2007 Abstracts: Relationship Between the Dynamic Geometry and Wall Thickness of the Proximal Aortic Neck of Abdominal Aortic Aneurysms Using IVUS: Implications For Endovascular Aneurysm Repair

Background: It is commonly assumed that the aortic wall deforms uniformly (i.e. expands and contracts concentrically) and has uniform wall thickness about the circumference. The purpose of this study was to evaluate aortic wall motion and thickness in the infrarenal aortic neck of patients with AAA who were undergoing endovascular repair (EVAR) and to compare the dynamic measurements of intravascular ultrasound to static measurements of CTA.
Methods: Between September 2004 and 2006, a total of 155 patients underwent EVAR in which 25 EVAR patients where studied with pre-operative CTA with three-dimensional reconstructions on a Vitrea Workstation and intraoperative IVUS of the proximal aortic neck. Infrarenal aortic neck dimensions on CTA were obtained at 1 mm intervals but for the purposes of this study all dimensions on CTA were obtained 1 mm below the lowest renal artery. IVUS analysis of the proximal aortic neck was obtained with a 10-second recorded data loop of aortic wall motion 1 mm below the lowest renal artery. A DICOM viewer was used to view the recorded loop of aortic movement and evaluated with SCION PCI Frame Grabber to determine aortic dimensions and wall thickness. IVUS diameters (n=250 measurements) were recorded through a full continuous cardiac cycle from the epicenter of the lumen (maintaining the left renal vein in its normal anatomic configuration) in an anterior posterior (AP) direction and 90 degrees perpendicular to this direction (lateral movement).
Results: There was significant variation in infrarenal aortic wall movement about the circumference with 1.7 ± 0.6 mm (range, 0.6-2.7 mm) displacement in the AP direction and 0.9 ± 0.5 mm (range, 0.3-1.5 mm) displacement in the lateral direction (P<0.001). Aortic wall thickness was greater in the region of increased AP wall motion than in the area of lesser lateral wall motion (2.3 ± 0.6 mm vs.1.2 ± 0.3 mm, P<0.001). There was no difference between the IVUS and CTA aortic neck measurements [25.5 mm vs. 25.3 mm (P=NS)] during the midpoint of the cardiac cycle. However, at peak systole, IVUS recorded a greater diameter than CTA (26.4 v 25.3 mm (P<0.001)] and at end-diastole, IVUS recorded a smaller diameter than CTA (24.7 v 25.3 mm (P=0.01).
Conclusions: The infrarenal neck of aortic aneurysms deforms anisotropically during the cardiac cycle. The greatest displacement is in the AP direction and corresponds with a significantly greater wall thickness in this area. The magnitude of cyclic change in aortic diameter can be as high as 11%. These findings may be of importance in understanding type I endoleaks and migration following EVAR. Further evaluation of proximal aortic neck wall motion following EVAR is warranted to determine the interaction of various stent designs and the aortic wall.