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Modelling and application of mitral valve dynamics for reproducing the flow in the left ventricle of the human heart
Full text linkThe fluid dynamics in the left ventricle of the human heart is considered an important player for the prediction of long term cardiovascular outcome. To this end, numerical simulations represent an important tool for integrating the existing medical imaging technology and uncover physical flow phenomena.
This study presents a computational method for the fluid dynamics inside the left ventricle designed to be efficiently integrated in clinical scenarios. It includes an original model of the mitral valve dynamics, which describes an asymptotic behavior for tissues with no elastic stiffness other than the constrain of the geometry obtained from medical imaging; in particular, the model provides an asymptotic description without requiring details of tissue properties that may not be measurable in vivo. The advantages of this model with respect to a valveless orifice and its limitations with respect to a complete tissue modeling are verified. Its performances are then analyzed in details to ensure a correct interpretation of results. It represents a potential option when information about tissue mechanical properties is insufficient for the implementations of a full fluid-structure interaction approach.
Geometries of left ventricle (LV) and mitral valve (MV) are extracted from 4D-transesophageal echocardiography. MV geometries are extracted in open and closed configurations and the intraventricular fluid dynamics is reproduced by a dedicated approach to direct numerical simulation (DNS) that includes flow-tissue interaction for the MV leaflet (Collia et al. 2019).
This approach is applied to normal and pathological ventricles to investigate the dynamics of the MV during the cardiac cycle: how it interacts with the ventricular flow and how it affects clinical measurements.
The dynamics of mitral opening at the onset of diastole, as well as the closure at the transition between diastole and systole, is governed by the high pressure gradients associated with the bulk cardiac flow. On the opposite, during the flow diastasis in the middle of the diastolic filling, valvular motion is primarily influenced by the intraventricular circulation that gives an increased tendency to close in enlarged ventricles. This observation provides a physical interpretation to echocardiographic measurements commonly employed in the clinical diagnostic process.
Results demonstrated the properties of false regurgitation, blood that did not cross the open MV orifice and returns into the atrium during the backward motion of the MV leaflets, whose entity should be accounted when evaluating small regurgitation (Collia et al. 2019). The regurgitating volume is found to be proportional to the effective orifice area, with the limited dependence of the LV geometry and type of prolapse. These affect the percentage of old blood returning to the atrium which may be associated with thrombogenic risk.
This non-invasive method is useful for the assessment of blood flow, to improve early detection of cardiac dysfunctions and for provide a concrete helpful in clinical routines
Mitral valve asymmetry in healthy, pathological, and repaired cases
Full text linkThe entry of blood into the left ventricle is regulated by the two valve leaflets. Mitral valve prolapse is the primary cause of mitral regurgitation. Mitral valve repair is the gold standard therapeutic procedure for patients with degenerative mitral valve regurgitation and follows two fundamental principles: restoring a good coaptation surface of the flap and correcting annular dilation. This study presents a first step in the direction of addressing the influence of valve geometry on valve fluid dynamics and mitral regurgitation. To this end, it develops a systematic analysis to identify how the level of regurgitation and the efficiency of flow transit in the left ventricle depend on the degree of asymmetry of the leaflets. The analysis is performed starting from a mathematically designed mitral valve and then extended to the actual valves extracted from medical imaging. The specific objective is to evaluate the changes in mitral regurgitation associated with the symmetrical properties of the mitral valve. The broader aim is to begin building physics-based means for evaluating repair options and prosthetic design. Results showed that valve shape does not affect flow; sub-volumes are similar to inflow and vary to outflow due to the presence of false regurgitation under healthy/repaired conditions and regurgitation under pathological conditions affecting the amount of direct flow, delayed and finally the Stroke volume. The best valve asymmetry point was found to be 0.25, while the optimal range was between 0.4 and 0.2, giving an important suggestion to valve surgery
The Influence of Mitral Valve Asymmetry for an Improved Choice of Valve Repair or Replacement
Full text linkThe study of valve asymmetry represents an important avenue for modern cardiac surgery. The correct choice of leaflet reconstruction may indicate a new path in the quality and long-term survival of patients. A systematic investigation was performed with a total of 25 numerical simulations using a healthy ventricle and an ideal valve with varying degrees of valve asymmetry. An overall assessment is made in terms of vorticity, kinetic energy, dissipated energy, and hemodynamic forces. The results indicate that the optimal asymmetry to consider for a valve repair or prosthetic design is between 0.2 and 0.4 with an optimal point of about 0.3. Out of this range, the heart is subjected to an excessive workload, which can only worsen the patient’s state of health
Simplified mitral valve modeling for prospective clinical application of left ventricular fluid dynamics
Full text linkThe fluid dynamics inside the left ventricle of the human heart is considered a potential indicator of long term cardiovascular outcome. In this respect, numerical simulations can play an important role for integrating existing technology to reproduce flow details and even conditions associated to virtual therapeutic solutions. Nevertheless, numerical models encounter serious practical difficulties in describing the interaction between flow and surrounding tissues due to the limited information inherently available in real clinical applications.
