- الرئيسية
- وقائع المؤتمر
- QScience Proceedings
- وقائع المؤتمر
5th Biennial Conference on Heart Valve Biology and Tissue Engineering
- تاريخ المؤتمر: 18-20 May 2012
- الموقع: Mykonos Island, Greece
- رقم المجلد: 2012
- المنشور: ٠١ مايو ٢٠١٢
41 - 60 of 86 نتائج
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Pressure and Angiotensin II Influence the Mechanical Properties of Aortic Valves
المؤلفون: Valtresa Myles, Jun Liao and James N. WarnockAbstractElevated cyclic pressure and angiotensin II (Ang II) both promote aortic valve collagen synthesis, an early hallmark of aortic sclerosis. In the current study, it was hypothesized that the increased collagen production induced by either elevated pressure or Ang II would increase tissue stiffness. Porcine aortic valve leaflets were randomly assigned to four groups; leaflets in group 1 were treated with 10-6M Ang II, leaflets in group 2 were exposed to 80 or 120 mmHg cyclic pressure, leaflets in group 3 were treated with Ang II and exposed to cyclic pressure and leaflets in group 4 were the control. Biaxial testing was performed after 24 and 48 hours on 10mm x 10mm samples of tissue dissected from the central region of the leaflets. Four fiducial markers arranged in an approximately 4 mm x 4 mm square were placed in the center of the extracted portion of the leaflets to track tissue strain. A membrane tension (force/unit length) was applied along each axis and increased slowly from a pre-stress tension of ~0.5 N/m to a peak tension of 60 N/m. The samples were preconditioned for ten contiguous cycles, following an equibiaxial protocol of TCC:TRR = 60:60 N/m, where TCC and TRR are the tensions applied in the circumferential and radial directions, respectively. Tissue extensibility was characterized by maximum stretch along the circumferential direction (λcc) and maximum stretch along the radial direction (λrr), at an equibiaxial tension of 60 N/m. Leaflet stiffness was greater in the circumferential direction than in the radial direction, which is consistent with previous studies. The peak stretches of native valves were calculated as 1.05±0.02 and 1.40±0.02 in the circumferential and radial directions, respectively. There was no significant difference in the stiffness of native valves compared to those exposed to 0mmHg(-Ang II) or 80mmHg(-Ang II) for 24 or 48 hours. Elevated pressure increased stiffness in both directions after 24 and 48 hours. After 24 and 48 hours Ang II significantly increased stiffness in the radial direction. The combination of Ang II and elevated pressure increased stiffness in radial direction after 24 and 48 hours. Increased stiffness may be due to remodeling of the ECM. Excessive collagen production is known to hinder valve function, eventually resulting in aortic stenosis. In conclusion, the results of the present study demonstrated that both elevated pressure and Ang II play a role in the increased stiffness of aortic valve leaflets.
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Adaptation of the Mitral Valve to Chronic Ischemic Left Ventricular Failure in Swine
المؤلفون: Bryant V. McIver, Samiya Hussain, Vinod Thourani and Muralidhar PadalaAbstractMitral valve leaflets in patients with chronic heart failure are severely stiffened, larger and fibrotic; though the mechanisms underlying such biological remodeling are currently unknown. Chronic tethering of the valve leaflets by the enlarged left ventricle, and the presence of mitral regurgitation (MR), is hypothesized to trigger fibrotic pathways that induce such remodeling. In this study, we sought to develop a large animal model of chronic ischemic left ventricular failure in which moderate leaflet tethering and MR can be repeatedly induced, and the remodeling of the mitral valve can be assessed in vivo using echocardiography. Inferior wall myocardial infarction (IMI) was induced in 16 farm swine (30-35kg) using a percutaneous transfemoral approach. Coronary angiography was performed to selectively identify the marginal branches of the left circumflex artery perfusing the inferior left ventricular (LV) wall and the posterior papillary muscle (PPM), and then these regions were infarcted using a 2 mm balloon catheter and injection of 100% ethyl alcohol into the corresponding branch. Transthoracic 2D echocardiography (TTE) of the left heart was performed pre and post-IMI in all animals, at 4 weeks and at 8 weeks. MR percentage (MR jet/left atrial area), mitral leaflet length and thickness, and mitral annular geometry were measured. Leaflet length and thickness were measured at end diastole to assure measurements were taken in the maximal unloaded state. All animals survived the procedure without complications (0% mortality). At baseline, the swine had mild MR (10.7±8%) which significantly increased to 26.6±9% immediately post-op (p<0.05), declined slightly to 23.8±7.8% at 4 weeks, and increased to 32.2±13.9% at 8 weeks. Mitral annular diameter increased from 2.7±0.5 cm pre-op, to 3.1±0.5 cm post-op (p<0.05), then to 3.8±0.4 cm (p<0.005) and 3.9±0.6 cm (p<0.005), at 4 and 8 weeks, respectively. Posterior leaflet length was 1.8±0.2 cm pre-op, and then stabilized at 1.9±0.2 cm post-op, at 4 weeks, and at 8 weeks. Anterior leaflet length was 2.3±0.3 cm at baseline and increased with the increase in MR, to 2.5±0.4 cm post-op, 2.8±0.5 cm at 4 weeks (p<0.05), and 3.2±0.2 cm at 8 weeks (p<0.001). Posterior leaflet thickness increased from 0.36±0.05 cm at baseline, to 0.37±0.07 cm post-op, and then to 0.4±0.05 cm and 0.41±0.03 cm at 4 weeks and 8 weeks, respectively. Anterior leaflet thickness was 0.4±0.05 cm at baseline and post-operatively, and 0.47±0.03 cm (p<0.01) and 0.45±0.07 cm at 4 weeks and 8 weeks, respectively. Changes in mitral leaflet geometry correspond with changes in the mitral valvular apparatus after IMI in a chronic swine model. As the mitral annulus increases in diameter, significant lengthening is seen from the anterior leaflet, while the posterior leaflet remains relatively static and tethered. The anterior leaflet thickens by 4 weeks, while the posterior leaflet remains stable.
