经导管瓣膜瓣中瓣模式下流体力学性能体外测试及评价

目的 随着生物瓣膜毁损病例的增加,越来越多的经导管瓣膜作为瓣中瓣被应用于二次瓣膜置换手术,但其流体力学性能鲜有报道.本文将经导管瓣膜安装在生物瓣膜内形成瓣中瓣结构,并对其流体力学性能进行体外测试及评价.方法 将经导管瓣膜(23 mm、27 mm、29 mm)分别安装在对应规格生物瓣膜(23 mm、27 mm、29 mm)中形成瓣中瓣,进行稳态前向流实验、稳态反向泄漏实验、脉动流实验,对其流体力学性能进行评价,并与同规格生物瓣膜流体力学性能进行对比.结果 稳态前向流实验中,同一规格的经导管瓣中瓣跨瓣压差随着前向流量的增大而增大.稳态反向泄漏实验中,同一规格的经导管瓣中瓣泄漏量随着反向压力的增大而增大.脉动流实验中,经导管瓣中瓣和生物瓣膜的平均跨瓣压差、返流百分比和有效瓣口面积变化趋势相同.对于同一规格的经导管瓣中瓣和生物瓣膜,随着心输出量的增加,跨瓣压差增大,返流百分比减小,有效瓣口面积增大;在同一心输出量下,随着经导管瓣中瓣和生物瓣膜规格的增大,跨瓣压差减小,返流百分比增大,有效瓣口面积增大.结论 经导管瓣中瓣体外脉动流性能指标满足YY/T1449.3—2016标准中经导管瓣膜的性能要求,且其脉动流性能与同规格生物瓣膜相比无明显差异.该经导管瓣中瓣具有良好的血流动力学性能.     

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。     

一种新型抗钙化处理的人工生物瓣膜流体力学性能

目的 评价一种新型生物瓣膜的体外流体力学性能,并与传统生物瓣膜及机械瓣膜进行比较.方法 将测试瓣膜分成三组:新型生物瓣组(GA SOB处理牛心包瓣),传统生物瓣组(单纯GA处理牛心包瓣),机械瓣组(双叶瓣),每组分别选21号、25号、29号三种型号,采用清华大学TH-1200脉动流测试仪,按照ISO5840瓣膜检测标准进行流体性能检测,包括跨瓣压差、返流量、返流百分比及有效开口面积,并进行组间的分析、比较.结果 新型生物瓣膜的前向流跨瓣压差较传统生物瓣小17%～30%,较机械瓣小23%～50%;新型生物瓣的有效开口面积较传统生物瓣和机械瓣分别大13%～37%和36%～50%;新型生物瓣的返流量较传统生物瓣大1.2～2.0 mL,约3%～6%;较机械瓣小0.9～2.8mL,约1.3%～5%.结论 新型人工生物瓣膜具有良好的血流动力学性能.     

一种新型干性生物瓣膜耐久性能体外测试及评价

随着生物技术的发展,近年来出现了通过覆盖钙结合位点的牛心包处理技术,并以此为瓣叶材料制备出干性生物瓣膜。由于干性生物瓣膜临床应用时间短,尚缺少长期耐久性数据。本研究采用体外加速方法,对一种干性生物瓣膜耐久性能进行测试及评价。选取23和32 mm这两个规格干性生物瓣膜进行体外耐久性能测试。通过瓣膜脉动流实验、瓣叶热力学分析和显微镜下胶原纤维观察,对其耐久性能进行评价。经过2亿次循环(模拟临床使用5年),干性生物瓣膜流体力学性能无明显变化,其中23 mm规格干性生物瓣膜平均跨瓣压差有所升高,但仍处于同规格生物瓣膜较低水平;32 mm规格干性生物瓣膜平均跨瓣压差几乎没有变化。有效瓣口面积基本一致,返流百分比无明显变化,说明干性生物瓣膜未发生明显的狭窄和返流,能量损失无明显变化,说明瓣膜的效能无明显降低。瓣叶材料的热力学变性温度由96.6℃降至91.2℃;在双光子共聚焦显微镜下观察,同样测试条件下亮度变暗,但胶原纤维形状未发生变化,仍是卷曲的立体结构,说明胶原纤维含量降低,化学键部分丢失,与热变性温度表现一致。干性生物瓣膜耐久性能实验后,微观结构发生一定变化,但仍具有良好的流体力学性能。     

