Influence of Hypertension on the Regional Elastic Properties of Ascending Thoracic Aortic Aneurysms with Bicuspid Aortic Valves

Although hypertension may affect the local biomechanical properties of ascending thoracic aortic aneurysms (ATAAs) with a bicuspid aortic valve (BAV), a comparison of the regional elastic properties of BAV-ATAAs between patients with and without hypertension is not yet available due to lack of biomechanical data. In this study, we collected 25 fresh ATAA samples from 17 hypertensive and 8 age-matched non-hypertensive patients who underwent elective aortic surgery. Biomechanical tests were executed to investigate the regional failure stresses and biaxial mechanical properties of BAV-ATAAs with and without hypertension. A material model was fitted to the biaxial experimental data to obtain model parameters in different regions. Histological analysis was performed to investigate the underlying microstructure and determine the percentages of elastic and collagen fibers. The circumferential failure stresses in the anterior, lateral, and posterior regions were significantly lower for the hypertensive group than in the non-hypertensive group. Regarding equibiaxial stresses, the hypertensive BAV-ATAAs showed significantly higher longitudinal tissue stiffness in the anterior and lateral regions than the non-hypertensive BAV-ATAAs. Collagen fiber hyperplasia was observed in the anterior and lateral regions of both the hypertensive and non-hypertensive BAV-ATAAs. However, in these regions the laminar structure of the elastic fibers was disrupted in several places and apoptotic smooth muscle cells were observed in the group with hypertension compared to the group without hypertension. In addition, the circumferential failure stresses in the anterior and lateral regions were significantly increased in the hypertensive group and strongly correlated with the collagen content. These results suggest that the regional elastic properties of the hypertensive BAV-ATAAs are more deteriorated than those of the non-hypertensive samples. Due to the significant impact on circumferential tensile strength, the hypertensive BAV patients may be at a higher risk of ATAA rupture than non-hypertensive BAV patients.