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Valve Calcification on Computed Tomography Can Estimate Aortic Stenosis Severity
AbstractUse of coronary CT angiography for screening of coronary artery disease is being advocated for the general population. Meanwhile, CT is increasingly used in preoperative planning for transcatheter aortic valve replacement (TAVR). Although aortic stenosis (AS) severity can be evaluated by measurement of incidentally found aortic valve calcification on CT, it has not been validated by pathologic specimens and can be confounded by calcification of adjacent structures as well as motion artefact. Micro-computed tomography (microCT) provides ultra-high resolution imaging of small structures to yield excellent estimation of tissue calcification. In this study, excised aortic valves from patients with confirmed AS were used to determine if the amount of calcium on microCT correlated with severity of aortic stenosis. Thirty-five aortic valves excised during surgical valve replacement underwent micro-CT imaging with resolution of 76μm in the axial direction. Amount of calcium was determined by absolute and proportional values of calcium volume. Correlation of calcium volume and preoperative mean aortic valve gradient (MAVG), peak transaortic velocity (Vmax), and aortic valve area (AVA) on echocardiography was evaluated. For the patients who had a preoperative CT scan with acceptable image quality, the amount of valvular calcification was also measured by a well-experienced radiologist using modified Agatston algorithm. Mean amount of calcium across all valves was 603.2±398.5mm3, while mean ratio of calcium volume to total valve volume was 0.36±0.16. Mean aortic valve gradient correlated positively with both calcium volume and ratio (r=0.72, p<0.001). Vmax also positively correlated with calcium volume and ratio (r=0.69 and 0.76 respectively, p<0.001). A logarithmic curvilinear model was best fit to the correlation. Calcium volume of 480mm3 showed sensitivity and specificity of 0.76 and 0.83, respectively for severe AS diagnosis, while calcium ratio of 0.37 yielded sensitivity and specificity of 0.82 and 0.94, respectively. Calcium volume and its proportion to total valve volume were found to be good predictive parameters for severe AS when estimated radiologically. Calcium volume quantification may be a complimentary measure for AS severity evaluation in situations where aortic valve calcification is found incidentally on CT as well as in preoperative assessment of aortic valves for TAVR.
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Mitral Valve Mechanics Following Posterior Leaflet Patch Augmentation
AbstractAttention towards optimization of mitral valve repair methods is increasing. Patch augmentation strategy is used to treat functional ischemic mitral regurgitation (FIMR) and hypertrophic cardiomyopathy. When used to extend the anterior leaflet, the procedure decreases the forces exerted on the secondary chordae. The purpose of this study was therefore to investigate the force balance changes following patch augmentation of the posterior leaflet, with particular attention to the secondary chordae tendineae emanating from the posterior papillary muscle (PPM) in an FIMR simulated valve. Twelve mitral valves were obtained from 80kg pigs. An in vitro test setup simulating the left ventricle was used to hold the valves with a papillary muscle positioning system. Water pressure within the ventricular chamber was regulated manually in order to simulate different static pressures during valve closure. An oval shaped porcine pericardial patch measuring 17x29mm was introduced into the posterior leaflet approximately 2mm from the annulus and extending circumferentially from the middle of P2 to the end of the P3 scallop. In order to simulate a healthy valve, retraction of the patch was performed using sutures, which were then released to simulate patch repair. Data were acquired with and without PPM displacement to simulate the effect from one of the main contributors of FIMR, before and after patch augmentation, giving four simulation scenarios. The PPM was displaced 12mm posteriorly and 5mm apically. Dedicated miniature transducers were used to record the forces exerted on the secondary chordae tendineae. Three-way ANOVA was used to analyze the measurements. The effect of displacing the posterior papillary muscle (p < .010) and implementing patch augmentation (p<.004) are significant and independent of each other. The overall effect of displacing the PPM induced tethering on the secondary chordae tendineae from the PPM to the posterior leaflet resulting in a force increase of 28.2 %. The overall effect of implementing the patch augmentation into the posterior leaflet induced a decrease in force of 24.8 % for the healthy and PPM displaced simulations together. The repairing effect of the patch augmentation is found by comparing the specific scenarios. A 40 % increase is induced by displacing the PPM and a 31 % decrease is found by implementing the patch augmentation, leaving the repaired tethering force a mere 9 % higher than that of the healthy measurements. Posterior leaflet patch augmentation significantly reduced the forces exerted onto the secondary chordae tendineae from the PPM in both healthy and PPM displaced valves. As changes in chordal tension leads to redistribution of the total stress exerted on the valve, patch augmentation may have adverse long term influence on mitral.
