Relationship Between Mechanical and Hydrodynamic Properties of Bioprosthesis Produced from Canine Aortic Valve

Static tensile, stress relaxation, and hydrodynamic tests were carried out to investigate the relationships between the mechanical deformation or stiffness of heart valve leaflets and the opening behavior of bioprosthetic valves. The specimens used were fresh and glutaraldehyde (GA)-treated canine aortic valves. The tensile strength depended on the fiber orientation in the leaflet. The deformability of fresh and 0.05% GA-treated tissues was significantly larger than that induced by 0.1-5.0% GA concentrations according to the stress-strain curves. The stress relaxation function, which expresses the viscoelastic property, did not show significant differences in the 0.05-5.0% range of GA concentrations. In the hydrodynamic tests, the opening resistance of fresh and 0.05% GA-treated valves was less than that of 0.1-5.0% GA-treated valves. Thus, it was shown that the hydrodynamic valve functions were closely related to the material properties of aortic valve leaflets.     

Severe stenosis of bioprosthetic valve due to late valve thrombosis

The typical cause of bioprosthetic valve dysfunction over years is calcification of leaflets, pannus formation, or tears due to structural degeneration. Thrombosis is rare as the valves get endothelialized early on, and, hence, anticoagulation is not recommended beyond 6 months after valve replacement. While bioprosthetic valve thrombosis is unusual (0.03% to 0.34%/year), it can be associated with significant mortality and morbidity. Here, we present a case of a middle-aged man with history of bioprosthetic mitral valve who presented with syncopal episode and was referred to us for mitral valve replacement for tentative bioprosthetic valve degeneration and stenosis. However, preoperative work up revealed prosthetic valve thrombosis which was successfully treated with anticoagulation.     

种植人体活性细胞的生物心脏瓣膜

目前,种植人体活性细胞的生物心脏瓣膜主要有组织工程心脏瓣膜和种植人体活性细胞的猪主动脉瓣两种。组织工程心脏瓣膜是在人体可吸收的聚二醇酸纤维支架上种植人体同种活性细胞,先种植成纤维细胞,再种植单层内皮细胞包裹瓣叶。种植人体活性细胞的猪主动脉瓣是在清除原有细胞的组织内重建人体同种活性细胞。清除新鲜猪主动脉瓣呐原有细胞的方法是将瓣膜先经高、低渗溶液处理,然后用酶溶液处理。细胞经培养分离后,将成纤维细胞植入经处理的瓣膜组织,再植入内皮细胞。种植人体活性细胞的生物心脏瓣膜不会促使受者产生有害的免疫反应,并具有再生能力。     

An assessment of the mechanical properties of leaflets from four second-generation porcine bioprostheses with biaxial testing techniques

The aim of this study was to determine whether second-generation porcine bioprostheses, glutaraldehyde fixed at pressures said to be less than 4 mm Hg, exhibit more natural leaflet material properties than earlier valves fixed at 80 to 100 mm Hg. Biaxial mechanical testing techniques were used to compare Carpentier-Edwards SAV, St. Jude Medical BioImplant, Hancock II, and Medtronic Intact bioprostheses (12 leaflets from four valves in each case) with fresh porcine aortic valves and high pressure-fixed Carpentier-Edwards 6625 bioprostheses (14 leaflets from five valves in each case). The circumferential extensibility of leaflets from Medtronic Intact bioprostheses and from fresh porcine aortic valves were not significantly different (p greater than 0.05), whereas leaflets from the other second-generation valves tested and from Carpentier-Edwards 6625 valves were highly inextensible in the circumferential direction. The radial material properties of leaflets from all bioprostheses differed from those of fresh porcine aortic valves, which were very extensible with a high pretransitional compliance. The radial extensibility and compliance of Hancock II, St. Jude Medical BioImplant, and Carpentier-Edwards 6625 leaflets were not significantly different (p greater than 0.05). In the radial direction, Carpentier-Edwards SAV and Medtronic Intact valve leaflets were substantially more extensible than Carpentier-Edwards 6625 leaflets (p less than 0.01), whereas Medtronic Intact leaflets were more compliant than all other bioprostheses. These data demonstrate (1) that second-generation porcine bioprosthetic valves do not necessarily exhibit more natural leaflet material properties than earlier high pressure-fixed xenografts and (2) that Medtronic Intact valve leaflets have material properties most closely approximating the fresh porcine aortic valve.     

