VISCOELASTIC BEHAVIOR OF THE ISOLATED GUINEA-PIG LEFT-VENTRICLE IN DIASTOLE

To determine left ventricular (LV) viscoelastic properties during acut e volume changes, the relaxation of LV pressure (2-Fr, Millar) at stea dy LV volume after a known volume change was measured in 14 isolated g uinea pig left ventricles arrested in diastole. The left ventricle was loaded and unloaded by manual injection and withdrawal of saline in 1 0 x 0.1-ml steps, controlling the steadiness of LV volume by measuring LV major and minor diameters (ultrasonic crystals). Cyclic stepwise v olume loading and unloading resulted in a hysteresis loop, the complex ity of which was caused by stress relaxation at each steady volume. Wi th the use of linear regression analysis, the gross elastic effect of the pressure signal was separated from the viscoelastic part, decompos ed into the fast and the slow component with time constants of relaxat ion equal to 1 and 20 s, respectively. The amplitudes of the fast and the slow component showed that 1) stress relaxation is more expressed at higher LV volume and 2) it is asymmetric, i.e., it is well expresse d during volume loading and almost absent during volume unloading. Bot h suggest that viscoelasticity of passive myocardium is not quasilinea r, 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 extracellular fluid filtration.