Design, Analysis and Testing of a Novel Mitral Valve for Transcatheter Implantation

Analysis of the haemodynamic changes caused by surgical and transcatheter aortic valve replacements by means fluid-structure interaction simulations

Aortic valve replacements, both surgical and transcatheter, are nowadays widely employed treatments. Although clinically effective, these procedures are correlated with potentially severe clinical complications which can be associated with the non-physiological haemodynamics that they establish. In this work, the fluid dynamics changes produced by surgical and transcatheter aortic valve replacements are analysed and compared with an ideal healthy native valve configuration, employing advanced fluid-structure interaction (FSI) simulations. The aim of the study is to investigate how existing treatments may affect the aortic valve function, and giving indications about how to improve current therapies. Simulations were performed using the commercial software LS-DYNA, where the FSI strategy is based on the coupling of a Lagrangian approach for the structures and a Eulerian approach for the fluid, whilst the coupling between the two domains is reached through a hybrid arbitrary-Lagrangian-Eulerian algorithm. Idealised geometries are used for the aortic root and leaflets. The aortic wall was modelled as linear elastic material, whilst leaflets were modelled as hyperelastic incompressible, using an Ogden's constitutive model. A combination of physiological flow velocity and pressure differences are applied as boundary conditions to model realistically the whole cardiac cycle. Results are analysed throughout the cardiac cycle in terms of leaflets kinematics, flow dynamics, pressure and valve performance parameters. Globally, surgical valves presented worse performance than transcatheter counterparts (reduced effective orifice area, increased transvalvular pressure drop and increased opening and closing times). The clinical parameters of transcatheter devices were improved and closer to those of the healthy native valve, although the vortical activity within the Valsalva's sinuses was substantially altered. Here, the presence of the partition obstructed the washing out, resulting in higher degree of blood stasis and potential blood damage. The implantation of prosthetic devices produces major haemodynamic changes which alters the valve dynamics and leads to diminished performance. Currently, the design of these substitutes is not optimised to mimic realistic native conditions, particularly in terms of valve opening behaviour. Although transcatheter devices provide systolic performance similar to that estimated for the healthy native aortic model, none of the prosthetic solutions appeared to be able to fully restore healthy physiological conditions.

Design and Analysis of a Polymeric Left Ventricular Simulator via Computational Modelling

Preclinical testing of medical devices is an essential step in the product life cycle, whereas testing of cardiovascular implants requires specialised testbeds or numerical simulations using computer software Ansys 2016. Existing test setups used to evaluate physiological scenarios and test cardiac implants such as mock circulatory systems or isolated beating heart platforms are driven by sophisticated hardware which comes at a high cost or raises ethical concerns. On the other hand, computational methods used to simulate blood flow in the cardiovascular system may be simplified or computationally expensive. Therefore, there is a need for low-cost, relatively simple and efficient test beds that can provide realistic conditions to simulate physiological scenarios and evaluate cardiovascular devices. In this study, the concept design of a novel left ventricular simulator made of latex rubber and actuated by pneumatic artificial muscles is presented. The designed left ventricular simulator is geometrically similar to a native left ventricle, whereas the basal diameter and long axis length are within an anatomical range. Finite element simulations evaluating left ventricular twisting and shortening predicted that the designed left ventricular simulator rotates approximately 17 degrees at the apex and the long axis shortens around 11 mm. Experimental results showed that the twist angle is 18 degrees and the left ventricular simulator shortens 5 mm. Twist angles and long axis shortening as in a native left ventricle show it is capable of functioning like a native left ventricle and simulating a variety of scenarios, and therefore has the potential to be used as a test platform.

A novel mono-physics particle-based approach for the simulation of cardiovascular fluid-structure interaction problems

BACKGROUND AND OBJECTIVE

Fluid-structure interaction (FSI) is required in the study of several cardiovascular engineering problems were the mutual interaction between the pulsatile blood flow and the tissue structures is essential to establish the biomechanics of the system. Traditional FSI methods are partitioned approaches where two independent solvers, one for the fluid and one for the structure, are asynchronously coupled. This process results into high computational costs. In this work, a new FSI scheme which avoids the coupling of different solvers is presented in the framework of the truly incompressible smoothed particle hydrodynamics (ISPH) method.

