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Volume of Distribution (Vd)

Description

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   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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Specialty

ICD-10