As outlined in Figure 1.1, blood pressure is jointly determined by cardiac output and total peripheral resistance. To answer several types of empirical questions relating to blood pressure, it is necessary to obtain corresponding measures of cardiac output or total peripheral resistance, the so-called parameters of hemodynamic functioning. Unfortunately, hemodynamic parameters are not convenient to measure.
The primary strategy for measuring cardiac output involves insertion of catheters into locations proximal to the heart so that dye can be injected into the right atrium of the heart and then detected as it is ejected from the left ventricle over a period of time. Obviously, this procedure is not easily conducted in clinical practice, nor can it be used for large-scale epidemiologic trials. Because total peripheral resistance reflects resistance to blood flow throughout the entire body, there is no known procedure for measuring it directly; however, it can be measured indirectly by dividing mean arterial pressure by cardiac output.
Although it may appear that reliable measures of cardiac output and total peripheral resistance are too cumbersome to be of any use, impedance cardiography has enabled clinical researchers to obtain measures of hemodynamic functioning with minimal hassle.