Abstract
The first use of indicator substances in the circulation was apparently by Haller (l) in 1761, who determined the circulation times through inflated and collapsed lungs using a colored liquid. Herring (l) and later Vierordt (1) further explored the determination of circulation times using Prussian Blue. | Stewart (2, 3) is credited with the first continuous monitoring of indicator concentrations and with the first quantitations of cardiac output by the indicator-dilution technique. Using a conductivity apparatus connected to telephone earphones, he injected 1.5% saline and collected an "integrated' blood sample from the time of appearance of the first indicator at the sampling site (as indicated by a change in tone from the earphones) until some point on the downslope.