Coronary clot removal by means of aspiration catheter : a numerical and experimental fluid dynamic study

The coronary arteries supply blood and oxygen to the heart tissues. If this flow becomes blocked, the downstream part of heart muscles begins to die. This leads to the heart attack and myocardial infarction (MI). The coronary arteries can become blocked by plaques which are the results of a slow process along the years called atherosclerosis. Although a coronary vessel is seldom completely blocked by a plaque, a complete blockage may happen if a clot (a result of hemostasis following tissue injury and abnormal vessel wall repair) lodges on the area. Furthermore, the plaque may also rupture and move and potentially cause a complete blockage. If the coronary vessel is blocked with a clot, several methods are available to tackle the issue: the coronary vessel could be treated with clot busting medicines or clot removing devices. In clot busting methods pharmacological methods are used to dissolve the clot. In the case of clot removing by means of devices, a very thin tube is guided toward the occluded vessel to mechanically remove the clot. Removal can occur according to different methods, including breaking down the clot and aspirating it. The most reliable method is based on the usage of coronary aspiration catheters. The aspiration catheters remove the clot only through the aspiration force. This method is the most preferred one as it has high effect and minimal clinical drawbacks. Although the results of aspiration catheter deployment have been investigated in a number of clinical trial studies, the quantitative prediction and comparison of catheter performances is also important in terms of aspiration ability. The main aim of the current study is to address this issue. More precisely, the purpose is to find out how an aspiration catheter aspirates a clot when used in different scenarios. In fact a wide range of aspiration catheters are available for the percutaneous coronary intervention (PCI) purposes. These catheters mainly differ in their geometry and design. In this thesis the effects of the geometrical differences are studied with regard to a number of catheters. The geometrical differences include size, tip shape, lumen diameter and hole design. The study of catheter geometrical parameters is combined with that of clot mechanical properties, which is a very important factor for the success of PCI for clot removal. To model the clot aspiration via coronary aspiration catheter, the first step was to define clot rheology. Ideally the blood should be collected from healthy human donor and rheology measurements immediately started. In practice the access to appropriate human blood is restricted and, furthermore, immediate rheology measurements right after blood collection are not feasible. Indeed, in this study the rheology measurements were based on porcine blood. The collected blood was anticoagulated and preserved in room temperature until transferred to the rheology laboratory where, after adding appropriate quantity of coagulation, the rheology of blood during clotting was measured. The choice of relying on properties of porcine blood clot as an alternative for human blood is reckoned reasonable due to the fact that in general porcine blood is similar to human blood in terms of structure, hemostatic properties, coagulation and ADP. However, there are still differences, for example porcine clot is hypercoagulable comparing to human blood. In order to measure rheological properties of clot, two types of measurements were performed: static measurements and dynamic measurements. In static measurements the blood is exposed to shear strain or shear stress in order to measure the viscosity. This method is a direct approach to find the clot viscosity, but suffers from several drawbacks. First, the blood is exposed to high shear strain. Second, the in vivo clotting procedure includes the formation of fibrin network, but static measurements may easily rupture this network and cause unreliable measurement. The dynamic rheology measurements, on the contrary, are much more reliable, as they are less likely to rupture the clot structure. In this study several measurements were done on incipient clots or clots which were initially exposed to mechanical precondition. The mechanical precondition was defined based on the frequency and shear strain as well as time period the clot had been exposed to, before the start of rheology measurements. The results indicated that the clot behavior is shear thinning and that can be described with a power law correlation. In order to model clot aspiration through a coronary aspiration catheter, a generic catheter was considered. Both clot and the aspiration catheter were modeled in a segment of the left coronary artery (LCA) close to the first bifurcation. By applying the pressure difference between the vessel outlets and catheter outlets, the clot position and movement was studied over time. To investigate the role of catheter geometry including tip diameter, tip shape, size, location and number of holes as well as clot rheological characteristics, a number of simulations were performed. The results indicated that in many cases part of the clot become trapped between the catheter wall and the vessel. Presence of lateral holes may enhance clot aspiration, in particular by aspirating part of clot which could not be aspirated by the main front hole. However, the presence of lateral hole played a negative role in many cases, due to the fact that holes may enhance aspiration of blood rather than clot. Indeed the hole design (location and size) is an important factor in clot aspiration performance. Only the holes located close to the trapped clot will enhance clot aspiration, while the holes located far from it will weaken clot aspiration performance. Furthermore hole size should be large enough to enhance the aspiration of trapped clot, however preventing aspiration of blood only. Moreover, the catheter with side holes has better performance for fresh clot (exhibiting lower viscosity) while older clots (with higher viscosity) should be preferentially treated with catheters without side holes. With regard to the tip design, the simulations indicated that the catheter with a right-angle tip performs better on clot aspiration, although most of the commercial catheters feature a beveled tip, due to the ease of insertion and minimal penetration force. The study on the role of pressure difference showed that it actually affects catheter performances depending on the viscosity of the clot. The catheter internal diameter has a great impact on both clot aspiration speed and success of complete aspiration. The final part of the work was related to the experimental characterization of different models of a commercial manual aspiration catheter provided by Invatec S.r.l., Roncadelle, BS, Italy. The performance of the catheter Diver C.E. MAX is much better than the other two aspiration catheters: Diver C.E. KIT and Diver C.E.. However this difference is more evident when the viscosity of fluid increases.