Segmental mechanical properties of the canine thoracic aorta in vivo

Abstract

The viscoelastic mechanical behavior of three well-defined segments of the canine thoracic aorta was described. These included segments of the ascending aorta (AA), the upper descending thoracic aorta (UDTA) and the middle descending thoracic aorta (MDTA). Ultrasonic dimension gauges and a catheter-tip manometer were used to measure changes in segment diameter, length and intravascular pressure during baseline, increased heart rate and increased pressure conditions. Change in segment volume (ΔV), a volumetric stiffness index (ΔP/ΔV) and a blood storage index (PΔV) were calculated for each segment during each condition. The ΔP/ΔV index significantly (α= 0.05) increased peripherally from the heart during baseline conditions. Average values from twelve dogs for ΔP/ΔV were 0.146 mmHg*mm^-3, 0.347 mmHg*mm^-3 and 0.739 mmHg*mm^-3 for the AA, UDTA and MDTA segments respectively. Longitudinal segment motion was a major factor in this increased stiffness. The total mean volume (V[T]), total change in volume (ΔV[T]) and a volumetric stiffness index (ΔP/ΔV[T]) were also calculated for the first two thirds of the thoracic aorta during baseline, increased heart rate and increased pressure conditions. The ΔP/ΔV[T] increased significantly (p < 0.03) from 10.7 mmHg*ml^-1 to 15.7 mmHg*ml^-1 using neosynephrine infusion. The first two thirds of the thoracic aorta contributes significantly to forward blood flow during diastole as determined by calculation of an average of 17% change in total volume for this segment in eight dogs.