In-vitro fluid dynamic characteristics of the abiomed trileaflet heart valve prosthesis

The need for better and longer lasting trileaflet valves has led to the design and development of the Abiomed polymeric trileaflet valve prosthesis. In-vitro fluid dynamic studies on sizes 25 and 21 mm valves in the aortic position indicate an overall improvement in performance compared to the Carpentier-Edwards and Ionescu-Shiley tissue valves in current clinical use. The pressure drop studies yielded effective orifice areas of 1.99 and 1.54 cm2, and performance indices of 0.41 and 0.45 for the Nos. 25 and 21 valves, respectively. Leaflet photography studies indicated that the two valve sizes had maximum opening areas of 225 and 145 mm2, respectively, at a normal resting cardiac output. Steady and pulsatile flow velocity measurements with a laser-Doppler anemometer (LDA) system indicate that the flow field downstream of the Abiomed valve is jetlike and turbulent. Maximum mean square axial velocity fluctuations of 55 and 83 cm/s, and turbulent shear stresses of 220 and 450 N/m2 were measured in the immediate vicinity of the nos. 25 and 21 valves, respectively. The Abiomed valves studied had been originally configured for use in valved conduits, and it is therefore our opinion that further improvements can be made to the valve and stent design, which would enhance its fluid dynamic performance.     

Numerical simulation of steady flow in a two-dimensional total artificial heart model.

In this paper, a numerical simulation of steady laminar and turbulent flow in a two-dimensional model for the total artificial heart is presented. A trileaflet polyurethane valve was simulated at the outflow orifice while the inflow orifice had a trileaflet or a flap valve. The finite analytic numerical method was employed to obtain solutions to the governing equations in the Cartesian coordinates. The closure for turbulence model was achieved by employing the k-epsilon-E model. The SIMPLER algorithm was used to solve the problem in primitive variables. The numerical solutions of the simulated model show that regions of relative stasis and trapped vortices were smaller within the ventricular chamber with the flap valve at the inflow orifice than that with the trileaflet valve. The predicted Reynolds stresses distal to the inflow valve within the ventricular chamber were also found to be smaller with the flap valve than with the trileaflet valve. These results also suggest a correlation between high turbulent stresses and the presence of thrombus in the vicinity of the valves in the total artificial hearts. The computed velocity vectors and turbulent stresses were comparable with previously reported in vitro measurements in artificial heart chambers. Analysis of the numerical solutions suggests that geometries similar to the flap valve (or a tilting disk valve) results in a better flow dynamics within the total artificial heart chamber compared to a trileaflet valve.     

Two-component laser velocimeter measurements downstream of heart valve prostheses in pulsatile flow

Elevated turbulent shear stresses resulting from disturbed blood flow through prosthetic heart valves can cause damage to red blood cells and platelets. The purpose of this study was to measure the turbulent shear stresses occurring downstream of aortic prosthetic valves during in-vitro pulsatile flow. By matching the indices of refraction of the blood analog fluid and model aorta, correlated, simultaneous two-component laser velocimeter measurements of the axial and radial velocity components were made immediately downstream of two aortic prosthetic valves. Velocity data were ensemble averaged over 200 or more cycles for a 15-ms window opened at peak systolic flow. The systolic duration for cardiac flows of 8.4 L/min was 200 ms. Ensemble-averaged total shear stress levels of 2820 dynes/cm2 and 2070 dynes/cm2 were found downstream of a trileaflet valve and a tilting disk valve, respectively. These shear stress levels decreased with axial distance downstream much faster for the tilting disk valve than for the trileaflet valve.     

Stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position.

In this paper, a finite element analysis of the stress distribution on the cusps of a polyurethane trileaflet heart valve prosthesis in the closed position is presented. The geometry of the valve was modified from a relationship proposed by Ghista and Reul (J. Biomechanics 10, 313-324, 1977). The effects of variations in stent height, leaflet thickness and coaptation area on the stress distribution were also analyzed. Analyses were performed with both rigid and flexible stents for the trileaflet valve in order to delineate the effect of stent flexibility on the leaflet stress distribution. The results showed that regions of stress concentration were present near the commissural attachment similar to those predicted with the bioprostheses. The stresses on the leaflets were reduced by increasing the stent height with both rigid and flexible stents. Selectively increasing the leaflet thickness near the commissures and also increasing the coaptation area did not prove to reduce the leaflet stresses when the stent flexibility was taken into account. The possible effect of high stresses on the structural integrity of polyurethane leaflets and its relationship with calcification is yet to be investigated.     

