General anaesthetic
A general anaesthetic (or anesthetic, see spelling differences) is a drug that brings about a reversible loss of consciousness. These drugs are generally administered by an anaesthesia provider to induce or maintain general anaesthesia to facilitate surgery. The biological mechanism(s) of the action of general anaesthetics are not well understood.
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[edit] Mode of administration
Drugs given to induce or maintain general anaesthesia are either given as:
- Gases or vapours (inhalational anaesthetics)
- Injections (intravenous anaesthetics)
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.
[edit] Inhalation
Inhalational anaesthetic substances are either volatile liquids or gases, 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 vapourized in the machine. 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).
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 anaesthetics today. They are often combined with nitrous oxide. Older, less popular, volatile anaesthetics, include halothane, enflurane, and methoxyflurane. Researchers are also actively exploring the use of xenon as an anaesthetic.
[edit] Injection
Injection anaesthetics are used for induction and maintenance of a state of unconsciousness. Anaesthetists prefer to use intravenous injections, as they are faster, generally less painful and more reliable than intramuscular or subcutaneous injections. Among the most widely used drugs are:
- Propofol
- Etomidate
- Barbiturates such as methohexital and thiopentone/thiopental
- Benzodiazepines such as midazolam
- Ketamine is used in the UK as "field anaesthesia", for instance at a road traffic incidents, and is more frequently used in the operative setting in the US.
[edit] Method of Action
[edit] Lipid theory
Overton and Meyer postulated 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.[1]
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.[2]
[edit] Ion channels
General anaesthetics inhibit excitatory functions of some central nervous system (CNS) receptors, such as neuronal nACh,[3] glutamate, or 5-HT3 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 presynaptically or decrease excitability of postsynaptic neuron.
[edit] Pharmacokinetics
[edit] Induction
Induction is between the onset of administration of anesthetic to surgical anesthesia. The speed of induction depends on the time taken for the drug to reach effective concentrations in the brain. IV anesthetic like Thiopental is generally used for induction.
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[edit] Elimination
Volatile anaesthetics are eliminated in the terminal phase via the lungs. A low blood:gas coefficient is therefore necessary for quick removal of the anaesthetic. When the oil:water coefficient is high, there will be little anaesthetic in the blood, so elimination will be slow, giving a prolonged hangover effect.
Some drugs are metabolised by the liver, so consequently their metabolites are often toxic (e.g. chloroform).
[edit] See also
[edit] Notes
- ^ "Mechanism of action of inhaled anaesthetic agents". Anesthesia UK. 2005. http://www.frca.co.uk/article.aspx?articleid=100341.
- ^ Ueda I (January 1999). "The window that is opened by optical isomers". Anesthesiology 90 (1): 336. PMID 9915358.
- ^ Weber M, Motin L, Gaul S, Beker F, Fink RH, Adams DJ (January 2005). "Intravenous anaesthetics inhibit nicotinic acetylcholine receptor-mediated currents and Ca2+ transients in rat intracardiac ganglion neurons". Br. J. Pharmacol. 144 (1): 98–107. doi:10.1038/sj.bjp.0705942. PMC 1575970. PMID 15644873. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1575970.
[edit] References
- Rod Flower; Humphrey P. Rang; Maureen M. Dale; Ritter, James M. (2007). Rang & Dale's pharmacology (6th ed.). Edinburgh: Churchill Livingstone. ISBN 0-443-06911-5.
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