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Distensibility of Decellularized and Glutaraldehyde-Preserved Aortic Full Roots as Evaluated by Computed Tomography
AbstractThe aortic root is a dynamic structure and its distensibility is considered an important factor in native and prosthetic heart valve function. Glutaraldehyde-preserved (GLUT) valves have limited durability possibly due to increased tissue stiffening. Decellularized deoxycholic acid-treated (DOA) valves exhibit excellent long-term performance in the pulmonary position in humans possibly due to preservation of valve tissue distensibility. We investigated aortic distensibility in DOA (n=8) and GLUT (n=3) aortic root prostheses in 60 kg pigs 2 weeks after orthotopic implantation. Five pigs served as controls. Using a dual source computed tomography scanner (Somatom Flash, Siemens Medical Solution, Forcheim, Germany) the cross sectional area at the level of Sinus of Valsalva (SoV), sino-tubular junction (STJ), and ascending aorta (AA), respectively, was measured in both diastole and systole. Distensibility was defined as the change in area from diastole (RR 95%) to systole (RR 15%). For assessment of the prosthetic- independent aortic distensibility we performed the same measurements in the descending aorta (AD) in all animals. Data were analyzed using students t-test and reported as a mean±SD. Native aortic distensibility was significant larger at the level of SoV (15.8%±4.9), STJ (46.7%±10.3), and AA (40.6%±9.0), compared with both DOA and GLUT aortic roots (p<0.05). No difference in distensibility between the DOA and GLUT aortic roots were observed: 8.8%±2.4 vs 6.2%±5.5 (SoV), 11.7%±3.0 vs 10.0%±7.1 (STJ), 14.9%±6.1 vs 11.8±1.7 (AA), and 15.6±1.5 vs 15.5±1.8 (DA) , respectively (p>0.05). There was no difference in mean distensibility between the three groups in the decending aorta. This is the first study to evaluate in-vivo distensibility of aortic root prostheses in vivo. Aortic root distensibility is reduced following implantation of DOA or GLUT prosthetic aortic valves. No difference was observed between DOA and GLUT valves. Long-term follow-up is needed in order to verify any changes in bioprosthetic aortic root distensibility over time.
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The Way to a New Generation of Bio-Prostheses
المؤلفون: Thomas Waldow, Katrin Plötze, Matthias König and Klaus MatschkeAbstractDiseases of heart valves often make a prosthetic replacement therapy necessary. Long term goal is the development of artificial leaflet material with defining properties of mechanical prostheses (i.e. durability) with complete biocompatibility without the need for any anticoagulation therapy whatsoever. In order to achieve this aim, objective efforts are being made to use a new material that is still in the process of development. It is a film-like tissue, made of pure carbon in the form of carbon-nanotubes, which are characterized by their excellent mechanical properties (tensile strength 65GPa in comparison to steel with 0.6Gpa). The woven material has also a very small mesh width, which can be influenced by the spinning method. Due to the nature of this material, which is associated with a high surface energy, there is a need for adaptions for the use in vivo according to the requirements. Plasma Enhanced Chemical Vapor Deposition PECVD has proven to be the appropriate method for such a functionalization. It is possible to deposit amorphous hydrocarbon coatings below 60°C and in a thickness range from a few nanometers up to several microns, while at the same time providing a very low Young's modulus that ensures to withstand mechanical stress. In addition, this method allows to influence regional properties e.g. by incorporating silicon into the matrix to prevent adhesion of thrombocytes and/or to add nitrogen to give the ability to endothelial cell growing. The first project step is to evaluate in high resolution and accuracy the parameter characteristic of native valves and prostheses. Therefore the elastic modulus, flexibility, extension, expansion, banding stress, banding elasticity, banding rigidity, reversed banding strength, cantilever load, surface tension, surface structure, surface tension and intensity of surface loading are under investigation. A mix of physical, chemical and visual analysis methods are used. The parameter characteristic of the nanotube material has to match these results. In cases of mismatch a technical adjustment, e.g. during weaving or coating, is possible. At least two big advantages of this new composite material exist: The possibility to produce a compound material with regional different main characteristics by using a two-phase coating process. And on the other hand the abandonment of exogenous substances during the production process and so the material has a distinct advantage over those currently used Teflon fabrics.
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Mechanical Properties of Bioprostheses Leaflets Compared to Human Aortic Valve
المؤلفون: Martins Kalejs, Peteris Stradins, Lacis Lacis, Iveta Ozolanta and Vladimir KasyanovAbstractHeart valve bioprostheses suffer from gradual tissue deterioration, which has a causal link with valve tissue mechanical properties. Limited data on mechanical properties of commercially available bioprostheses comparing them to native human aortic valves (AV) is available. Our objective was to determine the mechanical properties of several contemporary bioprostheses and compare them with native human and porcine aortic valves. Leaflets from 5 unchanged human AV, collected from cadaveric hearts and 5 porcine AV, and from 3 of each kind of bioprostheses - Medtronic Hancock II, Sorin Soprano and Medtronic Freestyle were analysed using uniaxial tensile tests in radial and circumferential directions. Data are presented as means ± standard deviation. In both tested directions there's a shift to the stress axis of stress-strain curve for HancockII prostheses and even more for Soprano prostheses when compared to native human valves. In circumferential direction modulus of elasticity (E) of native human AV is 15.34±3.84MPa, porcine AV - 9.7±1.3MPa, Freestyle - 9.0±3.0MPa, HancockII - 22.5±2.2MPa and Soprano - 29.5±6.0MPa. In radial direction E of native human AV is 1.98±0.15MPa, porcine AV - 1.0±0.2MPa, Freestyle - 0.8±0.3MPa, HancockII - 2.5±0.2MPa and Soprano - 15.8±5.4MPa. Xeno-aortic bioprostheses have a non-linear and anisotropic response to stress in uniaxial tensile tests similar to native AV leaflets. HancockII has gained mechanical strength but lost tissue elasticity compared to native valve tissue. Leaflets of Soprano prostheses are even more rigid and lack pronounced material anisotropy. These differences in mechanical properties may accelerate deterioration of bioprostheses, causing altered stress distribution within valve leaflets. These data provide important information about what mechanical properties future valve substitutes should conform to.