心室流腔角度对主动脉瓣影响的脉动流实验研究

目的探究左心室流腔与主动脉轴线所呈角度对主动脉瓣力学性能的影响。方法依据从华中科技大学同济医学院附属协和医院获得的患者CT图像上的心室流腔角度大小,通过3D打印技术制作心室流腔角度分别为0°、16.5°和30°的3组主动脉根部模型。然后将人工生物瓣安装在主动脉根部模型上,在Vivitro心脏-血管模拟实验系统中进行不同心输出量条件下的脉动流实验。心率设定为70次/min,脉动流流动速率分别为2、3、4、5、6、7 L/min的条件下,测试瓣膜的跨膜压差、反流比和有效开口面积。在每个脉动流流动速率条件下测试10次,取平均值。结果不同心室流腔角度模型之间,生物瓣的跨膜压差存在差异但均符合国家标准GB 12279—2008/ISO 5840:1996,即小于10 mmHg(1 mmHg=0.133 k Pa)。对于心输出量较低的情况,较小的心室流腔角度有助于反流比的下降,较大的心室流腔角度有助于增大瓣膜的有效开口面积;而对于心输出量较高的情况,较小的心室流腔角度有利于瓣膜有效开口面积的增大。结论手术时,医生可根据患者的各项参数大小选择合适的心室流腔角度。     

介入瓣膜瓣中瓣模式下耐久性能测试及评价

目的随着之前植入外科人工生物心脏瓣膜患者瓣膜毁损的病例增加,介入瓣膜瓣中瓣越来越多地被用于临床。由于其临床应用时间不长,目前没有长期临床数据可以参考,故实验采用体外加速方法,对一种介入瓣膜瓣中瓣耐久性能进行测试及评价。方法将23 mm、27 mm、29 mm 3个规格(牛心包材质)介入瓣膜作为瓣中瓣分别安装在23 mm、27 mm、29 mm对应规格外科生物心脏瓣膜中,通过预扩达到相同的内径后进行体外加速疲劳耐久性能测试,每5 000万次对其脉动流性能进行测试。2.0亿次疲劳测试后,对瓣叶进行热力学分析、双光子共聚焦显微镜下观察瓣叶胶原纤维结构。结果经过2.0亿次耐久性能测试,介入瓣膜瓣中瓣平均跨瓣压差无显著变化[23 mm:1.92～1.98 kPa(14.4～14.9 mmHg);27 mm:0.92～1.64 kPa(6.9～12.3 mmHg);29 mm:0.72～1.02 kPa(5.4～7.4 mmHg)],有效瓣口面积基本一致(23 mm:1.45～1.66 cm2;27 mm:1.88～2.17 cm2;29 mm:2.24～2.54 cm2),反流百分比减小(23 ...     

无支架心包二尖瓣的有限元分析

目的　通过无支架心包二尖瓣与有架三叶生物瓣对比，利用有限元法计算，观察在左心室舒张期瞬时跨瓣压差作用下的瓣叶静态应力分布。方法　采用八节点曲线薄壳单元，考虑大应变以及瓣叶闭合过程的接触，应用Newton-Raphson方法求解有限元非线性方程。结果　瓣膜开启状态下两种瓣膜应力水平均不高。在峰值跨瓣压差15.96 kPa作用下，无架心包二尖瓣第一主应力分布均匀，无明显应力集中、平均第一主应力为0.040~0.149 MPa；有架三叶瓣在临床上易发生撕裂的部位明显应力集中，最大第一主应力为2.352 MPa，平均第一主应力为0.223~0.724 MPa，明显高于无架心包二尖瓣。结论　(2)对于几何曲面形状不规则，高度接触的无支架心包二尖瓣，首次采用有限元方法进行应力计算是有效可行的；(2)本研究的有支架心包三叶瓣有限元模型得到的应力分布与临床结果一致；(3)无架心包二尖瓣较有架心包三叶瓣应力分布明显合理，有益于防止瓣叶撕裂和钙化、延长寿命。     

窦管交界锥度对生物瓣影响的脉动流实验研究EI北大核心CSCD

为了探究窦管交界直径呈现不同锥度角对于人工生物瓣膜性能的影响,对窦管交界不同锥度角的主动脉根部模型进行脉动流实验研究。通过三维打印方法制作窦管交界锥度角分别为0度、1度、3度和5度的4组主动脉根部模型。将人工生物瓣膜装配在主动脉根部模型上,在脉动循环模拟系统中进行不同心搏出量条件下的脉动流实验测试。心率设定为70次/分,脉动流流动速率分别为2~7 L/min。在每个脉动流流动速率条件下测试10个心动周期并将结果取平均值。实验结果显示,主动脉根部窦管交界不同锥度的模型,跨瓣压差符合国内标准,均在10 mm Hg之内;窦管交界锥度对于返流百分比存在影响,较小的窦管交界锥度有利于降低返流百分比;在心搏出量较小的情况下,较小的窦管交界锥度有利于增大有效开口面积,而在心搏出量较大的情况下,较大的窦管交界锥度有利于增大有效开口面积。实验结果表明,在临床手术过程中,对于心搏出量较低的情况,建议多考虑较小的窦管交界锥度;而对于心搏出量较高的情况,建议多考虑较大的窦管交界锥度。     

