APPARENT VOLUME OF DISTRIBUTION

APPARENT VOLUME OF DISTRIBUTION:-
Ø  Apparent volume of distribution and clearance are two separate and independent pharmacokinetic characteristics of a drug. Since they are closely related with physiologic mechanisms in the body, they are called as primary parameters.

        Vd =     Amount of drug in the body   =     X------ 12
                       Plasma drug concentration          C

Ø  Vd is a measure of the extent of distribution of drug and is expressed in litres. The best and simplest way of estimating Vd of a drug is administering it by rapid IV injection and using following equation:

        Vd =    Xo / Co = iv bolus dose / Co ------13

Ø  Equation 13 can only be used for the drugs that obey one compartment kinetics. This is because the Vd can only be estimated when distribution equilibrium is achieved between drug in plasma and that in tissues and such an equilibrium is instantaneously for a drug that follows one compartment kinetics.
Ø  A more general, more useful non compartmental method that can be applied to many compartment models for estimating the Vd is:

For drug given as IV bolus,
                        Vd(area)  =     Xo                                                                         -------(14.a)
                                           KE.AUC
For drug given as extravascularly

                         Vd(area)  =    FXo                                                                         -------(14.b)
                                           KE.AUC

Where Xo = dose administered and F= fraction of drug absorbed into systemic circulation.

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