Volume of distribution

This MedLibrary.org supplementary page on Volume of distribution is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:

Related Sponsors

BM Pharmacy Generic pharmaceuticals at unbeatable prices. Insomnia, men's health, hair health, pain control. bmpharmacy.com
Online Pharmacy Carisoprodol, tadalafil, sildenafil, vardenafil and more... xlpharmacy.com
MedBasket.com Discreet. Inexpensive. Fast shipping. Great selection. What more can we say? medbasket.com
Frugalmed 2000 reasons to shop with us first. frugalmed.com
Ads by Tiva

The volume of distribution (VD) , also known as apparent volume of distribution, is a pharmacological term used to quantify the distribution of a medication between plasma and the rest of 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 to produce the observed blood concentration. 1

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.

Contents

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 VD for that drug is known. The VD 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.

It is important to note that the volume of distribution cannot be smaller than the physiological volume of intravascular plasma, which is approximately 3L in humans.

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

The VD 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:

  • VP = plasma volume
  • VT = apparent tissue volume
  • fu = fraction unbound in plasma
  • fuT = fraction unbound in tissue

Examples

Further reading: Table of volume of distribution for drugs

Drug VD 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.

Sample values and equations

Variable Abbreviation(s) Example value Formula
Dose (loading dose, or steady state/maintenance) D (LD or MD) 1000 mg =Vd*C0
Volume of distribution Vd 25 L =D/C0
Concentration (initial or steady-state) C0 or Css 40.0 mg/L =D/Vd
Biological half-life T½ 14 hr =0.7/Ke
Elimination rate constant Ke 0.05/hr =0.7/(T½)
=Cl/Vd
Elimination rate,
or rate of infusion to balance elimination		 Kin 50 mg/hr =Css*Cl
Clearance Cl 1.25 L/hr =Vd*Ke
Bioavailability F 1 = \frac{[AUC]_{A}*dose_{B}}{[AUC]_{B}*dose_{A}}

Note that the "0.7" constant is a commonly used log approximation, but not the actual value. Another commonly used approximation is 0.693. -(ln(0.5)) = 0.69315.

References

  1. ^ "vetmed.vt.edu".

External links

v • d • e
Medication > Pharmacology
Pharmacokinetics
Pharmacodynamics
Agonism and antagonism
Other fields

Wikipedia content modification information:

  • This page was last modified on 15 October 2008, at 15:32.

Wikipedia Authorship and Review

Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.

Wikipedia Usage Guidelines

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Volume of distribution".

The URL for this specific entry is:

All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.