The influence of different inflow configurations on computational fluid dynamics in a novel three-leaflet mechanical heart valve prosthesis

The influence of different inflow configurations on computational fluid dynamics in a novel... Abstract OBJECTIVES A novel mechanical heart valve was developed with a special focus on avoiding anticoagulation. Computational fluid dynamics were used for the research design. Here, the effect of different anatomical inflow geometries on flow characteristics is evaluated. METHODS Flow and pressure simulations were performed on a novel 3-leaflet mechanical heart valve in a fully open position at 2 flow rates related to the peak and end-systolic flow. The computational fluid dynamics model was designed according to 4 different (1 cylindrical, 3 conical with increasing diameter) anatomical configurations of the left ventricular outflow tract derived from an inverse heart model. RESULTS With increasing inflow diameter, the flow velocity decreased for both flow rates, from 1543 mm/s in cylindrical configuration to 1475 mm/s in conical configuration for a flow rate of 18 l/min. However, there was no further decrease for the inflow diameters 38 and 48 mm. The velocity profile became broader with increasing inflow diameter and the maximal pressure decreased. At the leading edge, velocity almost stagnated, while the pressure increased and the reflection point moved downstream. No occurrence of dead space was observed with the different configurations and flow rates. CONCLUSIONS An analysis of different anatomical inflow configurations by computational fluid dynamic simulations showed a more homogenous velocity profile and lower flow velocity values with increasing inflow diameter up to 38 mm in this novel 3-leaflet mechanical heart valve. Mechanical heart valve, Computational fluid dynamics, Inflow characteristics INTRODUCTION Mechanical heart valve prostheses have the unsurmountable advantage of lifelong durability. However, the main drawback is the need for anticoagulation which limits its clinical applicability [1], although these valves seem to have a survival advantage compared to biological heart valve prostheses [2, 3]. Thus a new mechanical heart valve without the need for anticoagulation would open new windows of opportunity in the field of valve surgery. In the Department of Cardiac and Thoracic Vascular Surgery of the University of Lübeck, a novel 3-leaflet mechanical prosthesis was designed with the aim of avoiding anticoagulation (Fig. 1). In the development process, special focus was directed toward reducing the risk for clot formation, both by material improvements and also fluid dynamical optimizations. For example, hinges of the novel mechanical heart valve were placed in the centre of systolic flow, for maximal washout and to avoid regurgitation through pivots which have a high potential for thrombogenicity [4]. Also, leaflet geometry was optimized to avoid regions of turbulent areas or vortex formation which are critical for thrombus generation. Figure 1: View largeDownload slide On the top left the manufactured valve is shown. Schematic drawing of the top view of novel 3-leaflet mechanical heart valve prosthesis (Patent US9775708B2) (top right) and cross section A-A (bottom) with the leaflet (B), the leading edge (C) and trailing edge of the leaflet (D); (E) the valve centre area; (F) the peripheral area. Figure 1: View largeDownload slide On the top left the manufactured valve is shown. Schematic drawing of the top view of novel 3-leaflet mechanical heart valve prosthesis (Patent US9775708B2) (top right) and cross section A-A (bottom) with the leaflet (B), the leading edge (C) and trailing edge of the leaflet (D); (E) the valve centre area; (F) the peripheral area. Computational fluid dynamics (CFD) is a valuable tool in the development of such heart valve prostheses [5]. In this regard, CFD can be used to visualize and analyse the behaviour of the blood travelling through such a barrier in the flow [6, 7]. Velocity vectors and pressure profiles can be shown and optimized to reduce energy loss and the risk of thrombosis [8]. In the literature several CFD studies for mechanical heart valves are available; however, most of them do not apply anatomical configurations of the left ventricular outflow tract (LVOT) [9–12]. Thus, the aim of this study was to investigate a model for different anatomical inflow characteristics within a novel 3-leaflet mechanical heart valve with CFD. MATERIALS AND METHODS A three-dimensional (3D) computer-aided design-model of the novel 3-leaflet mechanical heart valve was created and also used for CFD simulations. The valve was considered to be fully open during most of the forward flow phase of the heart beat at an opening angle of 84°. Both the computer-aided design-model and the CFD simulations were performed with NX (Siemens Industry Software GmbH, Köln, Germany). Blood was modelled to be a Newtonian, incompressible and steady fluid with a density of ρ = 1.05 × 10³ kg/m³, dynamic viscosity of µ = 3.5 × 10−3 Ns/m2 and body temperature of 37.3°C [13]. Flow motion in the simulation was calculated by using Navier–Stokes equation [10, 14]. Flow rates at the inflow were considered to be 18 l/min and 3 l/min, with regard to the maximum systolic and a near end-systole flow. Aortic pressure was 125 mmHg, representing the peak pressure in mid-systole. To investigate the impact of the varying anatomy of the LVOT, different inflow configurations were implemented: a uniform (cylindrical) geometry with same inflow and outflow diameter (d = 22 mm) and 3 conical models with greater diameters of 28, 38 and 48 mm at a distance of 16 mm upstream the valve entrance. This resulted in inflow angles of 10.6°, 26.6° and 39.1°, respectively (Fig. 2). The anatomical configurations were based on measurements of an inverse heart model. This model was created with a pressurized porcine heart of normal size and filled with a resin (Technovit 7143 liquid, Heraeus Kulzer GmbH, Hanau, Germany). Although the LVOT was not circular but somewhat elliptical with the short diameter of 28 mm and the long diameter of 38 mm, conical shapes were used for simulation purposes. To simulate a dilated ventricle which is often associated with severe valve diseases, the 48 mm configuration was included. Figure 2: View largeDownload slide Illustration of different inflow configurations tested in the CFD model. α: inflow angle; CFD: computational fluid dynamics. Figure 2: View largeDownload slide Illustration of different inflow configurations tested in the CFD model. α: inflow angle; CFD: computational fluid dynamics. The outflow geometry remained unchanged for all simulations. Here, a sinus configuration with an entrance diameter of 28 mm and an outlet diameter of 22 mm were designed, also based on the inverse heart model and the ascending aorta with a length of about 100 mm to attain fully steady state flow. The basis of CFD simulations is the element size and type of grates which fit to the model. In order to mesh the leaflets edge, a 2-dimensional curved surface type TRI6 with size of 0.2 mm was implemented. For the meshing process, a 3D control volume mesh with about 9.2 million meshes and a mesh size of 0.5 mm (type TET10) was used to enhance the accuracy of data by covering and matching whole surface and volumes. For more accuracy of flow visualization from leaflets surface toward the control volume, 5 layers of meshing surfaces with the distance of 0.2 mm were applied. CFD simulation results were presented in velocity vector and total pressure distributions. Simulations were analysed in a plane through the centre of the valve and at the leading and trailing edge of the leaflet (Fig. 1). RESULTS Velocity profiles Simulations with a flow rate of 18 l/min resulted in peak velocities from 1543 mm/s in the cylindrical configuration down to 1475 mm/s in the conical configuration (Fig. 3, Table 1); 3 l/min flow showed also a reduction of the peak velocity with increasing inflow diameter, from approximately 273 mm/s to 245 mm/s (Table 1). Highest velocities were found in the centre of the valve orifice and in the edge area behind the leaflets. The velocity profile became more homogenous with increasing diameter and more pronounced at 18 l/min (Fig. 3). Table 1: Maximum velocities and pressure for all inflow configurations and both simulations (3 and 18 l/min), found in the centre of the valve orifice Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Table 1: Maximum velocities and pressure for all inflow configurations and both simulations (3 and 18 l/min), found in the centre of the valve orifice Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Figure 3: View largeDownload slide Velocity profiles at the centre line (A-A in Fig. 1) of the novel 3-leaflet mechanical heart valve for different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm, see Fig. 2) at 18 l/min simulated flow. Maximal velocity is reduced in the centre of the valve from (A) to (D) (red area) and as a result, a more homogenous profile is obtained, with nearly similar flow distribution between (C) and (D). Figure 3: View largeDownload slide Velocity profiles at the centre line (A-A in Fig. 1) of the novel 3-leaflet mechanical heart valve for different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm, see Fig. 2) at 18 l/min simulated flow. Maximal velocity is reduced in the centre of the valve from (A) to (D) (red area) and as a result, a more homogenous profile is obtained, with nearly similar flow distribution between (C) and (D). At the leading edge of the leaflet, smaller angles of streamlines were calculated for a larger diameter. Differences in the flow velocities at the leading edge were shown in Table 2. Analysis of the reflection point showed a variance in position for the different configurations. For the cylindrical configuration this point was near the bottom of the leaflet. For simulations with conical configurations, the stagnation point slightly travelled to the backside of the leaflet (Fig. 3). Table 2: Velocity values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7    Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7  Table 2: Velocity values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7    Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7  In contrast, velocity at the trailing edge increased with an increasing inflow diameter, from approximately 875 mm/s to 984 mm/s for 18 l/min (Fig. 3, Table 2). Further, the low flow area at that point decreased. Interestingly, there were no differences for inflow configurations of 38 mm and 48 mm. Similar results were obtained for simulations with 3 l/min (Table 2). Total pressure Simulations for a flow rate of 18 l/min showed that the pressure increased at the valve entrance for increased inflow diameters (Fig. 4). On the other hand, pressure at the centre of the valve orifice decreased (Table 1). With increasing inflow diameter, the pressure profile became broader towards the walls of the inflow model and also within the valve. Analysis for flow rates of 3 l/min yielded similar results (Table 