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Polymer Nanofiber Materials Matching the Mechanic Properties of Native Aortic Valve
المؤلفون: Martins Kalejs, Peteris Stradins, Viktorija Priedite, Romans Lacis, Iveta Ozolanta and Vladimir KasyanovAbstractPorous electrospun nanofiber materials are very promising as matrices for heart valve tissue engineering. Not only biocompatibility is important for this material but also the mechanical features – it has to be strong enough to withhold the pressure after implantation as well as deformable enough for better distribution of shear stress along its surface. Deformability is also crucial for stimulation of fibre production by fibroblasts on these matrices. Altogether 8 differing density variants of electrospun nanofiber materials from gelatine, polyurethane (PUR), polylactic acid (PLA) and polycaprolactone (PCL) were analysed using uniaxial tensile tests. Data were compared to mechanical properties of porcine aortic valve (AV) leaflets in radial and circumferential directions. Data are presented as means ± standard deviation. In circumferential direction modulus of elasticity (E) of native porcine AV is 9.7±1.3MPa and - 1.0±0.2MPa in radial. Ultimate stress and strain is 44.8±5.9% and 2.3±0.6 MPa in circumferential and 95.6±31.4% and 0.5±0.2MPa in radial direction for native leaflets. Closest of the materials to match the mechanical properties of porcine AV in circumferential direction was PUR with density 6.2 g/sqm showing E of 3.9±0.5 MPa, ultimate stress and strain - 5.3±1.68MPa and 141.8±43.9MPa respectively. Closest to match radial direction was gelatine with density 5.7 g/sqm showing E of 0.64±0.14 MPa, ultimate stress and strain - 0.38±0.05MPa and 82.53±10.20MPa respectively. Native AV leaflets have a non-linear and anisotropic response to stress in uniaxial tensile tests. Hence to model as precisely as possible their mechanical properties we suggest to use a combined material made in a sandwich fashion with layers of gelatine on the outside and PUR in the middle with their fibbers predominantly orientated in perpendicular directions. The other tested materials PLA and PCL either lacked strength to mimic leaflets in circumferential direction or deformability required for the radial direction.
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Functional Mitral Regurgitation Is a Main Determinant Of Adverse Outcome In Patients With Heart Failure Due To Non-Ischemic Dilated Cardiomyopathy.
المؤلفون: Toniolo Mauro, Rossi Andrea, Cicoira Mariantonietta, Bergamini Corinna and Vassanelli CorradoAbstractIschemic mitral regurgitation has been recently demonstrated to carry important prognostic information in patients with left ventricular dysfunction due to coronary artery disease. There is no information regarding the prognostic role of functional mitral regurgitation in patients with non ischemic dilated cardiomyopathy. Patients with stable heart failure due to non-ischemic dilated cardiomyopathy were prospectively enrolled. All patients underwent a comprehensive echocardiographic assessment. Left ventricular diastolic (LVD) , systolic (LVS) diameters, left atrial diameter (LAD), ejection fraction (EF) and restrictive mitral filling pattern (RMP) was measured. Mitral regurgitant volume (RV) was measured by means of proximal isovelocity surface area method. The end point of the study was death or hospitalization for worsening heart failure. 80 patients (mean age 61±9 years; 82% male) were enrolled. 10 patients reached the end point of the study. At univariate Cox analysis, the echocardiographic variables associated with outcome were: EF (HR 0.84 95% CI 0.75 0.94; p=0.002), RMP (HR 5.2 95% CI 1.4 19.7; p=0.01) and RV (HR 1.046 95% CI 1.02 1.07; p=0.0005), LVS/BSA (HR 1.2 95% CI 1.02 1.4; p=0.03). At multivariate analysis RV remained the only variables independently associated with outcome (p=0.04). Result did not change when LVS/BSA substituted EF in the model. Receiving operator characteristics analysis documented that the area under the curve for RV in identifying patients with adverse outcome was 0.84±0.06 (95% CI 0.74 0.91) and the best cut off value for RV was 28 ml (sensitivity 80% 95% CI 44 97 and specificity 87% 95% CI 77 94). Patients with RV<28 had a survival rate of 95% after 6 years from the index echocardiogram compared with 22% in those with RV> 28 (longrank 23; p<0.0001). In patients with non-ischemic dilated cardiomyopathy, RV was a main predictor of death or hospitalization for worsening heart failure.
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Prevalence of Severe Aortic Stenosis With Low Flow in Patients With and Without Left Ventricular Dysfunction. A Consecutive Echocardiographic Population
المؤلفون: Toniolo Mauro, Rossi Andrea, Cicoira Mariantonietta, Bergamini Corinna and Vassanelli CorradoAbstractPatients with aortic stenosis (AS) may have a severely reduced aortic valve area (AVA) and a paradoxically low mean gradient (MG). Although this condition is well known in presence of left ventricular (LV) dysfunction, it has recently been observed that it can be associated with normal ejection fraction (EF) as well. Since the prevalence of this condition with respect to LV function is not well defined, we aimed to evaluate the distribution of patients with severely reduced valve area and low MG in a group of patients regardless of EF over a set period of time. We retrospectively identified consecutive patients with severe AS (defined as aortic valve area <0.6 cmq/mq) from our echo data-base. Low MG was defined as < 40 mmHg. Left ventricular systolic dysfunction was considered as EF< 50%. 167 patients with AVA <0.6 cmq/mq formed the study population. 94 (56%) patients were characterized by high MG and 73 (44%) by low MG. Among patients with low MG, 38 were characterized by normal EF and 35 by reduced EF. Differences among groups are shown in Table 1. In this echocardiographic series of consecutive patients, the prevalence of low MG despite severely reduced AVA was high. The distribution of low MG was similar in presence of reduced and preserved EF (21 and 23% of the overall population respectively).
