Abstract

In the evaluation of the mechanical function of the ventricle, attention has been devoted to the analysis of the heart as a muscle rather than as a pump. Several contractile properties of isolated mammalian cardiac muscle are now well characterized. Yet the application of these muscle concepts to the intact heart remains controversial due to the many assumptions in modeling the ventricle, failure to account for some conceptual limitations, and the complex physiologic loading of a muscle-pump system, etc. (1–3). The ventricle is not called on to lift a constant load but to eject a viscous fluid into a viscoelastic vascular system (2, 3). Recently, we analyzed the properties of isolated cardiac muscle as an integral part of an ejecting ventricle (4). The imposed loading included inertial, resistive, and capacitive components of the cardiovascular system (Figs. 1 and 2). During this “dynamic” loading, as long as loading was dynamically increasing or decreasing, velocity of shortening was respectively lower or higher at any muscle length and total load, compared to velocity at the same length and load under static (constant preload and aiterload only) loading. Only when df/dl was zero, was velocity of shortening the same under static and dynamic loading (Fig. 3).