Afterload as a primary determinant of ventricular performance

The importance of aortic blood pressure as a determinant of left ventricular performance was systematically evaluated in the intact cat heart, and compared with the effects of afterloading on work performed by the isotonically contracting isolated cat papillary muscle. In the papillary muscle, at any one muscle length and state of contractility, work was determined by the afterload with which the muscle shortened. In the intact heart, blood pressure (BP) may be considered a gross approximation of the afterload encountered by the contracting ventricle. It was found that at any given initial left ventricular end-diastolic pressure (LVEDP), work performed was a function of the BP. At a constant BP, norepinephrine increased the work performed at any LVEDP by augmenting stroke volume. This work could be further increased by a concomitant increase of BP. Over a broad range, stroke volume was independent of the BP. The state of contractility of the myocardium sets the limits of work; however, work performance remains largely dependent on the BP (afterload). Thus, ventricular performance at a given constant inotropic state is the product of two largely independent variables, the preload (establishing initial muscle length) and the afterload.