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.

        V_d =     Amount of drug in the body   =     X------ 12

                       Plasma drug concentration          C

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

        V_d =    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 V_d 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 V_d is:

For drug given as IV bolus,

                        V_d(area)  =     X_o                                                                         -------(14.a)

                                           K_E.AUC

For drug given as extravascularly

                         V_d(area)  =    FX_o                                                                         -------(14.b)

                                           K_E.AUC

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