Transfer function comparisons of rat aortic wall image and intravascular pressure harmonic responses

This study examined the relationship between rat (mature Sprague-Dawley males) thoracic aortic wall and intraluminal pressure responses to a dynamic pressure input. High speed video image (Do) of outer wall area and intravascular pressure (Po) responses of the in vitro aorta were digitized and computer recorded during swept frequency pressure input (2-200 Hz; +/- 10 mm Hg) that was superimposed on static pressures from 20 to 200 mm Hg (Pi). Analysis included Fast Fourier transform (FFT) for Do/Pi and Po/Pi transfer functions and focused on comparison of coefficients from FANSIM (TUTSIM Products) polynomial equation fit to Bode plots for mean data of multiple aortas. The working hypothesis was that Do/Pi = Po/Pi. In FANSIM division by B0 of the general transfer function equality (A1s + A0)/(B2s2 + B1s + B0) yields (a1s + a0)/(b2s2 + b1s + 1); which was the form analyzed. Graphic and statistical comparisons indicated no difference for coefficients a1, a0, b2, and b1 between Do/Pi and Po/Pi. Coefficients b2, and b1 varied with change in level of static pressure. Values for a1 for both Do/Pi and Pi/Po remained relatively constant and appeared independent of static pressure. These results indicated appropriateness of the transfer function form and suggested that: b2, represented inertia of wall and intraluminal fluid mass; b1, represented wall and fluid viscosity influence; a1, represented influence of fluid viscosity and a0, represented influence of wall elasticity.