In Vitro Comparison of Velocity Profiles and Turbulent Shear Distal to Polyurethane Trileaflet and Pericardial Prosthetic Valves

A comparative study of flow dynamics past biomer trileaflet valves and a pericardial bioprosthetic valve under steady and physiological pulsatile flow conditions in vitro is reported in this paper. The velocity profiles and the turbulent shear stresses distal to the valves were measured using laser Doppler anemometry. The authors' results showed that the velocity profiles distal to the trileaflet valves were similar to that measured distal to the pericardial valve. Higher magnitudes of absolute turbulent shear stresses were measured distal to the synthetic valves in comparison to the pericardial valves. However, when the stresses were nondimensionalized with respect to the orifice diameter at the inlet aspect, the stresses were comparable for all of the three valves. With design modifications to increase the orifice diameter at the inlet aspect of the polyurethane valves, the turbulent stresses distal to the valves can be minimized. Such in vitro studies on the flow dynamics past the polyurethane valves can provide information towards design changes to improve the performance characteristics of these valves. Polyurethane valves with flow characteristics comparable to the pericardial valves can be manufactured relatively inexpensively compared to mechanical or tissue valve prosthesis. Hence, the synthetic valves may be a viable alternative for short-term use in total artificial heart devices as a bridge to transplant.     

Hydrodynamic characteristics of bileaflet mechanical heart valves in an artificial heart: cavitation and closing velocity

The aim of this study was to investigate the possibility of using the bileaflet valves in an electrohydraulic total artificial heart (EHTAH). Three kinds of bileaflet valves, namely the ATS valve (ATS Medical Inc., Minneapolis, MN, USA), the St. Jude valve (St. Jude Medical Inc., St. Paul, MN, USA), and the Sorin Bicarbon valve (Sorin Biomedica, Vercelli, Italy), were mounted in the mitral position on an inclined 45 degrees plane in an EHTAH. The pressure waves near the valve surface, the valve-closing velocity, and a high-speed camera were employed to investigate the mechanism for bileaflet valve cavitation. The cavitation bubbles in the bileaflet valves were concentrated along the leaflet tip. The cavitation intensity increased with an increase in the valve-closing velocity. It was established that squeeze flow holds the key to bileaflet valve cavitation. At lower heart rates, the delay time of the asynchronous closure motion between the two leaflets of the Sorin Bicarbon valve was greater than that of the other bileaflet valves. At higher heart rates, no significant difference was observed among the bileaflet valves.     

Long-term results of bioprosthetic mitral valve replacement: the pericardial perspective

Pericardial valve bioprostheses were introduced in early 1970s and were widely used in the 1980s. The longterm results with the Ionescu-Shiley valve, the first commercially available pericardial valve, were disappointing because of high rate cusp tears during the first decade after implantation. The enthusiasm for this type of bioprosthetic valve was further hampered by the premature failure of the Hancock pericardial valve. The long-term results of aortic valve replacement with the Carpentier-Edwards pericardial valve, which was introduced in 1981, indicated that that valve was durable and the issue of cusp tears had been resolved by an appropriate design. This knowledge prompted surgeons to revisit the merits of pericardial valves for mitral valve replacement and several other pericardial valves are now commercially available. The largest data on long-term results are with the Carpentier-Edwards pericardial mitral valve. The reported freedom from structure valve failure ranged from 69% to 85% at 10 years in patient population with mean age of 60 to 70 years. Young age is a major determinant of valve failure, which is largely due to calcification. There are also long-term data, albeit more limited on the Sorin Pericarbon and Mitroflow valves used for mitral valve replacement. This paper review the published experience with various pericardial bioprosthetic valves used for mitral valve replacement during the past 3 decades.     

The role of pannus in the longevity of an Ionescu-Shiley pericardial bioprosthesis

As the population ages, bioprosthetic heart valves are increasingly being used to replace diseased native valves. Bioprosthetic valve durability depends on patient age and other factors, but rarely exceeds 15 years. Explanted bioprosthetic valves commonly show tissue degeneration, tears, and calcification. Host tissue overgrowth (pannus), to the extent of interfering with their function, is another finding in bioprostheses that have been in place for long periods. We present a case in which a bovine pericardial valve was explanted after more than 20 years of implantation. The longevity of this pericardial valve may have been related to excessive pannus growth, which most likely protected the valve from earlier failure.     

Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

Bioprosthetic heart valves still have poor long-term durability due to calcification and mechanical failure. The function and performance of bioprostheses is known to depend on the collagen architecture and mechanical behavior of the target tissue. So it is necessary to select an appropriate tissue for such prostheses. In this study, porcine, equine, and bovine pericardia were compared histologically and mechanically. The specimens were analyzed under light microscopy. The planar biaxial tests were performed on the tissue samples by applying synchronic loads along the axial (fiber direction) and perpendicular directions. The measured biaxial data were then fitted into both the modified Mooney-Rivlin model and the anisotropic four parameter Fung-type model. The modified Mooney-Rivlin model was applied to the modeling of the bovine, equine, and porcine pericardia using finite element analysis. The equine pericardium illustrated a wavy collagen bundle architecture similar to bovine pericardium, whereas the collagen bundles in the porcine pericardium were thinner and structured. Wavy pericardia may be preferable candidates for transcutaneous aortic valves because they are less likely to be delaminated during crimping. Based on the biaxial tensile test, the specimens indicated some degree of anisotropy; the anisotropy rates of the equine specimens were almost identical, and higher than the other two specimens. In general, porcine pericardium appeared stiffer, based on the greater strain energy magnitude and the average slope of the stress–stretch curves. Moreover, it was less distensible (due to lower areal strain) than the other two pericardial tissues. Furthermore, the porcine model induced localized high stress regions during the systolic and diastolic phases of the cardiac cycle. However, increased mechanical stress on the bioprosthetic leaflets may cause tissue degeneration and reduce the long-term durability of the valve. Based on our observations, the pericardial specimens behaved as anisotropic and nonlinear tissues—well-characterized by both the modified Mooney-Rivlin and the Fung-type models. The results indicate that, compared to bovine pericardium, equine tissue is mechanically and histologically more appropriate for manufacturing heart valve prostheses. The results of this study can be used in the design and manufacture of bioprosthetic heart valves.     

去细胞猪主动脉瓣叶的获取和内皮细胞的种植

目的　探讨猪主动脉瓣叶去细胞后作为组织工程心脏瓣膜支架的可行性。　方法　经胰酶 - EDTA、表面活性剂和核酸酶处理 ,去除猪主动脉瓣叶的细胞成分 ,测定瓣叶去细胞前、后的生物力学特性 ,并在其表面种植新生牛主动脉内皮细胞 (BAECs) ;分别行大鼠皮下包埋实验。　结果　猪主动脉瓣叶中的细胞成分能完全去除 ,获得完整无细胞的纤维网状支架 ,断裂强度和断裂伸长率无明显变化 ;种植的 BAECs在去细胞瓣叶表面可形成一层连续的细胞层 ,其分泌前列环素 (PGI2 )的能力同直接种植在 2 4孔板中的比较 ,差异无统计学意义 (P>0 .0 5 )。　结论　猪主动脉瓣去细胞后获得的纤维支架可以用来构建组织工程瓣膜 ,适宜于血管内皮细胞的生长。     

On Stress Reduction in Bioprosthetic Heart Valve Leaflets by the Use of a Flexible Stent

It has been postulated that flexible stent posts can reduce tensile stress at the commissures of tissue heart valves by about 90% when compared with the same valve mounted on a rigid stent. We have used a detailed computer model to investigate the role of flexible stent posts in reducing stress in the leaflets of three types of bioprosthetic heart valves: the bovine pericardial and the high- and zero-pressure fixed porcine valves. The models use stress/strain data from biaxial experiments to characterize the tissue properties and are subjected to a back pressure of 120 mmHg. We found that strain was reduced linearly with stent post deflection and that this was a purely static process--it did not require the load to be applied impulsively. This finding was in close agreement with earlier experimental studies, which measured the same strain reduction whether the valve was loaded quasi-statically or at physiological rates. In addition, we found that for this mechanism to be effective the valve must have good coaptation at the center and the tissue should be stiff; in other cases, the advantages of strain reduction through the use of a flexible stent are considerably diminished.     

Stresses of Natural versus Prosthetic Aortic Valve Leaflets in Vivo

During normal function of the aortic valve, the aortic leaflets undergo not only cyclic loading and unloading but also cyclic reversal of their curvature. The stresses induced in the leaflet due to these variations have been computed using a new concept based on the structure of the leaflet. Membrane stresses have been related to the pressure difference across the leaflet and bending stresses to the leaflet curvature. Total stresses were obtained by adding the two stresses. Total stresses in bioprosthetic and synthetic leaflets also were computed using the same approach. In systole, the natural leaflet is subjected to much lower total stress than a bioprosthetic or a synthetic leaflet. The natural leaflet is not subjected to compressive stresses during the cardiac cycle, whereas bioprosthetic and synthetic leaflets must sustain compressive stresses during systole. The differences in stress patterns of these leaflets indicate that there is a difference in their longevity.     

