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Overview :
The apparent volume of distribution (Vd) is a term used to
refer to the extent of a drug's extravascular distribution at equilibrium. It is
a hypothetical volume into which a quantity of drug would distribute if its
concentration in the entire volume were the same as that in the plasma. The
volume of distribution is the major determinant of the loading dose.
Vd =
= ((amount of drug in the body) / (plasma concentration of
drug))
The volume of distribution expresses the constant
relationship between the amount of drug in the body and the plasma concentration
at equilibrium.
Its importance lies in that it determines the fraction
available to the organs of elimination.
The volume of distribution is dependent on the size of the
patient. It appears to more closely correspond to body weight (sometimes ideal
body weight) than to body surface area.
Approximation of
Vd
The Vd for the elimination phase of a 2-compartment drug with
first order kinetics given intravenously can be approximated by estimating the
<hypothetical concentration of drug at time 0> by extrapolation of the equilibrium
phase plot to zero time. Since the amount of drug in the body at time 0 is the
dose administered:
Vd =
= ((dose) / (hypothetical concentration of drug at time
0))
Simple 1st Order
Models
For simple 1st-order models,
volume of distribution =
= ((clearance) * (drug half-life) / (0.693)) =
= ((clearance) * (drug half-life) * (1.44))
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