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

Multiscale structure and function of the aortic valve apparatus

Whereas studying the aortic valve in isolation has facilitated the development of life-saving procedures and technologies, the dynamic interplay of the aortic valve and its surrounding structures is vital to preserving their function across the wide range of conditions encountered in an active lifestyle. Our view is that these structures should be viewed as an integrated functional unit, here referred to as the aortic valve apparatus (AVA). The coupling of the aortic valve and root, left ventricular outflow tract, and blood circulation is crucial for AVA's functions: unidirectional flow out of the left ventricle, coronary perfusion, reservoir function, and support of left ventricular function. In this review, we explore the multiscale biological and physical phenomena that underlie the simultaneous fulfillment of these functions. A brief overview of the tools used to investigate the AVA, such as medical imaging modalities, experimental methods, and computational modeling, specifically fluid-structure interaction (FSI) simulations, is included. Some pathologies affecting the AVA are explored, and insights are provided on treatments and interventions that aim to maintain quality of life. The concepts explained in this article support the idea of AVA being an integrated functional unit and help identify unanswered research questions. Incorporating phenomena through the molecular, micro, meso, and whole tissue scales is crucial for understanding the sophisticated normal functions and diseases of the AVA.

A Fibrin-Thrombin Based In Vitro Perfusion System to Study Flow-Related Prosthetic Heart Valves Thrombosis

Prosthetic heart valve (PHV) replacement has increased the survival rate and quality of life for heart valve-diseased patients. However, PHV thrombosis remains a critical problem associated with these procedures. To better understand the PHV flow-related thrombosis problem, appropriate experimental models need to be developed. In this study, we present an in vitro fibrin clot model that mimics clot accumulation in PHVs under relevant hydrodynamic conditions while allowing real-time imaging. We created 3D-printed mechanical aortic valve models that were inserted into a transparent glass aorta model and connected to a system that simulates human aortic flow pulse and pressures. Thrombin was gradually injected into a circulating fibrinogen solution to induce fibrin clot formation, and clot accumulation was quantified via image analysis. The results of valves positioned in a normal versus a tilted configuration showed that clot accumulation correlated with the local flow features and was mainly present in areas of low shear and high residence time, where recirculating flows are dominant, as supported by computational fluid dynamic simulations. Overall, our work suggests that the developed method may provide data on flow-related clot accumulation in PHVs and may contribute to exploring new approaches and valve designs to reduce valve thrombosis.

Hemodynamics of the VenusP Valve System™-an in vitro study

This study aims to evaluate the fluid dynamic characteristics of the VenusP Valve System™ under varying cardiac outputs in vitro. A thorough hemodynamic study of the valve under physiological cardiac conditions was conducted and served as an independent assessment of the performance of the valve. Flow fields downstream of the valve near the pulmonary bifurcation were quantitatively studied by two-dimensional Particle Image Velocimetry (PIV). The obtained flow field was analyzed for potential regions of flow stasis and recirculation, and elevated shear stress and turbulence. High-speed en face imaging capturing the leaflet motion provided data for leaflet kinematic modeling. The experimental conditions for PIV studies were in accordance with ISO 5840-1:2021 standard, and two valves with different lengths and different orientations were studied. Results show good hemodynamics performance for the tested valves according to ISO 5840 standard without significant regions of flow stasis. Observed shear stress values are all well below established hemolysis limits.

A parametric study on pulse duplicator design and valve hemodynamics

BACKGROUND

In vitro assessment is mandatory for artificial heart valve development. This study aims to investigate the effects of pulse duplicator features on valve responsiveness, conduct a sensitivity analysis across valve prosthesis types, and contribute on the development of versatile pulse duplicator systems able to perform reliable prosthetic aortic valve assessment under physiologic hemodynamic conditions.

METHODS

A reference pulse duplicator was established based on literature. Further optimization process led to new designs that underwent a parametric study, also involving different aortic valve prostheses. These designs were evaluated on criteria such as mean pressure differential and pulse pressure (assessed from high-fidelity pressure measurements), valve opening and closing behavior, flow, and regurgitation. Finally, the resulting optimized setup was tested under five different hemodynamic settings simulating a range of physiologic and pathologic conditions.

RESULTS

The results show that both, pulse duplicator design and valve type significantly influence aortic and ventricular pressure, flow, and valve kinematic response. The optimal design comprised key features such as a compliance chamber and restrictor for diastolic pressure maintenance and narrow pulse pressure. Additionally, an atrial reservoir was included to prevent atrial-aortic interference, and a bioprosthetic valve was used in mitral position to avoid delayed valve closing effects.

CONCLUSION

This study showed that individual pulse duplicator features can have a significant effect on valve's responsiveness. The optimized versatile pulse duplicator replicated physiologic and pathologic aortic valve hemodynamic conditions, serving as a reliable characterization tool for assessing and optimizing aortic valve performance.

Influence of Polymer Stiffness and Geometric Design on Fluid Mechanics in Tissue-Engineered Pulmonary Valve Scaffolds

There is still much unknown about the fluid mechanical response to cardiac valve scaffolds, even as their implementation in the clinic is on the horizon. Specifically, while degradable polymer valve scaffolds are currently being tested in the pulmonary valve position, their material and mechanical properties have not been fully elucidated. Optimizing these properties are important determinants not only of acute function, but long-term remodeling prospects. This study aimed to characterize fluid profiles downstream of electrospun valve scaffolds under dynamic pulmonary conditions. Valve scaffold design was changed by either blending poly(carbonate urethane) urea (PCUU) with poly(ε-caprolactone) (PCL) to modulate material stiffness or by changing the geometric design of the valve scaffolds. Specifically, two designs were utilized: one modeled after a clinically used bioprosthetic valve design (termed Mk1 design), and another using a geometrically "optimized" design (termed Mk2) based on anatomical data. Particle image velocimetry results showed that material stiffness only had a mild impact on fluid mechanics, measured by velocity magnitude, vorticity, viscous shear stress, Reynolds shear stress, and turbulent kinetic energy. However, comparing the two geometric designs yielded a much greater impact, with the Mk2 valve groups containing the highest PCUU/PCL ratio demonstrating the overall best performance. This report highlights the easily manipulable design features of polymeric valve scaffolds and demonstrates their relative significance for valve function.

