In vitro comparison of velocity profiles and turbulent shear distal to polyurethane trileaflet and pericardial prosthetic valves

Hydrodynamic Function of a Biostable Polyurethane Flexible Heart Valve after Six Months in Sheep

Survival to six months for sheep with a non-biostable polyurethane mitral heart valve prosthesis has been reported previously, however, with surface degradation and accumulation of calcified fibrin/thrombus that impaired leaflet motion and compromised hydrodynamic function. Newly available biostable polyurethanes may overcome this problem.

Six adult sheep with biostable polyurethane trileaflet heart valve prostheses of documented hydrodynamic performance, implanted in the mitral position, were allowed to survive for 6 months. Explanted valves were photographed, resubmitted to hydrodynamic function testing, and studied by light and electron microscopy.

Explanted valves were structurally intact and differed little in appearance from their preimplant state. Hydrodynamic testing showed no deterioration in pressure gradient or energy losses compared with pre-implant values.

Biostable polyurethanes demonstrated improved blood compatibility leaving leaflets flexible and valve function unimpaired. Biostable polyurethanes may thus improve prospects for prolonged function of synthetic heart valve prostheses.

Effect of Arched Leaflets and Stent Profile on the Hemodynamics of Tri-Leaflet Flexible Polymeric Heart Valves

Polymeric heart valves (PHV) can be engineered to serve as alternatives for existing prosthetic valves due to higher durability and hemodynamics similar to bioprosthetic valves. The purpose of this study is to evaluate the effect of geometry on PHVs coaptation and hemodynamic performance. The two geometric factors considered are stent profile and leaflet arch length, which were varied across six valve configurations. Three models were created with height to diameter ratio of 0.6, 0.7, and 0.88. The other three models were designed by altering arch height to stent diameter ratio, to be 0, 0.081, and 0.116. Particle image velocimetry experiments were conducted on each PHV to characterize velocity, vorticity, turbulent characteristics, effective orifice area, and regurgitant fraction. This study revealed that the presence of arches as well as higher stent profile reduced regurgitant flow down to 5%, while peak systole downstream velocity reduced to 58% and Reynolds Shear Stress values reduced 40%. Further, earlier reattachment of the forward flow jet was observed in PHVs with leaflet arches. These findings indicate that although both geometric factors help diminish the commissural gap during diastole, leaflet arches induce a larger jet opening, yielding to earlier flow reattachment and lower energy dissipation.

Polymeric trileaflet prosthetic heart valves: evolution and path to clinical reality

Present prosthetic heart valves, while hemodynamically effective, remain limited by progressive structural deterioration of tissue valves or the burden of chronic anticoagulation for mechanical valves. An idealized valve prosthesis would eliminate these limitations. Polymeric heart valves (PHVs), fabricated from advanced polymeric materials, offer the potential of durability and hemocompatibility. Unfortunately, the clinical realization of PHVs to date has been hampered by findings of in vivo calcification, degradation and thrombosis. Here, the authors review the evolution of PHVs, evaluate the state of the art of this technology and propose a pathway towards clinical reality. In particular, the authors discuss the development of a novel aortic PHV that may be deployed via transcatheter implantation, as well as its optimization via device thrombogenicity emulation.

Recent Advances in Polymeric Heart Valves Research

Heart valve replacement is fast becoming a routine surgery worldwide, and heart valve prostheses are today considered among the most widely used cardiovascular devices. Mechanical and bioprostheses have been the traditional choices to the replacement surgeries. However, such valves continue to expose patients to risks including thrombosis, infection and limited valve durability. In recent years, advances in polymer science give rise to an important new class of artificial heart valve made predominantly of polyurethane-based materials, which show improved biocompatibility and biostability. These polymeric heart valves have demonstrated excellent hemodynamic performance and good durability with excellent fatigue stress resistance. Advancements in the designs and manufacturing methods also suggested improved in the durability of polymeric heart valves. Animal studies with these valves have also shown good biocompatibility with minimal calcification of the valve leaflets. With these promising progresses, polymeric heart valves could be a viable alternative in the heart valve replacement surgeries in the near future.

