In vivo viscoelastic behavior in the human aorta.

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Abstract
To characterize the viscoelastic properties of the human aorta in vivo, the pressure-diameter relation was determined in the abdominal aorta in 15 subjects. Diameter was measured noninvasively with a highly sensitive ultrasonic displacement meter, while intra-aortic pressure was measured with a catheter tip micromanometer inserted from the femoral artery. The frequency-dependent changes in the pressure-strain modulus (Ep) of the aorta and the phase lag of diameter to pressure were calculated by frequency analysis of these wave forms at the mean blood pressure of 109 mm Hg. The Ep and the phase lag at the fundamental frequency component (1.2 +/- 0.3 Hz, mean +/- SD) were (1.52 +/- 0.57) x 10(6) dyne/cm2 and -6.7 degrees +/- 2.1 degrees, respectively. Although the phase lag at the fundamental frequency was in good agreement with the published in vitro data, the calculated phase lags above the second harmonic were inconsistent, which was probably due to the nonlinearity in the pressure-diameter relation. To ...