The Effects of Aortic Stenosis on the Hemodynamic Flow Properties using Computational Fluid Dynamics

Transcription

1 The Effects of Aortic Stenosis on the Hemodynamic Flow Properties using Computational Fluid Dynamics Adi Azriff Basri 1, Muhammad Zuber 1, Mohamad Shukri Zakaria 1,2, Ernnie Illyani Basri 1, Ahmad Fazli Abdul Aziz 3, Rosli Mohd Ali 4, Masaaki Tamagawa 5, Kamarul Arifin Ahmad 6 1 Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 2 Faculty of Mechanical Engineering, UniversitiTeknikal Malaysia Melaka, Hang Tuah Jaya 76100, Durian Tunggal, Melaka, Malaysia. 3 Department of Medicine, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia,43400 Serdang, Selangor, Malaysia 4 Department of Cardiology, National Hear Institute, 145, JalanTunRazak, Kuala Lumpur, Wilayah Persekutuan Kuala Lumpur, Malaysia 5 Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Japan 6 Department of Mechanical Engineering, P.O Box 800 King Saud University Riyadh, Saudi Arabia Corresponding Author :adi.azriff@gmail.com Received [19 th August 2016]; Revised [2 nd ]; Accepted [14 th ] Abstract: In this study, the state-of-the-art computational fluid dynamics was employed to investigate the relationship between aortic stenosis and its effect on hemodynamics. The normal aortic valve opening (fully open) is compared with a 62.5º valve opening (aortic stenosis condition). From the result, the aortic stenosis shows anincreased in velocity at the aortic branches (brachiocephalic artery, left subclavian artery, and common carotid artery). Meanwhile, the wall shear stress of the leaflet bottom surface shows a higher value than the leaflet top surface during the systole phase. In addition, the total mass flow rate entering the aortic branches reduced by 2.9% for the stenosis case. Keywords: Aortic stenosis, Patient specific - computational fluid dynamics, hemodynamic of aortic stenosis, wall shear stress leaflet 33

2 I. INTRODUCTION Aortic stenosis is one of the major problems associated with the aortic valve failure. It is frequently observed with the elderly heart patients and estimated to be up to 5% for the age group of 75 years and above [1],[2]. The existence of this condition is predominantly observed in the developed countries and is predicted to increase significantly in coming years due to the aging of the population and awareness [3],[4]. There are more than 67,500 surgical aortic valve replacement (SAVR) cases annually in the US solely [5]. This problem occurs due to the calcification of leaflet or possibly caused by the occurrence of rheumatic disease, inflammation and congenital diseases [6].The narrowing of the valve during the systolic state increases the resistance of blood flow and may generate larger pressure drop across the valve. Thus, aortic stenosis can severely hamper the normal flow of blood due to the non-opening of aortic valve. Indirectly, this may cause disturbance of the blood circulation to the entire human system, whichlead toseveral other complications. In this study, the performance of the normal valve opening and the 62.5º valve opening (aortic stenotic condition) are being compared. Then, the effect of stenosed leaflet on the shear stress developed and the disturbance of the blood flow in the brachiocephalic artery, left subclavian artery and common carotid artery are investigated using the state-of-the-art computational fluid dynamics study (CFD). A three dimensional (3D) patient specific aortic model is generated using MIMICS software (Materialise Sdn Bhd)and simulation is carried out using CFX software (ANSYS 14.5). II.METHODOLOGY A. MR Image Protocol and 3D Aortic Valve Design The (3D) aortic model has been developed based on a patient specific MRI scanned data. A 71 years old male patient with the annulus diameter of 27.3 mm has been used for this study. The DICOM format of MRI scan is imported in the MIMICS software. By applying appropriate threshold, the exact 3D model of the aortic arch consists of the ascending aorta, the aortic arch, the brachiocephalic artery, the left subclavian artery, the common carotid artery and the descending aorta (Figure 1) is generated. This 3D model is then transferred to the CATIA V5 to insert the tricuspid aortic valve with 62.5 valve opening, which defined as the aortic stenotic condition. Next, CFD meshing is accomplished using ANSYS workbench version 14.5 for carrying out CFD simulations. 34

3 Common carotid artery Brachiocephalic artery Aortic arch Left subclavian artery Aortic valve location aorta Ascending aorta Descending aorta Figure 1 Aorta model of patient-specific obtained from MR images B. Mesh Dependency The mesh developed is tested for its grid independency by studying in three difference tetrahedral meshes in the range of ~100k to ~1300k elements prepared in the ANSYS workbench. These meshes are evaluated according to the maximum velocity, the pressure and the wall shear stress (WSS). It is observed that the mesh with~800k tetrahedral elements is suitable and shows the minimum changes in the functional parameters considered. C. Boundary Condition In the CFD, fluid flow simulation is produced by solving the governing Navier-Stokes equation of fluid motion considering the continuity equation and the Navier-Stokes equation. Meanwhile, the k-ω standard is additionally applied by taking into account the turbulent nature of the flow studied. The inlet velocity and the outlet pressure of pulsatile blood flow are referred to the study done by Torii et al.[7], Vignon Clementel et al.[8], [9] and Khader et al.[10]. As referred to Figure 2A and Figure 2B,the time varying velocity and the pressure waveform are selected as the inlet velocity and outlet pressure of the fluid model. The inlet is assumed to be Newtonian and the incompressible fluid with density of blood also assumed to be 1060 kg/m3 and viscosity of Pa/s [11], [12]. 35

