The influence of mechanical properties on wall stress and distensibility of the dilated ascending aorta

Objectives: We sought to determine how intrinsic mechanical properties of dilated ascending aorta influence in vivo distensibility and wall stress, potential contributing factors to the risk of aortic rupture and dissection. Methods: Fresh tissue from patients undergoing ascending aortic replacement for Marfan syndrome (n = 8, age 38 +/- 14 years), bicuspid aortic valve with associated aneurysm (n = 20, age 55 +/- 12 years), or idiopathic degenerative aneurysm (P = 7, age 64 +/- 9 years) was analyzed for elastic properties and circumferential residual stress. Histologic specimens were graded for elastin fragmentation, cystic changes, fibrosis, and necrosis. Distensibility and wall stresses were predicted with a cylindrical mathematical model of the aorta. Results: Elastic properties of aortic tissue were nonlinear and extensibility decreased with age. Moderate or severe elastin fragmentation was prevalent in tissue from patients with the Marfan syndrome and more common in tissue from younger patients. Model-predicted mean circumferential stress did not vary significantly with age or patient group, but increased with aortic lumen diameter and systolic blood pressure. Model-predicted distensibility decreased with age in bicuspid aortic valve and degenerative aneurysm groups. Differences in distensibility were correlated with differences in elastic properties. Conclusions: The age dependence of elastic properties of dilated ascending aorta is consistent with that of normal aorta observed in previous studies. Increases in mean circumferential stress with blood pressure and diameter support the clinical importance of blood pressure control and serial evaluation of aortic diameter. Declining wall strength with age may increase the risk of rupture or dissection.