Structural evaluation of porcine heart valve prostheses with radiofrequency ultrasound.

BACKGROUND: Bioprosthetic heart valve use is limited by progressive degeneration. Early degenerative changes are often occult, making assessment of tissue integrity difficult. Ultrasound tissue characterization may detect alterations in tissue structure and allow early detection of leaflet degeneration. METHODS: Using a modified echocardiographic unit (Acuson), radiofrequency (RF) integrated backscatter amplitude (IBA) (integral/RF/dt) was measured in 38 leaflets from nine explanted and six control porcine valves. Regions of interest in each leaflet were studied using four ultrasound frequencies. Radiographic gray scale mean and leaflet thickness were measured at each region of interest. Percent collagen and mineral were calculated for each region of interest using color-image processing of histologic sections and compared to IBA. RESULTS: IBA values for control vs. explanted leaflets were (mean value+/-standard deviation): 8.2+/-4.69 dB vs. -4.7+/-4.64 dB at 7.0 MHz; -5.8+/-4.34 dB vs. -3.1+/-5.34 dB at 5.0 MHz; -3.8+/-3.38 dB vs. -2.1+/-3.18 dB at 3.5 MHz; and -9.0+/-4.58 dB vs. -7.1+/-4.25 dB at 2.5 MHz. Collagen content was 27.7+/-8.50% vs. 33.2+/-10.90%, mineral content was 0.1+/-0.10% vs. 2.1+/-4.30%, and radiographic gray scale mean was 150.6+/-1.96 vs. 145.3+/-5.14 for control vs. explanted leaflets, respectively. For control and explanted leaflets IBA, collagen content, mineral content, and radiographic gray scale mean were different (control vs. explanted P<0.05). Leaflet thickness was also noted to be different between the two groups. IBA was different among explanted leaflets with low, medium, and high mineral content. CONCLUSION: IBA was found to be a useful technique to differentiate normal from explanted porcine prosthetic valves in vitro. This method may be useful in the serial assessment of bioprosthetic leaflet degenerative properties in vivo.     

Glycosaminoglycan-targeted fixation for improved bioprosthetic heart valve stabilization

Numerous crosslinking chemistries and methodologies have been investigated as alternative fixatives to glutaraldehyde (GLUT) for the stabilization of bioprosthetic heart valves (BHVs). Particular attention has been paid to valve leaflet collagen and elastin stability following fixation. However, the stability of glycosaminoglycans (GAGs), the primary component of the spongiosa layer of the BHV, has been largely overlooked despite recent evidence provided by our group illustrating their structural and functional importance. In the present study we investigate the ability of two different crosslinking chemistries: sodium metaperiodate (NaIO(4)) followed by GLUT (PG) and 1-Ethyl-3-(3 dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) followed by GLUT (ENG) to stabilize GAGs within BHV leaflets and compare resulting leaflet characteristics with that of GLUT-treated tissue. Incubation of fixed leaflets in GAG-degrading enzymes illustrated in vitro resistance of GAGs towards degradation in PG and ENG treated tissue while GLUT fixation alone was not effective in preventing GAG loss from BHV leaflets. Following subdermal implantation, significant amounts of GAGs were retained in leaflets in the ENG group in comparison to GLUT-treated tissue, although GAG loss was evident in all groups. Utilizing GAG-targeted fixation did not alter calcification potential of the leaflets while collagen stability was maintained at levels similar to that observed in conventional GLUT-treated tissue.     

