comp

Volume of Distribution

Volume of distribution (V) relates the amount of drug in the body to the concentration of drug (C) in blood or plasma:

The volume of distribution may be defined with respect to blood, plasma, or water (unbound drug), depending on the con-centration used in equation (1) (C = Cb, Cp, or Cu).

That the V calculated from equation (1) is an apparent volume may be appreciated by comparing the volumes of distribution of drugs such as digoxin or chloroquine (Table 3–1) with some of the physical volumes of the body (Table 3–2). Volume of distri-bution can vastly exceed any physical volume in the body because it is the volume apparently necessary to contain the amount of drug homogeneously at the concentration found in the blood, plasma, or water. Drugs with very high volumes of distribution have much higher concentrations in extravascular tissue than in the vascular compartment, ie, they are not homogeneously distrib-uted. Drugs that are completely retained within the vascular com-partment, on the other hand, have a minimum possible volume of distribution equal to the blood component in which they are dis-tributed, eg, 0.04 L/kg body weight or 2.8 L/70 kg (Table 3–2) for a drug that is restricted to the plasma compartment.