Volume of distribution may be explained using the models above. These model compartments are defined as having a consistent concentration throughout. Therefore, if the amount of drug in the compartment is known, measuring the concentration of the drug will enable calculation of the apparent volume through which the drug is distributed. The model works on the basis that the drug is equally distributed throughout that calculated volume. Usually, the drug dose given is known and the plasma concentration that would exist if distribution occurred without elimination can be calculated. The apparent volume of distribution does not exist as an identifiable anatomical entity but rather is a mathematical concept representing the composite result of multiple volumes with differing solubilities for a given drug.

Figure PK.9. Mapleson's water analogue models. The levels illustrate the situation in the early part of maintenance Reproduced with permission from Mapleson, Pharmacokinetics of inhaled anaesthetics.

Roberts C, Hug CC Jr (eds). Pharmacokinetics of Anaesthesia. Oxford, Blackwell, 1984, pp 89-111.

Figure PK.9. Mapleson's water analogue models. The levels illustrate the situation in the early part of maintenance Reproduced with permission from Mapleson, Pharmacokinetics of inhaled anaesthetics.

Roberts C, Hug CC Jr (eds). Pharmacokinetics of Anaesthesia. Oxford, Blackwell, 1984, pp 89-111.

The apparent volume of distribution is, thus, used as a tool to describe the way in which drugs are distributed. Drugs that are mainly confined to plasma have low volumes of distribution, while drugs that are highly tissue bound have a high volume of distribution. The volume of distribution may easily exceed the total volume of the body. High tissue binding results in a low plasma concentration. This low concentration spread evenly throughout the calculated volume for a given dose results in a very large value for volume of distribution.

Elimination Kinetics

Zero- and first-order kinetics are used to describe the elimination characteristics of a drug. Most drugs are eliminated with first-order kinetics.

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