1). Figure 4: View largeDownload slide Pressure profiles of different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm) at the centre line of the novel 3-leaflet mechanical heart valve at 18 l/min (see A-A in Fig. 1). Pressure decreased for increasing inflow diameter from (A) to (D) in the centre of the valve, but increased in the periphery. Figure 4: View largeDownload slide Pressure profiles of different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm) at the centre line of the novel 3-leaflet mechanical heart valve at 18 l/min (see A-A in Fig. 1). Pressure decreased for increasing inflow diameter from (A) to (D) in the centre of the valve, but increased in the periphery. At the leading edge, a small spot of high pressure was observed (Fig. 5, Table 3) in all configurations and flow rates. Similar observations were made for the trailing edge, where a low pressure spot was determined (Table 3). Downstream of the leaflets trailing edge, the pressure increased for larger inflow diameters (Fig. 4), especially at the periphery compared to the centre line. The pressure distribution was similar for inflow diameters of 38 and 48 mm. Table 3: Pressure values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4    Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4  Table 3: Pressure values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4    Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4  Figure 5: View largeDownload slide Static pressure distribution in the vicinity of the leaflet. At the leading edge of the leaflet, a spot of high pressure is observed. The arrow indicates the point and the direction of the flow streamlines. This spot correlates with the stagnation point in the velocity profiles. (A) Cylindrical configuration, (B) conical configuration. Figure 5: View largeDownload slide Static pressure distribution in the vicinity of the leaflet. At the leading edge of the leaflet, a spot of high pressure is observed. The arrow indicates the point and the direction of the flow streamlines. This spot correlates with the stagnation point in the velocity profiles. (A) Cylindrical configuration, (B) conical configuration. DISCUSSION In this study we could show that an increase in the left ventricular outflow diameter of up to 38 mm showed a more homogenous velocity profile and lower flow velocity. Further the application of different inflow configurations (cylindrical/conical shapes) on CFD simulations in a novel 3-leaflet mechanical heart valve led to alterations of flow velocity and pressure characteristics, which are useful for design development. Different inflow configurations Different inflow shapes were adapted to varying physiological configurations (Fig. 2). In many CFD studies of heart valves, only the outflow configuration, which is the aortic root with 3 sinuses, was anatomically shaped but the inflow remained cylindrical [9–12]. The ventricle of a heart, however, is a cavity with a diameter larger than the aortic valve annulus and root, and thus the LVOT obviously has a large conical shape, from which flow lines are differently directed towards the valve compared to a tube-like inflow. This may influence the pressure and flow distribution, which may have an effect on leaflet motion, thrombus formation and shear stress. Flow characteristics In fact, changing the often applied uniform inflow in CFD simulation to a more anatomical [9–12], conical configuration resulted in a reduced peak flow velocity, and the velocity profile became more homogenous in the valve orifice of the new 3-leaflet mechanical heart valve. This is an interesting finding as one usually might assume that a more constricted flow profile for a conical inflow, with even higher velocities in the centre but the simulation showed the opposite. As velocity is directly interrelated with the pressure drop over the valve and thus energy loss, the observed change in profile depending on inflow configuration should be further investigated using actual flow visualizations like Particle Image Velocimetry or 4-dimensional flow magnetic resonance imaging, because it may influence prosthetic valve performance and thrombus generation. No further difference was found for the inflow diameter of 48 mm. More homogenous velocity distribution could be an indicator for lower shear stress, because differences of adjacent flow lines are obviously smaller. Shear stress is a known risk factor for blood damage causing haemolysis and leading to thrombus formation [11]. In our study, we observed a decrease in flow velocities at increasing inflow diameters. This may in turn have a positive effect on thrombus generation [15]. The changed inflow angle also altered the velocity profile at the leading edge of the valve’s leaflet (Fig. 3). A stagnation point resulted and shifted slightly downstream at the back side of the leaflet with increasing LVOT diameter. Due to this shift, the operating force changed and might affect the leaflets’ motion, supporting leaflet closure. At the leading edge, velocity decreased due to its impact on the leaflet as a barrier in the flow and also the pressure decreased at this point with increasing diameter. The area of high pressure impact increases and this point moved slightly downstream for larger inflow diameters due to the changed inflow angle of velocity vectors. On the other hand, velocity in the orifice at the back of the leaflet, especially close to the trailing edge, simultaneously increased for the conical configurations (Fig. 3) which forced the leaflet to open and might compensate the increased