On the multiscale modeling of heart valve biomechanics in health and disease

Theoretical models of the human heart valves are useful tools for understanding and characterizing the dynamics of healthy and diseased valves. Enabled by advances in numerical modeling and in a range of disciplines within experimental biomechanics, recent models of the heart valves have become increasingly comprehensive and accurate. In this paper, we first review the fundamentals of native heart valve physiology, composition and mechanics in health and disease. We will then furnish an overview of the development of theoretical and experimental methods in modeling heart valve biomechanics over the past three decades. Next, we will emphasize the necessity of using multiscale modeling approaches in order to provide a comprehensive description of heart valve biomechanics able to capture general heart valve behavior. Finally, we will offer an outlook for the future of valve multiscale modeling, the potential directions for further developments and the challenges involved.

A novel polymer for potential use in a trileaflet heart valve

A novel polyolefin, poly(styrene-b-isobutylene-b-styrene) (Quatromer), is being proposed as a viable polymer for use in trileaflet heart valves because of its oxidative stability. The current study was designed to assess the polymer's hemocompatibility and mechanical durability. Mechanical characterization included static tensile tests and dynamic tension-tension and bending fatigue tests, where the properties of isotropic and composite (polypropylene (PP) embedded) Quatromer specimens were compared with those of a polyurethane (PUR) approved for cardiovascular applications. It was found that by embedding PP fibers into the Quatromer matrix, the tensile and fatigue properties of the polymer could be improved, making them comparable, if not better than the PUR. The thrombotic potential of Quatromer was compared with the PUR, glutaraldehyde-fixed porcine valve material, and a positive and negative control by measuring platelet deposition with radiolabeled platelets in a parallel plate flow configuration. The porcine valve material was found to have significantly higher platelet deposition under all flow regimes, while no significant difference existed between Quatromer and PUR. In conclusion, Quatromer is shown to have suitable hemocompatibility and mechanical durability for use in polymer trileaflet heart valves, and fiber reinforcement can effectively be used to tailor the mechanical properties.

Fluid Dynamic Assessment of Three Polymeric Heart Valves Using Particle Image Velocimetry

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this paper we characterize the in vitro velocity and Reynolds Shear Stress (RSS) fields inside and downstream of three different prototype trileaflet polymeric heart valves. The fluid dynamic differences are then correlated with variations in valve design parameters. The three valves differ in leaflet thickness, ranging from 80 to 120 mum, and commisural design, either closed, opened, or semi-opened. The valves were subjected to aortic flow conditions and the velocity measured using three-dimensional stereo Particle Image Velocimetry. The peak forward flow phase in the three valves was characterized by a strong central orifice jet of approximately 2 m/s with a flat profile along the trailing edge of the leaflets. Leakage jets, with principle RSS magnitudes exceeding 4,500 dyn/cm(2), were observed in all valves with larger leaflet thicknesses and also corresponded to larger leakage volumes. Additional leakage jets were observed at the commissural region of valves with the open and the semi-open commissural designs. The results of the present study indicate that commissural design and leaflet thickness influence valve fluid dynamics and thus the thrombogenic potential of trileaflet polymeric valves.

A comparison of flow field structures of two tri-leaflet polymeric heart valves

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this in vitro study, the velocity fields inside and downstream of two different prototype tri-lealfet polymeric heart valves were studied. Experiments were conducted on two 23 mm prototype polymeric valves, provided by AorTech Europe, having open or closed commissure designs and leaflet thickness of 120 and 80 microm, respectively. A two-dimensional LDV system was used to measure the velocity fields in the vicinity of the two valves under simulated physiological conditions. Both commissural design and leaflet thickness were found to affect the flow characteristics. In particular, very high levels of Reynolds shear stress of 13,000 dynes/cm2 were found in the leakage flow of the open commisure design. Maximum leakage velocities in the open and closed designs were 3.6 m/s and 0.5 m/s respectively; the peak forward flow velocities were 2.0 m/s and 2.6 m/s, respectively. In both valve designs, shear stress levels exceeding 4,000 dyne/cm2 were observed at the trailing edge of the leaflets and in the leakage and central orifice jets during peak systole. Additionally, regions of low velocity flow conducive to thrombus formation were observed in diastole. The flow structures measured in these experiments are consistent with the location of thrombus formation observed in preliminary animal experiments.

