Regional and Directional Compliance of the Aortic Wall: ex vivo Dynamic Testing and Implications for Aortic Pathophysiology

Objective: To study the regional and directional compliance of the healthy aorta in the porcine model and to develop a model of the pathophysiology of aortic dissection. Methods: We developed a custom made pulse duplicator capable perfusing complete aortas ex vivo. Fresh porcine aortas (n = 13) were perfused with defined hemodynamic parameters, aortic compliance was measured optically. Additionally tissue strips of aortas were analyzed in a static tensile tester. Regional (ascendens, arch, descendens) and directional (transversal, longitudinal) compliance is studied under dynamic and static conditions. Results: The custom made pulse duplicator is capable perfusing the entire aorta with eligible hemodynamic parameters (pressures, resistance and frequencies) and a physiologic pulse curve (30% systole and 70% diastole). Aortic compliance is pressure and stress dependent, under hypertensive conditions, above 200 mm Hg the porcine aorta acts inelastic. Compliance of aortic tissue is highest in the ascending aorta and significantly decreases in the course of the vessel. The longitudinal compliance in the ascending aorta significantly exceeds the circumferential compliance, in the descending aorta there are no relevant differences in directional compliance. Under dynamic conditions, the compliance of the outer curvature in the ascending aorta significantly exceeds the compliance of the inner curvature. However, under static conditions, this effect is not verifiable. Conclusion: The new custom made pulse duplicator allows studying entire aortas under dynamic and preferably realistic conditions ex vivo. Though dynamic compliance data are largely comparable to data generated using the classic method of static tensile testing, we also find some relevant differences between the methods, particularly regarding the regional compliance of the inner- and outer curvatures of the ascending aorta. The longitudinal compliance of the ascending aorta and especially of the outer curvature is predominantly responsible for the Windkessel effect. The three dimensional configuration of the ascending aorta, its punctual fixation on the root and the supraaortic branches and its movement during the cardiac cycle determines regionally different mechanical stress and consecutive deformation of the aortic wall. Elongation and pronounced angulation of the ascending aorta may increase the stress on the outer curvature and might be important factors in the development of aortic dissection.