Time-varying myocardial elastance of canine left ventricle.

A formula was derived from an active cross-bridge model for expressing the "time-varying myocardial elastance" of the left ventricular (LV) wall. To assess the validity of this model's predictions of the behavior of the intact left ventricle, eight healthy beagles were instrumented with ultrasonic crystals to measure LV diameter and a micromanometer for LV pressure measurement. During ejecting beats starting from short end-diastolic myocardial lengths, the predicted values for myocardial elastance and force closely approximated the measured values for the instantaneous external load-muscle length ratio and the external load, respectively. However, in cycles starting from long end-diastolic lengths, the predicted myocardial elastance and force values deviated from the actual values during the shortening phase of LV contraction. The differences between myocardial elastance and external load-muscle length ratio as well as those between force and external load during the shortening phase (shortening deactivation) appeared to result from an internal load in the shortening myocardium that was closely related to the product of instantaneous myocardial force and shortening velocity. Thus this model may provide reasonable approximations of myocardial elastance and force in the intact LV wall. In addition, time-varying myocardial elastance might reflect time-dependent changes in calcium activation.