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heart valve poses a complex mechanical problem both for the physicians ... R.M. Ali is with the Department of Cardiology, National Heart Institute,. 50400, Kuala ...
2014 IEEE Conference on Biomedical Engineering and Sciences, 8 - 10 December 2014, Miri, Sarawak, Malaysia

Computational Fluid Dynamics Study of the Aortic Valve Opening on Hemodynamics Characteristics Adi A. Basri, Mohamed Zubair , Ahmad F.A. Aziz., Rosli M. Ali, Masaaki Tamagawa, Kamarul A. Ahmad

stresses. Computational modelling of the valve provides an alternate means of investigation that can quantify many types of data and can simulate a variety of situations.

Abstract— In this work, the 3D geometry of patient specific aorta was utilized to carry out CFD studies on the effect of different valve opening (45˚,62.5˚and fully opening) on the hemodynamic properties. The result shows that the lower valve opening induced jet flow and hampered the flow on the additional carotid arteries. Besides, the leaflets were subjected to extreme stress values having disastrous consequences. Consequently, stenosis which is characterized by weaker leaflets and low valve openings has serious impact on the well being of humans.

In this work, the effect of different valve opening (45˚,62.5˚ and fully opening) on the hemodynamics properties in terms of velocity, pressure and wall shear stresses are investigated using the state of art computational fluid dynamics method. The investigation will provide better understanding on the blood flow behaviour of the severity of aortic stenosis decease. A 3D patient specific aortic model is generated using MIMICS and simulation is carried out using Fluent (Ansys corporation).

I. INTRODUCTION The aortic valve is located in between the left ventricle and ascending aorta of the human heart. It consists of a passive leaflet tissue structure and facilitates the opening (systole) and closing (diastole) of the valve. This phenomenon is closely associated with the blood flow acting on the difference in pressure created due to the cardiac cycle and is therefore important in the investigation of the aortic heart valve performance and hemodynamics [1]. The aortic heart valve poses a complex mechanical problem both for the physicians repairing the valve and for the engineers seeking to design replacements for the living valve. It is reported that more than 26,000 people die annually in the United States alone due to the aortic valve disorder and it is estimated that by the year 2020 more than 800,000 people will require heart valve replacements annually [2] . Aortic stenosis is one of the major problems of aortic valve failure. This problem occurs due to the calcification of leaflet or possibly due the occurrence of rheumatic disease, inflammation and congenital disease [3]. Main issues with aortic valve stenosis is that, it may prevent the complete opening of the valves, obstruct the blood ejection and subsequently increase pressure drop across the valve.

II. METHOD A. MR Image Protocol and 3D Aortic Valve Design A patient specific MRI scan data was used to develop the three dimensional (3D) aortic model. The DICOM format of MRI scan was imported in MIMICS software and on providing appropriate threshold; an exact 3D image of aortic arch consisting of ascending aorta, aortic arch, right and left aorta subclavian, left common carotid and descending aorta (Figure 1) can be generated. This 3D model was then transferred to CATIA V5 to insert the tricuspid aortic valve at different valve openings (62.5° and 45°). Next, the aorta model was meshed in ANSYS workbench version 14.5 for carrying out Computational Fluid Dynamics (CFD) simulations. Common carotid Right aorta subclavian

Several in-vitro studies using the Doppler anemometry and MRI have examined the hemodynamic data of physiological and replacement valves[4,5].All of those experimental methods have limitations in fully characterizing mechanical behaviour of the assembled physiological valve, particularly data such as spatially and temporally detailed

Aortic valve location

aorta

Research supported by Aerospace Department,Universiti Putra Malaysia. A.A. Basri, M. Zuber, K.A. Ahmad are with the Aerospace Department, Universiti Putra Malaysia,43400 Serdang,Selangor Darul Ehsan, Malaysia (corresponding author phone: +603-8946 6405; fax:+6038656 7125; e-mail: [email protected]). A.F.A. Aziz is with the Department of Medicine, Universiti Putra Malaysia, 43400 Serdang, Selangor Darul Ehsan, Malaysia.(e-mail: [email protected]). R.M. Ali is with the Department of Cardiology, National Heart Institute, 50400, Kuala Lumpur, Malaysia. (e-mail: dr.rosli @ijn.com.my). M. Tamagawa is with the Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Japan.(e-mail: [email protected]).