In vitro hemodynamics of fabric composite membrane for cardiac valve prosthesis replacement

This study aimed to investigate the hemodynamics of a novel fabric composite that can be used as a substitute for bovine pericardium. The structure is composed of ultrahigh molecular weight polyethylene (UHMWPE) fabric coated with thermoplastic polyurethane (TPU) membranes on both sides. In vitro experiments were carried out on two composite valve samples with different specifications and a bovine pericardial one with the same dimension and structure. Hemodynamic properties including the effective orifice area (EOA) and regurgitant fraction (RF) were obtained and compared through pulsatile-flow testing in a pulse duplicator. Using the particle image velocimetry (PIV) technique, frames of the downstream velocity field in the aortic valve chamber were captured during cardiac cycles. Then, the field of Reynolds shear stress (RSS), viscous shear stress (VSS), and turbulent kinetic energy (TKE) at peak systole were calculated. A fluid-structure interaction (FSI) model has also been used to verify the pulsatile-flow testing. Compared with the bovine pericardial valve, composite valves have nosuperiority regarding EOA and RF due to their slightly higher rigidity. However, shear stresses of composite valves were lower than those of the bovine pericardial valve indicating more stable blood flows, which means that composite leaflets have the potential to reduce the risks of thrombosis and hemolysis induced by the mechanical contact between the blood flow and leaflets of valve prostheses.

Smartphone-based particle image velocimetry for cardiovascular flows applications: A focus on coronary arteries

An experimental set-up is presented for the in vitro characterization of the fluid dynamics in personalized phantoms of healthy and stenosed coronary arteries. The proposed set-up was fine-tuned with the aim of obtaining a compact, flexible, low-cost test-bench for biomedical applications. Technically, velocity vector fields were measured adopting a so-called smart-PIV approach, consisting of a smartphone camera and a low-power continuous laser (30 mW). Experiments were conducted in realistic healthy and stenosed 3D-printed phantoms of left anterior descending coronary artery reconstructed from angiographic images. Time resolved image acquisition was made possible by the combination of the image acquisition frame rate of last generation commercial smartphones and the flow regimes characterizing coronary hemodynamics (velocities in the order of 10 cm/s). Different flow regimes (Reynolds numbers ranging from 20 to 200) were analyzed. The smart-PIV approach was able to provide both qualitative flow visualizations and quantitative results. A comparison between smart-PIV and conventional PIV (i.e., the gold-standard experimental technique for bioflows characterization) measurements showed a good agreement in the measured velocity vector fields for both the healthy and the stenosed coronary phantoms. Displacement errors and uncertainties, estimated by applying the particle disparity method, confirmed the soundness of the proposed smart-PIV approach, as their values fell within the same range for both smart and conventional PIV measured data (≈5% for the normalized estimated displacement error and below 1.2 pixels for displacement uncertainty). In conclusion, smart-PIV represents an easy-to-implement, low-cost methodology for obtaining an adequately robust experimental characterization of cardiovascular flows. The proposed approach, to be intended as a proof of concept, candidates to become an easy-to-handle test bench suitable for use also outside of research labs, e.g., for educational or industrial purposes, or as first-line investigation to direct and guide subsequent conventional PIV measurements.

Flow assessment as a function of pump timing of tubular pulsatile pump for use as a ventricular assist device in a left heart simulator

INTRODUCTION

Although mechanical circulatory support saved many lives during the last decade, clinical observations have shown that the continuous flow pumps are associated with a much higher incidence of gastrointestinal bleeding and kidney problems, among others, compared with the earlier generation pulsatile pumps. However, the presence of several moving mechanical components made pulsatile pumps less durable, bulky, and prone to malfunction, ultimately leading to bias in favor of continuous flow designs.

OBJECTIVE

The aim of the current work is to create a prototype tubular pulsatile pump and to test the timing of the pump in a left heart simulator.

METHODS

A left heart simulator to mimic pumping from a failing heart was created. This was used to experimentally test the output of a prototype ventricular assist device relative to a failing heart in the form of flow and pressure. The effect of pulsation timing was quantified.

RESULTS

A failing heart was simulated with an average flow rate of 1.1 L/min and a systolic pressure of 47 mmHg. With the pump, the flow rate increases to 4.8 L/min and a systolic pressure of 110mmHg, in a copulsation mode, while activating for 300-400 ms. If the activation time is reduced, or increased, the pump becomes less effective. Load on the heart is reduced when the pump operates in a counterpulsation mode.

CONCLUSION

A pulsatile pump, like the one proposed, provides adequate output for mechanical circulatory support, while minimizing the number of moving parts that could otherwise lead to tribological wear.

Biomaterials in Valvular Heart Diseases

Valvular heart disease (VHD) occurs as the result of valvular malfunction, which can greatly reduce patient's quality of life and if left untreated may lead to death. Different treatment regiments are available for management of this defect, which can be helpful in reducing the symptoms. The global commitment to reduce VHD-related mortality rates has enhanced the need for new therapeutic approaches. During the past decade, development of innovative pharmacological and surgical approaches have dramatically improved the quality of life for VHD patients, yet the search for low cost, more effective, and less invasive approaches is ongoing. The gold standard approach for VHD management is to replace or repair the injured valvular tissue with natural or synthetic biomaterials. Application of these biomaterials for cardiac valve regeneration and repair holds a great promise for treatment of this type of heart disease. The focus of the present review is the current use of different types of biomaterials in treatment of valvular heart diseases.

Experimental analysis of pulsatile flow characteristics in prosthetic aortic valve models with stenosis

Bioprosthetic valves are widely used for aortic valve replacements for patients with severe aortic diseases. However, tissue-engineered leaflets normally deteriorate over time due to calcification, leading to life-threatening conditions that would require re-operation. The hemodynamics induced by a prosthetic stenosis is complicated and not fully understood. This in vitro experimental study focuses on the fluid dynamics of two aortic valve models with different prosthetic stenosis conditions. An in vitro cardiovascular flow simulator was utilized to provide the pulsatile physiological flow conditions. Phase-locked particle image velocimetry (PIV) and high-frequency pressure sensors were employed to measure the flow fields and pressure waveforms. Pressure data were evaluated for the two models representing moderate and severe stenosis conditions, respectively. The severe prosthetic stenosis induced a prolonged ejection period and increased acceleration time ratio. PIV results suggest the severe prosthetic stenosis resulted in a two-fold increase in peak jet velocity and a three-fold increase in peak turbulence kinetic energy compared to the moderate stenosis case. The severe stenosis also caused rapid expansion of the jet downstream of the valve orifice and increased eccentricity of the jet flow. The maximum Reynolds shear stress in the severe stenosis case was found similar to the bileaflet mechanical valve reported by previous literature, which was below the risk threshold of blood cell damage but could potentially increase the risks of platelet activation and aggregation.