4 A Figure 2A: Velocity inlet of pulsatile blood flow B Figure 2B: Pressure outlet of pulsatile blood flow III. RESULTS AND DISCUSSIONS A. Velocity Formulation of Normal and Aortic Stenosis Opening Figure 3A shows the velocity streamline plot for the normal aortic valve opening at the systolic phase. It is noticed that the blood flow passing through the aorta arch is smooth and no flow separation observed. The maximum velocity during this phase is found to be m/s, which is similar to H.A. Dwyer et al.[13]. On the other hand, the streamlines for the aortic stenosis case shows the flow separation at the arc region (Figure 3B). The maximum velocity is significantly increased to m/s. The aortic valve stenosis occurs due to the calcification of the leaflet tips, which hinder its movement inside the aorta. This fixated aortic valve poses a severe threat to the normal functioning of the artery. Meanwhile, the acute angle of the valve opening shows the increased of the velocity by 13.7% compared to the normal aortic valve opening. More severe stenosis occurs with the acute angle opening of 62.5 o and below, which may lead to the formation of jet flow in the vicinity. This may induce to the severe stresses on the leaflet and the aortic wall. Furthermore, the partial valve opening of 62.5 o, may causing a disturbance of the blood flowing into the carotid branches and, hence lead to the inefficient blood supply to the important organs of the body. Thus, this consequently may cause the platelet function abnormalities, the collagen binding activity and the diminish blood concentration as reported by the severe aortic stenosis patients with its probability as high as 67-92% [14]. 36

5 A Figure 3 A : Velocity streamline of normal aortic valve opening Figure 3 B : Velocity streamline of 62.5 degree valve opening B. Wall Shear Stress (WSS) Developed on Leaflet During Systole and Diastole The Wall Shear Stress (WSS) of the leaflet is also examined in this study referring to the systole and diastole phases. This is important to understand the relationship between the acute valve opening and the WSS effects on the leaflet, particularly at the tips of the leaflet. It is understood that the stress concentration at the leaflet is one of the factors which contribute to leaflet calcification and may lead to the structural failure[15],[16]. Figure 4 depicts the leaflet top and bottom surface, while Figure 5 and Figure 6 shows the WSS effect on the leaflet at the top surface and bottom surface during the systole and diastole phases with the respective Figure 5A, 5B,6A and 6B. It can be 37

6 seen that the maximum WSS is focused at the tips of leaflet for each phases. Referring to Figure 7, the bar chart shows the maximum WSS at the top surface and the bottom surface of the leaflet during the systole and diastole phases. During the peak systole phase, the WSS on the leaflet at the bottom surface ( Pa) is higher compared to the leaflet at top surface (341 Pa). This is because of the high flow velocity of the partial valve opening, induced an increased stress on the bottom surface. Moreover, the effective orifice area (EOA) of the leaflet is decreased with the concentration of force on the leaflet. This consequence produced the highest WSS at the bottom surface area particularly at the tips area compared to the top surface area. However, the situation is different for the diastole phase. The WSS on the leaflet at the top surface is higher than the leaflet at the bottom surface with the value of Pa and 25.3 Pa, respectively. During the diastole phase, the low velocity of the blood flow around the aortic arch produced higher pressure at the top surface of the leaflet. Leaflet top surface Leaflet bottom surface Figure 4: Leaflet top surface and leaflet bottom surface Figure 5 A : WSS of the leaflet bottom surface during systole phase Figure 5 B : WSS of the leaflet top surface during systole phase 38

7 Figure 6 A: WSS of the leaflet downward during diastole phase Figure 6 B: WSS of the leaflet upward during diastole phase Figure 7. Bar chart of maximum WSS of systole and diastole phase 39