METHODS

In the proposed FSI method, ISPH particles contribute to define both the fluid and structural domains and are solved together in a unified system. Solid particles, geometrically defined at the beginning of the simulation, are linked through spring bounds with elastic constant providing the material Young's modulus. At each iteration, internal elastic forces are calculated to restore the springs resting length. These forces are added in the predictor step of the fractional-step procedure used to solve the momentum and continuity equations for incompressible flows of all particles.

RESULTS

The method was validated with a benchmark test case consisting of a flexible beam immersed in a channel. Results showed good agreement with the system coupling approach of a well-established commercial software, ANSYS®, both in terms of fluid-dynamics and beam deformation. The approach was then applied to model a complex cardiovascular problem, consisting in the aortic valve operating function. The valve dynamics during opening and closing phases were compared qualitatively with literature results, demonstrating good consistency.

CONCLUSIONS

The method is computationally more efficient than traditional FSI strategies, and overcomes some of their main drawbacks, such as the impossibility of simulating the correct valve coaptation during the closing phase. Thanks to the incompressibility scheme, the proposed FSI method is appropriate to model biological soft tissues. The simplicity and flexibility of the approach also makes it suitable to be expanded for the modelling of thromboembolic phenomena.

Utilization of Engineering Advances for Detailed Biomechanical Characterization of the Mitral-Ventricular Relationship to Optimize Repair Strategies: A Comprehensive Review

The geometrical details and biomechanical relationships of the mitral valve-left ventricular apparatus are very complex and have posed as an area of research interest for decades. These characteristics play a major role in identifying and perfecting the optimal approaches to treat diseases of this system when the restoration of biomechanical and mechano-biological conditions becomes the main target. Over the years, engineering approaches have helped to revolutionize the field in this regard. Furthermore, advanced modelling modalities have contributed greatly to the development of novel devices and less invasive strategies. This article provides an overview and narrative of the evolution of mitral valve therapy with special focus on two diseases frequently encountered by cardiac surgeons and interventional cardiologists: ischemic and degenerative mitral regurgitation.

Use of 3D anatomical models in mock circulatory loops for cardiac medical device testing

INTRODUCTION

Mock circulatory loops (MCLs) are mechanical representations of the cardiovascular system largely used to test the hemodynamic performance of cardiovascular medical devices (MD). Thanks to 3 dimensional (3D) printing technologies, MCLs can nowadays also incorporate anatomical models so to offer enhanced testing capabilities. The aim of this review is to provide an overview on MCLs and to discuss the recent developments of 3D anatomical models for cardiovascular MD testing.

METHODS

The review first analyses the different techniques to develop 3D anatomical models, in both rigid and compliant materials. In the second section, the state of the art of MCLs with 3D models is discussed, along with the testing of different MDs: implantable blood pumps, heart valves, and imaging techniques. For each class of MD, the MCL is analyzed in terms of: the cardiovascular model embedded, the 3D model implemented (the anatomy represented, the material used, and the activation method), and the testing applications.

DISCUSSIONS AND CONCLUSIONS

MCLs serve the purpose of testing cardiovascular MDs in different (patho-)physiological scenarios. The addition of 3D anatomical models enables more realistic connections of the MD with the implantation site and enhances the testing capabilities of the MCL. Current attempts focus on the development of personalized MCLs to test MDs in patient-specific hemodynamic and anatomical scenarios. The main limitation of MCLs is the impossibility to assess the impact of a MD in the long-term and at a biological level, for which animal experiments are still needed.

