Trabeculated embryonic myocardium shows rapid stress relaxation and non-quasi-linear viscoelastic behavior

Passive viscoelastic behavior is important in embryonic cardiovascular func tion, influencing the rate and magnitude of contraction and relaxation. We hypothesized that if viscoelastic behavior is influenced by interstitial fl uid flow, then the stage-21 (3 1/2 d) and stage-24 (4 d) chick myocardium w ith large intertrabecular spaces will exhibit much different viscoelastic b ehavior than stage-16 (2 1/2 d) and stage-18 (3 d) compact myocardium and a non-quasi-linear response. Excised left ventricular sections were tested w ith ramp-and-hold stress relaxation tests at axial stretch ratios of 1.05 : 1.1 : 1.2 : 1.3. The measured stress relaxation was much more rapid than p reviously observed in the compact, non-trabeculated myocardium. The reduced relaxation curves depended significantly on the stretch level. A continuou s-spectrum quasi-linear relaxation Function described their shape well but the model-fit parameters also depended on the stretch level. Sinusoidal str etching of ventricular sections at rates from 0.2 to 25 Hz showed that the steepening of stress-strain curves with increasing strain rate was half as much as predicted by a quasi-linear model. Hysteresis ranged from 25-35%, v aried little with loading rate from 0.2 to 8 Hz, and was twice that predict ed from a quasi-linear model. Doubling the viscosity of the perfusate in st ress-relaxation tests produced increased stiffness and decreased relaxation rate. These results demonstrate that the passive viscoelastic behavior of the trabeculated embryonic myocardium is markedly different from that of yo unger, compact myocardium and is not quasi-linear. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.