Therefore, total body water as a percentage of body weight equals 50% in this woman. Substituting these values into the indicator dilution equation, we get Suppose during the equilibration period, urinary, respiratory, and cutaneous losses of D2O are 0.12 mL. A concentration of 0.001 mL D2O/mL plasma water is found. After a 2-hr equilibration period, a blood sample is withdrawn, and the plasma is separated and analyzed for D2O. We inject 30 mL of deuterium oxide (D2O) as an isotonic saline solution into an arm vein. For example, suppose we want to measure total body water in a 60-kg woman. To measure total body water, heavy water (deuterium oxide), tritiated water (HTO), or antipyrene (a drug that distributes throughout all of the body water) is used as an indicator. If there was loss of indicator from the fluid compartment, the amount lost is subtracted from the amount administered. Volume = Amount of indicator/ Concentration of indicator
The concentration of the indicator in the plasma at equilibrium is measured, and the distribution volume is calculated from this formula
After allowing sufficient time for uniform distribution of the indicator throughout the compartment, a plasma sample is collected.
A known amount of a substance (the indicator), which should be confined to the compartment of interest, is administered. The indicator dilution method can be used to determine the size of body fluid compartments (see Chapter 14).
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