A Constant - Change ( Single Injection ) Methodforthe Estimation of the Volume of Distribution of Substancesin Bodyfluid Compartments

There are at least two problems in estimating the volume of a body fluid compartment. First, a substance must be found which is distributed in the volume to be measured, and second, a method must be made available which will measure the distribution of the substance. In the literature, substance and method are rarely, if ever, separated. Kruhoffer (1), for example, states that an ideal indicator of the extracellular fluid space should possess the following properties: "1) When sufficient time has elapsed for complete distribution the material must be found everywhere in the extracellular fluid and exclusively there and the concentrations of it must be uniform throughout the interstitial fluid and throughout plasma water, the ratio between the concentrations in the two compartments being a known fixed figure. 2) It must not be eliminable from the extracellular space. 3) It must not possess so high an osmotic pressure, that it causes a perceptible displacement of water from the intracellular to the extracellular space. 4) not to be toxic." When complete distribution has been attained for a non-electrolytic indicator of this kind, the extracellular fluid volume may be calculated as follows: