Ansys Heart Valve Analysis

Journal Of Applied Biotechnology & Bioengineering

Hemodynamics of a Bileaflet Mechanical Heart Valve

*Corresponding Author: Fardin Khalili, Biomedical

of Mechanical and Aerospace Engineering, College of Florida, ENGR 1, Room 428, 12760 Pegasus Blvd, Orlando, FL 32816, USA, Fax (407) 823-0208; Tel: (616) 920-2661; Received: October 30, 2016 | Published: March 27, 2017

Abstract

Heart disease is one of leading causes of mortality worldwide. Healthy heart valves are key for proper heart function. When these valves dysfunction, a replacement is often necessary in severe cases. The current study presents an investigation of the pulsatile blood flow through a bileaflet mechanical heart valve (BMHV) where one leaflet is healthy and can fully open and the other leaflet cannot fully open with different levels of dysfunction. To better understand the implications that a dysfunctional leaflet has on the blood flow through these valves, analysis of flow characteristics such as velocity, pressure drop, wall shear stress and vorticity profiles was performed. Results suggested that leaflet dysfunction caused increased local velocities, separation regions and wall shear stresses. For example, the maximum velocity increased from 2.53 m/s to 4.9 m/s when dysfunction increased from 0% to 100%. The pressure drop increased (by up to 300%) with dysfunctionality. Results suggested that leaflet dysfunction also caused increased wall shear stresses on the valve frame where higher stresses developed around the hinges (at 75% and 100% dysfunctions). Analysis also showed that increased dysfunctionality of one leaflet led to higher net shear forces on both the healthy and dysfunctional leaflets (by up to 200% and 600%, respectively).

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Keywords: Pulsatile blood flow; Dysfunctional leaflet; Cardiovascular modeling;

Shear Forces; Cd-Adapco

Abbreviations: BMHV: Bileaflet Mechanical Heart Valve; CFD: Computational Fluid Dynamics; LDA: Laser Doppler Anemometry; PIV: Particle Image Velocimetry; WSS: Wall Shear Stresses

Introduction

Heart disease is one of the leading causes of death. Treatment of certain types of heart disease involves surgically implanting mechanical heart valves. Bileaflet mechanical heart valves (BMHVs) are commonly used for valve replacement because their design can minimize flow disturbances . However, the rate of artificial valve dysfunction is 0.2-6% patient/year . Analysis of blood flow characteristics such as velocity, vortex formation and turbulent stresses are of importance to identify potential blood cell damage , especially due to complex and unsteady flow in the valve hinges . Woo et al. and Hasenkam showed that the high-velocity flow through the valve resulted in higher shear stress levels at the valve hinges and downstream of the valve and consequently, restricts the leaflets motion. Moreover, high velocity and shear stresses were observed in leakage jets from BMHV hinges which were associated with platelet activation and also thrombosis due to increased residence time in the hinges.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

These life-threatening complications may lead to dysfunction in one or both leaflets of BMHVs . Hence, analysis of cardiac

Sounds And Flow Dynamics Are Active Research

areas, which may lead to better identification of the complications and dysfunction issues and consequently, help to improve valve design.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Previous studies analyzed the blood flow for different BMHV configurations. King et al. reported that the leaflet opening angle affects the blood flow behavior and concluded that a larger leaflet opening angle reduced the pressure drops across the valve.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Pulsatile Newtonian blood flow was studied for fixed leaflets at different levels of dysfunction . Another study illustrated some of the characteristics of pulsatile Newtonian and non-Newtonian blood flow through moving healthy leaflets and aortic root sinuses .

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Previous studies of BMHV have shown that the blood flow moves through three orifices and creates three jets that cause stronger shear stresses than the case of natural valves . Furthermore, studies suggested that vortices become more prominent and wall shear stresses increase with leaflet dysfunction . Increased wall shear stresses are important to detect as it can contribute thrombus formation . Moreover, a leaflet dysfunction can cause backflow. Several different techniques to study blood flow in BMHV were implemented in previous studies. This included CFD (Computational Fluid Dynamics) and experimental methods such as PIV (Particle Image Velocimetry), Video Analysis, LDA (Laser Doppler Anemometry) . In the current study, CFD is used to model the valves and blood flow as the software allows relatively easy and economical conduction of parametric studies.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

The objective of this study is to investigate the flow around BMHV

At Different Levels Of Leaflet Dysfunction By:

a) Documenting the relevant flow structures using streamlines

And Vorticity Information

Hemodynamics of a Bileaflet Mechanical Heart Valve with Different Levels of Dysfunction

Copyright:

©2017 Khalili et al. Citation: Khalili F, Gamage PPT, Mansy HA (2017) Hemodynamics of a Bileaflet Mechanical Heart Valve with Different Levels of Dysfunction. J Appl

B) Quantifying The Changes In Pressure Drop

c) Calculating wall shear stresses on the valve frame and

Shear Forces On The Leaflets

d) Comparing the results of Newtonian and non-Newtonian fluids.

This Study May Provide Data That Can Enhance Our

understanding of the implications dysfunctional leaflets, which can lead to improved design of BMHV.

Models And Methods

In this study, the computational domain was divided into four regions sequentially in the flow direction: upstream, heart valve, aortic root sinuses and downstream. The heart valve geometry (Figure 1a) investigated was chosen to be similar to

Previous Studies . Figure 1B Shows The Asymmetric

aortic root sinuses geometry (with inlet diameter of 0.023 m), which was generated using information extracted from a previous experimental study and was represented as an epitrochoid. Creating a realistic geometry of the aortic sinuses is important for appropriate internal flow field analysis .

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

In the current study, the top (functional) leaflet was assumed to be fixed in the fully open position while the bottom leaflet was fixed at different levels of dysfunctionalities. Since the dynamics of the leaflet opening and closure were not simulated, the data presented in the results section will focus on the fully opening period. Figure 1c shows the side cross section of the BMHV at different levels of leaflet dysfunction of 0, 25, 50, 75 and 100%.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

The positive direction of the shear forces was chosen to be in the flow direction from inlet to outlet. In this study, a commercial CFD software (STAR-CCM+, CD-adapco, Siemens, Germany) was utilized to perform the unsteady simulation for one complete cardiac cycle (simulation time, T = 0.866s) with a time step = 0.5 ms and 25 iterations per time step. The in vitro CFD analyses were performed for a pulsatile flow through a three-dimensional BMHV with fixed leaflets. The inlet velocity corresponding to cardiac output of 5 L/min and heart rate of 70 bpm with systolic phase duration of 0.3 s as illustrated in . The peak inflow velocity was ~1.2 m/s and the density of blood was set to ρ = 1080 kg/ m3. The blood also was considered to be both Newtonian with the viscosity of 0.0035 kg/(m.s) and non-Newtonian based on generalized Carreau-Yasuda model . This lead to an inlet

Is The

frequency of pulsatile flow equals to 17.21. Moreover, high quality polyhedral mesh was generated in the flow domain, especially in the heart valve and aortic sinuses regions, close to the wall and the leaflet surfaces, Figure 2 in order to maintain y+ ≪ 1 (y+ = 0.46 at the peak flow).

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Validation

The normalized velocity profile along a line located 7 mm downstream of the healthy (0% dysfunction) valve is shown in Figure 3. The velocity profiles obtained in previous experimental study that considered a valve with similar geometry and flow conditions are also shown in the same figure. The results from the current study are in good agreement with the previous experimental and computational data. To quantify the difference between our computational and the experimental results, the root-mean-square (RMS) of the difference in velocity was calculated. The RMS of the velocity difference was 6.58% of the maximum velocity, suggesting agreement between the results of

B. Aortic Root Sinuses

c. Different levels of dysfunction. Figure 2: High quality mesh generated in the flow domain especially close to the leaflets and aortic sinuses.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Hemodynamics of a Bileaflet Mechanical Heart Valve with Different Levels of Dysfunction

Copyright:

©2017 Khalili et al. Citation: Khalili F, Gamage PPT, Mansy HA (2017) Hemodynamics of a Bileaflet Mechanical Heart Valve with Different Levels of Dysfunction. J Appl the current study and the measured values.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Results And Discussion

Figures 4a2 to 4a5 shows the streamlines at the peak systolic time of 90 ms, where the color represents the velocity magnitude. For 0% dysfunction (Figure 4a1), the blood flow seemed to have a narrower velocity range (i.e. more uniform velocity) compared to the higher levels of dysfunction (Figures 4a2 to 4a5). Figure 3a1 also shows relatively smaller flow separation in the wake region downstream of the leaflets as would be expected. The flow reattachment also happened closer to the exit of the aortic sinuses.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

The velocity magnitude in the orifices increased, especially in the bottom orifice, as leaflet dysfunction increased. Figure 4 also suggests that an increased leaflet dysfunction may increase the potential for development of higher levels of disturbances in the flow and possibly increased turbulence. This data also suggested that more intense vortical structures start to appear in the valve and sinus regions during the acceleration phase (e.g., 60 to 90 ms). Figure 4b1 to 4b5 shows vorticity at different levels of dysfunction. Vorticity increased with dysfunctions and spread downstream of the leaflets. Conversely, lower levels of vorticity occurred in the sinus downstream of the dysfunction leaflet at 100% dysfunctions, which can be because the obstruction caused by the dysfunction created a low velocity region behind that leaflet. While Figure 4 shows information for t= 90ms, flow structures were also examined for all times between 60 to 250 ms and were found similar to those shown in Figure 4.

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

Figure 3: Validation of the computational methods by comparing the velocity distribution in the current study with a previous experimental study .

Figure: System Model & Simulation Flow for Ansys Heart Valve Analysis

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