Variability of the viscoelastic constants along the aortic axis of the dog.

EASUREMENTS of the elasticity of isolated aortic specimens are complicated by the nonlinear mechanical behavior of such tissues. Nonlinearity occurs in both static and dynamic measurements. Krafka, 1-2 in a series of studies on the static elasticity of isolated aortic strips, computed 3 elastic moduli, each at a different elongation. These moduli differed from each other and varied independently under different conditions. Under dynamic conditions, 3 elastic moduli may be several times greater than static elastic moduli at the same mean elongation. Damping in such specimens appears to be frequency dependent. Nonlinearity of the mechanical properties of blood vessel walls has been attributed to a complex interaction among the various fiber types present. 1 " 0 King, 7 ' 8 on the other hand, has interpreted the static mechanical behavior of the aortic wall in terms of modern high polymer theory. Recently, this latter interpretation has been extended to include dynamic data. 9-10 The justification for the application of the polymer theory rests, in part, upon experimental observations that the elastic force in isolated aortic specimens is en-tropic over a considerable range of elonga-tion. 11 Anomalous thermoelasticity, nonlinear stress-strain curves and long range reversible extensibility are characteristics usually associated with rubber-like materials. Many biological tissues show qualitatively these same characteristics. 12 This report presents a study of 47 aortic strips from 34 mongrel dogs, using a poly-meric model. A statistical analysis of the variations in dynamic elasticity and damping which occur with location along the arterial axis and during stretch and retraction of aortic strips is presented. The analysis of the mechanical behavior involves rather complex statistical mechanical equations (Appendix). Solution of these equations for a wide range of values of the parameters involved have been obtained on an IBM card calculator and reduced to graphic form. From such graphs, the value of the elastomeric modulus and the damping coefficient can be obtained quickly from the experimental data of each aortic Method and Results The experimental method has been presented in detail elsewhere. 3 In brief, a specimen aortic strip is hung from an electromechanical vibrator of variable frequency in a chamber where the environment can be maintained at constant temperature and humidity. With a load on the specimen , the frequency of the vibrator is adjusted until maximum amplitude at resonance is observed by means of an oscilloscope. If the amplitude of vibration is less than 5 per cent …