Experimental analysis of pulsatile flow characteristics in prosthetic aortic valve models with stenosis

Highlights

• The pressure and flow characteristics of two stenosed prosthetic aortic valves were studied.

• Severe stenosis caused a prolonged ejection period and an increased acceleration to ejection time ratio.

• Severe stenosis caused a more eccentric jet and a two-fold increase in the peak velocity.

• Severe stenosis caused a three-fold increase in peak turbulence kinetic energy compared to moderate stenosis.

• The hemodynamic changes were associated with the stiffened leaflets rather than the stent base structure.

Abstract

Bioprosthetic valves are widely used for aortic valve replacements for patients with severe aortic diseases. However, tissue-engineered leaflets normally deteriorate over time due to calcification, leading to life-threatening conditions that would require re-operation. The hemodynamics induced by a prosthetic stenosis is complicated and not fully understood. This in vitro experimental study focuses on the fluid dynamics of two aortic valve models with different prosthetic stenosis conditions. An in vitro cardiovascular flow simulator was utilized to provide the pulsatile physiological flow conditions. Phase-locked particle image velocimetry (PIV) and high-frequency pressure sensors were employed to measure the flow fields and pressure waveforms. Pressure data were evaluated for the two models representing moderate and severe stenosis conditions, respectively. The severe prosthetic stenosis induced a prolonged ejection period and increased acceleration time ratio. PIV results suggest the severe prosthetic stenosis resulted in a two-fold increase in peak jet velocity and a three-fold increase in peak turbulence kinetic energy compared to the moderate stenosis case. The severe stenosis also caused rapid expansion of the jet downstream of the valve orifice and increased eccentricity of the jet flow. The maximum Reynolds shear stress in the severe stenosis case was found similar to the bileaflet mechanical valve reported by previous literature, which was below the risk threshold of blood cell damage but could potentially increase the risks of platelet activation and aggregation.

Introduction

Aortic stenosis (AS) occurs when the valve leaflet tissues become stiff, causing a narrowed valve orifice area and an increased pressure gradient required to pump blood through the valve. Untreated AS will cause increased risks of other cardiovascular complications and mortality in the long-term. The severity of AS normally progresses from mild (often asymptomatic) to severe with annualized rates dependent on hemodynamic severity [1]. For patients with severe AS, aortic valve replacements are often required via surgery, or minimally invasive transcatheter aortic valve replacement (TAVR) procedures for those who are not suitable for surgeries [2,3]. Two types of artificial valves currently dominate the market, i.e. mechanical and bioprosthetic valves. While mechanical valves (typically designed with rigid tilting disks) provide superior material strengths and lifelong durability, they also generate increased wall shear stress and turbulence due to the non-physiological interactions with the pulsatile blood flow [4,5]. These alterations in hemodynamics increase the risks of platelet activation and even hemolysis [6], [7], [8]. Hence, patients with mechanical valve implants are often burdened with lifelong uses of anticoagulant medicines. In contrast, bio-prosthetic valves are normally fabricated from porcine or bovine valve tissues with the natural three-cusp configurations [9]. As the anatomy and material properties of the bio-prostheses are close to those of the native valves, the hemodynamics are more favorable and the risks of thrombosis and hemolysis are low [10]. Recent innovations in clinical interventions, such as TAVR, further reduce procedure-related risks of bio-prosthetic valve implantation [11]. However, bio-prostheses suffer from material deteriorations and their durability is often much shorter than the mechanical valves [12]. A deeper understanding of valve hemodynamics is of critical importance for the clinical assessment, deployment, and management of aortic valve prostheses.

In recent years, interdisciplinary biomedical engineering research has made significant contributions to the fundamental hemodynamics and fluid-structure interactions of artificial heart valves. Particularly, many experimental studies employing Particle Image Velocimetry (PIV) have been conducted to examine the bi-leaflet mechanical valve [13], [14], [15], [16], [17], [18] and bio-prosthetic valve hemodynamics [19], [20], [21], [22], [23] under simulated physiological pulsatile flows. These in vitro studies revealed important hemodynamic characteristics including unsteady flow velocity [24], large-scale vortices [17], and Reynolds stresses [6], and assisted in clinical assessments of artificial valve implantation based on the precisely-controlled flow conditions and the improved spatial and temporal resolutions [25]. Balducci et al. [24] studied the flow field immediately downstream of an artificial valve using PIV and particle tracking technique. Their results revealed highly unsteady flow phenomenon and the presence of large-scale vortices in the sinus of Valsalva and leaflet wakes. Using PIV and direct numerical simulation, Dasi et al. [17] revealed the three-dimensional and unsteady nature of the pulsatile jet and its associated coherent flow features downstream of a mechanical heart valve. Li et al., [6] estimated the Reynolds stresses and the viscous dissipative stresses of a mechanical heart valve using PIV measurements and indicated that the valve-induced viscous stresses could potentially cause platelet damage.

The objective of this study is to quantitatively investigate the fluid dynamic characteristics of two polymeric aortic valve model with stenosis based on in vitro pulsatile flow experiments and phase-locked PIV measurements. As aforementioned, the major flaw of bio-prostheses lies in the fact that they normally start to deteriorate within ten years after implantation [12,26]. The major modes of valve deterioration are the cusp stiffening and calcification that leads to bio-prostheses stenosis [27]. The annualized rate of deterioration of bio-prostheses is estimated to be 0.3 mmHg per year in terms of pressure gradient [28]. In certain cases, life-threatening accelerated valve deterioration could cause rapid progression of prosthetic stenosis, requiring repeat surgery within as short as 5 years [27]. A reduced leaflet motion was even observed in patients with bioprosthetic valves who developed leaflet thrombosis shortly after the TAVR procedure [29]. These clinically-observed negative impacts of bio-prosthetic stenosis underscore the importance of understanding of the related hemodynamics for the improvements in the future bio-prosthesis designs.

In this study, we applied the phase-locked PIV and high-frequency pressure measurements to study the pulsatile flow through two polymeric valves inside a 1:1 compliant aortic root model. The first valve prosthetic model was fabricated using polydimethylsiloxane (PDMS) by molding the leaflets and the aortic root wall together as a whole. The second model was constructed by installing a stented fabric-reinforced silicone valve into the PDMS aortic root. Due to the different material selection, a different degree of stenosis was achieved as evidenced by pressure measurement data. The phase-locked ensemble-average flow field and derived unsteady flow information were analyzed and discussed to reveal the impacts of stenosis on the flow inside the aortic root.