Analysis of the GPATD : Geometrical Influence on Blood Clot Extraction using CFD Simulation

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1 2014 UKSim-AMSS 16th International Conference on Computer Modelling and Simulation Analysis of the GPATD : Geometrical Influence on Blood Clot Extraction using CFD Simulation Gregorio Romero, M.Luisa Martínez CITEF Railway Technology Research Centre Universidad Politécnica de Madrid Spain {gregorio.romero, luisa.mtzmuneta}@upm.es Gillian Pearce Keele University United Kingdom gpearce2011@gmail.com Julian Wong Department of Cardiac, Thoracic & Vascular Surgery National University Heart Centre Singapore julian_wong@nuhs.edu.sg Abstract In this work, we present the study of the influence of geometry on an experimental device recently developed in the UK, called the GP Thrombus Aspiration Device (GPTAD). This device has been designed to remove blood clots without the need to make contact with the clot itself, thereby potentially reducing the risk of problems such as downstream embolisation. To obtain the minimum pressure necessary to extract the clot and to optimize blood clot extraction, we simulate the performance of the GPTAD analyzing the pressure losses and pressure distribution taking into account the geometrical effects. Previous full models have been undertaken using the Bond Graph technique. However in this paper we include the analysis of the influence of the of the geometry device using Computational Fluid Dynamics simulation. We model a range of diameters for the GPTAD considering a 95% occlusion case, different lengths and diameters of catheter, and different GP geometric characteristics. In each case we determine the pressure losses and distribution just in front of the attached blood clot. Keywords-Biomedical engineering, Thrombectomy Devices, Computer Fluid Dynamics. I. INTRODUCTION The World Health Organization reports that 15 million people worldwide suffer stroke; and of these, 5 million die and a further 5 million are left permanently disabled, many severely impaired. Consequently stroke is a major cause of mortality world-wide. Most strokes are caused by a blood clot that occludes an artery in the cerebral circulation. Thrombolytic agents such as Alteplase are used to dissolve blood clots that arise in the cerebral arteries of the brain but there are limitations on their use. Recently screening for patients at risk of strokes and TIA s has come into being. If such plaques are detected in the carotid arteries (by Ultrasound), a Carotid endarterectomy (CEA) - a surgical operation - may be performed to remove the occlusive plaque. Over the past decade, other methods of treatment have been developed which include Thrombectomy Devices e.g. the GP Thrombus Aspiration Device (GPTAD - fig. 1) [1]. Such devices have the potential to be used as an alternative to thrombolytic agents or in conjunction with them to extract clots inwe different arteries e.g. in the middle cerebral artery of the brain, carotid, popliteal artery, etc. A clot of blood may also become attached to the plaque (deposition of fats and lipids that may arise within arteries) in a distal artery, and subsequently become detached and travel to the cerebral circulation giving rise to a stroke. In the case of 100% occlusion, it causes total blockage of the artery. In this work, we present the analysis and study of different GPTAD geometries and show it to be a highly effective method of simulating the device under a variety of conditions of percentage arterial occlusions. We take into account new factors such as the non return of the flow that exists between catheter and artery or the distance to the blood clot in the modelling. Such modelling is useful in optimizing the GPTAD and predicting the result of clot extraction under a variety of conditions. II. DEVICE RE-MODELLING GENERAL STUDY The objective of this work is to introduce a new model that can be used and assist in the final design of the GPTAD, investigating the potential performance of the device under different conditions of geometry and preparation. The method chosen initially for the representation and simulation in different studies undertaken previously by the authors was the Bond Graph technique [2], which allowed assimilating and comparing the model to an electric circuit made up of inductances, capacitances and resistances. Using this technique it was possible to obtain the results in a simple way by evaluating flows and efforts that join and connect the components of the model. Nevertheless, some factors like pressures distribution over the blood clot or total pressure losses are not easy to analyze by using the previous model [3]. Consequently, we need to study other factors by using other techniques like Computer Fluid Dynamics (CFD) /14 $ IEEE DOI /UKSim

