Rate of Drug Distribution
In many cases, groups of tissues with similar perfusion-partition ratios all equilibrate at essentially the same rate such that only one apparent phase of distribution is seen (rapid initial fall of concentration of intravenously injected drug, as in Figure 1-3B). It is as though the drug starts in a "central " volume (Figure 1-1), which consists of plasma and tissue reservoirs that are in rapid equilibrium with it, and distributes to a "final" volume, at which point concentrations in plasma decrease in a log-linear fashion with a rate constant of k (Figure 1-3B).
The volume of distribution at steady state (Vgg) represents the volume in which a drug would appear to be distributed during steady state if the drug existed throughout that volume at the same concentration as that in the measured fluid (plasma or blood). V,s also may be appreciated as shown in Equation (1-8), where Vc is the volume of distribution of drug in the central compartment and Vt is the volume term for drug in the tissue compartment:
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