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Left aorta subclavian Aortic arch

Ascending aorta

Descending aorta

Figure 1. Aorta model of patient-specific obtained from MR images

B. Mesh Dependency Mesh dependence study of the 3D aortic arch model was accomplished using the ANSYS workbench. The maximum velocity, pressure and WSS (wall shear stress) was compared

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2014 IEEE Conference on Biomedical Engineering and Sciences, 8 - 10 December 2014, Miri, Sarawak, Malaysia

between the tetrahedral meshes of ~100k to ~1300k elements. It was noted that, mesh with~800k tetrahedral elements was the best mesh for the simulation with little or no variations in the functional parameters considered. C. Boundary Condition Computational Fluid Dynamics (CFD) produces fluid flow simulation by solving governing Navier Stokes equation of fluid motion. The governing equations of flow that was considered in this study are the continuity equation and navier stokes equation. Besides, k-ω standard was additionally used to account for the turbulent nature of the flow studied. The inlet velocity of ascending aorta was 1 m/s. This was chosen based on the peak systole of blood pulsatile flow of left ventricle heart [6,7]. The inlet was assumed to be Newtonian and incompressible fluid with density of blood assumed to be 1060 kg/m3 and viscosity of 0.0035 Pa/s [8,9]. III. RESULT AND DISCUSSION A. Effective of Valve Opening and Velocity Formulation Fig. 2 shows the maximum velocity of blood flow through aortic valve at a plane 5mm above the valve location. The red line represents the current result of simulation while blue line represents the result produce by Harry A. Dwyer et al.[10]. The lower angle of the aortic valve opening has produced higher value of velocity. The fully opened valve generated the lowest value of maximum velocity at 1.33 m/s followed 2.33 m/s for 62.5˚ and 5.46 m/s for 45˚ valve opening. Although the result obtained are slightly lower than Harry A. Dwyer et al.[10] 1.47 m/s for fully opened valve and 2.352 m/s for sclerosis (38% orifice reduction) valve, they follow identical trend and are in close agreement with each other. The differences could be attributed to variations in anatomical geometry of different patient specific cases studied.

lower angle of valve opening. Consequently, the 45˚ valve opening shows the maximum velocity, followed by 62.5˚ and fully opened valve. It can also be seen that, smaller valve opening (45˚), hampered the flow into the carotid branches and flow is restored only at higher valve opening angles. This is significant because, at low valve angles, very high jet flows are produced which are detrimental to the delicate tricuspid valves which regulate the flow inside the aorta. The pulsatile blood flow from the left ventricle impinges the tricuspid valve, forcing it open. In the process, a jet flow is generated which gets subsided when the valve tend to open further, allowing the blood to freely pass through it. Secondly, the initial opening of the valve does not facilitate flow through the carotid arteries. This situation is very severe in case of stenosed arteries which demonstrate only partial valve opening. This prevents blood flow via the carotid branches to important organs of the body. In the real circumstances, most of cardiologist consent that the turbulent flow indicates the occurrence of either regurgitation or aortic stenosis problem [11]. Moreover, reduction of valve opening may led to platelet function abnormalities, collagen binding activity and diminish blood concentration. It is normally happen to the severe aortic stenosis patient with the probability of 67-92% [12].

A

B

C Figure 3. A- velocity streamline of 45 degree of valve opening, B- velocity streamline of 62.5 degree of valve opening, C- velocity streamline of fully opening valve Figure 2. Maximum velocity after aortic valve vs angle of valve opening

Fig. 3 (A, B, C) shows the blood flow velocity streamline at different angle of valve opening (45˚,62.5˚ and fully open). It can be clearly inferred from the plot that the angle of valve opening has a significant impact on the velocity pattern developed. The velocity magnitude shoots at

B. Effective of Valve Opening and Pressure Formulation The pressure contour around the aorta has been plotted to analyze the effect of valve opening on the pressure developed (refer fig. 4 A,B,C). There is considerable variation in the pressure across the tricuspid valve at the 45˚ valve opening.

