Dynamic local distensibility of living arteries and its relation to wave transmission

The dynamic local distensibility of the abdominal aorta was measured in 11 anesthetized dogs by recording simultaneously phasic pressure and instantaneous intravascular cross-sectional area, utilizing a special transducer. Axial motion of the vessel wall was recorded using a modification of the same transducer. A nonlinear relationship was found to exist between area and pressure in most cases studied. Fourier analysis was performed on data from eight experiments in order to obtain frequency characteristics of distensibility. In roughly half of the cases, Fourier analysis revealed that pressure variations displayed a phase lead over area variations for frequencies up to 10 Hz. This phenomenon was ascribed to viscoelastic properties of the vessel wall and the magnitude of the phase leads roughly matched those found in vitro by others. The behavior of the vessel wall in these instances was correctly predicted by the dynamic formula for distensibility, derived by others from wave transmission theory in which absence of axial wall motion is assumed. In these experiments, axial motion of the wall was found to be virtually absent. In the other half of the cases, the reverse situation was obtained: a phase lead of area variations over pressure variations for frequencies up to 15 Hz. In those cases a craniocaudal axial displacement of the vessel wall was observed with each systole, amounting to around 1 mm. The finding of the phase leads was partially explained by a dynamic formula for distensibility, developed by us from the theory of wave transmission in which free axial motion of the wall is a chosen boundary condition. The sign and order of magnitude of the phase leads were correctly predicted by the theoretical formula, but there was a disagreement on the frequency range in which they occurred. We concluded that additional forces, not yet considered in theoretical treatments, are operative on the aortic wall, which account for this lack of agreement. The frequency dependent properties of distensibility in vivo cannot be compared to those obtained in vitro in those cases in which there is axial displacement of the vessel wall of the same order of magnitude as the radial extensions.