A multilayered valve leaflet promotes cell-laden collagen type I production and aortic valve hemodynamics

Patients with aortic heart valve disease are limited to valve replacements that lack the ability to grow and remodel. This presents a major challenge for pediatric patients who require a valve capable of somatic growth and at a smaller size. A patient-specific heart valve capable of growth and remodeling while maintaining proper valve function would address this major issue. Here, we recreate the native valve leaflet structure composed of poly-ε-caprolactone (PCL) and cell-laden gelatin-methacrylate/poly (ethylene glycol) diacrylate (GelMA/PEGDA) hydrogels using 3D printing and molding, and then evaluate the ability of the multilayered scaffold to produce collagen matrix under physiological shear stress conditions. We also characterized the valve hemodynamics under aortic physiological flow conditions. The valve's fibrosa layer was replicated by 3D printing PCL in a circumferential direction similar to collagen alignment in the native leaflet, and GelMA/PEGDA sustained and promoted cell viability in the spongiosa/ventricularis layers. We found that collagen type I production can be increased in the multilayered scaffold when it is exposed to pulsatile shear stress conditions over static conditions. When the PCL component was mounted onto a valve ring and tested under physiological aortic valve conditions, the hemodynamics were comparable to commercially available valves. Our results demonstrate that a structurally representative valve leaflet can be generated using 3D printing and that the PCL layer of the leaflet can sustain proper valve function under physiological aortic valve conditions.

In vitro Assessment of the Impacts of Leaflet Design on the Hemodynamic Characteristics of ePTFE Pulmonary Prosthetic Valves

Prosthetic pulmonary valves are widely used in the management procedures of various congenital heart diseases, including the surgical pulmonary valve replacement (PVR) and right ventricular outflow tract reconstruction (RVOT). The discouraging long-term outcomes of standard prostheses, including homografts and bioprosthetic, constrained their indications. Recent developments in the expanded-polytetrafluoroethylene (ePTFE) pulmonary prosthetic valves provide promising alternatives. In this study, the hemodynamic characteristics of bileaflet and trileaflet ePTFE valve designs were experimentally evaluated. The in vitro tests were performed under the right ventricle (RV) flow conditions by using an in vitro RV circulatory system and particle image velocimetry (PIV). The leaflet kinetics, trans-valvular pressure gradients, effective orifice areas, regurgitant fractions, energy losses, velocity fields, and Reynolds shear stress (RSS) in both prostheses were evaluated. The opening of the bileaflet and trileaflet valve takes 0.060 and 0.088 s, respectively. The closing of the former takes 0.140 s, in contrast to 0.176 s of the latter. The trans-valvular pressure is 6.8 mmHg in the bileaflet valve vs. 7.9 mmHg in the trileaflet valve. The effective orifice area is 1.83 cm2 in the bileaflet valve and 1.72 cm2 in the trileaflet valve. The regurgitant fraction and energy loss of bileaflet are 7.13% and 82 mJ, which are 7.84% and 101.64 mJ in its bileaflet counterpart. The maximum RSS of 48.0 and 49.2 Pa occur at the systole peak in the bileaflet and trileaflet valve, respectively. A higher average RSS level is found in the bileaflet valve. The results from this preliminary study indicate that the current bileaflet prosthetic valve design is capable of providing a better overall hemodynamic performance than the trileaflet design.

Mechanical considerations for polymeric heart valve development: Biomechanics, materials, design and manufacturing

The native human heart valve leaflet contains a layered microstructure comprising a hierarchical arrangement of collagen, elastin, proteoglycans and various cell types. Here, we review the various experimental methods that have been employed to probe this intricate microstructure and which attempt to elucidate the mechanisms that govern the leaflet's mechanical properties. These methods include uniaxial, biaxial, and flexural tests, coupled with microstructural characterization techniques such as small angle X-ray scattering (SAXS), small angle light scattering (SALS), and polarized light microscopy. These experiments have revealed complex elastic and viscoelastic mechanisms that are highly directional and dependent upon loading conditions and biochemistry. Of all engineering materials, polymers and polymer-based composites are best able to mimic the tissue-level mechanical behavior of the native leaflet. This similarity to native tissue permits the fabrication of polymeric valves with physiological flow patterns, reducing the risk of thrombosis compared to mechanical valves and in some cases surpassing the in vivo durability of bioprosthetic valves. Earlier work on polymeric valves simply assumed the mechanical properties of the polymer material to be linear elastic, while more recent studies have considered the full hyperelastic stress-strain response. These material models have been incorporated into computational models for the optimization of valve geometry, with the goal of minimizing internal stresses and improving durability. The latter portion of this review recounts these developments in polymeric heart valves, with a focus on mechanical testing of polymers, valve geometry, and manufacturing methods.

Fabrication of Low-Cost Patient-Specific Vascular Models for Particle Image Velocimetry

PURPOSE

Particle image velocimetry (PIV), an in vitro experimentation technique that optically measures velocity components to analyze fluid velocity fields, has become increasingly popular to study flow dynamics in various vascular territories. However, it can be difficult and expensive to create patient-specific clear models for PIV due to the importance of refractive index matching of the model and the fluid. We aim to implement and test the use of poly-vinyl alcohol (PVA) in a lost-core casting technique to create low-cost, patient-specific models for PIV.

METHODS

Anonymized patient vascular anatomies were segmented and processed in Mimics/3Matic to create patient-specific cores from 3D digital subtraction angiographies. The cores were 3D-printed with PVA and post-processed with a 80:20 water:glue mixture to smooth the surface. Two silicones, Sylgard 184 and Solaris, were used to encapsulate the model and the PVA core was dissolved using warm water. Computed tomography scans were used to evaluate geometric accuracy using circumferences and surface differences in the model.

RESULTS

Mean geometric differences in circumference along the inlet centerline and the mean surface difference in the aneurysm between the final Silicone Model and the desired STL Print geometry were statistically insignificant (0.6 mm, 95% CI [- 1.4, 2.8] and 0.3 mm 95% CI [- 0.1, 0.7], respectively). Particle illumination within each model was successful. The cost of one 10 cm × 10 cm × 5 cm model was $69.

CONCLUSION

This technique was successful to implement and test the use of PVA in a lost-core casting technique to create low-cost, patient-specific in vitro models for PIV experimentation.

