General anaesthetic

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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.

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.

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.

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.

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
• Benzodiazepine such as midazolam
• 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: Theories of general anaesthetic action

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.

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.

Ion channels

General anaesthetics inhibit excitatory functions of some central nervous system (CNS) receptors, such as neuronal nACh, glutamate, or 5-HT3 receptors. Some general anaesthetics also excite inhibitory receptors, notably GABA_A receptors and TREK. GABA_A is a major target of the IV anaesthetics thiopental and propofol.

General anaesthetics may decrease transmitter release presynaptically or decrease excitability of postsynaptic neuron.

Pharmacokinetics

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).

See also

• Local anaesthesia
• Mechanical ventilation
• Intraoperative awareness

Notes

• 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.{{cite book}}: CS1 maint: multiple names: authors list (link)