An Experimental Study of Steady Flow Patterns of a New Trileaflet Mechanical Aortic Valve

Hemodynamic research shows that thrombosis formation is closely tied to flow field turbulent stress. Design limitations cause flow separation at leaflet edges and the annular valve base, vortex mixing downstream, and high turbulent shear stress. The trileaflet design opens like a physiologic valve with central flow. Leaflet curvature approximates a completely circular orifice, maximizing effective flow area of the open valve. Semicircular aortic sinuses downstream of the valve allow vortex formation to help leaflet closure. The new trileaflet design was hemodynamically evaluated via digital particle image velocimetry and laser-Doppler anemometry. Measurements were made during peak flow of the fully open valve, immediately downstream of the valve, and compared with the 27-mm St. Jude Medical (SJM) bileaflet valve. The trileaflet valve central flow produces sufficient pressure to inhibit separation shear layers. Absence of downstream turbulent wake eddies indicates smooth, physiologic blood flow. In contrast, SJM produces strong turbulence because of unsteady separated shear layers where the jet flow meets the aortic sinus wall, resulting in higher turbulent shear stresses detrimental to blood cells. The trileaflet valve simulates the physiologic valve better than previous designs, produces smoother flow, and allows large scale recirculation in the aortic sinuses to help valve closure.

Effect of the sinus of valsalva on the closing motion of bileaflet prosthetic heart valves

Conventional bileaflet prosthetic mechanical heart valves close passively with backflow. Naturally, the valve has problems associated with closure, such as backflow, water hammer effect, and fracture of the leaflet. On the other hand, in the case of the natural aortic valve, the vortex flow in the sinus of Valsalva pushes the leaflet to close, and the valve starts the closing motion earlier than the prosthetic valve as the forward flow decelerates. This closing mechanism is thought to decrease backflow at valve closure. In this study, we propose a new bileaflet mechanical valve resembling a drawbridge in shape, and the prototype valve was designed so that the leaflet closes with the help of the vortex flow in the sinus. The test valve was made of aluminum alloy, and its closing motion was compared to that of the CarboMedics (CM) valve. Both valves were driven by a computer controlled hydraulic mock circulator and were photographed at 648 frames/s by a high speed charge-coupled device (CCD) camera. Each frame of the valve motion image was analyzed with a personal computer, and the opening angles were measured. The flow rate was set as 5.0 L/min. The system was pulsed with 70 bpm, and the systolic/diastolic ratio was 0.3. Glycerin water was used as the circulation fluid at room temperature, and polystyrene particles were used to visualize the streamline. The model of the sinus of Valsalva was made of transparent silicone rubber. As a result, high speed video analysis showed that the test valve started the closing motion 41 ms earlier than the CM valve, and streamline analysis showed that the test valve had a closing mechanism similar to the natural one with the effect of vortex flow. The structure of the test valve was thought to be effective for soft closure and could solve problems associated with closure.

Polyurethane: material for the next generation of heart valve prostheses?

OBJECTIVES

The prospects for a durable, athrombogenic, synthetic, flexible leaflet heart valve are enhanced by the recent availability of novel, biostable polyurethanes. As a forerunner to evaluation of such biostable valves, a prototype trileaflet polyurethane valve (utilising conventional material of known in vitro behaviour) was compared with mechanical and bioprosthetic valves for assessment of in vivo function, durability, thromboembolic potential and calcification.

METHODS

Polyurethane (PU), ATS bileaflet mechanical, and Carpentier-Edwards porcine (CE) valves were implanted in the mitral position of growing sheep. Counting of high-intensity transient signals (HITS) in the carotid arteries, echocardiographic assessment of valve function, and examination of blood smears for platelet aggregates were undertaken during the 6-month anticoagulant-free survival period. Valve structure and hydrodynamic performance were assessed following elective sacrifice.