8 C. Mass Flow Rate entering the Right and Left Subclavian and Common CarotidArteries The mass flow rate is estimated by comparing the normal valve opening with the aortic stenosis valve opening. Both cases have same flow rate at the inlet and the values obtained for the systolic phase is tabulated in Table 1. It can be observed that, the mass flow rate at brachiocephalic artery, common carotid, and the left subclavian has reduced particularly at the left subclavian artery. The reduction in the mass of blood entering the aorta sub arteries is about 2.9%. This reduction for the blood supply is significant because these sub-arteries provide blood to the important organs of our body such as head, neck and arm of the body. TABLE I. MASS FLOW RATE OF EACH BOUNDARY CONDITIONS Boundary Conditions/ Mass flow rate Normal opening (kg/s) 62.5degree opening (kg/s) Inlet Main Outlet Brachiocephalic Artery Common Carotid Left Subclavian IV. CONCLUSION The severity of aortic stenosis is explored in this study. The inappropriate aortic valve opening during the systole and diastole phases lead to the inefficient blood flow in the aortic arch branches. Increased velocity at the aortic branches with reduction in the mass flow rate is observed. Moreover, the bottom surface of the leaflet during the systole phase reported the highest WSS. This study has shown the usage of MRI and CFD in investigating the relationship between aortic stenosis and the hemodynamics effect of the blood flow. 40

9 REFERENCES [1] S. S. Subcommittee, AHA statistical update, Circulation, vol. 115, pp. e69 e171, [2] 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 , [3] V. T. Nkomo, J. M. Gardin, T. N. Skelton, J. S. Gottdiener, C. G. Scott, and M. Enriquez-Sarano, Burden of valvular heart diseases: a population-based study, The Lancet, vol. 368, no. 9540, pp , [4] R. L. Osnabrugge, D. Mylotte, S. J. Head, N. M. Van Mieghem, V. T. Nkomo, C. M. LeReun, A. J. Bogers, N. Piazza, and A. P. Kappetein, Aortic Stenosis in the Elderly: Disease Prevalence and Number of Candidates for Transcatheter Aortic Valve Replacement: A meta-analysis and modelling study, Journal of the American College of Cardiology, [5] M. A. Clark, F. G. Duhay, A. K. Thompson, M. J. Keyes, L. G. Svensson, R. O. Bonow, B. T. Stockwell, and D. J. Cohen, Clinical and economic outcomes after surgical aortic valve replacement in Medicare patients, Risk management and healthcare policy, vol. 5, p. 117, [6] R. van Loon, Towards computational modelling of aortic stenosis, International Journal for Numerical Methods in Biomedical Engineering, vol. 26, no. 3 4, pp , [7] M. Oshima, R. Torii, and T. Takagi, Image-based simulation of blood flow and arterial wall interaction for cerebral aneurysms. Springer, [8] C. A. Figueroa, I. E. Vignon-Clementel, K. E. Jansen, T. J. Hughes, and C. A. Taylor, A coupled momentum method for modeling blood flow in three-dimensional deformable arteries, Computer methods in applied mechanics and engineering, vol. 195, no. 41, pp , [9] I. E. Vignon-Clementel, C. Alberto Figueroa, K. E. Jansen, and C. A. Taylor, Outflow boundary conditions for three-dimensional finite element modeling of blood flow and pressure in arteries, Computer methods in applied mechanics and engineering, vol. 195, no. 29, pp , [10] A. S. Khader, M. Zubair, R. B. Pai, V. Rao, and G. S. Kamath, A Comparative Study of Transient Flow through Cerebral Aneurysms using CFD, World Academy of Science, Engineering and Technology, no. 60, pp , [11] G. Marom, H.-S. Kim, M. Rosenfeld, E. Raanani, and R. Haj-Ali, Fully coupled fluid-structure interaction model of congenital bicuspid aortic valves: effect of asymmetry on hemodynamics, Medical \& biological engineering \& computing, pp. 1 10, [12] G. Marom, H.-S. Kim, M. Rosenfeld, E. Raanani, and R. Haj-Ali, Effect of asymmetry on hemodynamics in fluid-structure interaction model of congenital bicuspid aortic valves, in Engineering in Medicine and Biology Society (EMBC), 2012 Annual International Conference of the IEEE, 2012, pp [13] H. A. Dwyer, P. B. Matthews, A. Azadani, N. Jaussaud, L. Ge, T. S. Guy, and E. E. Tseng, Computational fluid dynamics simulation of transcatheter aortic valve degeneration, Interactive cardiovascular and thoracic surgery, vol. 9, no. 2, pp ,

10 [14] A. Vincentelli, S. Susen, T. Le Tourneau, I. Six, O. Fabre, F. Juthier, A. Bauters, C. Decoene, J. Goudemand, A. Prat, and others, Acquired von Willebrand syndrome in aortic stenosis, New England Journal of Medicine, vol. 349, no. 4, pp , [15] K. B. Chandran, Role of computational simulations in heart valve dynamics and design of valvular prostheses, Cardiovascular engineering and technology, vol. 1, no. 1, pp , [16] M. Thubrikar, J. Deck, J. Aouad, and S. Nolan, Role of mechanical stress in calcification of aortic bioprosthetic valves., The Journal of thoracic and cardiovascular surgery, vol. 86, no. 1, pp ,