Increased passive stiffness of short-term pressure-overload hypertrophied myocardium in cat

The passive stress-strain relationship of right ventricular papillary muscles from 10 normal and 9 experimental cats with short-term pressure-overload right ventricular hypertrophy-failure was examined by plotting the logarithm of instantaneous stress (ln sigma) against the natural strain calculated as ln(l/l0) where l = instantaneous length and l0 = length at zero force. Such a stress-strain relationship was well approximated by a linear relationship. The slope K obtained from this linear relationship was higher in the hypertrophy-failure muscles (normal, 15.01 +/- 0.87 (SEM); hypertrophy-failure, 31.79 +/- 4.09; P less than 0.005). The value of the intercept, ln C was similar in the two groups (normal, -4.33 +/- 0.20; hypertrophy-failure, -4.71 +/- 0.10). This analysis indicates the the ln sigma-natural strain relationship is linear in the papillary muscle and the slope of this relationship, an index of stiffness, is increased in hypertrophy-failure muscles. Using a three-element muscle model, it is shown that increased diastolic stiffness may contribute to the decreased systolic performance.