RLO: Volume of Distribution

Vd defined

The volume of distribution (Vd), also known as the apparent volume of distribution, is a calculated theoretical value. It is used clinically when trying to determine the loading dose necessary for a desired blood concentration of a drug, and is also used for estimating a blood concentration in the treatment of overdose.

Simply defined, the volume of distribution is the volume of plasma that would be necessary to account for the total amount of drug in the patient's body, if that drug were present throughout the body at the same concentration as found in the plasma. This value is usually further divided by the patient's body weight, and the result expressed in terms of litres per kilogram.

Let's look at an illustration to see what this actually means...

An average 60 kg human will have a plasma volume of 3.0 litres. Following injection, most drugs will show some distribution into the tissues and extracellular fluid. Some drug will be retained in the plasma, but may be bound to plasma proteins. Remember: only the free drug is able to exert its effect.

Drugs that remain confined to the plasma will exhibit a volume of distribution similar to plasma volume, which can also be expressed in terms of litres per kilogram of body weight : 3 litres of plasma divided by 60 kilograms gives us a plasma volume of 0.05 litres per kilogram. For drugs which accumulate outside the plasma, the volume of distribution may exceed total plasma volume.


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