Viscoelastic behavior of the isolated guinea-pig left ventricle in diastole. Starc, Vito, Edward L. Yellin, Srdjan D. Nikolic. School of Medicine, University of Ljubljana, Slovenia, Albert Einstein College of Medicine, New York and Stanford University Medical School, Stanford
APStracts 3:0163H, 1996.
To determine left ventricular (LV) viscoelastic properties during acute volume changes, the relaxation of LV pressure (Millar 2F) at steady LV volume after a known volume change was measured in 14 isolated guinea pig left ventricles arrested in diastole. The left ventricle was loaded and unloaded by manual injection and withdrawal of saline in 10 x 0.1 ml steps, controlling the steadiness of LV volume by measuring LV major and minor diameters (ultrasonic crystals). Cyclic stepwise volume loading and unloading resulted in a hysteresis loop, the complexity of which was caused by stress relaxation at each steady volume. Using linear regression analysis, the gross elastic effect of the pressure signal was separated from the viscoelastic part, decomposed into the fast and the slow component with time constants of relaxation 1 and 20 sec. respectively. The amplitudes of the fast and the slow component showed that (i) stress relaxation is more expressed at higher LV volume, and (ii) it is asymmetric, i.e. it is well-expressed during volume loading and almost absent during volume unloading. Both suggest that viscoelasticity of passive myocardium is not quasilinear, when the left ventricle is subjected to aperiodic volume loading to a high LV volume. The asymmetric viscoelastic behavior is consistent with the hypothesis of the extracellular fluid filtration. 

Received 10 January 1995; accepted in final form 4 April 1996.
APS Manuscript Number H22-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 1 May 96