Leaflet arrest in St Jude Medical and CarboMedics valves: an experimental study

Objective: Two patients who suffered acute failure of their St Jude Medical Masters aortic valve prostheses due to leaflet arrest that were unrelated to suture material are presented. It was hypothesized that the valves failed because force applied to bear upon the valve annulus caused the hinge mechanism to become restricted or to arrest. Methods: A study was designed to measure the force that would cause leaflet arrest in three sizes of St Jude Medical Masters valves and CarboMedics mechanical valves. A specially manufactured pushrod device was used to apply a variable force to the sewing cuff, low annulus level, or to the pivot guards of all the valves. Results: For every valve size tested, the St Jude Medical Masters valves required significantly less force applied than did the CarboMedics valves to cause one or both leaflets to arrest (P<0.0004). Conclusions: A force applied on the valve ring of mechanical valves can cause leaflet arrest. The force required to arrest the leaflets is within an order of magnitude of that measured in other in vivo animal studies. We conclude that force on the valve rings in patients after aortic valve surgery could cause leaflet malfunction and even arrest in some patients.     

Ventricular assist device in patients with prosthetic heart valves

Ventricular assist device (VAD) support inpatients with a prosthetic heart valve had previously been considered a relative contraindication due to an increased risk of thromboembolic complications. We report our clinical experience of VAD implantation in patients with prosthetic heart valves, including both mechanical and bioprosthetic valves. The clinical records of 133 consecutive patients who underwent VAD implantation at a single institution from January 2002 through June 2009 were retrospectively reviewed. Six of these patients had a prosthetic valve in place at the time of device implantation. Patient demographics,operative characteristics, and postoperative complications were reviewed.Of the six patients,four were male.The mean age was 57.8 years (range 35–66 years). The various prosthetic cardiac valves included a mechanical aortic valve (n = 2), a bioprosthetic aortic valve (n = 3), and a mechanical mitral valve (n = 1).The indications for VAD support included bridge to transplantation (n = 2), bridge to recovery (n = 1), and postcardiotomy ventricular failure(n = 3). Three patients underwent left ventricular assist device placement and three received a right ventricular assist device. Postoperatively, standard anticoagulation management began with a heparin infusion (if possible)followed by oral anticoagulation.The 30-day mortality was50% (3/6). The mean duration of support among survivors was 194.3 days (range 7–369 days) compared with 16.0 days(range 4–29 days) for nonsurvivors. Of the three survivors,two were successfully bridged to heart transplantation and one recovered native ventricular function.Among the three nonsurvivors,acute renal failure developed in each case, and two developed heparin-induced thrombocytopenia. This study suggests that VAD placement in patients with a prosthethic heart valve, either mechanical or bioprosthetic,appears to be a reasonable option.     

Comparison of outcomes after mitral valve replacement with a mechanical versus a bioprosthetic valve in patients between forty and sixty years of age

Background  The American College of Cardiology/American Heart Association (ACC/AHA), guidelines for choice of prosthetic valve based on patients’ age are difficult to apply to the developing world because of a lower life expectancy and difficulty in maintaining correct levels of anticoagulation for a variety of reasons. While there is general agreement on the choice of prosthetic valves for patients below 40 years of age (mechanical) and above 60 years of age (biologic), the 40 to 60 age group remains a grey zone. The goal of our study was to compare outcomes after mitral valve replacement with a mechanical versus a bioprosthetic valve in patients between forty and sixty years of age. Methods  From Jan 2003 to July 2008, 250 patients between the ages of 40 and 60, undergoing mitral valve replacement at our institution were randomized to receive either a mechanical or a bioprosthetic valve. Outcomes in the form of incidence of valve thrombosis and thromboembolism, bleeding complications, incidence of prosthetic valve endocarditis and survival were compared in the two groups. Results  Out of 250 patients, 135 patients received mechanical valve and 115 patients were implanted with a bioprosthetic valve. Patients were followed up for a mean period of 3 years (range 6 months to 4.8 years). The incidence of valve thrombosis was higher in mechanical valve as compared to bioprosthetic valve (6% vs. 0.9%, p= 0.04). Similarly there was a higher incidence of thromboembolism in mechanical valves as compared to bioprosthetic valves (4.5% vs. 0%, p=0.03). Bleeding complications occurred more frequently in mechanical than bioprosthetic valve (6% vs. 0.9%, p=0.04). There was no significant difference in the incidence of prosthetic valve endocarditis (2.2% vs. 2.7%, p >0.05) or survival at three years (96.2% vs. 97.2%, p > 0.05) in the two groups. Conclusions  Patients in the age group of 40 to 60 years undergoing mitral valve replacement with a mechanical valve have a higher incidence of thrombotic and bleeding complications as compared to bioprosthetic valve, even though short term survival is similar. This favours implantation of a bioprosthetic valve in this age group.     

