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 function requires the two phases of this cycle to be normal. Diastolic properties determine the degree of filling of the ventricles and the strength 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 result, hydraulic energy is imparted to blood (external ventricular work) that manifests itself as blood pressure and flow. Given the cyclic nature of cardiac activity, the external ventricular work has steady and pulsatile components. The steady component is energy lost during steady flow because of vascular resistance, and the pulsatile work is that lost in arterial pulsations and mainly depends on the aortic impedance. Thus, the characteristics of the arterial tree will determine the relative contribution of these two components to blood flow and the efficency of the heart. In addition, the arterial tree modifies the different waves (pressure and flow) traveling in the circulation. These modifications have important consequences for cardiac function. The ventricle and the arterial tree constitute a coupled biological system, and its overall performance is a function of the behavior of each unit at any given moment.