Three-Dimensional Fluid-Structure Interaction Simulation of Bileaflet Mechanical Heart Valve Flow Dynamics

The wall shear stress induced by the leaflet motion during the valve-closing phase has been implicated with thrombus initiation with prosthetic valves. Detailed flow dynamic analysis in the vicinity of the leaflets and the housing during the valve-closure phase is of interest in understanding this relationship. A three-dimensional unsteady flow analysis past bileaflet valve prosthesis in the mitral position is presented incorporating a fluid-structure interaction algorithm for leaflet motion during the valve-closing phase. Arbitrary Lagrangian-Eulerian method is employed for incorporating the leaflet motion. The forces exerted by the fluid on the leaflets are computed and applied to the leaflet equation of motion to predict the leaflet position. Relatively large velocities are computed in the valve clearance region between the valve housing and the leaflet edge with the resulting relatively large wall shear stresses at the leaflet edge during the impact-rebound duration. Negative pressure transients are computed on the surface of the leaflets on the atrial side of the valve, with larger magnitudes at the leaflet edge during the closing and rebound as well. Vortical flow development is observed on the inflow (atrial) side during the valve impact-rebound phase in a location central to the leaflet and away from the clearance region where cavitation bubbles have been visualized in previously reported experimental studies.     

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.     

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

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

Calcification and stress distribution in bovine pericardial heart valves.

There is a strong relationship between mechanical stress and calcification in biological prosthetic heart valves. A dynamic in vitro calcification test has been used to study the relationship between stress distributions in the leaflets of bovine pericardial valves and the deposition of calcium over the leaflet surfaces. Intuitive stress regions have been defined over the leaflet surfaces. Calcium uptake by the leaflets has been assayed directly by ashing of leaflet material and analysis of the ash by atomic absorption spectrophotometry. Calcium and phosphorus distribution over the leaflet surface has been analyzed using energy-dispersive x-ray analysis by scanning electron microscope and data points assigned to the appropriate stress region. The uptake of calcium is assessed by comparing stress regions, surfaces, and the degree of calcification of the valve. Differences between stress regions and surfaces are significant. Uptake of calcium in these valves appears to be strongly related to the degree and type of stress present in the valve leaflets.     

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

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

     

Mechanical properties of a porcine aortic valve fixed with a naturally occurring crosslinking agent.

The study investigates the mechanical properties of porcine aortic valve leaflets fixed with a naturally occurring crosslinking agent, genipin, at distinct pressure heads. Fresh and the glutaraldehyde-fixed counterparts were used as controls. Subsequent to fixation, the changes in leaflet collagen crimps and its surface morphology were investigated by light microscopy and scanning electron microscopy (SEM). Also, the crosslinking characteristics of each studied group were determined by measuring its fixation index and denaturation temperature. In the mechanical testing, tissue strips made from each studied group were examined in both the circumferential and radial directions. Histological and SEM comparisons between fresh porcine aortic valve leaflet and those fixed at medium or high pressure revealed that the following changes may occur: elimination of the natural collagen crimping, and extensive loss of the endothelial layer. The denaturation temperatures of the glutaraldehyde-fixed leaflets were significantly greater than the genipin-fixed leaflets; however, their fixation indices were comparable. Generally, fixation pressure did not affect the crosslinking characteristics of the genipin- and glutaraldehyde-fixed leaflets. It was found that fixation of porcine aortic valves in genipin or glutaraldehyde did not alter the mechanical anisotropy observed in fresh valve leaflets. This indicated that the intramolecular and intermolecular crosslinks introduced into the collagen fibrils during fixation is of secondary importance to the presence of structural and mechanical anisotropy in fresh leaflet. Tissue fixation in genipin or glutaraldehyde may produce distinct crosslinking structures. However, the difference in crosslinking structure between the genipin- and glutaraldehyde-fixed leaflets did not seem to cause any significant discrepancies in their mechanical properties when compared at the same fixation pressure. Nevertheless, regardless of the crosslinking agent used, changes in mechanical properties and ruptured patterns were observed when the valve leaflets were fixed at distinct pressures.     

