Effect of perfusion pressure on diastolic stress-strain relations of isolated rat papillary muscle.

The effect of perfusion on diastolic muscle properties was investigated in six isolated right ventricular papillary muscles from rat hearts perfused with a crystalloid solution via the septal artery. Stress-strain relations were obtained at different perfusion pressures. Increased perfusion pressure caused an increase of stress at large strains but a decrease of stress at low strains. Thus stress-free strain increased with increasing perfusion pressure. Stress-strain relations of a given muscle at different perfusion pressures (range 12-122 cmH2O) intersected at a single "crossover" strain. Muscle stiffness, defined as the slope of the stress-strain relation, increased at all strains. Muscle diameter measurements indicated that the observed changes of the stress-strain relation occurred in association with vascular filling rather than with formation of edema. To explain the findings, the papillary muscle was modeled by two parallel compartments: muscle cells (together with the extracellular matrix) and vasculature. Perfusion was assumed to have an effect on the axial vascular properties but not on muscle cells. Combination of the stress-strain data of the muscle compartment and the vascular compartment (taken from literature) predicted stress-strain relations similar to those obtained in our perfused papillary muscles. We conclude that increased muscle stiffness at increased perfusion pressure is mainly caused by pressure-dependent changes in mechanical behavior of the vascular compartment.