Comparison of rat aortic, carotid & femoral artery viscoelastic properties using harmonic analysis.

Viscoelastic properties of rat (Wistar Kyota) large (6 aorta), medium (12 carotid) and small (8 femoral) in vitro artery segments, were contrasted over a wide range of static and dynamic pressures. Relationship of change in static pressure (delta dyne/mm2) to diameter (delta mm) was used to estimate a segment's incremental elasticity (KD) at each pressure level. Dynamic intravascular pressure response (Po) was recorded during swept frequency pressure (2-200 Hz; +/- 10 mm Hg) inputs as superimposed on mean pressure steps of 40, 80, 120, 160 and 200 mm Hg (P(i)). Analysis of dynamic data included Fast Fouier Transform of Po/P(i) with FANSIM (TUTSIM Products) curve fit to Bode plots. Curve fit coefficients were used to estimate properties of natural frequency (omega n) damping, viscosity and inertia. Statistical analysis employed ANOVA and SNK multiple comparison procedures. Results indicated that as step-pressure was increased diameter, KD and omega n increased proportionately in all segments. Values of KD and omega n were always highest in femoral and lowest in aortic segments. In all segments damping decreased inversely with increasing pressure while, viscosity and inertia were lowest between 80 and 160 mm Hg. These results documented distinct viscoelastic properties for the three arteries as well as, differences in their response characteristics.

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.

Harmonic analysis of intravascular pressure response as an index of arterial wall mechanical properties.

The ability to routinely assess mechanical properties of large blood vessels, like the aorta, before an aneurysm or rupture occurs, could benefit diagnostic and therapeutic procedures and save lives. In this study, images of the wall area and intravascular pressure (IP) responses of in vitro rat aorta were recorded during swept frequency pressure input (2-200 Hz; +/- 10 mm Hg) superimposed on mean pressures from 20 to 160 mm Hg. Data analysis included Fast Fourier transform (FFT) of input and responses. Wall and IP responses were underdamped with respective resonance frequencies (Wn) that varied as a function of mean input pressure and the nonlinear nature of wall elasticity. Results indicated closely coupled wall and IP responses and suggested that the IP response may be an adequate index of wall elasticity without need of a direct measure of wall displacement. We considered results to be a key step towards development of a clinical tool which would facilitate analysis of mechanical properties of in vivo conducting vessels.

Synchronizing blood vessel response CCD imagery to swept frequency dynamic pressure signals.

Harmonic analysis of the pressure and wall responses of a blood vessel exposed to a dynamic pressure input signal required the development of a software application which could properly synchronize the data gathered by two separate microcomputers. In order to accomplish this task, the Pressure-Image Editor was developed. The first computer is used to generate a swept frequency sinusoidal dynamic pressure input signal while at the same time monitoring the resulting response pressures. The second computer is used to record the physical (visual) response of the artery to the pressure signal via a high speed CCD camera and video digitizer. Using the Pressure-Image Editor, 256 animated images along with 65,536 pressure points can be combined and synchronized based on the camera frame rate, input trigger frequency, and any internal timing delays. The Pressure-Image Editor is a object-oriented application written in C++ and includes a window based graphical user interface.