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Fluid-Structure Interaction Simulation of an Aortic Bi-Leaflet Mechanical Heart Valve in a Patient-Specific Left Heart
المؤلفون: Trung Bao Le and Fotis SotiropoulosAbstractA large-scale kinematic model is developed for animating the left ventricle (LV) wall to drive the fluid-structure interaction (FSI) between the ensuing blood flow and a mechanical heart valve prosthesis implanted in the aortic position of an anatomic LV/aorta configuration. The kinematic model is of lumped type and employs a cell-based, FitzHugh-Nagumo framework to simulate the motion of the LV wall in response to an electrical wavefront propagating along the heart wall. The emerging large-scale LV wall motion exhibits complex contractile mechanisms that include contraction (twist) and expansion (untwist). The kinematic model is shown to yield global LV motion parameters that are well within the physiologic range throughout the cardiac cycle. The FSI between the leaflets of the mechanical heart valve and the blood flow driven by the dynamic LV wall motion and mitral inflow is simulated using the curvilinear immersed boundary (CURVIB) method (Ge et al., J. Comp. Physics., 2007 and Borazjani et al., J. Comp. Physics, .2008) implemented in conjunction with a domain decomposition approach. The computational results show that the simulated flow patterns are in good qualitative agreement with in vivo observations. The simulations also reveal complex kinematics of the valve leaflets, thus, underscoring the need for patient-specific simulations of heart valve prosthesis and other cardiac devices.
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Functional Performance and Biomechanics of Decellularized Human Aortic Valves
المؤلفون: Stacy M. G. Arnold, Steven Goldstein and David C. GaleAbstractAllograft decellularization based on detergents and/or enzymes to reduce antigenicity has been reported to damage tissue structure. We applied to human aortic valves a non-detergent/non-protease decellularization treatment prior to cryopreservation. To ensure that the method did not negatively impact valve structure, we submitted decellularized and conventional human aortic valves to pulsatile flow characterizations and accelerated wear testing (to 80 million cycles) under both normal and elevated aortic valve flow conditions. The biomechanical properties of decellularized aortic valve tissues were compared to those of non-decellularized aortic valves. Valve performance was assessed using six conventional and six decellularized human aortic valves (internal valve diameter of 21mm±1mm). Valves were placed in accelerated wear testers, at a cycle rate of 200beats/min as specified in ISO 5840. All valves underwent pulsatile flow characterization before testing and at 20 million cycle intervals up to 80 million loading cycles (pressure>100mmHg). Images captured during pulsatile testing at peak systole and diastole were used to evaluate proper valve function, full leaflet coaptation and wear related damage. Tissue biomechanics was evaluated pair-wise using ten bisected valves, half conventionally treated, half decellularized. For each, conduit and leaflet circumferential ultimate tensile strength (UTS), and conduit and myocardium suture retention strength were evaluated. Compared to conventional human aortic heart valves, the decellularized valves showed comparable valve performance based on effective orifice area before accelerated testing (1.57±0.3 vs 1.51±0.2 cm2, decellularized and conventional, respectively) and similar non-significant decrease after reaching an accumulated 80 million cycles (1.40±0.2 vs. 1.45±0.3 cm2). Both populations met the minimum performance requirements specified in ISO 5840 for effective orifice area and regurgitant fraction. There was no significant increase in retrograde flow due to post-wear leakage. Biomechanical testing demonstrated leaflet and conduit circumferential UTS and conduit and myocardium suture retention strength of decellularized valves were equal to or greater than the conventional valves. From these measurements we conclude that there is no quantifiable impact on valve functionality, performance or biomechanics due to the SynerGraft® (non-detergent based) decellularization process.
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The Congenital Bicuspid Aortic Valve can Experience High Frequency Unsteady Shear Stresses on its Leaflet Surface
AbstractBicuspid aortic valve (BAV) is a common congenital malformation affecting 1-2% of the population and is highly correlated to early calcification of the valve leaflets. Two widely held hypothesis for this correlation are (1) altered shape of the BAV results in altered fluid mechanical environment, leading to pro-calcification biology responses, and (2) inherent genetic defects results in pre-disposition of the tissues to calcify. In the current study, we tested the first hypothesis with porcine valve models in an in vitro flow loop. One BAV model and one tricuspid aortic valve (TAV) model were constructed using healthy porcine AV leaflets and tested in a physiological pulsatile flow loop. Fluid velocities near the center of the aortic surface of the valve leaflets were measured with Laser Doppler Velocimetry at a spatial resolution of 89 microns, and ensemble average shear stresses were calculated at various time points in the cardiac cycle. Unsteadiness of flow near the valve leaflets was quantified with variance analysis and power spectral analysis. Particle Image Velocimetry was used to visualize flow fields downstream of the valves and in the sinuses. The leaflets of the BAV model experienced shear stresses on the aortic surface with magnitudes similar to that of the TAV. However, flow near the BAV leaflets had high frequency unsteadiness components, especially during mid- to late- systole, and had high cycle-to-cycle magnitude variability, indicating that shear stresses will have similar unsteadiness and magnitude variability. These are most likely due to the stenosis in the BAV and the skewed forward flow, which collided with the aorta wall. In conclusion, our study indicated that some BAVs could experience high frequency unsteadiness and cycle-to-cycle magnitude variability on the valve leaflets because of its geometry. We speculate that, together with genetic factors, such adverse mechanical force environment could play a role in causing early calcification in the BAV leaflets.
