The dynamics of ventricular contraction: force, length, and shortening.

The heart functions as a muscular pump. The determinants of muscle fiber shortening, and consequently the extent of wall shortening, regular ventricular stroke volume. This concept of ventricular function permits the unification of the pumping characteristics of the ventricle with the behavior of its constituent muscle fibers. The isovolumetric force-length relation of the muscular wall describes the maximum force that can be sustained by any given fiber length. The "slope" of this nearly linear maximum force-length relation is determined by the contractile state of the myocardium. Because this relation determines the maximum force sustained by a given fiber length it establishes the limit to fiber shortening in the ejecting ventricle. Hence, when the instantaneous shortening load becomes maximum for the instantaneous shortening length, shortening will cease. During ejection, the rate and extent of fiber shortening are determined by the accompanying trajectory of instantaneous force (i.e., a function of chamber dimension and pressure), the instantaneous shortening length, and the contractile state of the myocardium. The interrelationship between force, length, and shortening, which may be used to describe the behavior of the muscular pump and alterations in its contractile state, indicate that the myocardium is capable of adjusting to instantaneous variations in force and length during its contraction. Furthermore, these properties of the muscular pump provide a framework for the derivation of the traditional ventricular function curve and an explanation of the mechanical disadvantage at which the failing heart operates.