The congenital bicuspid aortic valve can experience high-frequency unsteady shear stresses on its leaflet surface

The bicuspid aortic valve (BAV) is a common congenital malformation of the aortic valve (AV) affecting 1% to 2% of the population. The BAV is predisposed to early degenerative calcification of valve leaflets, and BAV patients constitute 50% of AV stenosis patients. Although evidence shows that genetic defects can play a role in calcification of the BAV leaflets, we hypothesize that drastic changes in the mechanical environment of the BAV elicit pathological responses from the valve and might be concurrently responsible for early calcification. An in vitro model of the BAV was constructed by surgically manipulating a native trileaflet porcine AV. The BAV valve model and a trileaflet AV (TAV) model were tested in an in vitro pulsatile flow loop mimicking physiological hemodynamics. Laser Doppler velocimetry was used to make measurements of fluid shear stresses on the leaflet of the valve models using previously established methodologies. Furthermore, particle image velocimetry was used to visualize the flow fields downstream of the valves and in the sinuses. In the BAV model, flow near the leaflets and fluid shear stresses on the leaflets were much more unsteady than for the TAV model, most likely due to the moderate stenosis in the BAV and the skewed forward flow jet that collided with the aorta wall. This additional unsteadiness occurred during mid- to late-systole and was composed of cycle-to-cycle magnitude variability as well as high-frequency fluctuations about the mean shear stress. It has been demonstrated that the BAV geometry can lead to unsteady shear stresses under physiological flow and pressure conditions. Such altered shear stresses could play a role in accelerated calcification in BAVs.     

Effect of systolic flow rate on the prediction of effective prosthetic valve orifice area

The Gorlin equation for the hemodynamic assessment of valve area is commonly used in cardiac catheterization laboratories. A study was performed to test the prediction capabilities of the Gorlin formula as well as the Aaslid and Gabbay formula for the effective orifice area of prosthetic heart valves. Pressure gradient, flow, and valve opening area measurements were performed on four 27 mm valve prostheses (two mechanical bileaflet designs, St. Jude and Edwards-Duromedics, an Edwards pericardial tissue valve, and a trileaflet polyurethane valve) each mounted in the aortic position of an in vitro pulse duplicator. With the known valve orifice area, a different discharge coefficient was computed for each of the four valves and three orifice area formulas. After some theoretical considerations, it was proposed that the discharge coefficient would be a function of the flow rate through the valve. All discharge coefficients were observed to increase with increasing systolic flow rate. An empirical relationship of discharge coefficient as a linear function of systolic flow rate was determined through a regression analysis, with a different relationship for each valve and each orifice area formula. Using this relationship in the orifice area formulas improved the accuracy of the prediction of the effective orifice area with all three formulas performing equally well.     

Velocity and turbulence measurements past mitral valve prostheses in a model left ventricle

Thrombogenesis and hemolysis have both been linked to the flow dynamics past heart valve prostheses. To learn more about the particular flow dynamics past mitral valve prostheses in the left ventricle under controlled experimental conditions, an in vitro study was performed. The experimental methods included velocity and turbulent shear stress measurements past caged-ball, tilting disc, bileaflet, and polyurethane trileaflet mitral valves in an acrylic rigid model of the left ventricle using laser Doppler anemometry. The results indicate that all four prosthetic heart valves studied create at least mildly disturbed flow fields. The effect of the left ventricular geometry on the flow development is to produce a stabilizing vortex which engulfs the entire left ventricular cavity, depending on the orientation of the valve. The measured turbulent shear stress magnitudes for all four valves did not exceed the reported value for hemolytic damage. However, the measured turbulent shear stresses were near or exceeded the critical shear stress reported in the literature for platelet lysis, a known precursor to thrombus formation.     

Numerical simulation of steady turbulent flow through trileaflet aortic heart valves—I. Computational scheme and methodology

A numerical simulation model and technique are described to simulate steady turbulent blood flow through trileaflet tissue valves of varying degrees of stenosis. The aortic trileaflet tissue valve design was chosen as the subject of this study, since it is the only popular valve in current clinical use which is approximately axisymmetric. An axisymmetric geometry is computationally more convenient since it involves only two dimensional equations. The geometry and dimensions of the aorta were designed from angiographic studies and measurements made from cadavers. The valve dimensions were obtained from tissue leaflet photography studies conducted on tissue bioprostheses of varying degrees of stenosis. The nonrectangular nature of this valve necessitated the use of a body conforming grid. Thompson's method coupled with a Chimera grid system was chosen for this purpose. The Chimera grid was used to avoid a grid with highly skewed cells. Turbulence was simulated by using the k- model with the wall function method. This decision was made after comparing the k- model's performance with that of lower order models, and after considering the increased computer time requirements and decreased stability of more complex models, such as the Reynolds stress model. The results of the study which are very encouraging and compare favorably with in vitro experimental data, are described in Part II of the paper.     