forces at the leading edge. In this study, however, only CFD simulation was applied, which did not incorporate fluid-structure interaction, and so conclusions concerning these effects need confirmation by functional analysis. Limitations Some limitations of the study should be considered. Simulation was only done in a steady state; however, from the aortic flow course 2 steady states with the most expected significance were chosen. Although the systolic flow may have the potential for washing all valve components and thus avoiding stagnation zones, the diastolic flow phase may also be important in this respect. This is important in the modern bileaflet mechanical heart valves; however, our valve design is completely different from contemporary designs which have pivots washed during diastole, causing the same kind of regurgitation. In our design there is no diastole leakage through the pivots. The diastole flow phase is the subject of further analysis. Simulations were performed for the fully open position because no movement of the leaflets was detected in previous in vitro measurements, even though velocity might have an impact on the movement what might change the profile. No obvious differences between conical inflow diameters of 38 and 48 mm were found, and therefore no larger configurations were chosen. We only demonstrated inflow configurations with circular shapes; however, the physiologically more-elliptical shape of the LVOT may also change the results. In general, there is a great variety of the LVOT geometries; nevertheless, the applied inflow configurations well reflect physiological geometries and also flow behaviours which are similar to those found in magnetic resonance velocity mapping of healthy volunteers [16, 17]. In the model, the downstream geometry was designed as a straight tube, whereas the aorta has a curved geometry, which would also have an influence on velocity vectors and pressure distribution, and the compliance of the downstream aorta, in particular, was also not considered. Furthermore, it must be considered that many other factors may influence flow velocity through the aortic valve, including coronary height and flow, size and shape of ascending aorta, size and shape of sinus of Valsalva that were not investigated in our study. However, the conditions in all experiments were the same, except inflow geometry. CONCLUSION In conclusion, CFD models using different anatomical inflow geometries revealed differences in cylindrical and conical configurations for the novel 3-leaflet mechnical heart valve. Furthermore, it was shown that increasing inflow diameters led to decreased flow velocity and maximal pressue, which may have a positive effect on shear stress and thromogenicity of the prosthesis. The geometry of the LVOT is much more complex in the wide range of clinical configurations and especially the dynamic behaviour could have influence on flow dynamics. What we focussed on was only a small fraction of possible clinical conditions. Funding This work was supported by the German Federal Ministry of Education and Research [grant number 13GW055B]. Conflict of interest: Hans-Hinrich Sievers is patent holder of the novel mechanical heart valve described in this article. REFERENCES 1 Beckmann A, Funkat AK, Lewandowski J, Frie M, Ernst M, Hekmat K. German Heart Surgery Report 2016: the Annual Updated Registry of the German Society for Thoracic and Cardiovascular Surgery. Thorac Cardiovasc Surg  2017; 65: 505– 18. Google Scholar CrossRef Search ADS PubMed  2 Goldstone AB, Chiu P, Baiocchi M, Lingala B, Patrick WL, Fischbein MP et al.   Mechanical or biologic prostheses for aortic-valve and mitral-valve replacement. N Engl J Med  2017; 377: 1847– 57. Google Scholar CrossRef Search ADS PubMed  3 Chiang YP, Chikwe J, Moskowitz AJ, Itagaki S, Adams DH, Egorova NN. Survival and long-term outcomes following bioprosthetic vs mechanical aortic valve replacement in patients aged 50 to 69 years. JAMA  2014; 312: 1323. Google Scholar CrossRef Search ADS PubMed  4 Alemu Y, Girdhar G, Xenos M, Sheriff J, Jesty J, Einav S et al.   Design optimization of a mechanical heart valve for reducing valve thrombogenicity—a case study with ATS valve. 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Google Scholar CrossRef Search ADS PubMed  14 Grigioni M, Daniele C, Del Gaudio C, Morbiducci U, Balducci A, D’Avenio G et al.   Three-dimensional numeric simulation of flow through an aortic bileaflet valve in a realistic model of aortic root. ASAIO J  2005; 51: 176– 83. Google Scholar CrossRef Search ADS PubMed  15 Loughnane S, Quinlan NJ. High-resolution measurements of velocity and shear stress in leakage jets from bileaflet mechanical heart valve hinge models. J Biomech Eng  2015; doi:10.1115/1.4031350. 16 Kilner PJ, Yang GZ, Mohiaddin RH, Firmin DN, Longmore DB. Helical and retrograde secondary flow patterns in the aortic arch studied by three-directional magnetic resonance velocity mapping. Circulation  1993; 88: 2235– 47. Google Scholar CrossRef Search ADS PubMed  17 Kilner PJ, Yang GZ, Wilkes AJ, Mohiaddin RH, Firmin DN, Yacoub MH. Asymmetric redirection of flow through the heart. Nature  2000; 404: 759– 61. Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Interactive CardioVascular and Thoracic Surgery Oxford University Press

The influence of different inflow configurations on computational fluid dynamics in a novel three-leaflet mechanical heart valve prosthesis

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© The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved.
