ARTERIAL-WALL MECHANICS IN CONSCIOUS DOGS - ASSESSMENT OF VISCOUS, INERTIAL, AND ELASTIC-MODULI TO CHARACTERIZE AORTIC-WALL BEHAVIOR

To evaluate arterial physiopathology, complete arterial wall mechanical characterization is necessary. This study presents a model for determining the elastic response of elastin (sigma(E) where sigma is stress), collagen (sigma(C)), and smooth muscle (sigma(SM)) fibers and viscous (sigma(n)) and inertial (sigma(M)) aortic wall behaviors. Our work assumes that the total stress developed by the wall to resist stretching is governed by the elastic modulus of elastin fibers (E(E)), the elastic modulus of collagen (E(C)) affected by the fraction of collagen fibers (f(C)) recruited to support wall stress, and the elastic modulus of the maximally contracted vascular smooth muscle (E(SM)) affected by an activation function (f(A)). We constructed the constitutive equation of the aortic wall on the basis of three different hookean materials and two nonlinear functions, f(A) and f(C): sigma = sigma(E) + sigma(C) + sigma(SM) + sigma(eta) + sigma(M) = E(E) . (epsilon - epsilon(0E)) + E(C) . f(C) . epsilon + E(SM) . f(A) . epsilon + eta . d epsilon/dt + M . d(2) epsilon/dt(2) where epsilon is strain and epsilon(0E) is strain al zero stress. Stress-strain relations in the control state and during activation of smooth muscle phenylephrine, 5 mu g . kg(-1) . min(-1) IV) were obtained by transient occlusions of the descending aorta and the inferior vena cava in 15 conscious dogs by using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. The f(C) was not linear with strain, and at the onset of significant collagen participation in the elastic response (break point of the stress-strain relation), 6.02 +/- 2.6% collagen fibers were recruited at 23% of stretching of the unstressed diameter. The f(A) exhibited a skewed unimodal curve with a maximum level of activation at 28.3 +/- 7.9% of stretching. The aortic wall dynamic behavior was modified by activation increasing viscous (eta) and inertial (M) moduli from the control to active state (viscous, 3.8 +/- 1.3 x 10(4) to 7.8 +/- 1.1x10(4) dyne . s . cm(-2), P < .0005; inertial, 61 +/- 42 to 91 +/- 23 dyne . s(2) . cm(-2), P < .05). Finally, the purely elastic stress-strain relation was assessed by subtracting the viscous and inertial behaviors.