提取方法,结合线性判别分析(LDA),对不同的机械瓣心音进行分类.同时,与基于局部最优基特征的分类器分类结果进行比较.分析结果表明,机械瓣心音的各阶Hilbert边界谱具有非常明显不同的分布,基于HHT的分类器识别率达到了97.3％,较基于局部最优基特征分类器的识别率(91.3％)更高.对于人造机械瓣心音而言,HHT是一种有效的分析处理手段.     

Appraisal of power loads on elements of artificial heart valves through the results of echocardiogram processing

Conclusions 1. Analysis of echocardiograms of patients with implanted prosthetic heart valves shows that the fibrous ring (along with the valve seat attached to it) is movable, which is an important factor for the calculation of valves and the designing of test stands. 2. The velocity vectors of the fibrous ring and the ball at the moment of impact of the ball and the shell are directed opposite to each other, which demonstrates the necessity of summing their moduli in the conduct of calculations. 3. Obtaining experimental data on the dynamics of prosthetic heart valves that have been implanted into patients allows evaluation of the adequacy of test-stand trials of valves. 4. As calculations based on the analysis of the energy balance equation for all elements of the valve have shown, the value of the stress for impact of the ball with the shell (taking account of the movability of the fibrous ring) is, on the average, equal to 0.352 N; for rigid mounting, the value is 1.48 N. Therefore, testing of prosthetic valves for reliability under conditions that do not correspond to physiological conditions may lead to incorrect conclusions. A. N. Bakulev Institute of Cardiovascular Surgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Meditsinskaya Tekhnika, No. 4, pp. 9–12, July–August, 1979.     

Estimation of the Shear Stress on the Surface of an Aortic Valve Leaflet

The limited durability of xenograft heart valves and the limited supply of allografts have sparked interest in tissue engineered replacement valves. A bioreactor for tissue engineered valves must operate at conditions that optimize the biosynthetic abilities of seeded cells while promoting their adherence to the leaflet matrix. An important parameter is shear stress, which is known to influence cellular behavior and may thus be crucial in bioreactor optimization. Therefore, an accurate estimate of the shear stress on the leaflet surface would not only improve our understanding of the mechanical environment of aortic valve leaflets, but it would also aid in bioreactor design. To estimate the shear stress on the leaflet surface, two-component laser-Doppler velocimetry measurements have been conducted inside a transparent polyurethane valve with a trileaflet structure similar to the native aortic valve. Steady flow rates of 7.5, 15.0, and 22.5 L/min were examined to cover the complete range possible during the cardiac cycle. The laminar shear stresses were calculated by linear regression of four axial velocity measurements near the surface of the leaflet. The maximum shear stress recorded was 79 dyne/cm², in agreement with boundary layer theory and previous experimental and computational studies. This study has provided a range of shear stresses to be explored in bioreactor design and has defined a maximum shear stress at which cells must remain adherent upon a tissue engineered construct. © 1999 Biomedical Engineering Society. PAC99: 8719Rr, 8768+z, 8719Hh, 4262Be, 4727Nz, 0630Gv     

A quantitative method for thein vitro study of sounds produced by prosthetic aortic heart valves Part I: analytical considerations

A method is presented for analysing sounds producedin vitro by prosthetic aortic heart valves, and a one-dimensional harmonic model is described. Procedures for estimating physical parameters of the model are outlined for the case of transient and nontransient sounds, and a computational method is described for making comparisons between two general sounds. The fast Fourier transform provides a satisfactory means for the basic transformation to the frequency domain. Useful representations of the acoustical information that are considered are the original time and amplitude plots. power-density spectra, power-distribution functions, a three-dimensional surface of power-frequency-time, sections of these three-dimensional surfaces, and a three-dimensional power-distribution surfaces. Note is made that each representation is useful for indicating specific acoustical characteristics which may be important when either comparing or describing sounds. The spectra provide an accurate means for estimating the parameters of the model and provide clearer comparisons when compared to the time-amplitude plots. This fact is most clearly shown by the three-dimensional powerdifference surface. This surface provides a very convenient means for the overall comparison of two sounds.     