Geometric modeling of functional trileaflet aortic valves: development and clinical applications

The dimensions of the aortic valve components condition its ability to prevent blood from flowing back into the heart. While the theoretical parameters for best trileaflet valve performance have already been established, an effective approach to describe other less optimal, but functional models has been lacking. Our goal was to establish a method to determine by how much the dimensions of the aortic valve components can vary while still maintaining proper function. Measurements were made on silicone rubber casts of human aortic valves to document the range of dimensional variability encountered in normal adult valves. Analytical equations were written to describe a fully three-dimensional geometric model of a trileaflet valve in both the open and closed positions. A complete set of analytical, numerical and graphical tools was developed to explore a range of component dimensions within functional aortic valves. A list of geometric guidelines was established to ensure safe operation of the valve during the cardiac cycle, with practical safety margins. The geometry-based model presented here allows determining quickly if a certain set of valve component dimensions results in a functional valve. This is of great interest to designers of new prosthetic heart valve models, as well as to surgeons involved in valve-sparing surgery.     

Turbulent stress measurements downstream of six mechanical aortic valves in a pulsatile flow model

In a pulsatile flow model aortic Bj?rk-Shiley Standard, Convex-Concave and Monostrut valves were investigated together with the Hall-Kaster (Medtronic-Hall), St Jude Medical and Starr-Edwards Silastic Ball valve using hot-film anemometry. Three-dimensional visualization of average systolic Reynolds normal stresses (RNS) reflected the design of the valves. Mean average RNS were used for comparison of the fluid dynamic performance along with Velocity Energy Ratio (VER100) and Turbulence Energy Ratio (TER) as a relative turbulence intensity for pulsatile flow. Mean average RNS ranged from 13.2 to 37.6 Nm-2 for all the valves with the highest levels for the Bj?rk-Shiley Standard and Starr-Edwards Ball valve and lowest values for the St Jude Medical valve and with the Hall-Kaster (Medtronic-Hall), Bj?rk-Shiley Convex-Concave and Monostrut valves in between.     

Experimental study of physiological pulsatile flow past valve prosthesis in a model of human aorta--II. Tilting disc valves and the effect of orientation

In Part II of this two paper sequence, pulsatile flow development past a tilting disc valve in a model human aorta has been studied using quantitative laser Doppler techniques. The valve was mounted in three different orientations with respect to the aortic root in this study. Under pulsatile flow, the region of flow reversal induced near the wall of the minor flow orifice extends to more than one tissue annulus diameter downstream from the valve into the ascending aorta. In a plane perpendicular to the tilt axis, a bi-helical secondary flow is induced distal to the valve. This secondary flow is further compounded by the multiple curvatures in the aorta. Hence the valve orientation affects the velocity profiles as far downstream as the mid-arch region as well as in the brachio-cephalic arterial branch. In the mid-arch region, a flow reversal along the entire cross-section is observed in early diastole for all the three orientations of the disc valve.     

Numerical simulation of steady turbulent flow through trileaflet aortic heart valves—II. Results on five models

Turbulent flow simulations are run for five aortic trileaflet valve geometries, ranging from a valve leaflet orifice area of 1.1 cm² (Model A1—very stenotic) to 5.0 cm² (Model A5—natural valve). The simulated data compares well with experimental measurements made downstream of various aortic trileaflet valves by Woo (PhD Thesis, 1984). The location and approximate width and length of recirculation regions are correctly predicted. The less stenotic valve models reattach at the end of the aortic sinus region, 1.1 diameters downstream of the valve. The central jet exiting the less stenotic valve models is not significantly different from fully developed flow, and therefore recovers very quickly downstream of the reattachment point. The more stenotic valves disturb the flow to a greater degree, generating recirculation regions large enough to escape the sinuses and reattach further downstream. Peak turbulent shear stress values downstream of the aortic valve models which approximated prosthetic valves are 125 and 300 N m⁻², very near experimental observations of 150 to 350 N m⁻². The predicted Reynolds stress profiles also present the correct shape, a double peak profile, with the location of the peak occuring at the location of maximum velocity gradient, which occurs near the recirculation region. The pressure drop across model A2 (leaflet orifice area 1.6 cm²) is 20 mmHg at 1.6 diameters downstream. This compares well with values ranging from 19.5 to 26.2 mmHg for valves of similar orifice areas. The pressure drop decreases with decreasing valve stenosis, to a negligible value across the least stenotic valve model. Based on the good agreement between experimental measurements of velocity, shear stress and pressure drop, compared to the simulated data, the model has the potential to be a valuable tool in the analysis of heart valve designs.     

