A constituent-based model of age-related changes in conduit arteries

Abstract

Tsamis A, Rachev A, Stergiopulos N. A constituent-based model of age-related changes in conduit arteries. Am J Physiol Heart Circ Physiol 301: H1286 -H1301, 2011. First published July 1, 2011; doi:10.1152/ajpheart.00570.2010.-In the present report, a constituent-based theoretical model of age-related changes in geometry and mechanical properties of conduit arteries is proposed. The model was based on the premise that given the time course of the load on an artery and the accumulation of advanced glycation end-products in the arterial tissue, the initial geometric dimensions and properties of the arterial tissue can be predicted by a solution of a boundary value problem for the governing equations that follow from finite elasticity, structure-based constitutive modeling within the constrained mixture theory, continuum damage theory, and global growth approach for stress-induced structure-based remodeling. An illustrative example of the age-related changes in geometry, structure, composition, and mechanical properties of a human thoracic aorta is considered. Model predictions were in good qualitative agreement with available experimental data in the literature. Limitations and perspectives for refining the model are discussed. arterial remodeling; vascular mechanics; elastin fatigue damage; collagen cross-linking AGING AFFECTS THE MECHANICAL FUNCTION of human conduit arteries such as the aorta, the iliac arteries, and the carotid arteries. It manifests as a change in the vessel geometry and material properties of the vascular tissue, accompanied by alterations in loading conditions. Geometric and structural alterations in the tissue of conduit vessels with progressing age cause a decrease in the total arterial compliance (66), which, in turn, leads to an increase in pulse pressure. The increase in pulse pressure has been shown to be the strongest predictor for cardiovascular mortality, for it augments the mechanical load on the left ventricle (9). Furthermore, the decrease in diastolic pressure observed in late middle age, which reduces the coronary arterial perfusion, raises the demands on the left ventricle (25).Geometric changes observed in aging are associated with an increase in arterial volume (7) characterized by increased lumen area (63) and wall thickening (43) and decreased axial stretch associated with loss in longitudinal stiffness (65). An increase in the opening angle has been observed, which was obtained after residual stress was relieved in an isolated arterial ring by a radial cut (54). Simultaneously, the content of collagen increases and the number of smooth muscle (SM) cells decreases (55). Furthermore, mean aortic pressure gradually increases with advanced age (27). The interrelation among the aforementioned processes is unknown. Additionally, the stiffness of the elastin structure gradually increases with advanced age due to a nonatherosclerotic mineralization mechanism called the medial elastocalcinosis (MEC), during which calcium salts bind to the elastin of the aortic media (15). Also, ela...

Fiber micro-architecture in the longitudinal-radial and circumferential-radial planes of ascending thoracic aortic aneurysm media

“…This finding is compatible with increased tensile stretch at the inflection point of Θ -strips of adventitial-medial half of BAV-ATAA (1.28 ± 0.03) when compared with CTRL-ATA (1.13 ± 0.02) (Fig. 5), since more undulated fibers permit increased stretch before all fibers become engaged (straightened) (Tsamis et al, 2011). Nakashima et al (1990b), who assessed histologically the media of ATA dissecting aneurysms of type-A, reported that medial microstructure was characterized by decreased number of interlaminar elastin fibers.…”

Extracellular matrix fiber microarchitecture is region-specific in bicuspid aortic valve-associated ascending aortopathy

“…This finding is consistent with the increased tensile stretch noted at the inflection point of Θ strips of the outer delaminated half of BAV-ATAA when compared with a nonaneurysmal ascending aorta, because more undulated fibers permit increased stretch before fibers become gradually engaged (ie, straightened). 30 Furthermore, we found 2 discrepancies in the architecture of RAD-oriented fibers across the media of TAV-ATAA with increasing age. Collagen AAU about the Z-axis was found to be increased within the outer-media.…”

A custom image-based analysis tool for quantifying elastin and collagen micro-architecture in the wall of the human aorta from multi-photon microscopy

“…Intersections density can impact the effective stiffness of the wall components and subsequently affect the mean and variance of the probability density function of fiber engagement. Attaching fibers to one another can stiffen the fiber ensemble by shifting the recruitment of the fibers to lower strains (decreased mean) and forcing the engagement to occur more abruptly (decreased variance) (Tsamis et al ., 2011). The end goal would be to model the fiber micro-architecture using finite-element methods.…”