2 Figure 1. Representation of the GPTAD sited in the artery In the work presented in this paper, some catheter diameters and distances between the GPTAD device and the blood clot are compared for a 95% occlusion to highlight differences between these situations. We select this level of occlusion since many thrombosis that occur in arteries exhibit this high level of occlusion by blood clots. In cases with a low occlusion, the GPTAD would not be useful, since blood would be sucked principally instead of extracting the clot. Until now, all our studies have been in models using simplified catheters, however such models have not allowed the device to be developed to the required level of accuracy. In addition to the components earlier described that are common to any catheter, the GPTAD also uses a suction pump to extract the thrombus that blocks the artery. Also in reality, in blood clot removal a balloon catheter would be placed in the artery to prevent fragments of blood clot and/or plaque being transported along the artery to other sites, which could potentially cause further problems. Such balloon catheters are routinely used in other such medical devices and procedures. directed along the central axis of the GPTAD and that there are relatively low forces on the periphery of the device. In our simulations we use a suction pressure of 2000 Pa. This is of the order of magnitude required to remove blood clots in laboratory based plastic tube models of arteries. In our model we use blood as the fluid that exists in arteries and in the space between the end of the GPTAD and the blood clot, to make the model realistic, and to simulate the conditions present in arteries. We also use the parameters for the density of blood 1060 kg/m3, the viscosity/stickiness of blood. There exist diverse studies that analyze the stickiness of the human blood according to the established conditions and it is known that the stickiness diminishes on having increased the temperature of the blood, coming to a minimal value of 0,002 kg/m-s. [4] The viscosity-temperature relationship is shown in Figure 3. Figure 3. Viscosity - temperature relationship Figure 2. Example of blockade balloon In this analysis, we concentrate on the development of the GPTAD and catheter in which it is located. The GPTAD has a length of 10 mm and is situated in the tip of the catheter. The GPTAD may be made of a range of diameters to fit into arteries of different diameters. Finally, we consider the risk of rupture of the arterial wall in the clot removal procedure. Previous work has tended to indicate that the maximum force of clot extraction is III. DEVICE PARTICULAR CASES The first design we use is a cylindrical model has used in previous studies using Bond Graph modelling without any helix or external element. We also design two catheters with conical geometry, the first one with a cone that becomes closer towards the clot and other that expands in this direction. In addition, we analyse a catheter with a geometry with double conicity, so that the diameter at the two ends of the GPTAD is the same but in the center the diameter is less. We also simulate the GPTAD containing a helix, that directs the suction pressure to the central axis of the device. We use the program Catia V5 for the design of the models, since it has good compatibility with Ansys, and has been used in previous FE analysis. In all out models we use a GPTAD of 10mm length. We use a helix that has a step of 1mm. The GPTD is positioned in the end of the catheter. Table I summerises our designs. 101

3 Design TABLE I. DEVICE DESIGNS Initial diameter Exit diameter Cylindrical 1,5 mm 1,5 mm Divergent nozzle 1,5 mm 2,1mm Convergent nozzle 1,5 mm 0,7mm To represent the geometry that exists at the moment suction pressure is applied, we define (i) a geometry that represents the blood in the tip of the catheter and (ii) a cylinder (equal for all the designs), which represents the blood accumulated between the top of the catheter and the thrombus. Finally we define a small semi-cylinder that represents the zone of the artery that is not occluded by the clot. Double nozzle 1,5 mm 1,5 mm Cylindrical with helix 1,5 mm 1,5 mm Divergent nozzle with helix 1,5 mm 2,1mm Convergent nozzle with helix 1,5 mm 0,7mm All the models that we use in the next sections have a length (GPTAD) of 10mm, which corresponds to being located in the last 10mm of the catheter. For purposes of comparison we use similar geometries in all the designs i..e. all the designs have the same diameter at the entry and only the exit diameters change. Although this is a relatively shirt length, the subtle changes in these variants in geometry would lead to large changes in the functional capability of the system. Table 1 shoes the various designs we modelled. In order to obtain valid conclusions for this analysis, it is necessary to evaluate the results in the same way in all the models. We therefore obtained the suction pressure at the same points in all the models. The first