CARDIAC MECHANICAL ENERGY AND EFFECTS ON THE ARTERIAL TREE

Blood flow pulsatility is the result of the heart's activity as a pump unable to develop steady flow, and its interaction with the arterial tree. Thus, the heart is a cyclic energy generator whose adequate func tion requires the two phases of this cycle to be normal. Diastolic pro perties determine the degree of filling of the ventricles and the stre ngth of the following systole. Systole, in turn, must generate enough energy to overcome forces opposing ejection. These can be divided into internal (the mechanical characteristics of the ventricle itself) and external loads (the characteristics of the arterial tree). As a resul t, hydraulic energy is imparted to blood (external ventricular work) t hat manifests itself as blood pressure and flow. Given the cyclic natu re of cardiac activity, the external ventricular work has steady and p ulsatile components. The steady component is energy lost during steady flow because of vascular resistance, and the pulsatile work is that l ost in arterial pulsations and mainly depends on the aortic impedance. Thus, the characteristics of the arterial tree will determine the rel ative contribution of these two components to blood flow and the effic ency of the heart. In addition, the arterial tree modifies the differe nt waves (pressure and flow) traveling in the circulation. These modif ications have important consequences for cardiac function. The ventric le and the arterial tree constitute a coupled biological system, and i ts overall performance is a function of the behavior of each unit at a ny given moment. Copyright (C) 1997 by W.B. Saunders Company.