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Autologous Bone Marrow Mononuclear Cell-Based Tissue Engineered Heart Valves: First Experiences with a One-step Intervention in Primates
AbstractA living heart valve with regeneration capacity based on autologous cells and minimally invasive implantation technology would represent a substantial improvement upon contemporary heart valve prostheses. This study investigates the feasibility of injectable, marrow stromal cell-based, autologous, living tissue engineered heart valves (TEHV) generated and implanted in a one-step intervention in non-human primates. Trileaflet heart valves were fabricated from non-woven biodegradable synthetic composite scaffolds and integrated into self-expanding nitinol stents. During the same intervention autologous bone marrow-derived mononuclear cells were harvested, seeded onto the scaffold matrix, and implanted transapically as pulmonary valve replacements into non-human primates (n=6). The transapical implantations were successful in all animals and the overall procedure time from cell harvest to TEHV implantation was 118±17 min. In vivo functionality assessed by echocardiography revealed preserved valvular structures and adequate functionality up to 4 weeks post implantation. Substantial cellular remodeling and in-growth into the scaffold materials resulted in layered, endothelialized tissues as visualized by histology and immunohistochemistry. Biomechanical analysis showed non-linear stress-strain curves of the leaflets, indicating replacement of the initial biodegradable matrix by living tissue. Here we provide a novel concept demonstrating that heart valve tissue engineering based on a minimally invasive technique for both cell harvest and valve delivery as a one-step intervention is feasible in non-human primates. This innovative approach may overcome the limitations of contemporary surgical and interventional bioprosthetic heart valve prostheses.
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The role of the epigenetic factor in valvulogenesis in zebrafish in vivo
المؤلفون: Hae Jin Kang and Morteza GharibAbstractIn this study, we focused on wall shear stress as an epigenetic factor to initiate valve formation. Fast or oscillatory flow is induced in an embryonic zebrafish vessel to mimic the flow in atrioventricular canal (AVC). By creating an environment similar to AVC in non-cardiac region in vivo, we propose to seek a better understanding of the mechanisms associated with the induction of valvulogenesis signaling pathways. Polydimethylsiloxane is injected at the vessel wall of embryonic zebrafish, creating fast or oscillatory flow, therefore modulating local wall shear stress (WSS). Particle image velocimetry (PIV) technique is used to measure blood flow velocity. Whole mount in situ hybridization is performed to detect kruppel-like factor 2a (klf2a) expression. Klf2a is used as a marker gene since it responds to shear stress and is crucial for valve formation. Using PIV, WSS is calculated. In situ hybridization showed klf2a expression in the vessel as a result of either high WSS or oscillatory WSS. Klf2a is not expressed in the vessel during normal embryonic development. With a new gel injection technique, local WSS in the zebrafish vessel has changed, inducing expression of klf2a, an important gene for the initiation of valve formation. Expression of different genes such as TGF-β family that are involved in valve formation will be studied further to establish an innovative model to explore valvulogenesis in vivo.
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Shear- and Side-dependent microRNAs and Messenger RNAs in Aortic Valvular Endothelium
المؤلفون: Casey J. Holliday-Ankeny, Randall F. Ankeny, Zannatul Ferdous, Robert M. Nerem and Hanjoong JoAbstractAortic valve (AV) disease is a major cause of cardiovascular-linked deaths globally. In addition, AV disease is a strong risk factor for additional cardiovascular events; however, the mechanism by which it initiates and progresses is not well-understood. We hypothesize that low and oscillatory flow is present on the fibrosa side of the AV and stimulates ECs to differentially regulate microRNA (miRNA) and mRNAs and influence AV disease progression. This hypothesis was tested employing both in vitro and in vivo approaches, high throughput microarray and pathway analyses, as well as a variety of functional assays. First, we isolated and characterized side-dependent, human aortic valvular endothelial cells (HAVECs) isolated from transplant recipient AVs. We found that HAVECs express both endothelial cell markers (VE-Cadherin, vWF, and PECAM) as well as smooth muscle cell markers (SMA and basic calponin). Further, HAVECs align in the direction of the flow as well as uptake acetylated LDL. Using microarray analysis on sheared, side-specific HAVECs, we identified side- and shear-induced changes in miRNA and mRNA expression profiles. More specifically, we identified over 1000 shear-responsive mRNAs which showed robust validation (93% of those tested). We then used Ingenuity Pathway Analysis to identify key miRNAs, including those with many relationships to other genes (for example, thrombospondin and IκB) and those that are members of over-represented pathways and processes (for example, sulfur metabolism). Furthermore, we validated five shear-sensitive miRNAs: miR-139-3p, miR-148a, miR-187, miR-192, and miR-486-5p and one side-dependent miRNA, miR-370. To prioritize these miRNAs, we performed in silico analysis to group these key miRNAs by cellular functions related to AV disease (including tissue remodeling, inflammation, and calcification). Additional miRNAs of interest (including miR-7 and miR-506) were determined through analysis of overrepresented miRNA binding sequences in the shear-sensitive mRNA array. Next, to compare our in vitro HAVEC results in vivo, we developed a method to isolate endothelial-enriched, side-dependent total RNA and identify and validate side-dependent (fibrosa vs. ventricularis) miRNAs in porcine aortic valvular endothelium. From this analysis, we discovered and validated eight side-dependent miRNAs in porcine endothelial-enriched AV RNA, including one miRNA previously identified in vitro, miR-486-5p, as well as a shear-responsive miRNA cluster, miR-199a/214. Through microarray studies and in silico analysis, we have prioritized key miRNAs which may serve as master regulators of AV disease. Better understanding of AV biology and disease in terms of gene-regulation under different hemodynamic conditions will facilitate the design of a tissue-engineered valve and provide alternative treatment options.
