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Volume of distribution and peak plasma concentration

Hello, could somebody explain what they mean? Thanks. :biggrin:

Volume of distribution (Vd) is the theoretical volume that a drug occupies in the body based on it's solubility and ease of ionisation/charge and size, this includes all 'body compartments' i.e. fat, intracellular fluid, extracellular fluid (including blood plasma). This is if it is equally distributed (hence the theoretical part as this often does not happen in reality).

It may not be the actual volume that it takes up, e.g. Vd can be a greater volume than the actual total volume of the body, this is because it may be much more soluble in some compartments (especially fat if it is lipophilic) and so a greater density of drug is stored there in reality. It can also be affected by how readily the drug binds to plasma proteins.

Vd depends on the size of dose given and the plasma concentration achieved, and so can be calculated from these (Vd = dose/plasma conc).

Peak plasma concentration is exactly that, it is the maximum concentration of a drug that can be achieved in blood plasma, and occurs when input of drug (oral/IV etc) is equal to elimination (removal of drug from plasma by liver and kidneys).

Reply 2

thegodofgod

Original post by ClumsyChild

Hello, could somebody explain what they mean? Thanks. :biggrin:

As above. V_d is usually expressed in litres (L), but sometimes is expressed as litres per kilogram of body weight (L/kg), in which case you have to multiply the L/kg by the patient's body weight. The V_d is constant for a drug molecule and is not altered by any other factors.

Just to add to the above post, the formula for V_d is: V_d = A / C_p, where A is the total amount of drug in the body (not just the dose that is given, but it takes previous doses and elimination into account too), and C_p is the plasma concentration of the drug at the time of the calculation. A is usually expressed in milligrams (mg) and C_p in milligrams per litre (mg/L). If you cancel out the units, you get units of L for V_d, which is correct. Bear in mind though that this can change, e.g. with digoxin, the usual plasma concentration will be 1-2 ng/mL, which is equivalent to 1-2 mcg/L. This means that you need to be careful with units when carrying out these calculations as you could very easily be out by a factor of 1000.

Hope this helps!