RESIDUAL STRAIN IN RAT LEFT-VENTRICLE

Residual stress in an organ is defined as the stress that remains when all external loads are removed. Residual stress has generally been ignored in published papers on left ventricular wall stress. To take residual stress into account in the analysis of stress distributions in a beating heart, one must first measure the residual strain in the no-load state of the heart. Residual strains in equatorial cross-sectional rings (2-3 mm thick) of five potassium-arrested rat left ventricles were measured. The effects of friction and external loading were reduced by submersing the specimen in fluid, and a hypothermic, hyperkalemic arresting solution containing nifedipine and EGTA was used to delay the onset of ischemic contracture. Stainless steel microspheres (60-100 μm) were lightly embedded on the surface of the slices, and the coordinates of the microspheres were digitized from photographs taken before and after a radial cut was made through the left ventricular free wall. Two-dimensional strains computed from the deformation of a slice after one radial cut were defined as the residual strains in that slice. It was found that the distributions of the principal residual stretch ratios were asymmetric with respect to the radial cut: in areas where substantial transmural strain gradients existed, the distributions of strain components were different on the two sides of the radial cut. A second radial cut produced deformations significantly smaller than those produced from the first radial cut. Hence, a slice with one radial cut may be considered stress free. Our results show that the circumferential residual strain was negative in the endocardial region (stretch ratios were significantly less than 1.00, p < 0.001 for five slices), while those in the epicardial region were either positive or negative with a much smaller magnitude. This residual strain distribution suggests that, in general, there is compressive circumferential residual stress at the inner layers of the ventricle. A residual stress distribution of this nature may reduce the endocardial peak in circumferential tensile stress at end diastole predicted by analytical models using nonlinear material properties.