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Apparent Volume of Distribution |  |
| The pharmacokinetic measure used to indicate the pattern of distribution of a drug in plasma and in the different tissues, as well as the size of the compartment into which a drug would seem to have distributed in relation to its concentration in plasma, is known as the apparent volume of distribution (Vd). It is usually reported as liters (L) or as liters per kilogram (L/kg) if corrected for the body weight of the animal. The apparent Vd for a drug is determined by its degree of
water or lipid solubility, the extent of plasma- and tissue-protein binding, and the perfusion of tissues. Drugs that tend to maintain high concentrations in the plasma because of low lipid solubility, extensive binding to plasma proteins, and diminished tissue binding have low Vd. The reverse is true for drugs with high apparent Vd. The value of Vd is characteristic for a drug and is usually constant over a wide dose range for a given species of animal. However, a number of
clinically significant factors can influence the Vd. Included among these are age; functional status of the kidneys, liver, and heart; fluid accumulations; concentration of plasma proteins; acid-base status; inflammatory processes or necrosis; and any other causes for alteration in the degree of plasma-protein binding. Vd is used to determine dose. A dose necessary to achieve desired plasma concentration can be calculated from the formula D = C × Vd × body wt (in kg), in which D is
the dose and C is the required plasma concentration for a given drug. |