Bioprosthetic heart valve rupture associated with trauma

Disruption of a bovine pericardial bioprosthetic aortic heart valve occurred in a motor vehicle accident, and was treated by valve replacement for progressive aortic insufficiency. Leaflet rupture was through areas of noninflammatory tissue degeneration, corresponding to regions of repeated mechanical stress and trauma that occur during the normal function of tissue valves. Patients with bioprosthetic heart valves may be predisposed to traumatic valve injury. Early diagnosis and replacement of these disrupted valves should be accomplished to avoid sudden, unpredictable heart failure.     

An improved pericardial bioprosthetic heart valve : Design and laboratory evaluation

The design and development of a new three-leaflet pericardial valve are described and its function and durability evaluated in the laboratory. The unique design allows the leaflets to be located on small radially projecting pins on a pericardial covered frame. Leaflets are easily interchanged on the frame to allow each set of leaflets to be matched during a function test prior to final assembly. The valve has similar hydrodynamic function to other pericardial valves and accelerated fatigue tests have shown improved durability with the elimination of leaflet tears caused by abrasion at the edge of the frame.     

Primary tissue failure in pericardial heart valves

A number of centers have recorded a significant incidence of primary tissue failure with the standard Ionescu-Shiley pericardial valve. In most cases severe regurgitation was caused by leaflet tears adjacent to the edge of the cloth-covered stent. Our early clinical experience (up to 4 years' follow-up) with two new pericardial valves (Ionescu-Shiley low-profile and Hancock pericardial valves) has shown that primary tissue failure also occurs in these new valves. In vitro accelerated fatigue studies on seven of these valves (size 29 mm) showed that in vitro premature leaflet failure was caused by abrasion of the leaflet on the cloth-covering at the edge of the stent. Clinically, endothelialization and host tissue ingrowth on the cloth and the leaflets at the edge of the frame greatly reduced the amount of abrasion and the incidence of tissue failure. In seven of the eight explanted valves studied, leaflet tears occurred at the top of the stent posts where there was less endothelialization and tissue ingrowth, close to the points where sutures pass through the leaflets. It is likely that both abrasion and stress concentration around these sutures contributed to the tissue failures in the clinical valves.     

Hemodynamic and pathologic evaluation of a unileaflet pericardial bioprosthetic valve

This study investigated the in vivo hemodynamics and pathologic changes of a unileaflet pericardial bioprosthetic valve 3 to 5 months after implantation in juvenile sheep. Group 1 had 10 sheep with tricuspid valve replacement. Group 2 had nine sheep with mitral valve replacement. Group 3 served as a control with 10 sheep that had tricuspid valve replacement with a trileaflet porcine bioprosthesis. Hemodynamic performance was satisfactory in all three groups despite prominent pathologic changes, particularly in unileaflet valves. Intrinsic cuspal calcification was present in 66% of the unileaflet tricuspid, 88% unileaflet mitral, and 25% porcine tricuspid valves. Neither cuspal tearing nor perforations were found. However, cuspal stretching and redundancy of the mobile cusp was present in six tricuspid, seven mitral unileaflet valves, and no porcine valves. Gross pericardial redundancy correlated with the microscopic appearance of distorted and separated collagen bundles. These findings suggest that multiple modes of primary tissue failure may limit the durability of this unileaflet pericardial valve.     

Computational evaluation of platelet activation induced by a bioprosthetic heart valve

It is known that bioprosthetic heart valves (BHVs) have better hemodynamics and lower thromboembolic events compared with their mechanical counterparts; however, patients implanted with BHVs still face the potential of such complications. The risk of a clinical thromboembolism is on average 0.7% per year in patients with tissue valves in sinus rhythm. In this study, we developed a computational fluid dynamic (CFD) model of a BHV implanted in an aortic root and investigated the BHV-induced platelet activation using a damage accumulation model previously applied to mechanical valves. The CFD model was validated against published experimental data, including the flow velocity profile across the valve and the transvalvular pressure drop, and close matches were obtained. Hemodynamic performance measures such as flow velocity, turbulent kinetic energy, and wall shear stress were explored. Lagrangian particle tracking was used to calculate the extent of platelet activation for central bulk flow and flow in the vicinity of the leaflets. A peak flow of 2.22 m/s was observed at 40 msec after peak systole in the vicinity of a fold at the base of the leaflets. With the platelet activation expressed as 0-100% of activation threshold levels, mean damage on one pass was 2.489 × 10(-7)% and maximum damage on one pass was 8.778 × 10(-4)%. Our results suggested that the potential for BHV-induced platelet activation was low and that the leaflet's fully open geometry might play a role in the extent of blood element damage.