The Effect of Endografts on the Propagation of Aortic Dissection: A Fluid-Structure Interaction (FSI) Finite-Element Model of Flow through the Descending Thoracic Aorta

Abstract

Tears in the lining of the aorta are termed aortic dissections. Aortic dissections affect 12,000 new patients per year in the US and are associated with significant morbidity and mortality. The primary management method is an invasive and difficult surgery to replace the dissected portion of the aorta with a synthetic graft. However, recent randomized trials on the use of minimally invasive stent-grafts to treat aortic dissection have proved promising. Though the precise etiology of aortic dissections is unclear, the underlying pathophysiology may be related to hypertension induced injury in an aorta that has lost compliance due to aging. Though stent-grafts can effectively protect the initial location of the dissection, stent-grafts are essentially stiff tubes and after implantation, residual portions of the native aorta proximal and distal to the stent-graft are needed to dissipate systolic pressure. These regions of the native aorta proximal to the stent-graft required to limit pressure differences are exposed to larger stresses thus increasing the potential of aortic tearing. Stent-graft-induced new entry, SINE, has been recorded in 30\% of patients treated with a stent-graft for aortic dissection. It is hypothesized that the stiffness of the stent-graft itself may exacerbate pressure in the proximal and distal native aorta and may be responsible for stent-graft induced new tears. A mathematical lumped parameter model and computational finite-element model were developed to evaluate this. The lumped parameter model uses electrical analogs for blood flow and compliances of the native aorta and stent-graft respectively. The mathematical model predicts an increased peak pressure for a stiff stent-graft as compared to a compliant stent-graft. Additionally, the mathematical model predicts an increase in peak pressure with an increase in stent-graft length. Motivated by the need to incorporate more anatomically accurate material properties and capture complexities of the flow field, a three-dimensional computational model of the flow through the descending thoracic aorta and stent-graft was developed. For an aortic dissection patient treated with a stiff stent-graft, the computational model predicts a peak cardiac cycle pressure of 190 mmHg, as compared to the predicted pressure of 176 mmHg for an aortic dissection patient treated with a compliant stent-graft. The model suggests that the increase in stent-graft length increases peak pressure during the length of the cardiac cycle, but a change in stent-graft modulus and stent-graft position does not affect the peak pressure during the length of the cardiac cycle. The modeling developed in this thesis confirms that the stiffness of the stent-grafts used to treat aortic dissection affects pressures in the proximal native aorta and could contribute to the formation of stent-graft induced new entry. This information is critical to the development of future stent-grafts to treat aortic dissection.