Outflow Resistance as an Independent Determinant of Cardiac Performance

Abstract

In the modified heart-lung preparation of the dog, sudden increases in outflow resistance were imposed during the diastolic interval between two beats. The changes in cardiac performance which were recorded during the first beat at higher resistance represent the response to augmented resistance per se, since there was no change in the initial length or initial tension of the ventricular myocardial fibers. Increased resistance elicited an immediate elevation of the peak aortic systolic pressure. This pressure rise was a direct function of the magnitude of the outflow resistance, and the effect was enhanced at higher left atrial pressures. Peak aortic flow and stroke volume were inversely related to the magnitude of the outflow resistance. At slight to moderate increases of resistance, stroke work was relatively unaffected; with more severe augmentations of resistance, however, stroke work varied inversely with outflow resistance. Arterial compliance played an important role in the ventricular response. For any given increase of resistance, the reductions of peak aortic flow and ventricular power were more severe when the arterial system was more rigid. It is concluded that the limits of cardiac performance are determined by the initial conditions, such as fiber length or tension, previous activity, temperature, and nervous and humoral factors. Within these limits, however, ventricular performance is determined by the afterload, of which the outflow impedance is an integral component.