A CONSTANT-CHANGE (SINGLE INJECTION) METHOD FOR THE ESTIMATION OF THE VOLUME OF DISTRIBUTION OF SUBSTANCES IN BODY FLUID COMPARTMENTS

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Abstract
A method is presented for estimating the apparent volume of distribution of substances in body fluid compartments of intact animals. For a two compartment system, the volume of distribution of an indicator equals the urinary excretion of the indicator during a certain time interval divided by the change in plasma indicator concentration during the same time interval. The method is compared with the simple dilution, extrapolation and constant infusion methods, using inulin, mannitol and sucrose as indicators, and found to have the following advantages: (1) It is independent of the amount of indicator used, and (2) prior to the period of measurement, it is unrelated to the amount of indicator lost from the fluid compartment in which the indicator is distributed, (3) The indicator need not be completely excreted before the distribution of a substance can be determined following a second injection, (4) It offers the possibility of estimating small changes in the distribution of the indicator, (5) It requires that only the relative change in plasma concentration of the indicator be measured.
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