Diastolic stress-strain relation of nonexcised blood-perfused canine papillary muscle

We studied diastolic stiffness of 10 coronary-perfused twitching papillary muscles of the canine right ventricle. The muscle beat at a regular sinus rhythm of 122 +/- 20 (SD) beats/min at 37 degrees C. They were stretched slowly at a constant rate. Diastolic force increased exponentially with the stretch. Calculating Lagrangian stress (sigma) and strain (epsilon) from diastolic force and length, we found a linear relationship between ln sigma and epsilon within the physiological range of strain (0.025 less than epsilon less than 0.4). This indicates that the diastolic stress-strain relationship of the canine papillary muscle can be approximated by a single exponential curve: sigma = a.exp(b.epsilon). The mean +/- SD of the stiffness constant b was 18.0 +/- 3.2 (dimensionless). Our b values are comparable to those of dogs and human subjects, either indirectly assessed from the ventricular pressure-volume relationship or directly obtained in excised quiescent muscle specimens. Different coronary perfusion pressures (75-125 mmHg) in 10 muscles showed a statistically significant positive correlation to b values.