Potentiation of myocardial contractility by continual premature extra-activations.

• The strength of myocardial contraction is influenced directly and strongly by the intervals separating individual beats. The large number of studies on this phenomenon during the 95 years since its original recognition have recently been reviewed. Interval-dependent changes in the strength and time course of contraction result both from alterations in the duration of the active state and from changes in the degree of activation of the contractile element (shifts of the force-velocity curve). Every activation of mammalian myocardium appears to leave behind two temporary changes in the muscle that have opposing effects on the degree of activation in subsequent beats.' The extent of cumulation of both changes depends on rate and rhythm of contraction, and the interaction of their respective levels of cumulation can account for all interval-dependent variations in the degree of activation. In ventricular heart muscle of most mammalian species the degree of activation and the rate of tension development in an isometric contraction increase steadily with frequency over the entire physiologic range.'