A. Balducci

Numerical simulation of the dynamics of a bileaflet prosthetic heart valve using a fluid-structure interaction approach

The main purpose of this study is to reproduce in silico the dynamics of a bileaflet mechanical heart valve (MHV; St Jude Hemodynamic Plus, 27mm characteristic size) by means of a fully implicit fluid-structure interaction (FSI) method, and experimentally validate the results using an ultrafast cinematographic technique. The computational model was constructed to realistically reproduce the boundary condition (72 beats per minute (bpm), cardiac output 4.5l/min) and the geometry of the experimental setup, including the valve housing and the hinge configuration. The simulation was carried out coupling a commercial computational fluid dynamics (CFD) package based on finite-volume method with user-defined code for solving the structural domain, and exploiting the parallel performance of the whole numerical setup. Outputs are leaflets excursion from opening to closure and the fluid dynamics through the valve. Results put in evidence a favorable comparison between the computed and the experimental data: the model captures the main features of the leaflet motion during the systole. The use of parallel computing drastically limited the computational costs, showing a linear scaling on 16 processors (despite the massive use of user-defined subroutines to manage the FSI process). The favorable agreement obtained between in vitro and in silico results of the leaflet displacements confirms the consistency of the numerical method used, and candidates the application of FSI models to become a major tool to optimize the MHV design and eventually provides useful information to surgeons.

Numerical simulation of a realistic total cavo-pulmonary connection: Effect of unbalanced pulmonary resistances on hydrodynamic performance

Total cavo pulmonary connection (TCPC) is one of the surgical techniques adopted to compensate the failure of the right heart in pediatric patients. The main goal of this procedure is the realization of a configuration for the caval veins and for the pulmonary arteries that can guarantee as low as possible pressure losses and appropriate lung perfusion. Starting from this point of view, a realistic TCPC with extracardiac conduit (TECPC) is investigated by means of Computational Fluid Dynamics (CFD) to evaluate the pressure loss under different pressure conditions, simulating different vessel resistances, on the pulmonary arteries. A total flow of 3 L/min, with a distribution between the inferior vena cava (IVC) and the superior vena cava (SVC) equal to 6/4, was investigated; three different boundary conditions for the pressure were imposed, resulting in three simulations in steady-state conditions, to the right pulmonary artery (RPA) and to the left pulmonary artery (LPA), simulating a balanced (ΔPLPA-RPA=0 mmHg) and two unbalanced pulmonary resistances to blood flow (a pressure difference ΔPLPA-RPA = ±2 mmHg, respectively). The geometry for the TECPC was realized according to MRI derived physiological values for the vessels and for the configuration adopted for the anastomosis (the extra-cardiac conduit was inclined 22° towards the left pulmonary artery with respect to the IVC axis). The computed power losses agree with previous in vitro Particle Image Velocimetry investigations. The results show that a higher resistance on the LPA causes the greater pressure loss for the TECPC under study, while the minimum pressure loss can be achieved balancing the pulmonary resistances, subsequently obtaining a balanced flow repartition towards the lungs.

Critical aspects for a CFD simulation compared with PIV analysis of the flow field downstream a prosthetic heart valve

The investigation of the fluid dynamic field determined by the implantation of a medical device is of fundamental importance from a bioengeneering and a clinical point of view: it is well-known that the modification of the physiological fluidic condition could generate thrombogenic andlor hemolytic phenomena. Several techniques can be implemented for this study (PIV, PTV, CFD), but none of them can be regarded as completely exhaustive; intrinsic technical limitations can lead to approximate results. This paper presents a numerical analysis of the flow field downstream of a prosthetic bileaflet heart valve in aortic position, placed in a glass test chamber designed as the natural aortic root for in vitro experiment. PIV analysis was performed as experimental guideline and its results were compared to those obtained by a 2D CFD simulation in order to verify if a simplified numerical model could furnish results sufficiently close to the PIV ones. This comparison was performed for a fraction of the systolic period (where the valve can be considered fully open) assuming a systolic flow of 1 llmin at 70 bpm. The agreement of the results of these two methodologies suggests not only that a numerical model, although simplified, can be helpful as complementary to an experimental analysis (e. g., investigation of areas of the fluid domain where the PIV technique suffers from lack of resolution or illumination), but also indicate critical aspects to improve the numerical model itself and to refine the fluid dynamical assumptions to be implemented. At the same time it could be necessary to define a more accurate approach for CFD and PIV set-ups, being the latter much more related to the experimental condition. Transactions on Biomedicine and Health vol 6,

Evaluation of the Dynamics of a Bileaflet Heart Valve: Physical and Numerical Experiments

Clinical reports indicate that mechanical heart valves are still unable to eliminate problems mainly related to a non physiological fluid dynamics like thrombosis and coagulation complications [1]. Advanced experimental technique such as laser doppler anemometry (LDA) and particle image velocimetry (PIV), used to investigate the fluid dynamics of these devices, suffer from some intrinsic limitations (eg. access difficulties, light reflection, low resolution) [2]. In parallel with the increased performance at computing, the use of computational fluid dynamics has gained relevance as a powerful tool able to provide meaningful information of clinical and design aspects [3]. Key parameters in the assessment of blood damage potency (velocity patterns and turbulence, among them) are related to the behaviour of the valve in the flow field. The application of fluid structure interaction (FSI) models moves in the direction of greater accuracy in the reproduction of realistic flow condition in order to reach more in-depth insight into the hemodynamic of the virtual prototypes. The aim of this study is the investigation, in silico, of the bileaflet mechanical valve dynamics during the whole systolic phase. A 3D direct numerical simulation (DNS) was performed and an implicit fluid structure interaction model was used [4,5]. The results of the dynamics of the valve were validated with an experimental counterpart.Copyright