Endothelial retention and phenotype on carbonized cardiovascular implant surfaces

Heart valve disease is an increasing clinical burden for which there is no effective treatment outside of prosthetic replacement. Over the last 20 years, clinicians have increasingly preferred the use of biological prosthetics to mechanical valves despite their superior durability because of the lifelong anticoagulation therapy that is required. Mechanical valve surface engineering has largely focused on being as non-thrombogenic as possible, but despite decades of iteration has had insufficient impact on the anticoagulation burden. In this study, we systematically evaluate the potential for endothelialization of the pyrolytic carbon surface used in mechanical valves. We compared adsorbed adhesion ligand type (collagen I, fibronectin, laminin, and purified adhesion domain fragments GFOGER and FN_7-10) and concentration on endothelial adhesion rates and adhesion strength on Medtronic-Hall prosthetic valve surfaces. Regardless of ligand type or concentration, endothelial adhesion strengthening was insufficient for their intended ultra-high shear stress environment. We then hypothesized that microfabricated trenches would reduce shear stress to tolerable levels while maintaining endothelial access to the flow stream, thereby promoting a confluent and anticoagulant endothelial monolayer. Computational fluid dynamics simulations predicted an empirical relationship of channel width, depth, and spacing that would maintain interior surface shear stress within tolerable levels. Endothelial cells seeded to confluence in these channels retained a confluent monolayer when exposed to 600 dyn/cm² shear stress for 48 h regardless of applied adhesive ligand. Furthermore, sheared EC expressed a mature anti-coagulant profile, including endothelial nitric oxide synthase (eNOS), VE-cadherin, and significantly downregulated plasminogen activator inhibitor-1 (PAI-1). As a final test, channeled pyrolytic carbon surfaces with confluent EC reduced human platelet adhesion 1000-fold over pyrolytic carbon alone. These results advance a promising biohybrid approach to enable active moderation of local coagulative response in mechanical heart valves, which could significantly extend the utility of this important treatment for heart valve disease.     

High-Resolution Measurement of the Unsteady Velocity Field to Evaluate Blood Damage Induced by a Mechanical Heart Valve

We investigate the potential of prosthetic heart valves to generate abnormal flow and stress patterns, which can contribute to platelet activation and lysis according to blood damage accumulation mechanisms. High-resolution velocity measurements of the unsteady flow field, obtained with a standard particle image velocimetry system and a scaled-up model valve, are used to estimate the shear stresses arising downstream of the valve, accounting for flow features at scales less than one order of magnitude larger than blood cells. Velocity data at effective spatial and temporal resolution of 60 μm and 1.75 kHz, respectively, enabled accurate extraction of Lagrangian trajectories and loading histories experienced by blood cells. Non-physiological stresses up to 10 Pa were detected, while the development of vortex flow in the wake of the valve was observed to significantly increase the exposure time, favouring platelet activation. The loading histories, combined with empirical models for blood damage, reveal that platelet activation and lysis are promoted at different stages of the heart cycle. Shear stress and blood damage estimates are shown to be sensitive to measurement resolution.     

机械瓣心音频谱分析与基于改进LDB算法的识别

听诊是通过听取心脏所发出的声音来帮助诊断各种心脏疾病的一种有效手段。鉴于目前机械瓣的使用非常普遍,研究简单有效的机械瓣病变判别方法对于临床诊断来讲具有很大的意义。针对五种不同的机械瓣心音进行的分析表明,运用频谱仅能鉴别瓣周漏这一种机械瓣病变。虽然直接利用信号的时频成分进行机械瓣心音分类是可能的,但识别率只有84.0%。利用改进的局部最优基(LDB)算法来提取特征对机械瓣心音分类有着非常大的帮助,识别率达到了97.3%。与原始的LDB算法相比,实验表明改进后的LDB算法对提高识别率和降低计算复杂性都有着明显的优势。     

Wrinkle-induced tear in the mitral valve leaflet tissue: a computational model

Abstract

In this study, we offer a numerical platform to detect the locations of high-stress zones in the prosthetic heart valve, in the mitral position, during the closing phase due to existing wrinkles. The intended prosthetic valves in this study have the same shape as the native mitral valve but made of synthetic biomaterials. We assume the most high-risk locations for ruptures to either initiate or propagate are at the base of existing wrinkles. We developed a finite element model for the human mitral valve. A mesh model was effectively created to account for the uneven stress distribution and high-stress concentration zones in the valve tissue structure. The constitutive material model used in this study is anisotropic and hyperelastic such that the membrane elements are used for the leaflets and spar elements are utilised for the mitral valve cords for which it was assumed flexural stiffness is insignificant for both sets of elements. We developed a novel and effective computational model for the simulation of wrinkles in the valve leaflet during the closing phase. The proposed numerical model provided a quick but precise assessment for the detection of locations of rips and tears on the leaflet tissue during the closing phase. The proposed model is an essential step for the design of material and geometry of leaflets of prosthetic heart valves made of polymers or tissue materials in the mitral position.     

