Relaxation of epicardial strains after volume changes in the isolated guinea pig left ventricle

Cardiac muscle is a porous viscoelastic material, exhibiting stress relaxation and hysteresis after being passively stretched. We investigated whether these material properties are also manifested in relaxation of epicardial segment lengths of the passive diastolic left ventricle (LV). For this purpose LV pressure and biaxial epicardial strains were measured simultaneously in isolated guinea pig hearts, arrested in diastole and instrumented to manipulate LV volume. Our study confirmed the existence of epicardial strain relaxation in both axial and circumferential directions, though it was much less expressed than LV pressure relaxation. Since the volume calculated from the segment lengths also revealed relaxation phenomena, our findings suggest that epicardial strain relaxation was connected with exchange of fluid from the LV cavity into the tightened epicardial vessels and back and not with the transformation of the LV shape.