Editing Volume of distribution (section)

{{short description|Measuring the relative affinity of a drug between blood constituents and tissue constituents.}}
{{see|Distribution (pharmacology)}}

In [[pharmacology]], the '''volume of distribution''' (V<sub>D</sub>, also known as '''apparent volume of distribution''', literally, ''volume of dilution''<ref name="Ward_2012">{{cite book | vauthors = Ward RM, Kern SE, Lugo RA | title=Avery's Diseases of the Newborn | chapter=Pharmacokinetics, Pharmacodynamics, and Pharmacogenetics | publisher=Elsevier | year=2012 | isbn=978-1-4377-0134-0 | doi=10.1016/b978-1-4377-0134-0.10034-4 | pages=417–428}}</ref>) is the theoretical [[volume]] that would be necessary to contain the total amount of an administered [[drug]] at the same concentration that it is observed in the [[blood plasma]].<ref>{{cite web|url=http://sepia.unil.ch/pharmacology/?id=61|title=Volume of distribution|website=sepia.unil.ch|access-date=19 April 2018}}</ref> In other words, it is the ratio of ''amount of drug in a body (dose)'' to ''concentration of the drug that is measured in blood, plasma, and un-bound in interstitial fluid''.<ref name="Davis_2011">{{cite book | vauthors = Davis PJ, Bosenberg A, Davidson A, Jimenez N, Kharasch E, Lynn AM, Tofovic SP, Woelfel S | title=Smith's Anesthesia for Infants and Children | chapter=Pharmacology of Pediatric Anesthesia | publisher=Elsevier | year=2011 | isbn=978-0-323-06612-9 | doi=10.1016/b978-0-323-06612-9.00007-9 | pages=179–261}}</ref><ref name="urlUntitled Document">{{cite web | vauthors = Wilcke JR |url=http://cpharm.vetmed.vt.edu/VM8314/VM8314PharmaModeling.htm | archive-url = https://web.archive.org/web/20110716153737/http://cpharm.vetmed.vt.edu/VM8314/VM8314PharmaModeling.htm | archive-date = 16 July 2011 | title = Pharmacokinetic Modeling |work = Virginia-Maryland Regional College of Veterinary Medicine }}</ref>

The V<sub>D</sub> of a drug represents the degree to which a drug is distributed in body tissue rather than the plasma. V<sub>D</sub> is directly proportional with the amount of drug distributed into tissue; a higher V<sub>D</sub> indicates a greater amount of tissue distribution. A V<sub>D</sub> greater than the total volume of body water (approximately 42 liters in humans<ref>{{cite web|url=http://www.anaesthesiamcq.com/FluidBook/fl2_1.php|title=Fluid Physiology: 2.1 Fluid Compartments|website=www.anaesthesiamcq.com|access-date=19 April 2018}}</ref>) is possible, and would indicate that the drug is highly distributed into tissue. In other words, the volume of distribution is smaller in the drug staying in the plasma than that of a drug that is widely distributed in tissues.<ref name="Ilowite_2011">{{cite book | vauthors = Ilowite NT, Laxer RM | title=Textbook of Pediatric Rheumatology | chapter= Pharmacology and Drug Therapy | publisher= Elsevier | year=2011 | isbn=978-1-4160-6581-4 | doi=10.1016/b978-1-4160-6581-4.10006-8 | pages=71–126}}</ref>

In rough terms, drugs with a high lipid solubility (non-polar drugs), low rates of ionization, or low plasma protein binding capabilities have higher volumes of distribution than drugs which are more polar, more highly ionized or exhibit high plasma protein binding in the body's environment. Volume of distribution may be increased by [[kidney 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.