In vitro flow dynamics of four prosthetic aortic valves: a comparative analysis

The velocity fields downstream of four prosthetic heart valves were mapped in vitro over the entire cross-section of a model aortic root using laser Doppler anemometry. THe Bj?rk-Shiley 60 degrees convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.     

Prosthetic heart valves: waveform comparisons and average value disposition for pulsatile flow in vitro

With the development of the in vitro testing of heart valves, the standardization of the test methods becomes increasingly important and they should also be improved continuously. This paper discusses the problems of waveform comparison and average value dispositions. In the pulsatile model driven by pneumatics, the pressures before and after the valve, and the flow through it, are measured as three one-dimensional variates. The mean values are calculated according to the FDA and the ISO. A comparison and analysis of experimental waveforms indicate that, for basically the same ranges of pressures and flow rate, the flow curves of different types of valve are clearly different. The mean values and the waveforms in the time domain should be taken into consideration synthetically so that the pusatile characteristics of the valve can be more completely reflected. Using numerical filtering methods to treat the waveforms allows for better comparisons between the measured results taken from the different devices. By means of the constellation graphical method for treating mean values as multivariates, it is feasible to classify the valves and to judge their qualities under conditions of pulsatile flow.     

Techniques of aortic valve repair

Similar to mitral repair, newer methods of aortic valve reconstruction are achieving excellent outcomes with an 85% to 90% freedom from valve-related complications at 10 years. The goal of this review is to illustrate these newer and more stable techniques of aortic valve repair. Most patients with aortic insufficiency from either trileaflet or bicuspid aortic valves are candidates for repair, in addition to selected patients with mixed aortic stenosis/insufficiency and aortic root aneurysms. Initially, aggressive commissural annuloplasty is performed to reduce measured valve diameter to 19 to 21 mm. Leaflet prolapse is corrected with plication stitches placed in the free edge of each leaflet adjacent to the Nodulus Arantius. In this regard, the leaflet free edge functions as the chorda tendinea of the aortic valve, and shortening with plication stitches raises the leaflet to a proper "effective height." Leaflet defects are augmented with gluteraldehyde-fixed autologous pericardium, and mild-to-moderate strategically placed spicules of calcium are removed with the cavitron ultrasonic surgical aspirator. Using these methods, most insufficient aortic valves, and many with mixed lesions, can be satisfactorily repaired. Six cases are illustrated in this review, spanning the spectrum of pathologies from annular dilatation without leaflet defects, to standard congenital bicuspid valve with prolapse, to trileaflet prolapse, to unusual bicuspid pathology with calcification, to a moderately calcified trileaflet valve with mixed lesions, and to aortic root aneurysms with severe aortic insufficiency. All valves were repaired using the techniques described above with trivial residual leak and minimal gradients. All repairs have been followed with yearly echocardiography, and valve reconstruction with these methods is now quite stable with excellent late outcomes. Most insufficient aortic valves now can undergo stable repair with minimal late valve-related complications. Greater application of aortic valve repair seems indicated.     

The use of a single-fiber reactor for the enzymatic removal of amino acids from solutions

In this article we describe the use of bench-scale single-fiber dialyzers for the development and testing of an immobilized enzyme reactor for the treatment of leukemia. The treatment is based on the enzymatic removal of specific amino acids from the blood of leukemia patients. L-Lysine alpha-oxidase and catalase were coimmobilized within the void space of the porous region of asymmetric hollow-fiber membranes for the removal of L-lysine from simulated human plasma solutions. Hollow-fiber reactor performance was evaluated using a small single-fiber dialyzer (SFD) consisting of a single fiber encased in a protective glass shell. This small reactor affords ease of use, requires small amounts of chemicals and biochemicals, and gives useful reactor performance data. Single-fiber dialyzers were constructed using polyamide fibers with a molecular weight cutoff of 10,000 (PA10 fibers); these fibers demonstrated the best compatibility with and retention of the enzymes. The SFD performance in removing L-lysine from solution was evaluated under both steady and pulsatile flow operation. Pulsatile flow was tested for two reasons: (1) to enhance the radial mass transfer of lysine within the SFD and (2) to simulate the pulsatile flow of blood in dialysis treatment. The use of pulsatile flow increased lysine conversion by 15% over the steady-flow case. Approximately 40% of the lysine was removed from simulated plasma by the SFD in a 4-h experiment using pulsatile flow in the recycle mode.     

