The ability to measure output accurately is essential for performing physiological studies involving the heart and managing clinical problems in patients with heart disease or heart failure. Cardiac output is measured either by one of several applications of the Fick principle or by observing changes in the volume of the heart during the cardiac cycle.
Cardiac Output Can Be Measured Using Variations of the Principle of Mass Balance
The use of mass balance to measure cardiac output is best understood by considering the measurement of an unknown volume of liquid in a beaker (Fig. 14.9). The volume can be determined by dispersing a known quantity of dye throughout the liquid and then measuring the concentration of dye in a sample of liquid. Because mass is conserved, the quantity of dye (A) in the liquid is equal to the concentration of dye in the liquid (C) times the volume of liquid (V):
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