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Abstract

Abstract OBJECTIVES A novel mechanical heart valve was developed with a special focus on avoiding anticoagulation. Computational fluid dynamics were used for the research design. Here, the effect of different anatomical inflow geometries on flow characteristics is evaluated. METHODS Flow and pressure simulations were performed on a novel 3-leaflet mechanical heart valve in a fully open position at 2 flow rates related to the peak and end-systolic flow. The computational fluid dynamics model was designed according to 4 different (1 cylindrical, 3 conical with increasing diameter) anatomical configurations of the left ventricular outflow tract derived from an inverse heart model. RESULTS With increasing inflow diameter, the flow velocity decreased for both flow rates, from 1543 mm/s in cylindrical configuration to 1475 mm/s in conical configuration for a flow rate of 18 l/min. However, there was no further decrease for the inflow diameters 38 and 48 mm. The velocity profile became broader with increasing inflow diameter and the maximal pressure decreased. At the leading edge, velocity almost stagnated, while the pressure increased and the reflection point moved downstream. No occurrence of dead space was observed with the different configurations and flow rates. CONCLUSIONS An analysis of different anatomical inflow configurations by computational fluid dynamic simulations showed a more homogenous velocity profile and lower flow velocity values with increasing inflow diameter up to 38 mm in this novel 3-leaflet mechanical heart valve. Mechanical heart valve, Computational fluid dynamics, Inflow characteristics INTRODUCTION Mechanical heart valve prostheses have the unsurmountable advantage of lifelong durability. However, the main drawback is the need for anticoagulation which limits its clinical applicability [1], although these valves seem to have a survival advantage compared to biological heart valve prostheses [2, 3]. Thus a new mechanical heart valve without the need for anticoagulation would open new windows of opportunity in the field of valve surgery. In the Department of Cardiac and Thoracic Vascular Surgery of the University of Lübeck, a novel 3-leaflet mechanical prosthesis was designed with the aim of avoiding anticoagulation (Fig. 1). In the development process, special focus was directed toward reducing the risk for clot formation, both by material improvements and also fluid dynamical optimizations. For example, hinges of the novel mechanical heart valve were placed in the centre of systolic flow, for maximal washout and to avoid regurgitation through pivots which have a high potential for thrombogenicity [4]. Also, leaflet geometry was optimized to avoid regions of turbulent areas or vortex formation which are critical for thrombus generation. Figure 1: View largeDownload slide On the top left the manufactured valve is shown. Schematic drawing of the top view of novel 3-leaflet mechanical heart valve prosthesis (Patent US9775708B2) (top right) and cross section A-A (bottom) with the leaflet (B), the leading edge (C) and trailing edge of the leaflet (D); (E) the valve centre area; (F) the peripheral area. Figure 1: View largeDownload slide On the top left the manufactured valve is shown. Schematic drawing of the top view of novel 3-leaflet mechanical heart valve prosthesis (Patent US9775708B2) (top right) and cross section A-A (bottom) with the leaflet (B), the leading edge (C) and trailing edge of the leaflet (D); (E) the valve centre area; (F) the peripheral area. Computational fluid dynamics (CFD) is a valuable tool in the development of such heart valve prostheses [5]. In this regard, CFD can be used to visualize and analyse the behaviour of the blood travelling through such a barrier in the flow [6, 7]. Velocity vectors and pressure profiles can be shown and optimized to reduce energy loss and the risk of thrombosis [8]. In the literature several CFD studies for mechanical heart valves are available; however, most of them do not apply anatomical configurations of the left ventricular outflow tract (LVOT) [9–12]. Thus, the aim of this study was to investigate a model for different anatomical inflow characteristics within a novel 3-leaflet mechanical heart valve with CFD. MATERIALS AND METHODS A three-dimensional (3D) computer-aided design-model of the novel 3-leaflet mechanical heart valve was created and also used for CFD simulations. The valve was considered to be fully open during most of the forward flow phase of the heart beat at an opening angle of 84°. Both the computer-aided design-model and the CFD simulations were performed with NX (Siemens Industry Software GmbH, Köln, Germany). Blood was modelled to be a Newtonian, incompressible and steady fluid with a density of ρ = 1.05 × 10³ kg/m³, dynamic viscosity of µ = 3.5 × 10−3 Ns/m2 and body temperature of 37.3°C [13]. Flow motion in the simulation was calculated by using Navier–Stokes equation [10, 14]. Flow rates at the inflow were considered to be 18 l/min and 3 l/min, with regard to the maximum systolic and a near end-systole flow. Aortic pressure was 125 mmHg, representing the peak pressure in mid-systole. To