Discrete Subaortic Stenosis: Perspective Roadmap to a Complex Disease

Discrete subaortic stenosis (DSS) is a congenital heart disease that results in the formation of a fibro-membranous tissue, causing an increased pressure gradient in the left ventricular outflow tract (LVOT). While surgical resection of the membrane has shown some success in eliminating the obstruction, it poses significant risks associated with anesthesia, sternotomy, and heart bypass, and it remains associated with a high rate of recurrence. Although a genetic etiology had been initially proposed, the association between DSS and left ventricle (LV) geometrical abnormalities has provided more support to a hemodynamic etiology by which congenital or post-surgical LVOT geometric derangements could generate abnormal shear forces on the septal wall, triggering in turn a fibrotic response. Validating this hypothetical etiology and understanding the mechanobiological processes by which altered shear forces induce fibrosis in the LVOT are major knowledge gaps. This perspective paper describes the current state of knowledge of DSS, articulates the research needs to yield mechanistic insights into a significant pathologic process that is poorly understood, and proposes several strategies aimed at elucidating the potential mechanobiological synergies responsible for DSS pathogenesis. The proposed roadmap has the potential to improve DSS management by identifying early targets for prevention of the fibrotic lesion, and may also prove beneficial in other fibrotic cardiovascular diseases associated with altered flow.

Comparison of 4D Flow MRI and Particle Image Velocimetry Using an In Vitro Carotid Bifurcation Model

Four-dimensional (4D) Flow magnetic resonance imaging (MRI) enables the acquisition and assessment of complex hemodynamics in vivo from different vascular territories. This study investigated the viability of stereoscopic and tomographic particle image velocimetry (stereo- and tomo-PIV, respectively) as experimental validation techniques for 4D Flow MRI. The experiments were performed using continuous and pulsatile flows through an idealized carotid artery bifurcation model. Transverse and longitudinal planes were extracted from the acquired velocity data sets at different regions of interest and were analyzed with a point-by-point comparison. An overall root-mean-square error (RMSE) was calculated resulting in errors as low as 0.06 and 0.03 m/s when comparing 4D Flow MRI with stereo- and tomo-PIV, respectively. Quantitative agreement between techniques was determined by evaluating the relationship for individual velocity components and their magnitudes. These resulted in correlation coefficients (R²) of 4D Flow MRI with stereo- and tomo-PIV, as low as 0.76 and 0.73, respectively. The 3D velocity measurements from PIV showed qualitative agreement when compared to 4D Flow MRI, especially with tomo-PIV due to the addition of volumetric velocity measurements. These results suggest that tomo-PIV can be used as a validation technique for 4D Flow MRI, serving as the basis for future validation protocols.

Impact of patient-specific morphologies on sinus flow stasis in transcatheter aortic valve replacement: An in vitro study

OBJECTIVE

The goal of this study is to evaluate how sinus flow patterns after transcatheter aortic valve replacement in realistic representative patient roots vary. Sinus flow can affect transcatheter aortic valve operation and likely leaflet thrombosis occurrence due to stasis and poor washout. How the interaction between transcatheter aortic valve and representative patient aortic roots affects sinus hemodynamics is important to establish for future individualization of transcatheter aortic valve replacement therapy.

METHODS

Two representative patient aortic roots were selected, segmented and 3-dimensional printed followed by deployment of Medtronic CoreValve (Medtronic Inc, Irvine, Calif) and Edwards SAPIEN (Edwards Lifesciences, Irvine Calif) transcatheter aortic valves. Sinus hemodynamics were assessed in vitro using high spatio-temporal resolution particle-image-velocimetry. Detailed sinus vortex tracking, shear stress probability density functions, and sinus washout were evaluated and assessed as a function of valve type and representative patient morphology as independent case studies.

RESULTS

Peak velocity in the sinus with SAPIEN valve was approximately 3 times higher than with CoreValve for both models (0.30 ± 0.02 m/s and 0.34 ± 0.041 m/s vs 0.13 ± 0.01 m/s and 0.10 ± 0.02 m/s) (P < .01). Between representative patient models, vorticity magnitudes were significantly different (75 ± 1.1 s^-1, 77 ± 3.2 s^-1, 109 ± 2.3 s^-1, and 250 ± 4.1 s^-1) (P < .01) regardless of valve type. Sinus blood washout characteristic as a function of cardiac cycles was strongly both patient related and valve specific. Fluid dynamics favored shear stresses and washout characteristics due to a smaller sinus and sinotubular junction, further amplified by the SAPIEN valve.

CONCLUSIONS

Sinus flow dynamics are highly sensitive to aortic root characteristics and transcatheter aortic valve aortic root interaction. Differences in sinus-flow washout and stasis regions between representative patient models may be reflected in different risks of leaflet thrombosis or valve degeneration.

Three-dimensional flow structures past a bio-prosthetic valve in an in-vitro model of the aortic root

The flow field past a prosthetic aortic valve comprises many details that indicate whether the prosthesis is functioning well or not. It is, however, not yet fully understood how an optimal flow scenario would look, i.e. which subtleties of the fluid dynamics in place are essential regarding the durability and compatibility of a prosthetic valve. In this study, we measured and analyzed the 3D flow field in the vicinity of a bio-prosthetic heart valve in function of the aortic root size. The measurements were conducted within aortic root phantoms of different size, mounted in a custom-built hydraulic setup, which mimicked physiological flow conditions in the aorta. Tomographic particle image velocimetry was used to measure the 3D instantaneous velocity field at various instances. Several 3D fields (e.g. instantaneous and mean velocity, 3D shear rate) were analyzed and compared focusing on the impact of the aortic root size, but also in order to gain general insight in the 3D flow structure past the bio-prosthetic valve. We found that the diameter of the aortic jet relative to the diameter of the ascending aorta is the most important parameter in determining the characteristics of the flow. A large aortic cross-section, relative to the cross-section of the aortic jet, was associated with higher levels of turbulence intensity and higher retrograde flow in the ascending aorta.

Time-Resolved PIV Technique for High Temporal Resolution Measurement of Mechanical Prosthetic Aortic Valve Fluid Dynamics

Prosthetic heart valves (PHVs) have been used to replace diseased native valves for more than five decades. Among these, mechanical PHVs are the most frequently implanted. Unfortunately, these devices still do not achieve ideal behavior and lead to many complications, many of which are related to fluid mechanics. The fluid dynamics of mechanical PHVs are particularly complex and the fine-scale characteristics of such flows call for very accurate experimental techniques. Adequate temporal resolution can be reached by applying time-resolved PIV, a high-resolution dynamic technique which is able to capture detailed chronological changes in the velocity field. The aim of this experimental study is to investigate the evolution of the flow field in a detailed time domain of a commercial bileaflet PHV in a mock-loop mimicking unsteady conditions, by means of time-resolved 2D Particle Image Velocimetry (PIV). The investigated flow field corresponded to the region immediately downstream of the valve plane. Spatial resolution as in “standard” PIV analysis of prosthetic valve fluid dynamics was used. The combination of a Nd:YLF high-repetition-rate double-cavity laser with a high frame rate CMOS camera allowed a detailed, highly temporally resolved acquisition (up to 10000 fps depending on the resolution) of the flow downstream of the PHV. Features that were observed include the non-homogeneity and unsteadiness of the phenomenon and the presence of large-scale vortices within the field, especially in the wake of the valve leaflets. Furthermore, we observed that highly temporally cycle-resolved analysis allowed the different behaviors exhibited by the bileaflet valve at closure to be captured in different acquired cardiac cycles.