Tissue Engineering of Human Heart Valve Leaflets: A Novel Bioreactor for a Strain-Based Conditioning Approach

Current mechanical conditioning approaches for heart valve tissue engineering concentrate on mimicking the opening and closing behavior of the leaflets, either or not in combination with tissue straining. This study describes a novel approach by mimicking only the diastolic phase of the cardiac cycle, resulting in tissue straining. A novel, yet simplified, bioreactor system was developed for this purpose by applying a dynamic pressure difference over a closed tissue engineered valve, thereby inducing dynamic strains within the leaflets. Besides the use of dynamic strains, the developing leaflet tissues were exposed to prestrain induced by the use of a stented geometry. To demonstrate the feasibility of this strain-based conditioning approach, human heart valve leaflets were engineered and their mechanial behavior evaluated. The actual dynamic strain magnitude in the leaflets over time was estimated using numerical analyses. Preliminary results showed superior tissue formation and non-linear tissue-like mechanical properties in the strained valves when compared to non-loaded tissue strips. In conclusion, the strain-based conditioning approach, using both prestrain and dynamic strains, offers new possibilities for bioreactor design and optimization of tissue properties towards a tissue-engineered aortic human heart valve replacement.     

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

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

An unusual complication associated with the Carpentier-Edwards porcine bioprosthesis

Primary tissue failure of bioprosthetic heart valves refers primarily to calcification of the leaflets of the bioprosthesis. A 75 year old patient underwent reoperation 15 years after mitral valve replacement with a Carpentier-Edwards porcine bioprosthesis. The extracted bioprosthetic valve was found to have one prolapsed leaflet and a small amount of calcification on all three leaflets without tear or perforation. The two commissures suspending the prolapsed leaflet were detached, causing mitral valve regurgitation.     

Hydrodynamic function of polyurethane prosthetic heart valves: influences of Young's modulus and leaflet thickness.

The development of flexible polyurethane heart valves has been hindered by material degradation in vivo. Low modulus polyurethane leaflets are regarded as desirable to achieve good hydrodynamic function. However, low modulus materials may suffer high strain accumulation, hence poor durability. Higher modulus materials may improve durability, but may have poor hydrodynamic function. This study examines the hydrodynamic behaviour of biostable polyurethane valves, varying Young's modulus from 5 to 63.6 MPa and mean leaflet thickness from 48-238 microm. Parameters studied included mean pressure gradient, energy losses and regurgitation over 5 equivalent cardiac outputs (3.6, 4.9, 6.4, 8.0 and 9.61 min(-1)) At low cardiac output, modulus was not significantly correlated with any parameter of valve opening. At 9.61 min(-1), modulus significantly influenced mean pressure gradient (p = 0.033). Mean leaflet thickness significantly correlated with mean pressure gradient and energy losses during forward flow at all cardiac outputs (p<0.001). This study demonstrates that, over a wide range of moduli, valve hydrodynamic function is not affected significantly by the material modulus. Leaflet thickness is a highly significant factor. Higher modulus elastomers in a range up to 32.5 MPa may be useful in prosthetic heart valve leaflet manufacture, retaining good hydrodynamic function while potentially extending the lifetime of the valve.     

Normal Physiological Conditions Maintain the Biological Characteristics of Porcine Aortic Heart Valves: An <Emphasis Type="Italic">Ex Vivo</Emphasis> Organ Culture Study

The aortic valve functions in a complex mechanical environment which leads to force-dependent cellular and tissue responses. Characterization of these responses provides a fundamental understanding of valve pathogenesis. The aim of this work was to study the biological characteristics of native porcine aortic valves cultured in an ex vivo pulsatile organ culture system capable of maintaining physiological pressures (120/80 mmHg) and cardiac output (4.2 l/min). Collagen, sGAG and elastin contents of the valve leaflets were measured and cusp morphology, cell phenotype, cell proliferation and apoptosis were examined. Presence of endothelial cells (ECs) on the leaflet surface was also evaluated. The differences in collagen, sGAG and elastin contents were not significant (p > 0.05) between the cultured and fresh valve leaflets. The cultured valves maintained the native ECM composition of the leaflets while preserving the morphology and cell phenotype. Cell phenotype in leaflets incubated statically under atmospheric conditions decreased compared to fresh and cultured valve leaflets, indicating the importance of mechanical forces in maintaining the natural biology of the valve leaflets. ECs were retained on the surfaces of cultured leaflets with no remodeling of the leaflets. The number of apoptotic cells in the cultured leaflets was significantly (p < 0.05) less than in the statically incubated leaflets and comparable to fresh leaflets. The sterile ex vivo organ culture system thus maintained the viability and native biological characteristics of the aortic valves that were cultured under dynamic conditions for a period of 48 h.     

