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Winning Abstracts from the 2008 Medical Student Abstract Competition: Hemodynamic Differences in the Infra-renal Aorta in Mice and Men: Implications for the Localization of Abdominal Aortic Aneurysms.

Author: Smbat Amirbekian, Emory University School of Medicine

Introduction
In humans, abdominal aortic aneurysms (AAA) develop inferior to the renal arteries. The aorta superior to the renal artery is remarkably spared. Conversely, mouse models exhibit AAA formation exclusively in the supra-renal aorta. Oscillatory wall shear stress (WSS) has been shown be important in the localization of atherosclerosis and AAA pathogenesis. Humans have a triphasic flow pattern in the infra-renal aorta that results in oscillatory WSS. The purpose of the current work was to investigate the hemodynamic environment of the supra& infra-renal abdominal aorta of normal mice using in-vivo MRI. We hypothesized that there is no reversal of blood flow in the infra-renal aorta of mice due to the lack of AAA formation in this segment.

Methods
In-vivo imaging was performed using a 4.7 Tesla Varian INOVA MRI. ECG-gating was used to acquire ten frames over the cardiac cycle. Using a phase contrast magnetic resonance imaging (PCMR) sequence, we obtained cine images, which allow measurement of both blood flow velocity and assessment of aortic wall motion. Ten images per cardiac cycle were obtained in a plane perpendicular to the aorta at the supra-renal and infra renal levels of 18 normal C57BL/6J mice. Flow vs. time curves were created at both locations.

Results
Supra-renal and infra-renal velocity measurements were successfully obtained in 18 of 20 mice. Time averaged infra-renal blood flow was 7.44.4ml/min. Through the entire cardiac cycle, flow was in the forward direction and there was no evidence of flow reversal at any time point. This difference in flow pattern between mice and humans may suggest differences in renal vasculature resistance and/or lower limb vessel capacitance. Supra-renal flow in mice, measured similarly as described above was 15.27.9ml/min and no reversal of flow was detected.

Conclusion
The absence of blood flow reversal in the mouse infra-renal aorta is a major difference in flow patterns between humans and mice. This difference in hemodynamic patterns between these two species may have important consequences for the etiology of AAA development. Performing similar hemodynamic studies in mouse AAA models may elucidate the effects of flow reversal on AAA formation. In conclusion, the mouse has become a commonly studied animal model for many cardiovascular diseases such as atherosclerosis and aneurysm development. However, there are differences that exist between many of these human diseases and their murine counterparts. Investigating these differences is a crucial part of determining the initiating factors in the pathogenesis of these diseases.

Back to July 2008 Issue of IMpact

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