chosen point is the center of the clot. The second fundamental point, due to it is the point at which extraction of the clot begins, is a point at the periphery of the clot (in the side opposite to the zone without any obstruction) - this point is chosen also because it yielded lots of information in the model concerning the forces of attachment of the blood clot to the artery wall. These points are shown in figure 4. Figure 5. CFD studied model Since it has been indicated previously, this semi-cylinder represents 5% of the entire area of the section of the artery, so that the occlusion is 95%. The only variable section in the models that serves to evaluate the design of the catheter is the first one, since the other two are similar in all the models. The zone that is without obstructing for the thrombus is represented by a length of 15mm, (since this is a common length in clots of this type). We use a velocity for blood flow due to circulation (in the body) of 0,4m/s. We use the Fluent Simulation package, which allows us to analyze the behaviour of the fluid system for our model. One of the most important reasons for using Ansys Fluent modelling is to enable us to ascertain the decisive model that should be used, since there exist many alternatives that allow us to solve any case. Amongst many existing possibilities in finding an established model, we can consider models of heat exchange, a model radiativo or a model multiphase. In the case of the present work, we consider using a viscous model. In this model diverse variants are used including the viscous model of k-epsilon. The model of k-epsilon has been selected because it has been shown to be a very trustworthy model widely approved in applications of this type and it therefore meets the needs in the present work. It makes use of a semi-empirical model proposed by Lave and Spalding based on two equations of transport of the kinetic energy. The kinetic energy (k), is associated with a valuation of dissipation ( ) and these parameters are obtained from equations (1) and (2), whereas the turbulent stickiness is defined in the equation (3): (1) Figure 4. Indication of the points of analyzing We also investigate the maximum speed that the blood experience during blood clot removal and the time taken to extract the blood clot. where: (2) (3) 102

4 G k represents the generation of kinetic turbulent energy due to the gradients of average speed; G b represents the generation of kinetic turbulent energy due to the buoyancy; Y M represents the contribution of the fluctuating dilation to the turbulence, at the speed of global dissipation; C 1, C 2, C 3 and C μ are constant, and k and are the numbers of Prandtl of the turbulence for k and respectively; and S k and S are terms defined by the user. For our simulation and dimensioning, we have used the following values: C 1 =1,44; C 2 =1,92; C 3 =0,09; k =1,0; =1,3 IV. DESIGN RESULTS To be able to evaluate the results obtained with Ansys Fluent, pressures have been analyzed in the different models together with the velocity of each of them. The results are shown in figures 6 and 7 respectively. The main selection for the optimum design of the device therefore appears to be the presence of a helix and pressure drops. The open nozzle design appears to deliver most pressure to the clot to facilitate clot extraction, although the pressure that the thrombus itself experiences is practically uniform at all points. Furthermore the pressure at the periphery of the device appears to be sufficient to separate the clot from the artery wall. The inclusion of the helix in the design appears to improve the design even more in that the decrease in pressure between the pint at the centre of the blood clot and the periphery is four times that for the case without the inclusion of a helix. The manufacture of the catheter with only one helix is relatively simple and low cost. As for the speed of the fluid during the procedure, the time taken for clot extraction big differences between the models do not exist. As a general conclusion, the ideal design for the device is a cylindrical catheter with a simple helix. To optimize the design of the helix that also incorporates a catheter, we change the step of the helix (pitch distance between turn on the helix), which then changes the angle of the helix. The step is changed because it is the key factor in the definition of a helix. We investigated using a step of 2mm instead of 1mm and also a step of 0,5mm. Our modelling showed that using a step of 1mm produced the best results for clot extraction. Figure 6. Pressure in the cylindrical model with helix Figure 7. Velocity in the cylindrical model with helix After performing several simulations, we observed that the biggest fall in pressure occurred in the catheter cylindrical form that incorporated a helix. In this design a difference of pressure is obtained of 2,7%, (far greater