In vitro pulsatile flow velocity and shear stress measurements in the vicinity of mechanical mitral heart valve prostheses

A three beam laser Doppler anemometer system was used to study the flow fields created by various types of mitral heart valve prostheses under physiological pulsatile flow conditions. The prosthetic valves studied were: Beall caged disc valve, Bjork-Shiley tilting disc valve, Medtronic-Hall tilting disc valve and St. Jude bileaflet valve. The results indicate that all four prosthetic valve designs studied create very disturbed flow fields with elevated turbulent shear stresses and regions of flow separation and/or stagnation. The observed elevated turbulent shear stresses could cause sublethal and/or lethal damage to red cells and platelets. The regions of flow separation and/or stagnation, could lead to thrombus formation and/or tissue overgrowth on the valve structure, as observed on clinically recovered prosthetic valves.     

The central axis prosthetic cardiac valve: an in vitro study of pressure drop assessment under steady-state flow conditions

In this work, a new mechanical prosthetic heart valve, the central axis valve, is presented. This new prosthesis has been tested in vitro, and compared with four other common prosthetic cardiac valves (Starr-Edwards 6120, Bjork-Shiley monostrut, Medtronic-Hall, and St Jude Medical valves). All valves studied have the same orifice diameter of 22 mm. The prostheses were installed inside a transparent mitral test chamber, which enables pressure drop measurement to be made under steady-state flow conditions using a blood analogue fluid. Pressure drop loss is one important factor affecting the overall performance of a prosthetic heart valve. Steady-state flow tests are essential to predict certain flow characteristics and pressure gradient loss before more complicated, expensive, and difficult-to-interpret pulsatile flow tests are conducted. All experiments were performed in vitro and at steady volumetric flow rates of 10 to 30 l/min. The Starr-Edwards SE 6120 showed the highest values for pressure drop. The St Jude Medical valve offers the minimum resistance to flow. The central axis valve comes second to the Starr-Edwards valve for this type of measurement. The new valve is promising. A complete valve evaluation programme, covering initial conceptional design through to clinical use, is in progress. Materials for the fabrication of the new valve are also under consideration.     

Effect of fiber orientation on the stress distribution within a leaflet of a polymer composite heart valve in the closed position

Polymer trileaflet valves offer natural hemodynamics with the potential for better durability than commercially available tissue valves. Strength and durability of polymer-based valves may be increased through fiber reinforcement. A finite element analysis of the mechanics of a statically loaded polymer trileaflet aortic heart valve has been conducted. A parametric analysis was performed to determine the effects of fiber orientation and volume density in a single and double ply model. A maximum stress value of 1.02MPa was obtained in the non-reinforced model for a transvalvular load (downstream-upstream) of 120mmHg. The maximum stress on the downstream side of the leaflet was approximately twice the maximum stress on the upstream side, and always occurred on the interface with the valve stent. The single ply model reduced the stress on the polymer matrix, with the maximum reduction of at least 64% occurring when the fiber orientation was such that the fibers ran perpendicular to the stent edge. The double ply model further reduced the stress on the polymer matrix, with the maximum reduction of greater than 86% now occurring when the fibers are oriented most perpendicular to one another.     

Laser anemometry measurements of pulsatile flow past aortic valve prostheses

Experimental results are presented on physiological pulsatile flow past caged ball and tilting disc aortic valve prostheses mounted in an axisymmetric chamber incorporated in a mock circulatory system. The measurements of velocity profiles and turbulent normal stresses during several times in a cardiac cycle were obtained using laser-Doppler anemometry. Our results show that with increased angle of opening for the tilting disc valves, a large but locally confined vortex is observed along the wall in the minor flow region throughout most of the cardiac cycle. The turbulent normal stresses measured downstream to the tilting disc in the minor flow region parallel to the tilt axis were found to be larger than those measured downstream to the caged ball valves. Comparison of measurements with steady flow at flow rates comparable to peak pulsatile flow rate show that the turbulent normal stresses are larger by a factor of two in pulsatile flow with a frequency of 1.2 Hz.