This study presents a computational method for the fluid dynamics inside the left ventricle designed to be efficiently integrated in clinical scenarios. It includes an original model of the mitral valve dynamics, which describes an asymptotic behavior for tissues with no elastic stiffness other than the constrain of the geometry obtained from medical imaging; in particular, the model provides an asymptotic description without requiring details of tissue properties that may not be measurable in vivo.
The advantages of this model with respect to a valveless orifice and its limitations with respect to a complete tissue modeling are verified. Its performances are then analyzed in details to ensure a correct interpretation of results. It represents a potential option when information about tissue mechanical properties is insufficient for the implementations of a full fluid-structure interaction approach
Influence of mitral valve elasticity on flow development in the left ventricle
Full text linkThe Mitral valve of the human heart has a great relevance for numerous cardiac pathologies; however, the knowledge of relationships between valvular properties and cardiac function is still limited. On one side, this is partly due to the limited resolution of clinical imaging technologies that do not allow routinely visualization of the valve during its motion. On the other, its modeling presents serious challenges either due to the strong flow–tissueinteraction or because the mechanical properties of its constitutive elements are complex and not measurable in vivo. This work introduces a parametric model of the Mitral valve where the interaction with the blood flow obeys global balances and the overall elastic properties are summarized into a single functional parameter. This is integrated into a numerical model of left ventricular fluid dynamics with the aim to study the effect of varying the valvular stiffness. Results show that the elasticity of the valve influences the amplitude of the mitral opening, while the timings of opening/closure are driven by the transmitral blood flow due to the ventricular dynamics. In addition, the increase of stiffness increases the transvalvular pressure gradients required to ensure the same flow. These results are discussed in relation to parameters for monitoring valvular stiffness that are accessible through clinical imaging
Surrogate models provide new insights on metrics based on blood flow for the assessment of left ventricular function
Full text linkRecent developments on the grading of cardiac pathologies suggest flow-related metrics for a deeper evaluation of cardiac function. Blood flow evaluation employs space-time resolved cardiovascular imaging tools, possibly integrated with direct numerical simulation (DNS) of intraventricular fluid dynamics in individual patients. If a patient-specific analysis is a promising method to reproduce flow details or to assist virtual therapeutic solutions, it becomes impracticable in nearly-real-time during a routine clinical activity. At the same time, the need to determine the existence of relationships between advanced flow-related quantities of interest (QoIs) and the diagnostic metrics used in the standard clinical practice requires the adoption of techniques able to generalize evidences emerging from a finite number of single cases. In this study, we focus on the left ventricular function and use a class of reduced-order models, relying on the Polynomial Chaos Expansion (PCE) technique to learn the dynamics of selected QoIs based on a set of synthetic cases analyzed with a high-fidelity model (DNS). The selected QoIs describe the left ventricle blood transit and the kinetic energy and vorticity at the peak of diastolic filling. The PCE-based surrogate models provide straightforward approximations of these QoIs in the space of widely used diagnostic metrics embedding relevant information on left ventricle geometry and function. These surrogates are directly employable in the clinical analysis as we demonstrate by assessing their robustness against independent patient-specific cases ranging from healthy to diseased conditions. The surrogate models are used to perform global sensitivity analysis at a negligible computational cost and provide insights on the impact of each diagnostic metric on the QoIs. Results also suggest how common flow transit parameters are principally dictated by ejection fraction
Analysis of mitral valve regurgitation by computational fluid dynamics
Full text linkThe clinical syndrome of mitral insufficiency is a common consequence of mitral valve (MV) prolapse, when the MV leaflets do not seal the closed orifice and blood regurgitates back to the atrium during ventricular contraction. There are different types of MV prolapse that may influence the degree of regurgitation also in relation to the left ventricle (LV) geometry. This study aims to provide some insight into the fluid dynamics of MV insufficiency in view of improving the different measurements available in the clinical setting. The analysis is performed by a computational fluid dynamics model coupled with an asymptotic model of the MV motion. The computational fluid dynamics solution uses the immersed boundary
