Peter Lin

Date of Award

3-4-2008

Document Type

Open Access Thesis

Degree Name

Medical Doctor (MD)

First Advisor

John Elefteriades

Abstract

Objective: Treatment decisions for aortic aneurysms are currently based on size criteria originally developed in the 1960s, even though we now have more sophisticated methods that can refine interventional criteria. In this project, we applied engineering principles in order to generate a comprehensive picture of the mechanical properties of descending thoracic aortic aneurysms, including their ability to deform in response to pressure, as well as the stresses that cause wall stretch or rupture. Our goal was to use these mechanical properties to understand, explain, and predict the tendency of descending aneurysms to rupture or dissect. Methods: Using an epi-aortic ultrasound probe intra-operatively, we measured aortic wall thickness during systole and diastole, circumference during systole and diastole, and blood pressure on 12 patients undergoing elective resection of their descending aortic aneurysms. From these measurements, we calculated the distensibility, wall stress, elastic modulus (Einc), and pulse wave velocity (PWV) for the neck (narrow portion) and belly (widest portion) of fusiform aneurysms. We compared these mechanical properties between the neck and belly of descending aortic aneurysms with a paired t-test, as well as between ascending and descending aortic aneurysms with an unpaired t-test. Results: The average aneurysm belly was 4.1 cm in diameter compared to 2.7 cm in the neck (p = 0.0002). Distensibility was higher in the neck than the belly (p = 0.02), the wall stress was higher in the belly (p = 0.01), and Einc was non-significantly higher in the belly (p = 0.08). There was no significant difference in PWV (p = 0.33). There were no significant differences in any of the mechanical properties between descending and ascending aortic aneurysms. Conclusion: Larger aneurysms are at increased risk of rupture because 1) they experience greater circumferential wall stress tending to expand the lumen, and 2) they are less distensible with a higher elastic modulus which indicates they have less reserve stretch capacity. We also showed that different sections of the same aneurysm behave differently but that the ascending and descending aortic aneurysms behave similarly. These findings have implications on the validity of using mechanical parameters to predict the natural course of aortic aneurysms. Finally, we demonstrated that there may be better ways to predict aortic rupture or dissection than current standards using diameter or growth rate alone.

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This is an Open Access Thesis.

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