| Abstract: | Several mechanical indices predicted from a model of cardiac muscle contraction are tested. In the in-vivo canine heart, dp/dtmax and t-dp/dt, the time interval from onset of contraction to dp/dtmax, were measured. The product of these parameters Ap = t-dp/dtmax was calculated. t-dp/dt was shortened when heart rate was elevated and remained constant when ventricular end diastolic volume was changed. Ap increased with augmentation of ventricular end diastolic volume. To achieve constant muscle length when heart rate is changed, analogous tension measurements (assigned as dT/dtmax, t-dT/dt and AT) of prestretched Walton Brodie strain-gauge arch had been taken instead of pressure measurements. In the experiments in which Tmax, maximal isometric tension, was not changed for various heart rates, AT was also unchanged. These results are consistent with the predictions that t-dp/dt and Ap can be used as two independent mechanical indices: 1) t-dp/dt for the evaluation of the "time-dependent properties" of contraction and 2) Ap, for the evaluation of the "quantitative properties" of contraction. The advantages of applying these two mechanical indices for use in the intact ejecting heart, instead of the well-established parameters Vmax and Po are emphasized. |