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The Effect of Side-specific Valve Endothelial Cells on Extracellular Matrix Production by Valve Interstitial Cells
المؤلفون: Napachanok Mongkoldhumrongkul, Magdi H. Yacoub and Adrian H. ChesterAbstractValve endothelial cells (VECs) play an important role in regulating the function of the underlying interstitial cells (VICs). However, the regulatory effects VECs from each side of the valve on the production of ECM proteins by VICs have not been studied. This study aims to determine the regulatory effects of VECs isolated from the aortic (aVECs) and ventricular (vVECs) surfaces of the aortic valve on the production of collagen, glycosaminoglycan (GAGs) and elastin by VICs. Side-specific VECs were co-cultured with VICs using Transwell® inserts and the effects on extracellular matrix production by VICs were investigated by quantifying the amount of secreted collagen and sulphate GAGs by VICs as well as examining gene expressions of collagen, GAGs and elastin components by VICs. Collagen production by VICs was significantly increased by co-culturing with aVEC and vVECs to 154.38 ± 13.71% (p=0.041) and 196.35 ± 16.59% (p=0.008), respectively, of the control (VIC culture without VECs). Furthermore, vVECs significantly enhanced production of sulphate GAGs by VICs to 217.18 ± 32.9% (p=0.008) above control whereas aVECs showed an increase of 150.08 ± 18.0%, which was non-significance. There was no significant difference between sulphate GAGs release in response to vVECs and aVECs. Only fibrillin 1, a component of elastin, gene expression was increased by co-culturing VICs with aVEC, 2.07 ± 0.34 (p=0.008), and vVECs, 2.13 ± 0.31 (p=0.03) fold increase above control. In contrast, media collected over a 48 hour period from aVEC or vVEC cultures and subsequently incubated with VICs (in the absence of VECs) showed no induction on the ECM production by VICs. In conclusion, aVECs and vVECs induce the ECM production and expression by releasing labile molecules which are degradable or lose their activities with time. Further experiments are necessary to identify the mediators that produce these effects and to determine how their release is modulated by the different flow patterns experienced by aVECs and vVECs. This study further highlights the complex interaction and communication between different cell types present in the valve cusps.
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Calcification of Aortic Valve leaflets is Shear Dependent and Side-specific
AbstractAortic valve (AV) sclerosis is a degenerative disease and is one of the leading causes of mortality in elderly population. AV experiences various mechanical stimuli such as pressure, shear and flexure with differing magnitudes on either sides of AV (fibrosa and ventricularis) with fibrosa side facing more dynamic environment. Normal hemodynamic conditions constantly renew and remodel the valve, whereas altered mechanical loading such as hypertension and altered shear stress can cause tissue inflammation that leads to calcification, preferentially on fibrosa side. AV calcification progressively leads to sclerosis and ultimately results in valve failure. However, the molecular and cellular processes that lead to inflammation and calcification are not very well understood. To understand the role of mechanics and underlying molecular mechanisms behind this preferential calcification, aortic side and ventricularis side of fresh porcine AVs were exposed to different shear stress patterns using an ex vivo cone and plate viscometer for 72 hours. Osteogenic medium was used to accelerate the calcification process ex vivo. To investigate the effect of shear stress magnitudes on fibrosa side, sine waveforms of amplitudes 5, 10 and 25 dynes/cm2 at a frequency of 1 Hz were used. To investigate the effects of shear stress frequency on fibrosa side, sine waveforms of 1 and 2 Hz at 10dynes/cm2 were used. Following exposure to shear, calcium levels of the samples were quantified using calcium Arsenazo assay. Von Kossa stain for mineralization was also done. Fresh porcine AVs were used as controls. Results indicated that low magnitude shear stress, 5 dyne/cm2 at 1 Hz elicited significant calcium levels on fibrosa side compared to other magnitudes and frequencies. To investigate if the oscillatory nature or the low magnitude was responsible for this high calcium response, fibrosa side was exposed to steady 5 dyne/cm2 under same experimental conditions as above. However, calcium levels at steady 5dyne/cm2 were comparable to fresh AV levels and thus non-significant. This result indicated that the magnitude in combination with the oscillatory nature of the sine 5dyne/cm2 triggered significant calcification levels on the fibrosa side of the AV leaflets. To test the side-specificity of this response, ventricularis side was also exposed to 5 dyne/cm2 under same experimental conditions as above. It is interesting to note that calcium levels on ventricularis side were not significant compared to that on the fibrosa side. This result further indicated that the expression of significant calcium levels in response to the low oscillatory shear is indeed side-specific. Thus our results suggest that the calcification of the AV leaflets is shear-dependent and side-specific. Shear dependent calcification in AV also suggests mechanobiological similarities with the atherosclerosis of blood vessels.
