Force-frequency relationship. A basis for a new index of cardiac contractility?

The way that contractility varies between beats—the force-frequency relationshipwas determined for rabbit papillary muscles held at different lengths. When the maximum rate of rise of tension in a contraction, Fmai, was used as a measure of contractility, the way contractility changed between contractions was independent of length. That is, when the values of Fmax obtained at one length were multiplied by an appropriate scaling factor, the force-frequency relationship determined at that length was indistinguishable from the force-frequency relationship determined at another length. If, however, the peak tension in a contraction was used as a measure of contractility, the force-frequency relationship generally was not independent of muscle length. Therefore, it is proposed that the ratio of two values of Fmax obtained by perturbing the rate of stimulation at any given length should be tried as a lengthindependent index of the inotropic state of the muscle.