5 - drugs used in general anaesthesia

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.

Drugs given to induce or maintain general anaesthesia are either given as:

• Gases or vapors (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.

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 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 anaesthetics 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 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 and diazepam (commonly known as Valium)
• 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. They share the property of being liquid at room temperature, but evaporating easily for administration by inhalation (some experts make a distinction between volatile and gas anesthetics on this basis, but both are treated in this article, since they probably do not differ in mechanism of action). 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