RESULTS

Twenty-eight animals survived surgery (ten ATS; ten CE; eight PU). At 6 months the mechanical valve group (n=9) showed highest numbers of HITS (mean 40/h, P=0.01 cf. porcine valves), and platelet aggregates (mean 62.22/standard field), but no thromboembolism, and no structural or functional change. The bioprosthetic group (n=6) showed low HITS (1/h) and fewer aggregates (41.67, P=1.00, not significant), calcification and severe pannus overgrowth with progressive stenosis. The PU valves (n=8) showed a small degree of fibrin attachment to leaflet surfaces, no pannus overgrowth, little change in haemodynamic performance, low levels of HITS (5/h) and platelet aggregates (17.50, P<0.01 cf. mechanical valves, P=0.23 cf. porcine valves), and no evidence of thromboembolism.

CONCLUSIONS

In the absence of valve-related death and morbidity, and retention of good haemodynamic function, the PU valve was superior to the bioprosthesis; lower HITS and aggregate counts in the PU valve imply lower thrombogenicity compared with the mechanical valve. A biostable polyurethane valve could offer clinical advantage with the promise of improved durability (cf. bioprostheses) and low thrombogenicity (cf. mechanical valves).

In vitro function and durability assessment of a novel polyurethane heart valve prosthesis

While flexible-leaflet, central-flow prosthetic heart valves promise relief from anticoagulation therapy, they continue to be restricted by inadequate durability. In consequence, a novel trileaflet valve, made entirely from polyurethane, has been developed. A batch of 6 consecutively manufactured polyurethane valves was subjected to hydrodynamic function and accelerated fatigue testing. Computerized data acquisition and control systems have been introduced to improve valve testing methodologies. In terms of hydrodynamic function, the polyurethane valve demonstrates transvalvular pressure gradients similar to those for a bioprosthetic valve (Carpentier-Edwards) and levels of retrograde flow significantly less than those for either the bioprosthetic valve or a bileaflet mechanical valve (St Jude Medical). The equivalent of 10 years of cycling without failure has been exceeded by all 6 polyurethane valves in accelerated fatigue tests with 2 valves remaining intact after 674 million cycles (equivalent to approximately 17 years) in continuing tests. Highspeed photography revealed considerable differences in leaflet motion between valves cycled at accelerated and physiological rates.

The effect of varying degrees of stenosis on the characteristics of turbulent pulsatile flow through heart valves

Many problems and complications associated with heart valves are related to the dynamic behavior of the valve and the resultant unsteady flow patterns. An accurate depiction of the spatial and temporal velocity and rms distributions imparts better understanding of flow related valve complications, and may be used as a guideline in valve design. While the generalized correlation between increased turbulence level and the severity of the stenosis is well established, few studies addressed the issue of the intermittent nature of turbulence and its timing in the cardiac cycle, and almost none assessed the effect of a progressive stenosis on the flow characteristics through heart valves. In this experimental work we simulated the type of flow which is present in normal and stenosed valves and conducted a comprehensive investigation of valve hemodynamics, valvular turbulence and morphology under varying degrees of stenosis. The characteristics of valves and stenoses were simulated closely, to achieve the flow conditions that initiate turbulent flow conditions. Laser Doppler anemometry (LDA) measurements were carried out in a pulse duplicator system distal to trileaflet polyurethane prosthetic heart valves, installed at mitral and aortic positions. The effect of the degree of the stenosis was comparatively studied through the structure of the turbulent jets emerging from normal and stenotic heart valves. Maximum turbulence level was achieved during the decelerating phase and correlated to the severity of the stenosis, followed by relaminarization of the flow during the acceleration phase. The intermittent nature of the turbulence emphasized the importance of realizing the timing of the turbulence production and its spatial location for optimizing current valve designs. The plug flow through the normal aortic valve prosthesis was replaced by jet like behavior for a 65% stenosis, with the jet becoming narrower and stronger for a 90% stenosis. The morphology of the velocity and turbulence waveforms was found to be governed by the stenosis geometry and the valve position (aortic, mitral).

Biological interactions: causes for risks and failures of biomaterials and devices

Biomaterials and devices have been used in a variety of applications ranging from disposable extracorporeal devices and soft and hard tissue augmentation, to total artificial internal organs. However, there are risks and failures in each application which are a result of undesirable biological interactions. This article deals with various biological interactions in each category of implants and devices used in medical applications.

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.

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.