窦管交界锥度对生物瓣影响的脉动流实验研究

为了探究窦管交界直径呈现不同锥度角对于人工生物瓣膜性能的影响,对窦管交界不同锥度角的主动脉根部模型进行脉动流实验研究。通过三维打印方法制作窦管交界锥度角分别为0度、1度、3度和5度的4组主动脉根部模型。将人工生物瓣膜装配在主动脉根部模型上,在脉动循环模拟系统中进行不同心搏出量条件下的脉动流实验测试。心率设定为70次/分,脉动流流动速率分别为2~7 L/min。在每个脉动流流动速率条件下测试10个心动周期并将结果取平均值。实验结果显示,主动脉根部窦管交界不同锥度的模型,跨瓣压差符合国内标准,均在10 mm Hg之内;窦管交界锥度对于返流百分比存在影响,较小的窦管交界锥度有利于降低返流百分比;在心搏出量较小的情况下,较小的窦管交界锥度有利于增大有效开口面积,而在心搏出量较大的情况下,较大的窦管交界锥度有利于增大有效开口面积。实验结果表明,在临床手术过程中,对于心搏出量较低的情况,建议多考虑较小的窦管交界锥度;而对于心搏出量较高的情况,建议多考虑较大的窦管交界锥度。     

应用于零维左心血液循环的二尖瓣模型的研究

提出一个可以准确合理地模拟二尖瓣动力学特性的瓣叶运动流阻模型。考虑影响二尖瓣瓣叶运动的跨瓣压差和血流推力,建立二尖瓣运动的控制方程,提出依赖于瓣叶打开角度θ的瓣叶运动流阻模型,把该模型应用于零维左心血液循环系统,得到血液动力学特性。在保持心输出量和反流分数一致的条件下,比较该模型、瞬态关闭的阶梯流阻模型和经验指定的时变流阻模型。结果发现,瓣叶运动流阻模型能反映瓣膜关闭过程中的血液动力学,如压差和流量的滞后性以及关闭流量,同时该模型可以通过调整单位转动惯量跨瓣压差影响系数K_p和血流影响系数K_b的大小,改变瓣膜打开过程和关闭过程所需时间,瓣膜打开和关闭时间分别为50.0和40.2 ms。该模型可弥补阶梯流阻模型中忽略瓣膜运动过程的瞬态关闭的缺点,同时也能避免时变流阻模型中关闭起始时间的不合理性。此模型较为合理准确地模拟二尖瓣关闭过程的动力学特性,且简单易控制。     

The closing velocity of mechanical heart valve leaflets

The closing motion of the occluder leaflets in bileaflet type mechanical heart valves (MHV) was monitored with a laser sweeping technique. The angular displacements of the leaflets were registered with precision of 0.2 μs steps. Experimental measurements were made using five 29 mm Edwards-Duromedics™ including three original specification (EDOS) and two modified specification (EDMS), and two 29 mm St Jude Medical® MHVs. The testing valve was installed in the mitral position of a physiologic pulsatile mock circulatory flow loop using water-glycerine solution as the testing fluid. Each valve was tested by: (1) direct mounting the valve on metal washers, and (2) mounting the valve with its sewing ring. Experiments were carried out at pulse rates of 70, 90, and 120 beats min⁻¹, with the corresponding cardiac output of 5, 6, and 7.5 litres min⁻¹, and maximum left ventricular pressure gradients ( ) of 1,800, 3,000 and 5,600 mm Hg s⁻¹, respectively. The maximum leaflet closing velocity of each of the tested valve types are presented. The difference in leaflet closing movements between the direct rigid mounting and the sewing ring mounting are discussed. The details of the laser sweeping technique are presented.     

