Rheological Properties of Atherosclerotic Aorta

Abstract

By using a tensile testing instrument (Shinko Tsushin, TOM 30-J), stress relaxation and stress-strain curves were measured of longitudinal and circumferential strips excised from various portions along the aorta in normal and WHHL rabbits aged from 8 to 30 months. The WHHL rabbit is an inbred strain developed from a mutant with inherited hyperlipidemia. Atherosclerotic changes with gross intimal thickening, sclerosis and atheromatous plaques increased progressively with advancing age.
There existed region and direction-dependent differences in the degree of stress relaxation in the normal aortas. In general, the degree of stress relaxation for longitudinal strips was lower than that for circumferential ones in each aortic portion and did not show any significant regional variation. The degree of stress relaxation of the circumferential strips was similar in the ascending and thoracic aortas but large in the abdominal aorta. In the atherosclerotic aorta, the stress relaxation was large in the thoracic and abdominal aortas compared with the normal aortas, and the difference in the former was larger than that in the latter. This indicates the mechanical effect of atherosclerotic changes which occurred in the intima.
Tension?strain curves were concave toward the tension-axis, indicating a decrease in wall distensibility with increasing strain. The slope of the curve progressively increased with age for all the samples from WHHL rabbits. Furthermore, a positive relationship was obtained between wall thickness and age with the correlation coefficients of 0.66 for the thoracic portion and 0.44 for the abdominal portion. Elastic modulus calculated from the stress?strain curves of the atherosclerotic aortas did not exhibit any distinct age?related changes in the young age group but became higher in the old age group. As a result, the decreased distensibility of the atherosclerotic wall may be attributed mainly to an increase in wall thickness caused by the intimal thickening. In advanced atherosclerotic aortas, wall stiffness explained by an increase in elastic modulus may be an additional factor of the increased wall distensibility.