By accurately capturing hemodynamically relevant time scales of motion, time-resolved PIV characterization can realistically be expected to help designers in improving PHV performance and in furnishing comprehensive validation with experimental data on fluid dynamics numeric modelling.

Integrated strategy for in vitro characterization of a bileaflet mechanical aortic valve

Background

Haemodynamic performance of heart valve prosthesis can be defined as its ability to fully open and completely close during the cardiac cycle, neither overloading heart work nor damaging blood particles when passing through the valve. In this perspective, global and local flow parameters, valve dynamics and blood damage safety of the prosthesis, as well as their mutual interactions, have all to be accounted for when assessing the device functionality. Even though all these issues have been and continue to be widely investigated, they are not usually studied through an integrated approach yet, i.e. by analyzing them simultaneously and highlighting their connections.

Results

An in vitro test campaign of flow through a bileaflet mechanical heart valve (Sorin Slimline 25 mm) was performed in a suitably arranged pulsatile mock loop able to reproduce human systemic pressure and flow curves. The valve was placed in an elastic, transparent, and anatomically accurate model of healthy aorta, and tested under several pulsatile flow conditions. Global and local hydrodynamics measurements and leaflet dynamics were analysed focusing on correlations between flow characteristics and valve motion. The haemolysis index due to the valve was estimated according to a literature power law model and related to hydrodynamic conditions, and a correlation between the spatial distribution of experimental shear stress and pannus/thrombotic deposits on mechanical valves was suggested. As main and general result, this study validates the potential of the integrated strategy for performance assessment of any prosthetic valve thanks to its capability of highlighting the complex interaction between the different physical mechanisms that govern transvalvular haemodynamics.

Conclusions

We have defined an in vitro procedure for a comprehensive analysis of aortic valve prosthesis performance; the rationale for this study was the belief that a proper and overall characterization of the device should be based on the simultaneous measurement of all different quantities of interest for haemodynamic performance and the analysis of their mutual interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-017-0314-2) contains supplementary material, which is available to authorized users.

Transcatheter aortic valves produce unphysiological flows which may contribute to thromboembolic events: An in-vitro study

PURPOSE

Transcatheter aortic valve implantation (TAVI) has been associated with large incidence of ischemic events, whose sources are still unclear. In fact, sub-acute complications cannot be directly related to the severity of the calcification in the host tissues, nor with catheter manipulation during the implant. A potential cause could be local flow perturbations introduced by the implantation approach, resulting in thrombo-embolic consequences. In particular, contrary to the surgical approach, TAVI preserves the presence of the native leaflets, which are expanded in the paravalvular space inside the Valsalva sinuses. The purpose of this study is to verify if this configuration can determine hemodynamic variations which may promote blood cell aggregation and thrombus formation.

METHODS

The study was performed in vitro, on idealized models of the patient anatomy before and after TAVI, reproducing a range of physiological operating conditions on a pulse duplicator. The fluid dynamics in the Valsalva sinuses was analyzed and characterized using phase resolved Particle Image Velocimetry.

RESULTS

Comparison of the flow downstream the valve clearly indicated major alterations in the fluid mechanics after TAVI, characterized by unphysiological conditions associated with extended stagnation zones at the base of the sinuses.

CONCLUSION

The prolonged stasis observed in the Valsalva sinuses for the configuration modelling the presence of transcatheter aortic valves provides a fluid dynamic environment favourable for red blood cell aggregation and thrombus formation, which may justify some of the recently reported thromboembolic and ischemic events. This suggests the adoption of anticoagulation therapies following TAVI, and some caution in the patients׳ selection.

Physiological vortices in the sinuses of Valsalva: An in vitro approach for bio-prosthetic valves

PURPOSE

The physiological flow dynamics within the Valsalva sinuses, in terms of global and local parameters, are still not fully understood. This study attempts to identify the physiological conditions as closely as possible, and to give an explanation of the different and sometime contradictory results in literature.

METHODS

An in vitro approach was implemented for testing porcine bio-prosthetic valves operating within different aortic root configurations. All tests were performed on a pulse duplicator, under physiological pressure and flow conditions. The fluid dynamics established in the various cases were analysed by means of 2D Particle Image Velocimetry, and related with the achieved hydrodynamic performance.

RESULTS

Each configuration is associated with substantially different flow dynamics, which significantly affects the valve performance. The configuration most closely replicating healthy native anatomy was characterised by the best hemodynamic performance, and any mismatch in size and position between the valve and the root produced substantial modification of the fluid dynamics downstream of the valve, hindering the hydrodynamic performance of the system. The worst conditions were observed for a configuration characterised by the total absence of the Valsalva sinuses.

CONCLUSION

This study provides an explanation for the different vortical structures described in the literature downstream of bioprosthetic valves, enlightening the experimental complications in valve testing. Most importantly, the results clearly identify the fluid mechanisms promoted by the Valsalva sinuses to enhance the ejection and closing phases, and this study exposes the importance of an optimal integration of the valve and root, to operate as a single system.

Experimental investigations on the fluid-mechanics of an electrospun heart valve by means of particle image velocimetry

End-stage failing heart valves are currently replaced by mechanical or biological prostheses. Both types positively contribute to restore the physiological function of native valves, but a number of drawbacks limits the expected performances. In order to improve the outcome, tissue engineering can offer an alternative approach to design and fabricate innovative heart valves capable to support the requested function and to promote the formation of a novel, viable and correctly operating physiological structure. This potential result is particularly critical if referred to the aortic valve, being the one mainly exposed to structural and functional degeneration. In this regard, the here proposed study presents the fabrication and in vitro characterization of a bioresorbable electrospun heart valve prosthesis using the particle image velocimetry technique either in physiological and pathological fluid dynamic conditions. The scaffold was designed to reproduce the aortic valve geometry, also mimicking the fibrous structure of the natural extracellular matrix. To evaluate its performances for possible implantation, the flow fields downstream the valve were accurately investigated and compared. The experimental results showed a correct functionality of the device, supported by the formation of vortex structures at the edge of the three cusps, with Reynolds stress values below the threshold for the risk of hemolysis (which can be comprised in the range 400-4000N/m(2) depending on the exposure period), and a good structural resistance to the mechanical loads generated by the driving pressure difference.