investigate the impact of the varying anatomy of the LVOT, different inflow configurations were implemented: a uniform (cylindrical) geometry with same inflow and outflow diameter (d = 22 mm) and 3 conical models with greater diameters of 28, 38 and 48 mm at a distance of 16 mm upstream the valve entrance. This resulted in inflow angles of 10.6°, 26.6° and 39.1°, respectively (Fig. 2). The anatomical configurations were based on measurements of an inverse heart model. This model was created with a pressurized porcine heart of normal size and filled with a resin (Technovit 7143 liquid, Heraeus Kulzer GmbH, Hanau, Germany). Although the LVOT was not circular but somewhat elliptical with the short diameter of 28 mm and the long diameter of 38 mm, conical shapes were used for simulation purposes. To simulate a dilated ventricle which is often associated with severe valve diseases, the 48 mm configuration was included. Figure 2: View largeDownload slide Illustration of different inflow configurations tested in the CFD model. α: inflow angle; CFD: computational fluid dynamics. Figure 2: View largeDownload slide Illustration of different inflow configurations tested in the CFD model. α: inflow angle; CFD: computational fluid dynamics. The outflow geometry remained unchanged for all simulations. Here, a sinus configuration with an entrance diameter of 28 mm and an outlet diameter of 22 mm were designed, also based on the inverse heart model and the ascending aorta with a length of about 100 mm to attain fully steady state flow. The basis of CFD simulations is the element size and type of grates which fit to the model. In order to mesh the leaflets edge, a 2-dimensional curved surface type TRI6 with size of 0.2 mm was implemented. For the meshing process, a 3D control volume mesh with about 9.2 million meshes and a mesh size of 0.5 mm (type TET10) was used to enhance the accuracy of data by covering and matching whole surface and volumes. For more accuracy of flow visualization from leaflets surface toward the control volume, 5 layers of meshing surfaces with the distance of 0.2 mm were applied. CFD simulation results were presented in velocity vector and total pressure distributions. Simulations were analysed in a plane through the centre of the valve and at the leading and trailing edge of the leaflet (Fig. 1). RESULTS Velocity profiles Simulations with a flow rate of 18 l/min resulted in peak velocities from 1543 mm/s in the cylindrical configuration down to 1475 mm/s in the conical configuration (Fig. 3, Table 1); 3 l/min flow showed also a reduction of the peak velocity with increasing inflow diameter, from approximately 273 mm/s to 245 mm/s (Table 1). Highest velocities were found in the centre of the valve orifice and in the edge area behind the leaflets. The velocity profile became more homogenous with increasing diameter and more pronounced at 18 l/min (Fig. 3). Table 1: Maximum velocities and pressure for all inflow configurations and both simulations (3 and 18 l/min), found in the centre of the valve orifice Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Table 1: Maximum velocities and pressure for all inflow configurations and both simulations (3 and 18 l/min), found in the centre of the valve orifice Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Inflow diameter (mm)  Maximum velocity for 3 l/min (mm/s)  Maximum pressure for 3 l/min (Pa)  Maximum velocity for 18 l/min (mm/s)  Maximum pressure for 18 l/min (Pa)  22  273.2  61.94  1543.2  1383  28  263.4  56.01  1499.4  1283  38  250.6  56.65  1476.2  1276  48  245.7  57.10  1475.4  1288  Figure 3: View largeDownload slide Velocity profiles at the centre line (A-A in Fig. 1) of the novel 3-leaflet mechanical heart valve for different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm, see Fig. 2) at 18 l/min simulated flow. Maximal velocity is reduced in the centre of the valve from (A) to (D) (red area) and as a result, a more homogenous profile is obtained, with nearly similar flow distribution between (C) and (D). Figure 3: View largeDownload slide Velocity profiles at the centre line (A-A in Fig. 1) of the novel 3-leaflet mechanical heart valve for different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm, see Fig. 2) at 18 l/min simulated flow. Maximal velocity is reduced in the centre of the valve from (A) to (D) (red area) and as a result, a more homogenous profile is obtained, with nearly similar flow distribution between (C) and (D). At the leading edge of the leaflet, smaller angles of streamlines were calculated for a larger diameter. Differences in the flow velocities at the leading edge were shown in Table 2. Analysis of the reflection point showed a variance in position for the different configurations. For the cylindrical configuration this point was near the bottom of the leaflet. For simulations with conical configurations, the stagnation point slightly travelled to the backside of the leaflet (Fig. 3). Table 2: Velocity values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7    Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7  Table 2: Velocity values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7    Velocity values for 3 l/min   Velocity values