Arterial impedance as ventricular afterload.

• "Afterload," defined as the external factors that oppose the shortening of muscle fibers, is as important a determinant of myocardial performance in the intact heart (1-3) as it is in the papillary muscle preparation (4). Many investigators are searching for an "index" of contractile state that will be independent of all external forces, but until there is more general agreement on such an index than is now the case afterload must be measured and taken into account in any evaluation of ventricular performance. An operational definition of afterload appropriate to the intact ventricle is more difficult to formulate, however, than one might at first suppose. In an isotonic experiment on a papillary muscle, the load is simply the weight the muscle is required to lift. Weight has the same dimensions as force, and, if inertial effects are neglected, both are equal to the mass of the object suspended from the muscle multiplied by the gravitational acceleration constant. The force is exerted on a cross section of muscle, and afterload is often expressed as force per unit area or stress. In this kind of experiment, the suspended weight is directly responsible for the muscular stress, and it makes no difference which is regarded as the "load." In the intact ventricle, however, the assumption that load and stress are synonymous terms leads to confusion. Stress in the ventricular wall is related to the external constraints on ventricular ejection, but the relationship is complicated by the globular shape of the contracting chamber and by the physical properties of the artery into which it ejects blood. The shape of the ventricle is significant because the pressure imparted to the blood by the myocardium is related to, but not the same as, tangential stress in the ventricular wall. The relationship between them would be relatively simple mathematically if the ventricle were of some regular