Abstract 474: Early Events in Dissecting Aneurysms Induced by Angiotensin-II Infusion: The Geometry-Driven Aspect of a Multifaceted Problem

While research on dissecting aneurysms in Angiotensin-II infused mice spans more than a decade, the temporal sequence of initial events still remains unclear. Recent findings in our group suggested that focal medial tears at the vicinity of suprarenal side branches are the primary event in disease formation. In this study we used a combined experimental-computational approach to investigate the hypothesis that initial events of dissecting AAAs originate at branching sites along the aorta. Male apolipoprotein-deficient mice were infused with Angiotensin-II (n=11) and saline 0.9% (n=6) for 3 days and scanned with contrast-enhanced microCT prior to sacrifice. One animal presented an in-vivo rupture during the microCT scan, and was rescanned after 2.5 hours to observe real-time morphological changes. In all other animals, the excised aortic tissue was imaged with Phase Contrast X-Ray Tomographic Microscopy (PCXTM) at 6.5um isotropic resolution. An automatic morphing code was developed to map the ex-vivo geometry onto the in vivo geometry, and a finite element simulation yielded a stress distribution that represents an estimation of the wall tension, not only due to the pressurization, but also due to the local stretch field. We found that the nanoparticulate microCT contrast agent had infiltrated the aortic wall in 11/11 Ang-II infused animals, while no infiltration was observed in 6/6 control mice. The infiltration affected at least one pair of intercostal arteries in 11/11 mice, and in 9/11 mice the coeliac region was also affected. Image-guided histology allowed us to determine the circumferential distribution of microlesions at branching sites, including disruption of elastin fibers, apoptotic cell appearance, subintimal leukocyte infiltration and intramural hematomas. In the animal whose aorta had ruptured during the in vivo scan, the initial hematoma had originated around 3 pairs of intercostal arteries and quickly propagated afterwards. Mouse-specific finite element simulations revealed a co-location of computed peak stresses at the vessel wall and histologically identified vascular damage. We conclude that the aortic geometry, and side branches in particular, play a pivotal role in the onset of dissecting AAA.