Heart Valves from Polyester Fibers vs. Biological Tissue: Comparative Study <Emphasis Type="Italic">In Vitro</Emphasis>

Transcatheter aortic valve implantation (TAVI) has become a popular alternative technique to surgical valve replacement for critical patients. Biological valve tissue has been used in TAVI procedures for over a decade, with over 100,000 implantations to date. However, with only 6 years follow up, little is known about the long-term durability of biological tissue. Moreover, the high cost of tissue harvesting and chemical treatment procedures favor the development of alternative synthetic valve leaflet materials. Textile polyester is one such material which provides outstanding folding and strength properties combined with proven biocompatibility, and could therefore be considered as a candidate to replace the biological valve leaflets in TAVI procedures. For that purpose, in addition to the mechanical properties, the hemodynamic properties of the synthetic material should be comparable to the properties of biological tissue. An ideal replacement heart valve would provide low static and dynamic regurgitation, ensure laminar flow across the valve, and limit the turbidity of flow downstream of the valve. The purpose of the present work is to compare in vitro the mechanical and hemodynamic performances of textile woven polyester valves with biological ones. Testing results indicate that textile valves trade elasticity for superior mechanical strength, relative to biological tissue. Despite this, the dynamic flexibility of textile valve leaflets strongly resembled what was seen with biological leaflets. Regurgitation, as well as slightly modified turbulent patterns, in textile valves was higher than biological valves due to the increased porosity, but, rapid tissue ingrowth post-implantation would likely mitigate this effect. Together these findings provide additional evidence favoring the use of textile polyester as a synthetic heart valve leaflet material.     

Tissue-engineered acellularized valve xenografts: a comparative animal study between plain acellularized xenografts and autologous endothelial cell seeded acellularized xenografts

BACKGROUND: Acellularized valve xenografts are considered a promising way of overcoming the inherent limitations of current prosthetic valves. The aim of this study was to compare the biological responses of an autologous endothelial cell seeded acellularized xenograft (AAX) and a plain acellularized xenograft (PAX) implanted in the pulmonary valve leaflet in the same animal. METHODS: Endothelial cells were isolated and cultured from the jugular vein of goats. Porcine valve leaflets were acellularized with Nacl-SDS, and for AAX, leaflets were then seeded with autologous endothelial cells. A PAX and an AAX were implanted as double pulmonary valve leaflet replacement in the same animal in a goat model (n = 6). After sacrifice, the implanted valve leaflet tissues were retrieved and analyzed visually and under a light microscope. RESULTS AND CONCLUSIONS: Six animals were sacrificed as scheduled during the short-term (6 and 24 hours), mid-term (1 week and 1 month) and long-term (3 and 6 months). Gross and ultrasonographic examinations revealed good valve function with no thrombosis but with slight thickening. Microscopic analysis of the leaflets showed abundant cellular ingrowth into the acellularized leaflets over time. The role of endothelial cell seeding remains controversial. This animal experiment demonstrates the practical feasibility of using acellularized valve xenografts.     

New J-3 flexible-leaflet polyurethane heart valve prosthesis with improved hydrodynamic performance.

