The volume of distribution (V_D) , also known as apparent volume of distribution, is a pharmacological term used to quantify the distribution of a drug throughout the body after oral or intravenous 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.

Volume of distribution may be increased by renal failure (due to fluid retention) and liver failure (due to altered body fluid and 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

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The volume of distribution is given by the following equation:

$\{V\_\{D\}\}\; =\; \backslash frac\{\backslash mathrm\{total\; \backslash \; amount\; \backslash \; of\; \backslash \; drug\; \backslash \; in\; \backslash \; the\; \backslash \; body\}\}\{\backslash mathrm\{drug\; \backslash \; blood\; \backslash \; concentration\}\}$

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

$\{V\_\{D\}\}\; =\; \{V\_\{P\}\}\; +\; \{V\_\{T\}\}\; \backslash left(\backslash frac\{fu\}\{fu\_\{t\}\}\backslash right)$

Where:

• V_P = plasma volume

• V_T = apparent tissue volume

• fu = fraction unbound in plasma

• fu_T = fraction unbound in tissue

Examples

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|- | Warfarin || V_D = 8L || Reflects a high degree of plasma protein binding. |- | Theophylline, Ethanol|| V_D = 30L || Represents distribution in total body water. |- | Chloroquine || V_D = 15000L || Shows highly lipophilic molecules which sequester into total body fat.

Drug

VD

Comments

External links

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• Overview at cornell.edu
• Overview at stanford.edu
• Overview at boomer.org

pharmacology | pharmacokinetics

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