for 18 l/min   Leading edge (mm/s)  Trailing edge (mm/s)  Leading edge (mm/s)  Trailing edge (mm/s)  22  45.55  136.6  514.8  900.7  28  43.91  131.7  375.3  874.9  38  41.80  125.3  246.4  984.3  48  40.97  122.9  246.0  983.7  In contrast, velocity at the trailing edge increased with an increasing inflow diameter, from approximately 875 mm/s to 984 mm/s for 18 l/min (Fig. 3, Table 2). Further, the low flow area at that point decreased. Interestingly, there were no differences for inflow configurations of 38 mm and 48 mm. Similar results were obtained for simulations with 3 l/min (Table 2). Total pressure Simulations for a flow rate of 18 l/min showed that the pressure increased at the valve entrance for increased inflow diameters (Fig. 4). On the other hand, pressure at the centre of the valve orifice decreased (Table 1). With increasing inflow diameter, the pressure profile became broader towards the walls of the inflow model and also within the valve. Analysis for flow rates of 3 l/min yielded similar results (Table 1). Figure 4: View largeDownload slide Pressure profiles of different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm) at the centre line of the novel 3-leaflet mechanical heart valve at 18 l/min (see A-A in Fig. 1). Pressure decreased for increasing inflow diameter from (A) to (D) in the centre of the valve, but increased in the periphery. Figure 4: View largeDownload slide Pressure profiles of different inflow configurations (A = 22 mm, B = 28 mm, C = 38 mm, D = 48 mm) at the centre line of the novel 3-leaflet mechanical heart valve at 18 l/min (see A-A in Fig. 1). Pressure decreased for increasing inflow diameter from (A) to (D) in the centre of the valve, but increased in the periphery. At the leading edge, a small spot of high pressure was observed (Fig. 5, Table 3) in all configurations and flow rates. Similar observations were made for the trailing edge, where a low pressure spot was determined (Table 3). Downstream of the leaflets trailing edge, the pressure increased for larger inflow diameters (Fig. 4), especially at the periphery compared to the centre line. The pressure distribution was similar for inflow diameters of 38 and 48 mm. Table 3: Pressure values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4    Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4  Table 3: Pressure values at the leading and trailing edge of the leaflet for all inflow configurations and both simulations (3 and 18 l/min)   Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4    Pressure values for 3 l/min   Pressure values for 18 l/min   Leading edge (Pa)  Trailing edge (Pa)  Leading edge (Pa)  Trailing edge (Pa)  22  59.52  14.88  1267  472.7  28  56.69  14.17  1215  452.5  38  55.92  13.98  1184  462.8  48  56.66  18.88  1174  436.4  Figure 5: View largeDownload slide Static pressure distribution in the vicinity of the leaflet. At the leading edge of the leaflet, a spot of high pressure is observed. The arrow indicates the point and the direction of the flow streamlines. This spot correlates with the stagnation point in the velocity profiles. (A) Cylindrical configuration, (B) conical configuration. Figure 5: View largeDownload slide Static pressure distribution in the vicinity of the leaflet. At the leading edge of the leaflet, a spot of high pressure is observed. The arrow indicates the point and the direction of the flow streamlines. This spot correlates with the stagnation point in the velocity profiles. (A) Cylindrical configuration, (B) conical configuration. DISCUSSION In this study we could show that an increase in the left ventricular outflow diameter of up to 38 mm showed a more homogenous velocity profile and lower flow velocity. Further the application of different inflow configurations (cylindrical/conical shapes) on CFD simulations in a novel 3-leaflet mechanical heart valve led to alterations of flow velocity and pressure characteristics, which are useful for design development. Different inflow configurations Different inflow shapes were adapted to varying physiological configurations (Fig. 2). In many CFD studies of heart valves, only the outflow configuration, which is the aortic root with 3 sinuses, was anatomically shaped but the inflow remained cylindrical [9–12]. The ventricle of a heart, however, is a cavity with a diameter larger than the aortic valve annulus and root, and thus the LVOT obviously has a large conical shape, from which flow lines are differently directed towards the valve compared to a tube-like inflow. This may influence the pressure and flow distribution, which may have an effect on leaflet motion, thrombus formation and shear stress. Flow characteristics In fact, changing the often applied uniform inflow in CFD simulation to a more anatomical [9–12], conical configuration resulted in a reduced peak flow velocity, and the velocity profile became more homogenous in the valve orifice of the new 3-leaflet mechanical heart valve. This is an interesting finding as one usually might assume that a more constricted flow profile for a conical inflow, with even higher velocities in the centre but the simulation showed the opposite. As velocity is directly interrelated with the pressure drop over