In-vitro analysis of haemodynamics in stented arteries. (2022)
Type of ContentTheses / Dissertations
Thesis DisciplineMechanical Engineering
Degree NameDoctor of Philosophy
Cardiovascular diseases (CVD) are the leading cause of death in the developed world. One of the most common management methods for CVD is through vascular implants such as stents to support arterial walls. However, determining the efficacy of stents can be difficult, particularly for high-risk stents, such as those used in the aorta. In-vitro modelling can provide safe insight into the haemodynamics changes within an artery due to specific stenting methods, without intrusive patient monitoring. The in-vitro studies presented in this thesis contribute to research on the haemodynamic changes within arteries using particle image velocimetry (PIV).
In-vitro modelling can be used to investigate haemodynamics of arterial geometry and stent implants. However, in-vitro model fidelity is reliant on precise matching of in-vivo conditions. Flow distribution and wall shear stress depend on the Reynolds and Womersley numbers. This thesis reviewed currently published Reynolds and Womersley numbers for 14 major arteries in the human body. The results were presented both in a table and graphically for ease of understanding and future use. The results identified a paucity of information in smaller distal arteries compared to major arteries such as the aorta.
Matching Reynolds and Womersley numbers for compliant in-vitro modelling may also be limited by model dimensional tolerances. A method for visualising the range of experimental conditions required for dynamic matching was developed and case studies for the ascending aorta and common carotid artery were presented. The assumed Sylgard 184 silicone would be used for phantom fabrication, and compared three working solutions: water/glycerine, water/glycerine/urea, and water/glycerine/sodium-iodide. To manufacture compliance matched silicone models of the ascending aorta and common carotid arteries, the models were scaled to 1.5x (ascending aorta) and 3x (common carotid) life scale, respectively. Modelling the ascending aorta with the comparatively high viscosity water/glycerine solution will lead to very high pump power demands. However, any of the working fluids considered could be dynamically matched with low pump demand for the common carotid model.
The Frozen Elephant Trunk (FET) stent is a hybrid endovascular device that may be implemented in the event of an aneurysm or aortic dissection of the aortic arch or superior descending aorta. However, the FET stent is a high risk stent. In particular, the Type 1B endoleak can lead to intrasaccular flow due to an incomplete distal fit between the stent and artery during systole. Chapter 5 developed an in-vitro modelling technique to enable the investigation of the known failure. Recirculation zones and an asymmetric endoleak were identified distal to the surrogate stent graft. The endoleak developed at the peak of systole and was sustained until the onset of diastole. The endoleak geometry indicated a potential variation in the phantom artery wall thickness or stent alignment. Recirculation was identified on the posterior dorsal line during late systole which may induce an inflammatory response in an artery. The identification of the Type 1B endoleak proved that in-vitro modelling can be used to investigate complex compliance changes and wall motions.
The kissing stent (KS) configuration is a low risk, stenting method often used to treat aorto-iliac occlusive disease (AIOD). However, long-term patency reduces by nearly 25% in the first five years potentially due to deleterious flow behaviour. The risk of harmful haemodynamics due to the KS configuration were investigated in-vitro. PIV experimentation identified peak proximal and distal velocity in-vitro was 0.71 m·s-1 and 1.90 m·s-1, respectively. A lumen wall collapse in the sagittal plane occurred during late systole to early diastole proximal the KS configuration. The collapse disturbed the flow proximal to the stented region producing potential recirculation zones and abnormal flow patterns. However, the systolic flow was as normal and undisturbed indicating the KS configuration is safe to use for repairing AIOD. The collapse had not been previously identified and would require further investigation.
Thoracic extra-anatomic bypasses (EAB) are grafted stents that may be used to prophylactically revascularize supra-aortic arteries that may require blockage during thoracic endovascular aortic repair (TEVAR) methods. However, prophylactic use of EAB may introduce a risk of failure due to abnormally low or disrupted flow, known as competitive flow, within the bypasses. Competitive flow within the bypasses between supra-aortic arteries has not been captured previously. PIV was used to assess each model configuration for flow abnormalities and potential for flow competition. The investigation found potential for competitive flow in the bypasses when just the left subclavian artery (LSA), the left carotid artery (LCCA), or none of the arteries are blocked. In contrast, when the LSA and LCCA were both blocked, there was no evidence of competitive flow. Flow stagnated at the initiation of systole within the BC bypass in the 2 configurations with an unblocked LCCA, along with notable recirculation zones and reciprocating flow occurring throughout the rest of systolic flow. Flow stagnated in the CS bypass at early systole when only the LCCA was blocked. A large recirculation was identifiable in the CS bypass when just the LSA was blocked, particularly after peak systole. The potential of competitive flow indicated prophylactic used of EAB in the supra-aortic arteries may require location of proximal arteries to limit the number of pathways blood flow can take.
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