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.

Long-Term Durability of Carpentier-Edwards Magna Ease Valve: A One Billion Cycle In Vitro Study

BACKGROUND

Durability and hemodynamic performance are top considerations in selecting a valve for valve replacement surgery. This study was conducted in order to evaluate the long-term mechanical durability and hydrodynamic performance of the Carpentier-Edwards PERIMOUNT Magna Ease Bioprostheses, through 1 billion cycles (equivalent to 25 years).

METHODS

In vitro valve hydrodynamic performance, durability, and quantitative flow visualization were conducted in accordance with ISO 5840:2005 heart valve standard. The study valves were subjected to accelerated valve cycling to an equivalent of 25 years of wear. Hydrodynamic evaluations at intervals of 100 million cycles (2.5 years) were performed on the study valves. New uncycled Magna Ease valves were used as hydrodynamic controls in this study. A quantitative assessment of the fluid motion downstream of the control and study valves was performed using particle image velocimetry. The results between the test and control valves were compared to assess valve performance after an equivalent of 25 years of wear.

RESULTS

All study valves met the ISO 5840 requirements for effective orifice area, 1.81 ± 0.06 cm(2) and 2.06 ± 0.17 cm(2), and regurgitant fraction, 1.11% ± 0.87% and 2.5% ± 2.34%, for the 21 mm and 23 mm study valves, respectively. The flow characterization of the control valves and the billion-cycle valves demonstrated that the valves exhibited similar flow characteristics. The velocity and shear stress fields were similar between the control and study valves.

CONCLUSIONS

The Magna Ease valves demonstrated excellent durability and hydrodynamic performance after an equivalent of 25 years of simulated in vitro wear. All study valves successfully endured 1 billion cycles of simulated wear, 5 times longer than the standard requirement for a tissue valve as stipulated in ISO 5840.

Fluid dynamic characterization of a polymeric heart valve prototype (Poli-Valve) tested under continuous and pulsatile flow conditions

PURPOSE

Only mechanical and biological heart valve prostheses are currently commercially available. The former show longer durability but require anticoagulant therapy; the latter display better fluid dynamic behavior but do not have adequate durability. New Polymeric Heart Valves (PHVs) could potentially combine the hemodynamic properties of biological valves with the durability of mechanical valves. This work presents a hydrodynamic evaluation of 2 groups of newly developed supra-annular, trileaflet prosthetic heart valves made from styrenic block copolymers (SBC): Poli-Valves.

METHODS

2 types of Poli-Valves made of SBC and differing in polystyrene fraction content were tested under continuous and pulsatile flow conditions as prescribed by ISO 5840 Standard. A pulse duplicator designed ad hoc allowed the valve prototypes to be tested at different flow rates and frequencies. Pressure and flow were recorded; pressure drops, effective orifice area (EOA), and regurgitant volume were computed to assess the behavior of the valve.

RESULTS

Both types of Poli-Valves met the minimum requirements in terms of regurgitation and EOA as specified by the ISO 5840 Standard. Results were compared with 5 mechanical heart valves (MHVs) and 5 tissue heart valves (THVs), currently available on the market.

CONCLUSIONS

Based on these results, PHVs based on styrenic block copolymers, as are Poli-Valves, can be considered a promising alternative for heart valve replacement in the near future.

Emerging Trends in Heart Valve Engineering: Part IV. Computational Modeling and Experimental Studies

In this final portion of an extensive review of heart valve engineering, we focus on the computational methods and experimental studies related to heart valves. The discussion begins with a thorough review of computational modeling and the governing equations of fluid and structural interaction. We then move onto multiscale and disease specific modeling. Finally, advanced methods related to in vitro testing of the heart valves are reviewed. This section of the review series is intended to illustrate application of computational methods and experimental studies and their interrelation for studying heart valves.

Three-dimensional macro-scale assessment of regional and temporal wall shear stress characteristics on aortic valve leaflets

The aortic valve (AV) achieves unidirectional blood flow between the left ventricle and the aorta. Although hemodynamic stresses have been shown to regulate valvular biology, the native wall shear stress (WSS) experienced by AV leaflets remains largely unknown. The objective of this study was to quantify computationally the macro-scale leaflet WSS environment using fluid-structure interaction modeling. An arbitrary Lagrangian-Eulerian approach was implemented to predict valvular flow and leaflet dynamics in a three-dimensional AV geometry subjected to physiologic transvalvular pressure. Local WSS characteristics were quantified in terms of temporal shear magnitude (TSM), oscillatory shear index (OSI) and temporal shear gradient (TSG). The dominant radial WSS predicted on the leaflets exhibited high amplitude and unidirectionality on the ventricularis (TSM>7.50 dyn/cm(2), OSI < 0.17, TSG>325.54 dyn/cm(2) s) but low amplitude and bidirectionality on the fibrosa (TSM < 2.73 dyn/cm(2), OSI>0.38, TSG < 191.17 dyn/cm(2) s). The radial WSS component computed in the leaflet base, belly and tip demonstrated strong regional variability (ventricularis TSM: 7.50-22.32 dyn/cm(2), fibrosa TSM: 1.26-2.73 dyn/cm(2)). While the circumferential WSS exhibited similar spatially dependent magnitude (ventricularis TSM: 1.41-3.40 dyn/cm(2), fibrosa TSM: 0.42-0.76 dyn/cm(2)) and side-specific amplitude (ventricularis TSG: 101.73-184.43 dyn/cm(2) s, fibrosa TSG: 41.92-54.10 dyn/cm(2) s), its temporal variations were consistently bidirectional (OSI>0.25). This study provides new insights into the role played by leaflet-blood flow interactions in valvular function and critical hemodynamic stress data for the assessment of the hemodynamic theory of AV disease.

A Novel Design of a Polymeric Aortic Valve

Introduction

In this paper we propose a novel method for developing a polymeric heart valve that could potentially offer an optimum solution for a heart valve substitute. The valve design proposed will provide superior hydrodynamic performance and excellent structural integrity. A full description of the design process is given together with an analysis of the hemodynamic performance using a 2-way strongly coupled Fluid Structure Interaction (FSI).

Method

A polymeric tri-leaflet heart valve is designed based on a patient's sinus of Valsalva (SOV) geometry. The design strategy aims to improve valve hemodynamic performance as well as valve durability by avoiding stress concentrations in the leaflets and reducing the maximum stress level. The valve dynamics and stress levels are also validated by comparing the predicted data to existing experimental and numerical data.