the valve and thus energy loss, the observed change in profile depending on inflow configuration should be further investigated using actual flow visualizations like Particle Image Velocimetry or 4-dimensional flow magnetic resonance imaging, because it may influence prosthetic valve performance and thrombus generation. No further difference was found for the inflow diameter of 48 mm. More homogenous velocity distribution could be an indicator for lower shear stress, because differences of adjacent flow lines are obviously smaller. Shear stress is a known risk factor for blood damage causing haemolysis and leading to thrombus formation [11]. In our study, we observed a decrease in flow velocities at increasing inflow diameters. This may in turn have a positive effect on thrombus generation [15]. The changed inflow angle also altered the velocity profile at the leading edge of the valve’s leaflet (Fig. 3). A stagnation point resulted and shifted slightly downstream at the back side of the leaflet with increasing LVOT diameter. Due to this shift, the operating force changed and might affect the leaflets’ motion, supporting leaflet closure. At the leading edge, velocity decreased due to its impact on the leaflet as a barrier in the flow and also the pressure decreased at this point with increasing diameter. The area of high pressure impact increases and this point moved slightly downstream for larger inflow diameters due to the changed inflow angle of velocity vectors. On the other hand, velocity in the orifice at the back of the leaflet, especially close to the trailing edge, simultaneously increased for the conical configurations (Fig. 3) which forced the leaflet to open and might compensate the increased forces at the leading edge. In this study, however, only CFD simulation was applied, which did not incorporate fluid-structure interaction, and so conclusions concerning these effects need confirmation by functional analysis. Limitations Some limitations of the study should be considered. Simulation was only done in a steady state; however, from the aortic flow course 2 steady states with the most expected significance were chosen. Although the systolic flow may have the potential for washing all valve components and thus avoiding stagnation zones, the diastolic flow phase may also be important in this respect. This is important in the modern bileaflet mechanical heart valves; however, our valve design is completely different from contemporary designs which have pivots washed during diastole, causing the same kind of regurgitation. In our design there is no diastole leakage through the pivots. The diastole flow phase is the subject of further analysis. Simulations were performed for the fully open position because no movement of the leaflets was detected in previous in vitro measurements, even though velocity might have an impact on the movement what might change the profile. No obvious differences between conical inflow diameters of 38 and 48 mm were found, and therefore no larger configurations were chosen. We only demonstrated inflow configurations with circular shapes; however, the physiologically more-elliptical shape of the LVOT may also change the results. In general, there is a great variety of the LVOT geometries; nevertheless, the applied inflow configurations well reflect physiological geometries and also flow behaviours which are similar to those found in magnetic resonance velocity mapping of healthy volunteers [16, 17]. In the model, the downstream geometry was designed as a straight tube, whereas the aorta has a curved geometry, which would also have an influence on velocity vectors and pressure distribution, and the compliance of the downstream aorta, in particular, was also not considered. Furthermore, it must be considered that many other factors may influence flow velocity through the aortic valve, including coronary height and flow, size and shape of ascending aorta, size and shape of sinus of Valsalva that were not investigated in our study. However, the conditions in all experiments were the same, except inflow geometry. CONCLUSION In conclusion, CFD models using different anatomical inflow geometries revealed differences in cylindrical and conical configurations for the novel 3-leaflet mechnical heart valve. Furthermore, it was shown that increasing inflow diameters led to decreased flow velocity and maximal pressue, which may have a positive effect on shear stress and thromogenicity of the prosthesis. The geometry of the LVOT is much more complex in the wide range of clinical configurations and especially the dynamic behaviour could have influence on flow dynamics. What we focussed on was only a small fraction of possible clinical conditions. Funding This work was supported by the German Federal Ministry of Education and Research [grant number 13GW055B]. Conflict of interest: Hans-Hinrich Sievers is patent holder of the novel mechanical heart valve described in this article. 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Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Interactive CardioVascular and Thoracic SurgeryOxford University Press

Published: Mar 30, 2018

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