method that is efficiently integrated with clinical imaging technologies. Healthy and dilated LVs obtained by multislice cardiac MRI are combined with simplified models of healthy and pathological MVs deduced from computed tomography and 4D-transesophageal echocardiography. The results demonstrated the properties of false regurgitation, blood that did not cross the open MV orifice and returns into the atrium during the backward motion of the MV leaflets, whose entity should be accounted when evaluating small regurgitation. The regurgitating volume is found to be proportional to the effective orifice area, with the limited dependence of the LV geometry and type of prolapse. These affect the percentage of old blood returning to the atrium which may be associated with thrombogenic risk
Comparative Analysis of Right Ventricle Fluid Dynamics
Full text linkThe right and left sides of the human heart operate with a common timing and pump the same amount of blood. Therefore, the right ventricle (RV) presents a function that is comparable to the left ventricle (LV) in terms of flow generation; nevertheless, the RV operates against a much lower arterial pressure (afterload) and requires a lower muscular strength. This study compares the fluid dynamics of the normal right and left ventricles to better understand the role of the RV streamlined geometry and provide some physics-based ground for the construction of clinical indicators for the right side. The analysis is performed by image-based direct numerical simulation, using the immersed boundary technique including the simplified models of tricuspid and mitral valves. Results demonstrated that the vortex formation process during early diastole is similar in the two ventricles, then the RV vorticity rapidly dissipates in the subvalvular region while the LV sustains a weak circulatory pattern at the center of the chamber. Afterwards, during the systolic contraction, the RV geometry allows an efficient transfer of mechanical work to the propelled blood; differently from the LV, this work is non-negligible in the global energetic balance. The varying behavior of the RV, from reservoir to conduct, during the different phases of the heartbeat is briefly discussed in conjunction to the development of possible dysfunctions
A scenario for heart failure during the filling phase
Full text linkHeart failure (HF) is a life-threating cardiac disease that develops progressively for the reduced ability of the left ventricle (LV) to pump blood into the circulation during systole. HF can also develop in patients with a preserved systolic function, typically in presence of hypertrophic cardiomyopathy (HCM). This type of HF is sometimes termed as diastolic HF, but its biomechanical origin is still unclear. This study employs a physics-based analysis of both the LV and left atrium (LA) in selected HCM patients and matched healthy subjects using 3D echocardiography and demonstrates that alteration on the LV side (stiffening) reduces the elastic recovery of the LA. Moreover, the analysis of the forces exchanged between the two chambers demonstrates that they result unbalanced, keeping the LA in a sustained stretched condition that leads to dilation. This scenario clarifies the diastolic root of the dysfunction that may likely be the cause of the spiraling of events progressing toward failure of both LA emptying and LV filling. This deeply interdisciplinary study provides a physics-based basis for both physics/engineering modeling of heart function and to cardiologists for the design of clinical studies
Interplay between Geometry, Fluid Dynamics, and Structure in the Ventricles of the Human Heart
Full text linkNatural structures conveying fluid flow exhibit an interplay between flow-mediated forces and long-term adaptation. This phenomenon is relevant in the cardiovascular system where the geometric remodelling of the heart chambers is the main mechanism underlying pathological progression leading to hearth failure. Cardiac adaptation is analyzed here in children with a single right ventricle (SRV) in their heart. In these patients, the left ventricle (LV) is not well-developed and the healthy right ventricle (RV) is surgically reconnected, early after birth, to take the functional role of the systemic ventricle. Such a condition represents a special model to investigate cardiac adaptation and this study takes advantage of the availability of an uncommon dataset (64 normal RV, 64 normal LV, 64 SRV with clinically normal function).
The ventricular functional performance is analyzed in terms of fluid dynamics and tissue deformation with the objective of verifying to which degree the SRV configuration adapts from the original RV and progresses toward the function of a LV.
Results show that SRV immediately assumes a larger volume and a wider geometry due to the higher operating pressure. However, the fluid dynamics is weakly turbulent and produces a reduced propulsion. The surrounding tissue develops muscular thickening with multi-directional orientation of myofibers that mimic a LV. However, the reduced flow performance and a lower structural consistency makes the SRV at higher risk of progressive dysfunctional adaptations.
This study demonstrates how the interplay between cardiac flow and tissue response represents the driving macroscopic factor underlying the development of heart failure. More in general, the combined evaluation of fluid dynamics and structural functional properties can be a requirement for the exploration of of the adaptation processes across the different time-scales
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