This paper describes the concept of effective mass, which provides a quantitative explanation for the forbidden indicator phenomenon. The forbidden indicator phenomenon pertains to the problems encountered when the concentration of an indicator used to measure volume or flow by the indicator-dilution technique approaches the concentration of the solution to which it is to be added. This phenomenon is of particular concern when applying the saline-dilution method with hypertonic saline, to measurement of cardiac output. Clinically, one would like to give as little additional salt to a patient as possible while maintaining a signal-to-noise ratio adequate to give reliable results. This paper develops an equation that permits accurate calculation of volume or flow by the indicator-dilution method, even when indicator solutions with concentrations approaching that of the original solution are used (i.e., the forbidden indicator). The validity of this equation is demonstrated experimentally using the saline-dilution method to measure flow in a hydraulic model of the circulation.
