In silico study of the ageing effect upon aortic valves

Abstract

A fluid–structure interaction (FSI) numerical model of the aortic valve was used to simulate and compare young and physiological aged operating conditions. The effect of normal ageing was considered by introducing alterations typically associated with senility: namely mild stiffening of the tissues and progressive dilation of the aortic chamber. The aim of this study is to provide a haemodynamic baseline which allows to assess the typical physiological variations associated with advancing age. Results were analysed in terms of leaflets kinematics, flow dynamics, pressure and valve performance parameters. The study indicates that the normal changes occurring with ageing, such as stiffening and progressive aortic root dilation, can result in substantial alterations in the haemodynamics and mechanical efficiency of the aortic valve. In particular, mild tissue stiffening and aortic root dilation reduce the valve efficiency over the cardiac cycle. The concomitant presence of both phenomena can lead to some mitigation of the impairment. The observed changes, which can be associated with normal and healthy ageing, need to be taken into consideration when evaluating the real pathological contribution of aortic valve diseases occurring in aged patients.

Introduction

Ageing is often associated with degenerative pathological conditions of the aortic valve and root such as senile aortic valve calcification (Tamburino and Ussia, 2012) or aneurysm, which result from the presence of an over-dilated aorta (Kirali and Günay, 2017). These pathologies have been the object of extensive studies to identify their effect on the hydrodynamic function (Amindari et al., 2017, Barannyk and Oshkai, 2015, Marom et al., 2013, Querzoli et al., 2014, Toninato et al., 2016, Yoganathan, 1988). However, the normal ageing process itself, even in healthy conditions, involves anatomical variations in the human aortic root (Mao et al., 2008, Roman et al., 1987, Sahasakul et al., 1988, Sugawara et al., 2008, Virmani et al., 1991, Vriz et al., 2014) which are not associated with diseased conditions and can therefore be considered entirely physiological. In fact, normal ageing leads to degenerative changes upon the tissues in the aortic root and valve, which result from a complex interplay between the haemodynamics and biological processes (Bäck et al., 2013). In particular, from the age of 40, the aortic root progressively expands its diameter at the sino-tubular junction (STJ) and sinuses region (Virmani et al., 1991, Vriz et al., 2014) by about 1 mm per decade (Craiem et al., 2012, Martin et al., 2013), whilst vascular tissues stiffen (Sugawara et al., 2008) and remodel (Guala et al., 2015) and aortic valve cusps (Sahasakul et al., 1988) thicken, especially after the fifth decade. The enlargement of the STJ region occurring with advancing age is not associated with an equivalent dilation of the annulus, causing an increasing mismatch between the dimensions of these two regions (Virmani et al., 1991, Vriz et al., 2014), which in the healthy young condition have similar size (Thubrikar, 1990). This process is typically associated with arteriosclerosis, the most widespread age-associated change in the vascular system, which in its general form consists in the progressive thickening and stiffening of the arterial walls. Its potential effect on the performance of the aortic valve is acknowledged by the international regulation for the assessment of cardiac valve prostheses (International Standard ISO 5840:2009), which requests that stentless valves are tested in chambers of two specified values of compliance. Similarly, valve cusps thicken with ageing in all regions, about doubling their average thickness when patients progress from 20 to 75 year old (Sahasakul et al., 1988).

All these variations may potentially lead to functional alterations. Therefore, in order to correctly interpret pathological conditions and their isolated impact on the aortic valve, it is essential to understand if and how normal ageing is expected to modify the operating mechanisms and the efficiency of these components.

This study attempts to provide a haemodynamic baseline for the physiological changes due to ageing, analysing independently the potential contribution of the main age related factors.