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

Abstract

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.

Keywords: Haemolysis index; Image velocimetry; Pulse duplicator; Valve leaflets dynamics.

Fig. 1

a Sketch of the experimental apparatus: 1 Piston pump; 2 ventricular chamber; 3 aortic chamber; 4 aorta; 5 mitral valve; R1 and R2 peripheral resistance; RC compliance flow regulator; C compliance chamber; S1 right atrial chamber, S2 left atrial chamber. b Set up of camera, laser sheet, valve and aortic root mutual position; aortic root model plus the adopted mechanical valve. c Measuring tool for leaflet tilting angles [right (α_R) and left (α_L)], and chosen time instants for leaflets dynamic measurements, in the ejection phase. The grey area represents the SV pumped into the aorta

Fig. 2

Comparison between the ventricular (p_v) and the aortic (p_a) pressure behavior from medical literature (red lines, [53]) and in vitro test with the mock loop (black lines)

Fig. 3

EOA as a function of the SV (white squares) for the fixed physiological T = 2.4 s, and as a function of the period (black dots), for SV = 64 ml (experiments numbered as reported in Table 1)

Fig. 4

Left (α_L, white dot) and right (α_R, black dot) leaflet tilting angles behavior in non-dimensional time t/T. a–c show the case SV = 54, 64 and 80 ml, respectively. d, e show the trend between the same leaflet but at different SV. T = 2.4 s was used for all results

Fig. 5

Phase averaged vector velocity field (black arrows) and non-dimensional vorticity 〈ωT〉 color map (red for counterclockwise vorticity and blue for clockwise vorticity) at different time instants (red dots on the flow rate curve) for the test case SV = 64 ml, T = 2.4 s. In particular, A, B and C are the three main jets formed downstream of the valve, A′ and B′ the evolution of A and B as the main eddies observed downstream the sinus

Fig. 6

Phase averaged velocity field and non-dimensional maximum viscous shear stress τ_tmax/ρU² (color map) at different time instants for the test case SV = 64 ml, T = 2.4 s

Fig. 7

Non-dimensional maximum shear stress averaged over the aortic root area ${\overline{\mathit{\tau }}}_{tmax}$/ρU² as a function of non-dimensional time t/T for different haemodynamic working conditions