C. Del Gaudio

Computational model of the fluid dynamics of a cannula inserted in a vessel: incidence of the presence of side holes in blood flow

Vascular access methods, performed by the insertion of cannulae into vessels, may disturb the physiological flow of blood, giving rise to non-physiological pressure variations and shear stresses. To date, the hydrodynamic behaviour of the cannulae has been evaluated comparing their pressure loss-flow rate relationships, as obtained from in vitro experiments using a monodimensional approach; this methodology neither furnish information about the local fluid dynamics nor the established flow field in specific clinical work conditions. Since the shear stress is a critical factor in the design of artificial circulatory devices, more knowledge should be necessary about the local values assumed by the haemodynamic parameters during cannulation. An alternative way to investigate the fluid dynamic as accurately as possible is given by numeric studies. A 3D model of cannula concentrically placed in a rigid wall vessel is presented, with the finite element methodology used to numerically simulate the steady-state flow field in two different venous cannulation case studies, with two cannulae having a central hole and two or four side holes, respectively, with the same boundary conditions. Lower velocity and shear stress peak values have been computed for the model with four side holes upstream of the central hole, in the region of the cannula where the inlet flows meet and towards cannulas outlet, due to the increased flow symmetry and inlet area with respect to the model with two side holes. Starting from the investigation of different cannula designs, numerically assessing the local fluid dynamics, indications can be drawn to support both the design phase and the device optimal clinical use, in order to limit risks of biomechanical origin. Thus the presence of four side holes implied, as a consequence of the greater inlet area and of the increased symmetry, a less disturbed blood flow, together with reduced shear stress values. Furthermore, results show that the numerical simulations furnished useful informations on the interaction between vessel and cannula, e.g. on the fluid dynamics establishing in the free luminal space left, in the vessel, by the inserted cannula.

Innovative technologies for the assessment of cardiovascular medical devices: state-of-the-art techniques for artificial heart valve testing

Prosthetic heart valves (PHVs) are engineered devices used for replacing diseased natural cardiac valves. This article presents several investigational techniques for the evaluation of the performance of these clinical devices, whose implantation is not completely free of drawbacks. The state-of-the-art in the technological approach for PHV testing is addressed. As the fluid dynamics of PHVs are particularly complex, the main focus will be on experimental velocimetric techniques and computational analysis. A methodology for the analysis of the valve’s signature, in terms of its characteristic sound in the opening and closing phases, is also presented. The aforementioned techniques are necessary to guarantee an operational life of the implanted device as free as possible from clinical complications. It can be realistically expected that this characterization will help designers in improving PHV performance.

Time dependent non-Newtonian numerical study of the flow field in a realistic model of aortic arch.

A three-dimensional time dependent numerical simulation was performed in a geometric model of aortic arch complete with a realistic aortic root and major branches originating from the arch, for a peak Reynolds number set at 2200 and Womersley number set at 20.4. The computational fluid dynamic analysis was aimed to provide spatial and temporal distribution of the shear stress all along the entire model together with the velocity patterns, related both to the non planar geometry of the aortic system here considered and to the pulsatility imposed on the numerical model to simulate physiologic conditions. A non-Newtonian evolving fluid was considered to account for the actual rheological nature of blood; a comparison on the incidence of wall shear stress, implementing a Newtonian fluid, was also made as reference. The spatial shear stress pattern, within the cardiac cycle, was shown to have higher values in correspondence to the inner wall of the aortic arch and the sites where the major vessels originated from the arch itself. The velocity patterns, on transversal sections of the aorta, resulted in highly skewed morphology. The resulting complex fluid dynamics, established in the aortic arch and in its branches, can be related to the possible endothelium response to mechanical stimuli, induced by wall shear stress, in the promotion of inflammatory events.

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.

Beat to beat analysis of mechanical heart valves by means of return map.

Three mechanical heart valves (two bileaflet prostheses and a tilting one) were investigated in a basic hardware setup in order to evaluate with a hydrophone their opening and closing action in time and in amplitude of each beat. The recorded signal was then segmented into the series of cycles xi(t) having a temporal duration equal to the working period imposed on the valve. Two return maps were defined, in order to evaluate the degree of dispersion of the resulting scatter plot: (i) the amplitude map xi(t) versus xi+1(t); (ii) the delay map for the closure of the valve within each beat versus the successive ones. To evaluate the results obtained, two indices were proposed based on both the degree of dispersion and the deviation of the regression line of the resulting scatter plot with respect to the bisector of the map plane. The tilting disc valve showed a lower degree of dispersion, both in the amplitude signal and in the closure time delays, with respect to the other two bileaflet heart valves. The methodology proposed here could be regarded as an alternative non-invasive tool to investigate the dynamic behaviour of prosthetic heart valves, especially in the case of their suspected failure.

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,

A study of discharge coefficient in bileaflet valves

The measurement of a cardiac valves area is a common procedure, usually performed with noninvasive, Doppler-based techniques. Such measurements are not, however, without problems: a potential source of errors is the value of a valves discharge coefficient. In-vitro pressure and flow measurements relative to the bileaflet valve of four brands were performed. A total of 12 valve samples was studied to cover the entire range of valve sizing. The data were used in the Gorlin formula for valve area measurements, and the dependence of the discharge coefficient on the internal diameter of the valve and the flow rate was accurately determined. The reported results can be used in a great number of follow-up clinical assessments to improve the accuracy of valvular orifice measurements.

Hydraulic properties of the Hemopump HP31: a study of the downstream pressure distribution

The Hemopump was commercialized as an useful tool for the left ventricle assistance. Bioengineers and clinicians showed great interest to develop applications and to analyze its hydraulic behaviour; in this work an application for axial pump in different conditions is presented. A study of the spatial pressure distribution generated by the impeller of the Hemopump is investigated in highly accurate steady-flow conditions. The experimental set up adopted for this study consists in a plexiglass test pipe (simulating an aortic conduit of 22-mm diameter) and allows the sampling of the pressure at the outlet of the pump in 16 points spaced 1/2 diameter each other. Keeping fixed the constant head at the inlet of the Hemopump and varying the constant head at its outlet, i.e. afterloads, in 11 step levels, it was possible to draw the characteristic flow curves versus delivered pressure for all the seven speed levels. A pressure range of about 35-130 mmHg and a flow range of about 0.7-3.7 l/min was experimented. The results show that the flow delivered by the Hempopump is fully developed after 5 pressure taps (about 55 mm), with no further varying along the test chamber. These data could be used to optimize the setting up of clinical experimental procedures.