STRAIN-SOFTENING IN RAT LEFT-VENTRICULAR MYOCARDIUM

We investigation whether strain softening (or the Mullins effect) may explain the reduced left ventricular stiffness previously associated w ith the strain-history-dependent preconditioning phenomenon. Passive p ressure-volume relations were measured in the isolated arrested rat he art during LV balloon inflation and deflation cycles. With inflation t o a new higher maximum pressure, the pressure-volume relation became l ess stiff, particularly in the low (diastolic) pressure range, without a significant change in unloaded vertricular volume. In five differen t lending protocols in which the maximum passive cycle pressure ranged from 10 to 120 mmHg, we measured influences at 10 mmHg in LV volume u p to 350 percent of inloaded volume that depended significantly on the history (p < 0.05) and magnitude (p < 0.01) of maximum previous press ure. Although a quasi-linear viscoelastic model based on the pressure- relaxation response could produce a nonlinear pressure-volume relation with hysteresis, it was unable to show any significant change in vent ricular stiffness with new maximum pressure. We incorporated a strain Softening theory proposed by Johnson and Beatty (1992) into the model by modifying the elastic response with a volume-amplification factor t hat depended an the maximum previous pressure. This model more accurat ely reproduced the experimentally observed behavior. Thus, the precond itioning behavior of tile myocardium is better explained by strain sof tening rather than viscoelasticity and may be duc to damage to elastic components, rather than the effects of viscous tissue components.