Experimental Investigation of the Steady Flow Downstream of the St. Jude Bileaflet Heart Valve: A Comparison Between Laser Doppler Velocimetry and Particle Image Velocimetry Techniques

This study investigates turbulent flow, based on high Reynolds number, downstream of a prosthetic heart valve using both laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Until now, LDV has been the more commonly used tool in investigating the flow characteristics associated with mechanical heart valves. The LDV technique allows point by point velocity measurements and provides enough statistical information to quantify turbulent structure. The main drawback of this technique is the time consuming nature of the data acquisition process in order to assess an entire flow field area. Another technique now used in fluid dynamics studies is the PIV measurement technique. This technique allows spatial and temporal measurement of the entire flow field. Using this technique, the instantaneous and average velocity flow fields can be investigated for different positions. This paper presents a comparison of PIV two-dimensional measurements to LDV measurements, performed under steady flow conditions, for a measurement plane parallel to the leaflets of a St. Jude Medical (SJM) bileaflet valve. Comparisons of mean velocity obtained by the two techniques are in good agreement except for where there is instability in the flow. For second moment quantities the comparisons were less agreeable. This suggests that the PIV technique has sufficient temporal and spatial resolution to estimate mean velocity depending on the degree of instability in the flow and also provides sufficient images needed to duplicate mean flow but not for higher moment turbulence quantities such as maximum turbulent shear stress. © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 4262Be, 8780-y     

Mechanical valve closing dynamics: Relationship between velocity of closing, pressure transients, and cavitation initiation

In this study, the closing dynamics of mechanical heart valves was experimentally analyzed with the valves mounted in the mitral position of anin vitro flow chamber simulating a single closing event. The average linear velocity of the edge of the leaflet during the final 2.065° of the traverse before closing was measured using a laser sweeping technique, and the negative pressure transients at 2 mm from the leaflet inflow surface in the fully closed position was recorded at the instant of valve closure. The cavitation number was computed for the various mechanical valves at a range of load at valve closure. The data were correlated with cavitation bubble visualization previously obtained with the same experimental set up. Cavitation incipience with mechanical valves was found to be independent of the flexibility of the valve holder. For the same loading rate at valve closure, valves with flexible (polyethylene) leaflets were found to close with comparable velocity to those with rigid (pyrolytic carbon) leaflets, but the negative pressure transients did not reach magnitudes close to the vapor pressure for the fluid with flexible leaflets. For the same leaflet closing velocity (and hence the cavitation number), valves with a seat stop or a seating lip in the region of maximum leaflet velocity were observed to cavitate earlier, suggesting that the effect of “squeeze flow” may be an important factor in cavitation incipience. This study was presented at the Biomedical Engineering Society's 1996 Annual Fall Meeting, October 1996.     

Physical principles of the Edinburgh prosthetic heart valve

A new type of prosthetic heart valve, at present under development, has been designed according to a set of general assumptions, here made explicit, concerning the interaction of flowing blood with artificial structures. On these assumptions, it is shown that the characteristics desirable in a prosthetic heart valve are supplied by a form of butterfly valve in which an occluder, capable of rotation about a nearly diametral axis in a smooth-bore housing, rapidly assumes a low-drag position aligned with the forward flow and closes rapidly on flow reversal. These desiderata, not furnished by conventional butterfly valves, are achieved in the Edinburgh design, in which an occluder of suitable aerofoil shape is mounted pivotally in a housing of conical or similarly divergent bore. Certain forms of carbon are suitable materials of construction. Preliminary data forin vivo (canine) performance are given for Edinburgh valves fabricated from graphite coated with pyrolytic carbon.