Mechanical Events Within the Arterial Wall in Pulsatile Blood Flow

Under the oscillatory forces of pulsatile flow, the arterial wall suffers considerable stresses and strains in the radial as well as in the longitudinal direction. The consequences of these forces depend critically on the viscoelastic makeup of the vessel wall material and on the degree of tethering imposed by surrounding tissue. In vivo measurements of the stresses and strains within the wall thickness are extremely difficult, but some remarkable images of the dynamics involved have been obtained recently and demonstrated significant longitudinal displacements and shear stresses within the wall, which were considered negligible in the past.;For the arterial wall material, we employ a viscoelastic model which allows a study of the dynamics of the wall with different ratios of viscosity to elasticity of the wall material to mimic changes in the properties of the arterial wall caused by disease or aging. Change in arterial stiffness is generally considered a risk factor for cardiovascular disease and, in various ways, has been associated with hypertension, diabetes, hyperlipidemia, atherosclerosis, and heart failure, likely because of altered dynamics of the wall and of the fluid-wall interplay in pulsatile flow.;The results indicate that the extent of displacement and shear stress within the depth of the vessel wall depend critically on the degree to which the wall is tethered to surrounding tissue and on the mechanical consistency of the wall material, particularly on the relative proportions of viscous and elastic content within the wall. In particular, loss of viscous consistency leads to higher shear stresses within the wall, thus putting higher loading on elastin and may ultimately lead to elastin fatigue. As elastin gradually fails, its load bearing function is presumably taken over by collagen which renders the vessel wall less elastic and more rigid as is indeed observed in the aging process. The results also indicate that high tethering leads to high stresses and low displacements within the vessel wall, while low tethering leads to low stresses and high displacements. Since both extremes would be damaging to wall tissue, particularly elastin, this suggest that moderate tethering would be optimum in the physiological setting.;The findings from the coupled fluid-solid study are consistent with the simplified model of longitudinal motion (Chapter 3, 4, 5) and throws new light on the range of validity of the long wave approximation. We found that the long wave approximation is valid when the wall is free of tethering, but for a fully tethered wall the wave length was found to become shorter and therefore the long wavelength approximation ceases to be valid and should be carefully considered in this case.;We present the results of a comprehensive mathematical study of longitudinal and radial displacements and stresses within the wall thickness induced by the pulsatile blood flow. We also explore the effects of perivascular tethering on the vessel wall, which can be different at different physiological sites. We present a new method of defining the degree of tethering in terms of displacements of its outer layer compared with the displacements of the outer layer of the untethered wall, the latter being obtained from a solution of the limiting problem of tethering at infinity. This new approach makes it possible for the first time to describe the effects of partial tethering in its full range, from zero to full tethering.;Keywords: vascular mechanics; arterial wall; viscoelasticity; tethering; wave speed...