Measurement of Body Fluid Compartments: Indicator-Dilution Technique:
A volume indicator is a substance which when injected intravenously (i.v.) will, over time, become uniformly distributed in one or more of the body fluid compartments; in PV, in ECF (PV + ISF) or in TBW ( PV + ISF + ICF). In each case the plasma concentration of the indicator will be its concentration throughout its volume of distribution.
It is not possible to measure the volume of all fluid compartments directly. Where direct measurement is possible, a known amount of an indicator substance may be injected intravenously and time allowed until it becomes evenly distributed in those compartments into which it can diffuse. The plasma concentration of the substance is then measured, and the volume (VI) in which the indicator has become diluted is calculated by dividing the indicator dose (DI) by its plasma concentration (PI). This procedure is known as the indicator-dilution technique and is illustrated in the first figure.
The ideal indicator for this method of measuring the volume of fluid compartments is:
It is important to realize, however, that ideal indicators do not exist. For this reason, different indicators may generate somewhat different values when used to estimate the same fluid compartment volume. Although non- metabolized indicators are available almost all those in use are excreted, primarily in the urine, during the equilibration period. The amount excreted must be measured by collecting the urine excreted between the time of injection and the time of blood sampling and measuring the urine indicator concentration. The urine concentration multiplied by the urine volume equals the amount of indicator excreted (EI). The injected dose must be adjusted by subtracting from the dose the amount of indicator excreted between the time of injection and the time of blood sampling. For reliable results it is important that the blood sample be obtained after equilibrium of concentration (uniform distribution of indicator) has occurred. Equilibrium times have been determined empirically for each indicator in common use as illustrated in the second figure.
Indicators for estimating fluid compartment volumes fall into three categories which are used for: