Static and Dynamic Mechanical Properties of the Carotid Artery From Normotensive and Hypertensive Rats

Abstract

Abstract-Several recent results obtained in hypertensive animals and subjects under in vivo isobaric conditions do not confirm the classic view of stiffer arteries in hypertensive subjects. We compared the mechanical behavior of in situ isolated common carotid arteries from normotensive Wistar-Kyoto rats (WKY) and age-matched spontaneously hypertensive rats (SHR) under both static and dynamic conditions for transmural pressure ranging from 50 to 200 mm Hg. Because of the nonlinear pressure-diameter relationship of large arteries, it is difficult to compare the mechanical properties of the large arterial wall in patients or animals with different arterial blood pressure. Furthermore, most of the experiments to assess the mechanical properties of the arterial wall are performed under static conditions, ie, with the pressure being varied by successive steps.2,3 However, under in vivo conditions with pulsatile pressure generated by the cardiac pump, the large arterial wall must be considered in terms of dynamic behavior, ie, taking into account the viscous, frequency-dependent properties of the vascular wall.New ultrasonic technologies have been developed that allow the precise measurement of both in vitro and in vivo instantaneous arterial diameter. 4 The application of these ultrasonic techniques to the calculation of mechanical properties of large arteries in human and experimental hypertension has led to conflicting results. In contrast to the classically reported stiffer arterial wall in hypertensives, similar or even increased arterial compliance and distensibility were calculated in hypertensive subjects and animals. 5,6 However, because of in vivo measurements in the operating arterial pressure range, it was impossible to compare the dynamic mechanical properties of the arterial wall in hypertensives and normotensives under isobaric conditions without an unverified hypothesis and theoretical models of the mechanical behavior of the arterial wall. In fact, to compare the arterial wall properties in vivo under isobaric conditions in normotensives and hypertensives, one must use calculations performed from pressure-diameter values recorded during the systole in the former and during the late diastole in the latter, ie, under markedly different dynamic conditions. It is also possible to modify the operating pressure in 1 group to reduce the arterial pressure in hypertensives and/or to increase it in normotensives, and therefore to compare wall properties in normotensive and hypertensive animals at comparable blood pressures in vivo. 7 However, the calculated parameters do not represent their actual operating values; furthermore, a direct effect of the drugs used to modify the arterial pressure on arterial smooth muscle tone and mechanical properties cannot be excluded.Therefore, the purpose of the present study was to (1) describe an experimental setup that would allow us to simultaneously evaluate in situ the static and dynamic properties of the CCA in normotensive WKY and SHR and (2) differentiate from the alt...

Assessment of the viscosity of the pulmonary artery wall

“…Ddyn and Dst did not seem to differ considerably in either subject. In systemic arteries, the pressure/diameter relation was found to shift markedly upwards during pressure responses, indicating significantly higher Dst than Ddyn [1,2,5]. The clinical characteristics of all the subjects are given in table 2.…”

“…In reality, however, part of the energy that corresponds to viscous deformation is dissipated within the arterial wall. Wall viscosity reduces the efficiency of heart/vessel coupling, and may place extra load on the ventricle [3].Wall viscosity is responsible for the difference that exists between the static (Dst) and dynamic distensibility (Ddyn) of a given vessel [1,2,4,5]. Under static conditions, the change in diameter in response to a given change in distending pressure is determined only by the elastic properties of the vessel wall.…”

“…Dst The arterial wall responds to stress through both elastic and viscous behaviour, the former being related to the elastin and the latter to the collagen and smooth muscle content of the vessel wall [1,2]. During systole, the elastic behaviour of the arterial wall allows its diameter to increase proportionally to pressure; in the case of a purely elastic arterial wall, the total amount of energy stored would be released during diastole.…”

“…Wall viscosity is responsible for the difference that exists between the static (Dst) and dynamic distensibility (Ddyn) of a given vessel [1,2,4,5]. Under static conditions, the change in diameter in response to a given change in distending pressure is determined only by the elastic properties of the vessel wall.…”

“…The greater the wall viscosity, the greater the difference between Dst and Ddyn. Dst has been found to be considerably higher than Ddyn in systemic arteries, the difference being accounted for by wall viscosity [1,2,4,5].…”