A synthetic leaflet heart valve with improved opening characteristics

A new geometry for the design of polyurethane leaflet heart valves has been investigated. The geometry termed the ‘alpharabola’ has a radius of curvature that increases from the centre of the leaflet at the free edge towards the base of the valve and perimeter of the leaflet. The hydrodynamic function and leaflet opening characteristics of the new valve design have been compared to a valve with a spherical leaflet geometry using the same material. The pressure and flow required to open alpharabola leaflets in steady flow tests was markedly lower than for spherical leaflets. Under pulsatile flow conditions with the valve leaflets fully open, the pressure drop across the alpharabola and spherical leaflets was similar, but much lower than in a porcine bioprosthesis. High speed photography showed that the alpharabola leaflets opened in less than 30 ms with the leaflet opening initiating in the base of the leaflet where the radius of curvature was larger. The synthetic leaflet valve has demonstrated short term durability in accelerated fatigue tests to 100 million cycles.     

Analysis of ATS leaflet behavior by in vitro experiment

It has been reported that the normally functional bileaflet valve ATS with open-pivot design does not exhibit a full opening motion either in the mitral or in the aortic position in patients. An in vitro experiment was conducted to investigate the mechanism of the ATS leaflet movement. ATS 29 mm for the mitral position was chosen in our experiment and SJM 29 mm was chosen as a control. Two pulsatile simulators were employed to investigate the factors affecting leaflet movement. Two different conduits to be incorporated downstream (in simulator I) and three different inlet coverings to alter the local flow field around the open pivot (in simulator II) were used. A high-speed video camera was employed to observe leaflet movment. The ATS valve could exhibit a fully open movement in straight conduit but could not fully open when an enlarging shape was incorporated downstream of the ATS valve. The covering of the ATS open pivot could make the leaflets fully open or increase the opening angle with the existene of the enlarging downstream shape. The enlargement downstream of the ATS valve, which induces a divergent transvalvular flow, is the main reason that the leaflets do not fully open. The local flow field around the open pivot, which induces an additional moment M_a, plays an important role in the movement of the ATS leaflets.     

Bioprosthetic Heart Valve Leaflet Deformation Monitored by Double-Pulse Stereo Photogrammetry

A double-pulse stereo photogrammetry technique has been developed for the dynamic assessment of the leaflet deformation of bioprosthetic heart valves under simulated physiological conditions. By using a specially designed triggering technique, which takes the advantage of the field transfer mechanisms of the charge coupled device camera, two consecutive images separated by a time interval as short as 5 ms were captured. This made it possible to investigate the realistic leaflet deformation during the valve opening and closing processes which typically last 25–45 ms. This technique was applied to assess a newly developed pericardial valve leaflet in a physiological pulse flow loop. Quantitative leaflet deformations of the valve opening and closing were generated from sequences of digital images. The results can later be applied to finite element analysis of bioprosthetic heart valve leaflet stress and strain during a complete cardiac cycle. © 2002 Biomedical Engineering Society. PAC02: 8719Hh, 8768+z, 8719Rr, 8719Uv     

Development and validation of implantable sensors for monitoring function of prosthetic heart valves:<Emphasis Type="Italic">in vitro</Emphasis> studies

The development of a ‘smart’ heart valve prosthesis, with the intrinsic ability to monitor thrombus formation, mechanical failure and local haemodynamics and to relay this information externally, would be of significant help to clinicians. The first step towards such a valve is development of the sensors and examination of whether sensor output provides predictive information on function. Custom-made piezo-electric sensors were mounted onto the housing of mechanical valves with various layers of simulated thrombus and bioprosthetic valves with normal and stiffened leaflets. Sensor output was examined using joint time-frequency analysis. Sensors were able to detect leaflet opening and closing with high fidelity for all types of valve. The frequency content of the closing sounds for the mechanical valves contained several peaks between 100 Hz and 10 kHz, whereas closing sounds for the bioprosthetic valve contained energy in a lower frequency range (<1 kHz). A frequency peak of 47±15 Hz was seen for the normal bioprosthetic valve; this peak increased to 115±12 Hz for the valve with visibly stiffened leaflets. Total low-frequency (80–3500 Hz) energy content diminished predictably with increasing levels of thrombus for the mechanical valves. Lastly, closing sound intensity correlated well with closing pressure dynamics (dp/dt) (y=190x−443; r=0.90), indicating that the sensors also provide information on haemodynamics. These studies provide initial evidence regarding the use of embedded sensors to detect prosthetic valve function. Efforts to encapsulate these sensors with telemetry into a custom valve are currently underway.     

