Apparent Volume of Distribution
Relates total amount of drug in body to the concentration of drug in PLASMA.
qT = Vd(Cp)
Units of Vd = ml, liter, liter/kg body weight
Vd = Dose/Co = Dose/[AUC(ke)]
where AUC is area under the curve of the plasma concentration versus time curve
Vd - does not correspond to any anatomical space
High Vd (e.g 20 liters/kg) - present in high concentration in tissue relative to plasma.
Low Vd (e.g. 0.1 liters/kg) - present in high concentration in plasma relative to other tissues.
Volume of plasma in a 70 kg male is approximately 2 liters.
Drug Type Vd Binds to plasma proteins Plasma volume (0.05 liters/kg) Not highly bound to plasma
but don't enter cell
Total extracellular water volume (0.2 liters/kg) Enter cells freely Total body water volume (0.55 liters/kg) Binds to tissues Extremely high Vd's >10 liters/kg