Volume of Distribution (Vd) - Pharmacokinetics

Volume of Distribution (Vd)

·        Volume of distribution = volume into which the drug appears to be uniformly distributed with a concentration equal to that of plasma

·        Relates the amount of drug in the total body relative to that in the plasma. Explains where drug goes – not what it does. In one compartment model:

o   Vd = dose/plasma concentration L/kg, or

o   Amount in body = Vd * conc

o   Vd = Cl/Ke

·        Examples:

o   Gentamycin has an apparent Vd of extracellular H2O

o   Digoxin has a Vd much larger than physical volume of body

o   Warfarin has a much smaller Vd than expected

·        Use Vd to calculate loading dose: LD = Vd * target conc

·        Vd can be altered in cardiac, liver and renal disease and in obesity

·        For obese patients, use total body weight for lipid soluble drugs and ideal weight for water-soluble