In Vitro Mitral Valve Model with Unrestricted Ventricular Access: Using Vacuum to Close the Valve and Enable Static Trans-Mitral Pressure

Current in vitro models of the left heart establish the pressure difference required to close the mitral valve by sealing and pressurizing the ventricular side of the valve, limiting important access to the subvalvular apparatus. This paper describes and evaluates a system that establishes physiological pressure differences across the valve using vacuum on the atrial side. The subvalvular apparatus is open to atmospheric pressure and accessible by tools and sensors, establishing a novel technique for experimentation on atrioventricular valves. Porcine mitral valves were excised and closed by vacuum within the atrial chamber. Images were used to document and analyze closure of the leaflets. Papillary muscle force and regurgitant flow rate were measured to be 4.07 N at 120 mmHg and approximately 12.1 ml/s respectively, both of which are within clinically relevant ranges. The relative ease of these measurements demonstrates the usefulness of improved ventricular access at peak pressure/force closure.

Transcatheter Heart Valves: A Biomaterials Perspective

Heart valve disease is prevalent throughout the world, and the number of heart valve replacements is expected to increase rapidly in the coming years. Transcatheter heart valve replacement (THVR) provides a safe and minimally invasive means for heart valve replacement in high-risk patients. The latest clinical data demonstrates that THVR is a practical solution for low-risk patients. Despite these promising results, there is no long-term (>20 years) durability data on transcatheter heart valves (THVs), raising concerns about material degeneration and long-term performance. This review presents a detailed account of the materials development for THVRs. It provides a brief overview of THVR, the native valve properties, the criteria for an ideal THV, and how these devices are tested. A comprehensive review of materials and their applications in THVR, including how these materials are fabricated, prepared, and assembled into THVs is presented, followed by a discussion of current and future THVR biomaterial trends. The field of THVR is proliferating, and this review serves as a guide for understanding the development of THVs from a materials science and engineering perspective.

Comparison of different transcatheter interventions for treatment of mitral regurgitation: A protocol for a network meta-analysis

BACKGROUND

The arrival of transcatheter mitral valve therapies has provided feasible and safe alternatives to medical and surgical treatments for mitral regurgitation. The aim of this study is to estimate the relative efficacy and safety of different transcatheter mitral valve therapies for mitral regurgitation patients through network meta-analysis.

METHODS

A systematic search will be performed using PubMed, EMBASE, the Cochrane Library, Web of Science, Chinese Biomedical Literature Database, and China National Knowledge Infrastructure to include random controlled trials and nonrandom controlled trials comparing the efficacy and safety of different transcatheter mitral valve techniques. The risk of bias for the included nonrandom controlled studies will be evaluated according to Risk of Bias in Non-randomized Studies - of Interventions. For random controlled trials, we will use Cochrane Handbook version 5.1.0 as the risk of bias tool. A Bayesian network meta-analysis will be conducted using R-4.0.3 software. Grading of recommendations assessment, development, and evaluation will be used to assess the quality of evidence.

RESULTS

The results of this network meta-analysis will be submitted to a peer-reviewed journal for publication.

CONCLUSION

This study will provide broad evidence of efficacy and safety of different transcatheter mitral valve therapies for treatment of mitral regurgitation and provide suggestions for clinical practice and future research.

PROTOCOL REGISTRATION NUMBER

INPLASY2020110034.

Thinner biological tissues induce leaflet flutter in aortic heart valve replacements

Valvular heart disease has recently become an increasing public health concern due to the high prevalence of valve degeneration in aging populations. For patients with severely impacted aortic valves that require replacement, catheter-based bioprosthetic valve deployment offers a minimally invasive treatment option that eliminates many of the risks associated with surgical valve replacement. Although recent percutaneous device advancements have incorporated thinner, more flexible biological tissues to streamline safer deployment through catheters, the impact of such tissues in the complex, mechanically demanding, and highly dynamic valvular system remains poorly understood. The present work utilized a validated computational fluid-structure interaction approach to isolate the behavior of thinner, more compliant aortic valve tissues in a physiologically realistic system. This computational study identified and quantified significant leaflet flutter induced by the use of thinner tissues that initiated blood flow disturbances and oscillatory leaflet strains. The aortic flow and valvular dynamics associated with these thinner valvular tissues have not been previously identified and provide essential information that can significantly advance fundamental knowledge about the cardiac system and support future medical device innovation. Considering the risks associated with such observed flutter phenomena, including blood damage and accelerated leaflet deterioration, this study demonstrates the potentially serious impact of introducing thinner, more flexible tissues into the cardiac system.

