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Volume of distribution

The volume of distribution (V_D) , also known as apparent volume of distribution, is a pharmacological term used to quantify the distribution of a medication throughout the body after oral or parenteral dosing. It is defined as the volume in which the amount of drug would need to be uniformly distributed in to produce the observed blood concentration.

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Volume of distribution may be increased by renal failure (due to fluid retention) and liver failure (due to altered body fluid and plasma protein binding). Conversely it may be decreased in dehydration.

The initial volume of distribution describes blood concentrations prior to attaining the apparent volume of distribution and uses the same formula.

Equations

The volume of distribution is given by the following equation:

${V_{D}} = \frac{\mathrm{total \ amount \ of \ drug \ in \ the \ body}}{\mathrm{drug \ blood \ concentration}}$

Therefore the dose required to give a certain plasma concentration can be determined if the V_D for that drug is known. The V_D is not a real volume; it is more a reflection of how a drug will distribute throughout the body depending on several physicochemical properties, e.g. solubility, charge, size, etc.

The units for Volume of Distribution are typically reported in (ml, liter, or liter)/kg body weight. The fact that V_D is a ratio of a theoretical volume to a fixed unit of body weight explains why the V_D for children is typically higher than that for adults, even though children are smaller and weigh less. As body composition changes with age, V_D decreases.

The V_D may also be used to determine how readily a drug will displace into the body tissue compartments relative to the blood:

${V_{D}} = {V_{P}} + {V_{T}} \left(\frac{fu}{fu_{t}}\right)$

Where:

V_P = plasma volume

V_T = apparent tissue volume

fu = fraction unbound in plasma

fu_T = fraction unbound in tissue

Examples

Further reading: Table of volume of distribution for drugs

Drug	V_D	Comments
Warfarin	8L	Reflects a high degree of plasma protein binding.
Theophylline, Ethanol	30L	Represents distribution in total body water.
Chloroquine	15000L	Shows highly lipophilic molecules which sequester into total body fat.
NXY-059	8L	Highly-charged hydrophilic molecule.