Results

The stress distribution in the valve structure is fully characterized throughout the simulation and Von Mises stress is found to be up to 5.32 Mpa during diastole. The results show that an effective orifice area (EOA) and a pressure drop of 3.22 cm^2, and 3.52 mmHg, respectively, can be achieved using the proposed design.

Conclusions

The optimized valve demonstrates high hemodynamic performance with no sign of damaging stress concentration in the entire cardiac cycle.

The Influence of the Aortic Root Geometry on Flow Characteristics of a Prosthetic Heart Valve

In this paper, performance of aortic heart valve prosthesis in different geometries of the aortic root is investigated experimentally. The objective of this investigation is to establish a set of parameters, which are associated with abnormal flow patterns due to the flow through a prosthetic heart valve implanted in the patients that had certain types of valve diseases prior to the valve replacement. Specific valve diseases were classified into two clinical categories and were correlated with the corresponding changes in aortic root geometry while keeping the aortic base diameter fixed. These categories correspond to aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. Experiments were performed at test conditions corresponding to 70 beats/min, 5.5 L/min target cardiac output, and a mean aortic pressure of 100 mmHg. By varying the aortic root geometry, while keeping the diameter of the orifice constant, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots.

An immersogeometric variational framework for fluid-structure interaction: application to bioprosthetic heart valves

In this paper, we develop a geometrically flexible technique for computational fluid-structure interaction (FSI). The motivating application is the simulation of tri-leaflet bioprosthetic heart valve function over the complete cardiac cycle. Due to the complex motion of the heart valve leaflets, the fluid domain undergoes large deformations, including changes of topology. The proposed method directly analyzes a spline-based surface representation of the structure by immersing it into a non-boundary-fitted discretization of the surrounding fluid domain. This places our method within an emerging class of computational techniques that aim to capture geometry on non-boundary-fitted analysis meshes. We introduce the term "immersogeometric analysis" to identify this paradigm. The framework starts with an augmented Lagrangian formulation for FSI that enforces kinematic constraints with a combination of Lagrange multipliers and penalty forces. For immersed volumetric objects, we formally eliminate the multiplier field by substituting a fluid-structure interface traction, arriving at Nitsche's method for enforcing Dirichlet boundary conditions on object surfaces. For immersed thin shell structures modeled geometrically as surfaces, the tractions from opposite sides cancel due to the continuity of the background fluid solution space, leaving a penalty method. Application to a bioprosthetic heart valve, where there is a large pressure jump across the leaflets, reveals shortcomings of the penalty approach. To counteract steep pressure gradients through the structure without the conditioning problems that accompany strong penalty forces, we resurrect the Lagrange multiplier field. Further, since the fluid discretization is not tailored to the structure geometry, there is a significant error in the approximation of pressure discontinuities across the shell. This error becomes especially troublesome in residual-based stabilized methods for incompressible flow, leading to problematic compressibility at practical levels of refinement. We modify existing stabilized methods to improve performance. To evaluate the accuracy of the proposed methods, we test them on benchmark problems and compare the results with those of established boundary-fitted techniques. Finally, we simulate the coupling of the bioprosthetic heart valve and the surrounding blood flow under physiological conditions, demonstrating the effectiveness of the proposed techniques in practical computations.

Steady flow hemodynamic and energy loss measurements in normal and simulated calcified tricuspid and bicuspid aortic valves

The bicuspid aortic valve (BAV), which forms with two leaflets instead of three as in the normal tricuspid aortic valve (TAV), is associated with a spectrum of secondary valvulopathies and aortopathies potentially triggered by hemodynamic abnormalities. While studies have demonstrated an intrinsic degree of stenosis and the existence of a skewed orifice jet in the BAV, the impact of those abnormalities on BAV hemodynamic performance and energy loss has not been examined. This steady-flow study presents the comparative in vitro assessment of the flow field and energy loss in a TAV and type-I BAV under normal and simulated calcified states. Particle-image velocimetry (PIV) measurements were performed to quantify velocity, vorticity, viscous, and Reynolds shear stress fields in normal and simulated calcified porcine TAV and BAV models at six flow rates spanning the systolic phase. The BAV model was created by suturing the two coronary leaflets of a porcine TAV. Calcification was simulated via deposition of glue beads in the base of the leaflets. Valvular performance was characterized in terms of geometric orifice area (GOA), pressure drop, effective orifice area (EOA), energy loss (EL), and energy loss index (ELI). The BAV generated an elliptical orifice and a jet skewed toward the noncoronary leaflet. In contrast, the TAV featured a circular orifice and a jet aligned along the valve long axis. While the BAV exhibited an intrinsic degree of stenosis (18% increase in maximum jet velocity and 7% decrease in EOA relative to the TAV at the maximum flow rate), it generated only a 3% increase in EL and its average ELI (2.10 cm2/m2) remained above the clinical threshold characterizing severe aortic stenosis. The presence of simulated calcific lesions normalized the alignment of the BAV jet and resulted in the loss of jet axisymmetry in the TAV. It also amplified the degree of stenosis in the TAV and BAV, as indicated by the 342% and 404% increase in EL, 70% and 51% reduction in ELI and 48% and 51% decrease in EOA, respectively, relative to the nontreated valve models at the maximum flow rate. This study indicates the ability of the BAV to function as a TAV despite its intrinsic degree of stenosis and suggests the weak dependence of pressure drop on orifice area in calcified valves.

SPATIO-TEMPORAL COMPLEXITY OF THE AORTIC SINUS VORTEX

The aortic sinus vortex is a classical flow structure of significant importance to aortic valve dynamics and the initiation and progression of calific aortic valve disease. We characterize the spatio-temporal characteristics of aortic sinus voxtex dynamics in relation to the viscosity of blood analog solution as well as heart rate. High resolution time-resolved (2KHz) particle image velocimetry was conducted to capture 2D particle streak videos and 2D instantaneous velocity and streamlines along the sinus midplane using a physiological but rigid aorta model fitted with a porcine bioprosthetic heart valve. Blood analog fluids used include a water-glycerin mixture and saline to elucidate the sensitivity of vortex dynamics to viscosity. Experiments were conducted to record 10 heart beats for each combination of blood analog and heart rate condition. Results show that the topological characteristics of the velocity field vary in time-scales as revealed using time bin averaged vectors and corresponding instantaneous streamlines. There exist small time-scale vortices and a large time-scale main vortex. A key flow structure observed is the counter vortex at the upstream end of the sinus adjacent to the base (lower half) of the leaflet. The spatio-temporal complexity of vortex dynamics is shown to be profoundly influenced by strong leaflet flutter during systole with a peak frequency of 200Hz and peak amplitude of 4 mm observed in the saline case. While fluid viscosity influences the length and time-scales as well as the introduction of leaflet flutter, heart rate influences the formation of counter vortex at the upstream end of the sinus. Higher heart rates are shown to reduce the strength of the counter vortex that can greatly influence the directionality and strength of shear stresses along the base of the leaflet. This study demonstrates the impact of heart rate and blood analog viscosity on aortic sinus hemodynamics.

