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 mechanica l characterization is necessary. This study presents a model for deter mining the elastic response of elastin (sigma(E) where sigma is stress ), collagen (sigma(C)), and smooth muscle (sigma(SM)) fibers and visco us (sigma(n)) and inertial (sigma(M)) aortic wall behaviors. Our work assumes that the total stress developed by the wall to resist stretchi ng is governed by the elastic modulus of elastin fibers (E(E)), the el astic modulus of collagen (E(C)) affected by the fraction of collagen fibers (f(C)) recruited to support wall stress, and the elastic modulu s of the maximally contracted vascular smooth muscle (E(SM)) affected by an activation function (f(A)). We constructed the constitutive equa tion of the aortic wall on the basis of three different hookean materi als and two nonlinear functions, f(A) and f(C): sigma = sigma(E) + sig ma(C) + sigma(SM) + sigma(eta) + sigma(M) = E(E) . (epsilon - epsilon( 0E)) + E(C) . f(C) . epsilon + E(SM) . f(A) . epsilon + eta . d epsilo n/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 descen ding thoracic aortic pressure (microtransducer) and diameter (sonomicr ometry) measurements. The f(C) was not linear with strain, and at the onset of significant collagen participation in the elastic response (b reak point of the stress-strain relation), 6.02 +/- 2.6% collagen fibe rs were recruited at 23% of stretching of the unstressed diameter. The f(A) exhibited a skewed unimodal curve with a maximum level of activa tion at 28.3 +/- 7.9% of stretching. The aortic wall dynamic behavior was modified by activation increasing viscous (eta) and inertial (M) m oduli from the control to active state (viscous, 3.8 +/- 1.3 x 10(4) t o 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 elast ic stress-strain relation was assessed by subtracting the viscous and inertial behaviors.