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Proteoglycan-Rich Leaflet Thickening in Diet-Induced Early Aortic Valve Disease
المؤلفون: Mark C. Blaser, Krista L. Sider, Andrea V. Kwong, Yu-Qing Zhou, R. Mark Henkelman and Craig A. SimmonsAbstractAortic valve disease (AVD) is a cell-mediated pathology without effective pharmacotherapy, and the early pathogenesis of AVD is drastically understudied. Examining the early stages of disease development may identify biomarkers and novel treatment strategies for use before an untreatable burden of calcification develops. We investigated the effects of a mildly atherogenic diet on early AVD development in mice, and the involvement of proteoglycans (PG) and Sox9 in this process. Male wild-type (WT) C57Bl/6J mice were fed a control diet or BioServ F3282, a high-fat, high-carbohydrate diet (HF/HC) with 58.7% kcal from fat (cholesterol <0.1% w/w) for four months (n = 4-6 per group). Aortic valve function was examined by high-resolution ultrasound biomicroscopy (UBM). Longitudinal aortic valve sections from formalin-fixed and paraffin-embedded hearts were stained by Movat’s pentachrome (morphological changes and ECM composition) and Osteosense 680 (calcification), then immunostained for α-smooth muscle actin (αSMA) and Sox9. Mice on the HF/HC diet for four months became significantly obese (46.7 ± 4.7 vs. 32.3 ± 0.8 g, p < 0.05) but did not develop cardiac hypertrophy. They developed mild but statistically significant hypercholesterolemia (4.7 ± 1.0 vs. 3.1 ± 0.4 mmol/L total cholesterol, p < 0.05). UBM revealed moderate decreases in aortic valve opening area along with increased left ventricular ejection time in HF/HC mice, while no mice exhibited aortic regurgitation. Significant valve thickening was found in the distal third of HF/HC leaflets (84.4 ± 10.4 vs. 37.3 ± 2.7 µm, p < 0.05), while proximal and medial regions were unaltered. Valve thickening was primarily due to PG deposition in HF/HC leaflets (11435 ± 7681 vs. 5448 ± 2948 µm2, p < 0.01), not an increase in collagen content (1729 ± 815 vs. 1771 ± 663 µm2, p = 0.87). Sox9 expression was increased in thickened HF/HC leaflets, and was most highly expressed in PG-rich lesions. HF/HC leaflets did not stain positive for αSMA, implying that changes in leaflet structure and function were not the result of actively synthetic myofibroblasts. Osteosense 680 staining was negative, denoting that the HF/HC diet did not induce leaflet microcalcification.
These studies reveal that a high-fat Western diet lacking cholesterol can induce changes in the ECM and functional properties of a WT mouse aortic valve. This early AVD is characterized by thickened leaflets with lesions that are PG-rich and have increased Sox9 expression. Although AVD is considered to be driven by pathological myofibroblast differentiation leading to fibrosis and calcification, the early changes observed herein occur independently of myofibroblast activation, and provide new insight into the initiation and pathogenesis of AVD.
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Angiotensin Receptor Blocker Has no Effect on Atherosclerotic Factors in AVS
المؤلفون: Zachary B. Armstrong, Derek R. Boughner, Colin P. Carruthers, Maria Drangova and Kem A. RogersAbstractAortic valve sclerosis (AVS) is a chronic progressive disease affecting 25% of the population over the age of 65. Despite this high prevalence, there are currently no preventative therapies which inhibit the progression of AVS. This study sought to determine the effects of an angiotensin II type 1 receptor blocker (ARB), alone or in combination with a statin, on AVS. Male New Zealand White rabbits were fed either regular chow (Control, n=5) or an atherogenic diet for a period of 18 months to induce AVS. Recognizing the clinical reality, therapy was introduced after disease onset. After 12 months, rabbits were block randomly assigned to four groups receiving either no treatment (Cholesterol, n=6), olmesartan medoxomil (Olmesartan, n=7), atorvastatin calcium (Atorvastatin, n=7), or a combination of both drugs (Combination, n=7) for the final 6 months. Magnetic resonance imaging (MRI) was used to monitor disease progress throughout the treatment period. After sacrifice, valve lesions were analyzed using histology and immunohistochemistry. In vivo disease monitoring yielded no discernible treatment effect. While Cholesterol cusps were significantly thicker than Control throughout the treatment period (0.465 ± 0.030 vs 0.388 ± 0.023mm for Cholesterol and Control, respectively, at 18 months), the various treatments had no positive effect on cusp thickness, and were all identical to Cholesterol at 18 months. Aortic valve area provided similar results; while significant disease was established (0.379 ± 0.033 vs 0.623 ± 0.074cm2 for Cholesterol and Control, respectively, at 18 months), there were no significant differences between treatment groups. Histological analysis of Cholesterol, Atorvastatin, Olmesartan, and Combination cusps revealed fibrosal thickening, lipid deposition, macrophage infiltration, and minor calcification. However, morphological analysis did not reveal significant differences in lesion composition among the treatment groups. Treatment efficacy was confirmed by analysis of aortic lesion area which revealed a significant reduction of atherosclerosis in Olmesartan-treated animals. Neither olmesartan medoxomil nor atorvastatin calcium, alone or in combination, provide demonstrable benefit in the treatment of established AVS despite success in the treatment of atherosclerosis.
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