Dynamic particle image velocimetry flow analysis of the flow field immediately downstream of bileaflet mechanical mitral prostheses

New dynamic particle image velocimetry (PIV) technology was applied to the study of the flow field associated with prosthetic heart valves. Four bileaflet prostheses, the St. Jude Medical (SJM) valve, the On-X valve with straight leaflets, the Jyros (JR) valve, and the Edwards MIRA (MIRA) valve with curved leaflets, were tested in the mitral position under pulsatile flow conditions to find the effect of the leaflet shape and overall valve design on the flow field, particularly in terms of the turbulent stress distribution, which may influence hemolysis, platelet activation, and thrombus formation. Comparison of the time-resolved flow fields associated with the opening, accelerating, peak, and closing phases of the diastolic flow revealed the effects of the leaflet shape and overall valve design on the flow field. Anatomically and antianatomically oriented bileaflet valves were also compared in the mitral position to study the effects of the orientation on the downstream flow field. The experimental program used a dynamic PIV system utilizing a high-speed, high-resolution video camera to map the true time-resolved velocity field inside the simulated ventricle. Based on the experimental data, the following general conclusions can be made. High-resolution dynamic PIV can capture true chronological changes in the velocity and turbulence fields. In the vertical measuring plane that passes the centers of both the aortic and mitral valves (A-A section), bileaflet valves show clear and simple circulatory flow patterns when the valve is installed in the antianatomical orientation. The SJM, the On-X, and the MIRA valves maintain a relatively high velocity through the central orifice. The curved leaflets of the JR valve generate higher velocities with a divergent flow during the accelerating and peak flow phases when the valve is installed in the anatomical orientation. In the velocity field directly below the mitral valve and normal to the previous measuring plane (B-B section), where characteristic differences in valve design on the three-dimensional flow should be visible, the symmetrical divergent nature of the flow generated by the two inclined half-disks installed in the antianatomical orientation was evident. The SJM valve, with a central downward flow near the valve, is contrasted with the JR valve, which has a peripherally strong downward circulation with higher turbulent stresses. The On-X valve has a strong central downward flow attributable to its large opening angle and flared inlet shape. The MIRA valve also has a relatively strong downward central flow. The MIRA valve, however, diverts the flow three-dimensionally due to its peripherally curved leaflets.     

In Vitro Study of Influence of Mimic Cardiac Rate on Hydrodynamics of the Different Mechanical Cardiac Valve Prostheses

To assess the influence of mimic cardiac rate on hydrodynamics of the different mechanical prosthetic cardiac valves. Methods. US-made CarboMedics bileaflet valve and China-made Jiuling bileaflet valve and C-L tilting disc valve have been tested in a pulsatile flow simulator in the aortic position. The testing condition was set at the mimic cardiac rate of 55 beats/min,75 beats/min,100beats/min and a constant mimic cardiac output of 4L/min. The mean pressure differences (△P),leakage volumes (LEV) and closing volumes(CLV) across each valve,and the effective orifice areas(EOA) have been analyzed. Results.Within the range of physiology,the AP,LEV and CLV were falling as the increasing of mimic cardiac rate,and the extent of variance was larger. The EOA was increasing with the increase of the mimic cardiac rate. It is a different response as the altering of the cardiac rate for the different type of the mechanical prosthetic cardiac valves.Conclusions.The change of the mimic cardiac rate can affect the hydrodynamics of the mechanical prosthetic cardiac valves. The hydrodynamics of the bileaflet valve prosthesis is better than the tilting disc valve.     

Dynamic In Vitro Quantification of Bioprosthetic Heart Valve Leaflet Motion Using Structured Light Projection

Quantification of heart valve leaflet deformation during the cardiac cycle is essential in understanding normal and pathological valvular function, as well as in the design of replacement heart valves. Due to the technical complexities involved, little work to date has been performed on dynamic valve leaflet motion. We have developed a novel experimental method utilizing a noncontacting structured laser-light projection technique to investigate dynamic leaflet motion. Using a simulated circulatory loop, a matrix of 150–200 laser light points were projected over the entire leaflet surface. To obtain unobstructed views of the leaflet surface, a stereo system of high-resolution boroscopes was used to track the light points at discrete temporal points during the cardiac cycle. The leaflet surface at each temporal point was reconstructed in three dimensions, and fit using our biquintic hermite finite element approach (Smith et al., Ann. Biomed. Eng. 26:598–611, 2001). To demonstrate our approach, we utilized a bovine pericardial bioprosthetic heart valve, which revealed regions of complex flexural deformation and substantially different shapes during the opening and closing phases. In conclusion, the current method has high spatial and temporal resolution and can reconstruct the entire surface of the cusp simultaneously. Because it is completely noncontacting, this approach is applicable to studies of fatigue and bioreactor technology for tissue engineered heart valves. © 2001 Biomedical Engineering Society. PAC01: 8719Hh, 8780-y, 4262Be, 8719St     

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.