General anaesthetic
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clean up, typos fixed: conciousness → consciousness using [[Project:AutoWikiBrowser|AWB]]
A '''general anaesthetic''' (or '''anesthetic''', see [[spelling differences]]) drug is an anaesthetic drug that brings about a reversible loss of [[consciousness]]. These drugs are generally administered by an anesthesia provider in order to induce or maintain [[general anaesthesia]] to facilitate [[surgery]].
==Mode of administration==
Drugs given to induce or maintain general [[anaesthesia]] are either given as:
* Gases or vapors ([[inhalational anaesthetic]]s)
* Injections ([[intravenous anaesthetic]]s)
Most commonly these two forms are combined, with an injection given to induce anaesthesia and a gas used to maintain it, although it is possible to deliver anaesthesia solely by inhalation or injection.
Inhalational anaesthetic substances are either [[Volatility (chemistry)|volatile]] [[liquid]]s or [[gas]]es and are usually delivered using an [[anaesthesia machine]]. An anaesthesia machine allows composing a mixture of oxygen, anaesthetics and ambient air, delivering it to the patient and monitoring patient and machine parameters. Liquid anaesthetics are vaporized in the machine.
Many compounds have been used for inhalation anaesthesia, but only a few are still in widespread use. [[Desflurane]], [[isoflurane]] and [[sevoflurane]] are the most widely used [[volatile anaesthetic]]s today. They are often combined with [[nitrous oxide]]. Older, less popular, volatile anesthetics, include [[halothane]], [[enflurane]], and [[methoxyflurane]]. Researchers are also actively exploring the use of [[xenon]] as an anaesthetic.
Injection anaesthetics are used for induction and maintenance of a state of unconsciousness. Anaesthetists prefer to use [[intravenous injection]]s as they are faster, generally less painful and more reliable than [[intramuscular injection| intramuscular]] or [[subcutaneous injection]]s. Among the most widely used drugs are:
* [[Propofol]]
* [[Etomidate]]
* [[Barbiturate]]s such as [[methohexital]] and [[thiopentone]]/[[thiopental]]
* [[Benzodiazepine]]s such as [[midazolam]] and [[diazepam]]
* [[Ketamine]] is used in the UK as "field anaesthesia", for instance at a road traffic incident, and is more frequently used in the operative setting in the US.
The volatile anaesthetics are a class of general anaesthetic drugs composed of gasses and liquids which evaporate easily for administration by inhalation. All of these agents share the property of being quite hydrophobic (i.e., as liquids, they are not freely miscible with in water, and as gases they dissolve in oils better than in water).
==Method of Action==
{{See also|Outdated theories of anaesthetic action}}
===Lipid theory===
Overton and Meyer postulated that general anaesthetics exert their action by acting on the [[plasma membrane]]. This is supported by evidence that the potency of the drug has a direct, positive correlation with the lipid solubility of the blood. <ref>Anesthesia UK, http://www.frca.co.uk/article.aspx?articleid=100341, `15 May 08</ref>
The mechanism of action was proposed to be increased fluidity of the membrane. The interpretation of the Overton and Meyer finding has been challenged and discredited.<Ref>The Window that Is Opened by Optical Isomers, I Ueda, Anesthesiology. 90(1):336, January 1999.</ref>
===Ion Channels===
General anaesthetics inhibit excitatory functions of some CNS receptors, such as glutamate or 5-HT receptors. Some general anaesthetics also excite inhibitory receptors, notably [[GABAA receptors]] and [[TREK]]. GABAA is a major target of the IV anaesthetics [[thiopental]] and [[propofol]].
General anaesthetics may decrease transmitter release pre-synptically or decrease excitability of post-synaptic neuron.
==Pharmacokinetics==
===Induction===
The arterial concentration and therefore the induction/recovery phase is governed by numerous factors, including:
:*blood:gas partition co-efficient
:*inspired air concentrations
:*breathing rate
The potency of the drug is governed by the oil:gas partition coefficient, which also influences distribution. High oil:gas coefficients cause quick redistribution into adipose tissue, at the expense of CNS anaesthetic action. It is this redistribution phase into fat which causes the recovery of consciousness. This is why the weight and fat ratio of a patient must be considered when administering. Multiple administrations cause a big rise in concentrations in adipose tissue to maintain the same arterial concentration. In the terminal phase of multiple doses the recovery will take a very long time.
Three redistribution phases exist:
:'''Fast redistribution phase''': drug diffuses into low blood flow tissues
:'''Intermediate phase''':diffusion out of lean tissues and into adipose tissue
:'''Terminal phase''': complete elimination from all tissue.
===Elimination===
Volatile anaesthetics are eliminated in the terminal phase via the lungs. A low blood:gas coefficient is therefore necessary for quick removal of anaesthetic. When oil:water coefficient is high, there will be little anaesthetic in the blood, so elimination will be slow, giving a prolonged hangover effect.
Some are drugs are metabolised by the liver, and their metabolites are often toxic (e.g. [[chloroform]])
== See also ==
* [[Local anesthesia]]
* [[Mechanical ventilation]]
* [[Intraoperative awareness]]
==Notes==
<references />
:Pharmacology, Rang and Dale, Churchill Livingstone; 6 edition (January 25, 2007), 0443069115
{{General anesthetics}}
[[Category:Pharmacology]]
[[Category:Anesthesia]]
[[it:Anestetico generale]]
[[hu:Általános érzéstelenítők]]