Viscoelastic characteristics (VEC) of old rat aorta (Wistar, 10 months) were obtained by sinusoidal excitation of intraluminal pressure (p) in cylindrical arterial preparations. The pressure excitation frequency (fexc) was swept in the range 3-30 Hz up and down at several mean-pressure levels while response volume oscillations were recorded and resonance curves were plotted. Natural frequency (f 0 ), dynamic modulus of elasticity (E ) and coefficient of viscosity (β) were estimated from resonance curves and the dependences of VEC on p were drawn. The results showed that f 0 decreased linearly with p whereas our previous data for young rat aorta (Wistar, 4 months) showed independence of f 0 on p. E increased nonlinearly with p with the values being higher in comparison to young rat aorta. This means stiffening of rat aorta with age in accordance with the known literature data. β-values increased linearly with p being higher in comparison to young rat aorta, demonstrative of raised intrinsic friction in the wall. VEC values were higher at decreasing fexc suggesting that the direction of excitation sweeping also determines the arterial wall biomechanical behaviour. It could be concluded that blood vessels VEC worsen with age, which endangers the arterial wall integrity, especially at higher intraluminal pressure.

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The arterial wall viscoelasticity plays an essential role in the vascular responsiveness to vasoactive drugs or pathologies. The aim of this investigation was to derive and compare resonance curve (RC), natural frequency (f(0)), dynamic modulus of elasticity (E'), and coefficient of viscosity (beta) of (i) vital and devitalized preparations of rat thoracic and abdominal aorta, (ii) human arterial prostheses, and to study the histomorphology of vital and devitalized rat aorta. The method of low frequency forced oscillations was employed. RC of vital preparations showed a hardening type of elasticity whereas in devitalized preparations it was of softening type. E' increased nonlinearly, f(0) decreased and beta increased linearly with equivalent intraluminal pressure (p(eqi)). Distensibility of abdominal aorta was lower than thoracic aorta. Distensibility decreased with increasing p(eqi). E', f(0), and beta increased significantly after devitalization. It was suggested that postmortem viscoelastic characteristics should not be used directly to specify the vital arteries viscoelasticity. RC of human prostheses showed a softening type of elasticity. Arterial prostheses have low circumferential distensibility with E'-values higher than reported in the literature for human arteries. The method of forced oscillations could be employed for studying the arterial wall biomechanics and viscoelasticity of arterial prostheses.

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These specific features are assumed as elastogram attributes in the second paper, in which a noninvasive determination of both elastogram and blood pressure is carried out.

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