Hydrodynamic Noise of Pulsating Jets through Bileaflet Mechanical Mitral Valve

Experimental research results of hydrodynamic noise of pulsating flow through a bileaflet mechanical mitral valve are presented. The pulsating flow of pure water corresponds to the diastolic mode of the cardiac rhythm heart. The valve was located between the model of the left atrium and the model of the left ventricle of the heart. A coordinate device, on which a block of miniature sensors of absolute pressure and pressure fluctuations was installed, was located inside the model of the left ventricle. It is found that the hydrodynamic noise of the pulsating side jet of the semiclosed valve is higher than for the open valve. The pressure fluctuation levels gradually decrease with the removal from the mitral valve. It is established that at the second harmonic of the pulsating flow frequency, the spectral levels of the hydrodynamic noise of the semiclosed bileaflet mechanical mitral valve are almost 5 times higher than the open valve. With the removal from the mitral valve, spectral levels of hydrodynamic noise are decreased, especially strongly at the frequency of the pulsating water flow and its higher harmonics.

Patient-specific computer simulation for transcatheter cardiac interventions: what a clinician needs to know

Patient-specific computer simulation consists of the assessment of the interaction of the device with the host based on the integration of the detailed geometric and biomechanical properties of the device and host. Hence, it allows the prediction of valve performance (efficacy) and complications (safety) and may consequently help the physician to select the valve/device that best fits the individual patient, thereby improving outcome. There is currently little awareness and information in clinical medicine on patient-specific computer simulation. In this paper, we describe the technical background and a number of illustrations to illustrate how patient-specific computer simulation may be used for catheter-based treatment planning of acquired heart disease.

Validation and Extension of a Fluid-Structure Interaction Model of the Healthy Aortic Valve

Purpose

The understanding of the optimum function of the healthy aortic valve is essential in interpreting the effect of pathologies in the region, and in devising effective treatments to restore the physiological functions. Still, there is no consensus on the operating mechanism that regulates the valve opening and closing dynamics. The aim of this study is to develop a numerical model that can support a better comprehension of the valve function and serve as a reference to identify the changes produced by specific pathologies and treatments.

Methods

A numerical model was developed and adapted to accurately replicate the conditions of a previous in vitro investigation into aortic valve dynamics, performed by means of particle image velocimetry (PIV). The resulting velocity fields of the two analyses were qualitatively and quantitatively compared to validate the numerical model. In order to simulate more physiological operating conditions, this was then modified to overcome the main limitations of the experimental setup, such as the presence of a supporting stent and the non-physiological properties of the fluid and vessels.

Results

The velocity fields of the initial model resulted in good agreement with those obtained from the PIV, with similar flow structures and about 90% of the computed velocities after valve opening within the standard deviation of the equivalent velocity measurements of the in vitro model. Once the experimental limitations were removed from the model, the valve opening dynamics changed substantially, with the leaflets opening into the sinuses to a much greater extent, enlarging the effective orifice area by 11%, and reducing greatly the vortical structures previously observed in proximity of the Valsalva sinuses wall.

Conclusions

The study suggests a new operating mechanism for the healthy aortic valve leaflets considerably different from what reported in the literature to date and largely more efficient in terms of hydrodynamic performance. This work also confirms the crucial role that numerical approaches, complemented with experimental findings, can play in overcoming some of the limitations inherent in experimental techniques, supporting the full understanding of complex physiological phenomena.

Electronic supplementary material

The online version of this article (doi:10.1007/s13239-018-00391-1) contains supplementary material, which is available to authorized users.