than falls in pressure obtained with the majority of the remaining designs). The manufacture of such a catheter with only one helix is much simpler and therefore less costly. The speed of the fluid during clot extraction and the time taken for clot extraction do not appear to vary much with different models. Figure 8. Velocity in the 2nd cylindrical model with helix Figure 8 shows the results of computing the velocity of blood flow through a GPTAD model with a helix Our results and analysis lead us to the conclusion that the maximum difference in pressure that arises between the center of the thrombus and the periphery is 2,7%. Bearing in mind that we would apply suction pressures of approximately Pa, there would obtain a difference of pressure between the center and the rim of a few 1100 Pa, which is considered to be a quite high value. V. GP BLOOD CLOT DISTANCE INFLUENCE We analysed the optimum distance for clot extraction (between the end of the GPTAD and the blood clot itself). We used three distances 1mm, 3mm and 5mm. Using these starting parameters it was possible to determine the optimum distance to position the GPTAD from the clot in order the optimize clot extraction in arteries of different diameters. 103

5 diameter 4.5mm. In this case we also observed that the fluid flow changed from turbulent to laminar, and the results showed that the optimum distance between clot and end of the GPTAD was 5mm. Figure 9. CFD models considering 1, 3 and 5 mm distance influence Beyond the optimum distance (for a diameter of artery with a GPTAD of a given size) suction would become less effective. From our analysis it was possible to show that for a GPTAD in and artery of diameter 2.5mm the optimum distance is 1mm. Figure 10 shows other such results. Figure 10. Velocity distribution with 5 mm distance At a clot to GPTAD distance of 3mm, we observed that the suction pressure was directed to the center of the clot. Beyond 3mm suction pressure tended to fall off in a GPTAD of a given diameter. Obviously the suction pressure generated depends on distance between the GPTAD and blood clot, and the diameter of the GPTAD in a given artery, e.g. a GPTAD placed in an artery of diameter 2.5mm generates maximum suction pressure over a distance (GPTAD to blood clot) of 1mm In case of the smallest artery, the communicating artery on the Circle of Willis, with a diameter of 2,5mm, we would use a catheter with a diameter of 1,5mm. The intermediate artery, (cerebral artery), has a diameter of 3,17mm, and in this case we would use a catheter which has diameter 2,5mm. We also observed that fluid flow (blood) changed from being turbulent to laminar at distances of separation (end of GPTAD to blood clot) of 3mm. The results showed as the ideal distance of use is about 3mm. Finally, the artery internal carotid which is an artery that supplies blood to the Circle of Willis in the brain has a diameter of 5,5mm. In this artery we would use a GPTAD of VI. CONCLUSIONS In our modelling we have investigated different geometries for the GPTAD, different step distances and different separation distances between the end of the GPTAD and the blood clot, all with a view to optimising blood clot extraction. Our work has revealed interesting results e,g, the suction pressure needs to be practically double that needed to remove the blood clot from the artery wall. Our work has also yielded useful information concerning the positioning of the GPTAD (and catheter containing it) from the blood clot in order best facilitate clot removal. We therefore conclude that CFD modelling is extremely useful in the optimisation of the GPTAD. Analyzing the finished system with the catheter, and the GPTAD, one concludes that the loss of load is nearly 100%, i.e. loss of 97,2% at the entrance to the GPTAD and a loss of 99,2% at the center of the clot. This is to be expected since one is working with pipes of 1,5mm in diameter and in such the primary losses generated by friction are very big. However, the same study in an artery of larger size would reveal low losses as explained by Darcy's equation. REFERENCES [1] Pearce G, Perkinson ND (2006) Biomechanical Probe. International Patent Corporate Treatise (WO ) published ; European patent (EP (A2)) published ; Japanese patent (JP (T)) Published ; Chinese patent (CN (A)) published [2] Karnopp, D.C., Margolis, D.L. and Rosemberg, R.C System Dynamics: A Unified Approach. John Wiley & Sons, Inc., Second edition. [3] Romero, G. et al Modelling and simulation of a Thrombectomy Probe applied to the Middle Cerebral Artery by using the Bond Graph technique. 9º International Conference on Bond Graph Modeling and Simulation - ICBGM 2010, pp Orlando (United States). [4] Zupancic, A. Et al The influence of temperature on rheologicalproperties of blood mixtures with different volumen expanders-implications in numerical arterial hemodynamics simulations. Springer-Verlag. 104