Volume of distribution

Volume of distribution

As shown under therapeutic drug concentration, drug action depends upon the concentration of the drug in the body. How do we determine the concentration when we administer a specific dose? The first thing we would like to know is the volume over which a drug can distribute. This is called the volume of distribution. In other words: the volume of distribution (Vd) of a drug is the volume of fluid required to contain the total amount of drug in the body at the same concentration as that of the plasma. It is a mathematically determined value and in general, the greater the Vd, the greater the diffusibility of the drug. It is impossible

to measure the volume of distribution of a drug, but it can be calculated. This volume comes from the division of the administered dose by the concentration measured in a certain body fluid, usually plasma (Vd = Dose / C). In the figure 30 units are dissolved in 100 litres. If we measure in the solution we find the concentration to be 0,3 units/litre. The volume of distribution is then 30 / 0,3 = 100 litres. C = Dose / Vd (or) Vd = Dose / C. See how different volumes will effect drug concentrations by clicking here and selecting the simple container model (module 1).

Volume of distribution

Volume of distribution

Although everybody has a different Vd for different drugs, an average apparent Vd is determined for new drugs during development. This can help with the dosing of new drugs in new patients. Example:Gentamicin: a dose of 180 mg is given by IV bolus to a person with a weight of 70 kg. The apparent volume of distribution of gentamicin is 0,31 L/kg. What is the initial concentration likely to be immediately after injection? 8.29 mg/L