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2014 IEEE Conference on Biomedical Engineering and Sciences, 8 - 10 December 2014, Miri, Sarawak, Malaysia

The average blood pressure just before the valve is about15.91kPa which decrease significantly to approximately 1.5 kPa just beyond the 45˚ valve opening. The pressure drop across the 62.5˚ valve opening exhibited reduced variation, and was found to be in the range of 3.276kPa to 1.068kPa. These result show that, the lower angle of valve opening produce higher blood pressure drop when compared to larger angle of valve openings.

A

B

A

B

Figure 5. A –Wall shear stress contour of 45 degree of valve opening, B – Wall shear stress contour of 62.5 of valve opening.

IV. CONCLUSION C Figure 4. A – pressure contour of 45 degree valve opening, B- press ure contour of 62.5 degree of valve opening, C- pressure contour of fully opening

C. Effective of Valve Opening on the Wall Shear Stress (WSS) Developed The profile and magnitude of wall shear stress induced on the leaflet tip of 62.5˚ and 45˚ valve opening has been examined (refer fig. 5 A,B). The leaflet at 45˚ valve opening exhibited average WSS in the range of 70 - 94 Pa, whereas for the 62.5˚ valve opening it was in the range of ~32 – 40 Pa. The stress developed for 62.5˚ of valve opening is identical with the values reported in the work of Harry A. Dwyer et al. for the sclerosis phase (38% of reduction orifice) [10]. The findings of this study are very important and shows that the smaller leaflet opening can introduce very high stress on the sensitive leaflets. This problem can exaggerate on application of continuous cyclic loads over a period of several years. The higher WSS on the leaflet will produce profound effect on endothelial cell synthetic activities which is the activity of antithrombotic (preventing thrombosis)[13,14]. In addition, the capability of red cells and platelets damage is higher under high shear stress condition in blood vessel [15]. The blood damage is one of the activator of thrombus formation which may lead to flow stagnations.

The combination of MRI and CFD simulation is an excellent tool for evaluating the relationship between aortic valve opening and their relevant effect on blood flow. Different valve openings exhibited various flow regimes in terms of velocity, pressure and also wall shear stress developed. It was found that the lower leaflet opening has detrimental effect on blood flow as well as induced higher stress on the leaflets. This work is important in understanding the relationship between blood flow behavior, WSS on the leaflet and the severity of aortic stenosis which has huge effects on blood damage and thrombus formation. REFERENCES [1] L. R. Croft and M. R. K. Mofrad, “Computational Modeling of Aortic Heart Valves,” in Computational Modeling in Biomechanics,Springer, 2010, pp. 221–252. [2] S. S. Subcommittee, “AHA statistical update,” Circulation, vol. 115, pp.e69–e171, 2007. [3] R. van Loon, “Towards computational modelling of aortic stenosis,” International Journal for Numerical Methods in Biomedical Engineering, vol. 26, no. 3–4, pp. 405–420, 2010. [4] A. P. Yoganathan, Y.-R. Woo, H.-W. Sung, and M. Jones, “Advances in prosthetic heart valves: fluid mechanics of aortic valve designs,” Journal of biomaterials applications, vol. 2, no. 4, pp. 579–614, 1987. [5] G.-Z. Yang, R. Merrifield, S. Masood, and P. J. Kilner, “Flow and myocardial interaction: an imaging perspective,” Philosophical Transactions of the Royal Society B: Biological Sciences, vol. 362, no. 1484, pp. 1329–1341, 2007. [6] C. Jamuna and M. Anburajan, “Design of patient specific prosthetic aortic valve and to study its computational fluid dynamics,” in Electronics Computer Technology (ICECT), 2011 3rd International Conference on, 2011, vol. 3, pp. 355–360.

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