The aim of this new three-leaflet valve development was to design a leaflet with minimum membrane stresses during performance. This is achieved by manufacturing the valve leaflets shaped almost flat in a medium opening position. Thus, the leaflets have two stable positions, one with maximum opening area and the other with favorable stress distribution in the closed position. The transition between the two end positions is achieved through a two-dimensional rolling motion without buckling and with minimum membrane stresses (bulge forces). The manufacturing technique is dip-coating in polyurethane. Hydrodynamic evaluation of the J-3 valve in steady and pulsatile flow showed minimum pressure drop compared to other commercially available valves. Laser-Doppler-anemometry studies indicated very low shear stresses in the flow field downstream of the valve. In durability tests prototypes have reached lifetimes of up to 17 years. In conclusion, the J-3 valve shows superior hydrodynamic performance thereby reducing potential thrombus formation. Minimization of stresses within the valve leaflets through design could reduce calcification.     

Biosynthetic activity in heart valve leaflets in response to in vitro flow environments

The development of bioreactors for tissue engineered heart valves would be aided by a thorough understanding of how mechanical forces impact cells within valve leaflets. The hypothesis of the present study is that flow may influence the biosynthetic activity of aortic valve leaflet cells. Porcine leaflets were exposed to one of several conditions for 48 h, including steady or pulsatile flow in a tubular flow system at 10 or 20 l/min, and steady shear stress in a parallel plate flow system at 1, 6, or 22 dyne/cm². Protein, glycosaminoglycan, and DNA synthesis increased during static incubation but remained at basal levels after exposure to flow. The modulation of synthetic activity was attributed to the presence of a shear stress on the leaflet surface, which may be transmitted to cells within the leaflet matrix through tensile forces. The -smooth muscle (-SM) actin distribution observed in fresh leaflets was proportionately decreased after exposure to antibiotics and not recovered by either static incubation or exposure to flow. These results indicate that exposure to flow maintains leaflet synthetic activity near normal levels, but that the inclusion of another force, such as bending or backpressure, may be necessary to preserve -SM actin immunoreactive cells. © 2001 Biomedical Engineering Society. PAC01: 8780Rb, 8719Hh, 8719Uv, 8715Rn, 8768+z, 8719Ff, 8714Gg, 8714Ee     

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 study of the aortic flow field of contemporary mechanical bileaflet prostheses

The characteristics of mechanical bileaflet valves, the leaflets of which open at the outside first, differ significantly from those of natural valves, whose leaflets open at the center first, and this fact affects the flow field down-stream of the valves. The direction of jet-type flows, which is influenced by this difference in valve features, and the existence of the sinus of Valsalva both affect the flow field inside the aorta in different ways, depending on the valve design. There may also be an influence on the coronary circulation, the entrance to which resides inside the sinus of Valsalva. A dynamic particle image velocimetry (PIV) study was conducted to analyze the influence of the design of prosthetic heart valves on the aortic flow field. Three contemporary bileaflet prostheses, the St. Jude Medical (SJM) valve, the On-X valve (with straight leaflets), and the MIRA valve (with curved leaflets), were tested inside a simulated aorta under pulsatile flow conditions. A dynamic PIV system was employed to analyze the aortic flow field resulting from the different valve designs. The two newer valves, the On-X and the MIRA valves, open more quickly than the SJM valve and provide a wider opening area when the valve is fully open. The SJM valve's outer orifices deflect the flow during the accelerating flow phase, whereas the newer designs deflect the flow less. The flow through the central orifice of the SJM valve has a lower velocity compared to the newer designs; the newer designs tend to have a strong flow through all orifices. The On-X valve generates a simple jet-type flow, whereas the MIRA valve (with circumferentially curved leaflets) generates a strong but three-dimensionally diffuse flow, resulting in a more complex flow field downstream of the aortic valve. The clinically more adapted 180 degrees orientation seems to provide a less diffuse flow than the 90 degrees orientation does. The small differences in leaflet design in the bileaflet valves generate noticeable differences in the aortic flow; the newer valves show strong flows through all orifices.     

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.