Passive biaxial mechanics and viscoelastic relaxation of the thoracic aorta in intermittent hypobaric hypoxia

Intermittent hypobaric hypoxia (IHH), a common exposure pattern in high-altitude occupational environments, can modulate arterial wall behavior and potentially affect pulsatile load buffering. In this study, we examined how acute and chronic IHH influence the passive elastic and viscoelastic properties of the descending thoracic aorta (DTA) in adult rats. Planar biaxial tensile tests under low strain-rate conditions and stress-relaxation protocols were combined with quantitative histology to assess both functional and structural adaptations. Most elastic parameters derived from the stress–stretch response remained unchanged across Control, acute IHH, and chronic IHH groups, indicating preservation of the overall passive biaxial mechanical response. Nevertheless, the chronic IHH group exhibited a significant reduction in the longitudinal high-stretch stiffness parameter (E2[jls-end-space/]), suggesting subtle alterations in the arterial response under elevated loading conditions. Stress-relaxation experiments revealed trends toward increased stress decay and dissipated energy in the chronic IHH group, particularly in the circumferential direction, although these differences did not reach statistical significance. Histological analysis showed reduced cell nuclei density and modest elevations in elastin and collagen content in hypoxia-exposed groups, consistent with low-grade remodeling that does not translate into detectable changes in elastic behavior. Collectively, these findings indicate that IHH induces microstructural modifications and meaningful alterations in time-dependent mechanics while largely preserving the low strain-rate biaxial mechanical response of the aortic wall. This work provides an integrated biomechanical framework for evaluating arterial remodeling under hypoxic stress and supports the view that the DTA exhibits a robust passive mechanical phenotype under intermittent hypobaric exposure.