An in vitro evaluation of the impact of eccentric deployment on transcatheter aortic valve hemodynamics

Patients with aortic stenosis present with calcium deposits on the native aortic valve, which can result in non-concentric expansion of Transcatheter Aortic Valve Replacement (TAVR) stents. The objective of this study is to evaluate whether eccentric deployment of TAVRs lead to turbulent blood flow and blood cell damage. Particle Image Velocimetry was used to quantitatively characterize fluid velocity fields, shear stress and turbulent kinetic energy downstream of TAVRs deployed in circular and eccentric orifices representative of deployed TAVRs in vivo. Effective orifice area (EOA) and mean transvalvular pressure gradient (TVG) values did not differ substantially in circular and eccentric deployed valves, with only a minor decrease in EOA observed in the eccentric valve (2.0 cm(2) for circular, 1.9 cm(2) for eccentric). Eccentric deployed TAVR lead to asymmetric systolic jet formation, with increased shear stresses (circular = 97 N/m(2) vs. eccentric = 119 N/m(2)) and regions of turbulence intensity (circular = 180 N/m(2) vs. eccentric = 230 N/m(2)) downstream that was not present in the circular deployed TAVR. The results of this study indicate that eccentric deployment of TAVRs can lead to altered flow characteristics and may potentially increase the hemolytic potential of the valve, which were not captured through hemodynamic evaluation alone.

Hemocompatibility and Hemodynamics of Novel Hyaluronan-Polyethylene Materials for Flexible Heart Valve Leaflets

Polymeric heart valves (PHVs) hold the promise to be more durable than bioprosthetic heart valves and less thrombogenic than mechanical heart valves. We introduce a new framework to manufacture hemocompatible polymeric leaflets for HV (PHV) applications using a novel material comprised of interpenetrating networks (IPNs) of hyaluronan (HA) and linear low density polyethylene (LLDPE). We establish and characterize the feasibility of the material as a substitute leaflet material through basic hemodynamic measurements in a trileaflet configuration, in addition to demonstrating superior platelet response and clotting characteristics. Plain LLDPE sheets were swollen in a solution of silylated-HA, the silylated-HA was then crosslinked to itself before it was reverted back to native HA via hydrolysis. Leaflets were characterized with respect to (1) bending stiffness, (2) hydrophilicity, (3) whole blood clotting, and (4) cell (platelet and leukocyte) adhesion under static conditions using fresh human blood. In vitro hemodynamic testing of prototype HA/LLDPE IPN PHVs was used to assess feasibility as functional HVs. Bending stiffness was not significantly different from natural fresh leaflets. HA/LLDPE IPNs were more hydrophilic than LLDPE controls. HA/LLDPE IPNs caused less whole blood clotting and reduced cell adhesion compared to the plain LLDPE control. Prototype PHVs made with HA/LLDPE IPNs demonstrated an acceptable regurgitation fraction of 4.77 ± 0.42%, and effective orifice area in the range 2.34 ± 0.5 cm2. These results demonstrate strong potential for IPNs between HA and polymers as future hemocompatible HV leaflets. Further studies are necessary to assess durability and calcification resistance.

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.

Effect of hypertension on the closing dynamics and Lagrangian blood damage index measure of the b-datum regurgitant jet in a bileaflet mechanical heart valve

We hypothesize that the formation of the closing vortex and subsequent b-datum regurgitation jet in bileaflet mechanical heart valves is governed by the magnitude of the driving mean aortic pressure (MAP), and that this sensitivity does impact the blood damage index (BDI) corresponding to platelet activation and lysis. High spatial resolution time resolved (1 kHz) as well as phase locked particle image velocimetry techniques captured the dynamic leaflet closure and regurgitation jet of a model 25 mm St. Jude Medical BMHV. Cell trajectories were estimated using Lagrangian particle tracking analysis while the leaflet kinematics was quantified by tracking the leaflet tip-position throughout closure. The non-principal as well as principal shear stress loading histories along each cell trajectory revealed BDI for platelet activation and lysis as a function of cell initial position, release time-point, and blood pressure. Results show that the leaflet closing time reduces by roughly 10 ms, in response to an increase in MAP by 40 mmHg. We report that higher MAP leads to a stronger b-datum vortex and jet formation. Platelet activation BDI lowers with a higher MAP due to reduction in exposure times despite an increase in principal shear stress experienced. Platelet lysis BDI however increases with higher MAP. Maximum BDI may occur for cells initially in the b-datum zone during the onset of leaflet closure. Our results provide a better understanding of BDI of the regurgitant b-datum jet and sheds light on the potential importance of blood damage testing under hypertensive conditions.

Fluid Dynamic Investigation of the ATS 3F Enable Sutureless Heart Valve

OBJECTIVE

: Currently, sutureless heart valves (SHV) reveal good clinical results during aortic valve replacement. The aim of this study was to evaluate the fluid dynamics of the ATS 3F Enable SHV in the ascending aorta and their influence on the aortic wall in an in vitro setup.

METHODS

: A two-dimensional particle image velocimetry study with an image rate of 15 Hz was conducted to evaluate the fluid dynamics of the SHV in the aortic flow field. The prosthesis (diameter, 23 mm) was placed inside a silicone mock aorta under pulsatile flow conditions. Velocities, vorticity, and strain rate were obtained and calculated with a fixed frequency (70 Hz) at constant stroke volume (70 mL).

RESULTS

: 3F Enable showed a jet flow type profile with a maximum velocity of 1.01 ± 0.13 m/s during peak flow phase (PFP). The jet flow was surrounded by ambilateral vortices with a slightly higher percentage of clockwise than counterclockwise vorticity (377 ± 57/s vs 389 ± 76/s), strain rate (370 ± 79/s for elongation vs -370 ± 102/s for contraction) was nearly similar. The point-of-interest analysis revealed a higher velocity for bottom compared with upper aortic wall (0.28 ± 0.07 m/s vs 0.31 ± 0.06 m/s, P = 0.024). All values were lower during acceleration and deceleration phase compared with PFP.

CONCLUSIONS

: The peak flow of the 3F Enable SHV seems to be little higher compared with native aortic valves, thus simulating nearly physiologic conditions. Vorticity and strain rate are high during PFP and low during other phases and might have an influence on the aortic wall as well.

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.