Aortic input impedance in cardiovascular disease

0 VER the past 2 decades considerable interest has centered around attempts to define and quantify hemodynamic consequences of cardiovascular disorders. In general, these attempts employed traditional hemodynamic measurements, like aortic and ventricular pressures, cardiac output, peripheral resistance, ventricular volumes, ejection fraction, etc., to assess effects of a wide variety of cardiovascular disorders. These data provide valuable information relative to some of the hemodynamic alterations that accompany many cardiovascular disorders. However, to extend our understanding of various cardiovascular disorders, other approaches are required. One approach to a more complete description of the cardiovascular effects of various disorders, relative to the total external load imposed on the ejecting left ventricle, is to use pulsatile ascending aortic pressureflow relations. Since blood pressure and flow waves generated by the left ventricle are modified in a frequency dependent fashion by the arterial system, data derived from classical analysis using only steady-flow hydraulic terms are limited. Thus, specific effects resulting from the vascular mechanical alterations, per se, cannot be obtained. Interpretation of steady-flow type data is further limited by dependence of pressure and flow waves on the caliber of both large and small vessels. Some cardiovascular disorders can affect different size vessels in divergent ways. In addition, the physical properties of blood vessels and even blood itself affect pressure and volume of blood flowing within the system. The basis for pulsatile pressure and flow wave analysis and representation by the aortic input impedance spectrum has been reviewed in detail.lm7 Elsewhere in this symposium, we developed the rational for consideration of this complex function as the total systemic vascular load that opposes left ventricular ejection. As noted in that section, aortic input impedance provides information about both pulsatile and nonpulsatile components of vascular load.8-‘3 This function is relatively unaltered by changes in classical determinants of left ventricular performance, such as heart rate, filling pressure, and myocardial contractility.4,5,9 Briefly summarized, if the left ventricle produces a nonpulsatile (steady) flow, work expended to move blood through the vascular system could be calculated from the product of mean flow through the system and pressure drop across the system. It could be concluded correctly that energy dissipating properties of this system resided in dimensions of vessels and blood viscosity. Because the left ventricle produces not a steady but an intermittent or pulsatile blood flow, other factors become important to the control and energy cost of blood flow through the vascular system. Two major factors are the viscoelasticity of blood vessel walls and pressure and flow wave reflections within the arterial system. Of lesser importance are longitudinal tethering of arteries, which affect propagation of pulse waves and effects of inertia of blood. Resistance to steady blood flow can be determined simply as the ratio of mean pressure to mean flow at entry to the system, hence the term input resistance. The left ventricle, however, does not pump directly into the resistance vessels. Only a fraction of the left ventricular volume, ejected during each systole, is immediately displaced peripherally into the anteriolar resistance bed. The remaining fraction of the ejected volume is taken up by the large, central distensible arteries and slowly dissipated peripherally throughout diastole. Total opposition to this systolic and diastolic (pulsatile) blood flow can be described by the relationship of pulsatile pressure and flow wave forms obtained throughout the cardiac cycle. This complex function is termed aortic input impedance. Since pulsatile pressure and flow waves are influenced by physical properties of the arterial system, various components of input impedance provide detailed