Document Type

Dissertation

Publication Date

11-2011

Abstract

Abdominal aortic aneurysms (AAA’s) are characterized by a permanent and irreversible enlargement of the abdominal aorta to at least 150 percent its expected normal size. Endovascular treatment of AAA’s with endografts has gained tremendous popularity in the last decade, largely due to decreased perioperative mortality and faster recovery times, when compared to open surgical repair.

Although endografts are an appealing treatment option for many patients, some device design and safety issues have yet to be addressed. One of the biggest problems with endografts is their tendency to shift their position in the patient anatomy over time. The forces that cause this shift and their relation to endograft design are not well understood. In addition, the degree of blood damage due to the insertion of an endograft into the abdominal aorta has not been quantified. The following studies were completed in order to investigate the issues described above.

1. Calculation of endograft displacement force in patient-specific device models. This study used electron-beam computed tomography (CT) image data to generate patient-specific anatomical models using novel segmentation techniques. The patient models were then virtually modeled to represent three different endograft designs, based on aortic stent-graft devices already available in the U.S. or currently in clinical trials. Computational Fluid Dynamics (CFD) simulations were run to characterize the hemodynamic factors for each patient and a total displacement force was calculated for each model. Results show that the location of the device bifurcation impacts the overall displacement force, with proximal bifurcation endograft designs generating a lesser force than distal bifurcation designs.

2. Characterization of blood damage due to endograft placement. Patient-specific computational models were created to represent endograft features that partially obstruct blood flow to the renal arteries, which is sometimes necessary to attain complete exclusion of the aneurysm after deploying the device. Findings show that the insertion of an endograft causes a two-fold increase in blood damage. However, the magnitude of blood damage is within acceptable safety standards.

3. Benchtop testing of red blood cell damage. A benchtop experimental setup was created to measure the damage to red blood cells under various flow conditions and flow obstructions. Samples were characterized at different time points using light scatter methods to determine cell volume and hemoglobin concentration. Results indicate that significant damage to red blood cells occurs only after prolonged exposure (>103 seconds) to high shear (>4000 dynes/cm2) conditions. In addition, the presence of flow obstructions creates red blood cell fragments, instead of destroying the cells entirely. Future directions for this work include additional CFD modeling of devices in more patients treated with different aortic stent graft designs to derive statistical significance relative to various design and anatomical features and extend the analysis to the evaluation of devices used for the treatment of thoracic aortic aneurysms (TAA’s). Follow-up studies on device migration studies can also be completed in the cohort of patients where the endograft displacement forces were calculated. Further, blood damage models that incorporate the fragmentation of red blood cells can be developed.

Comments

© 2011 Polina A. Segalova. This dissertation was submitted to the Department of Mechanical Engineering and the Committee on Graduate Studies of Stanford University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.

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