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.
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.
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).
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.
General anaesthetics may decrease transmitter release pre-synaptically or decrease excitability of post-synaptic neuron.
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:
Some drugs are metabolised by the liver, and so consequently their metabolites are often toxic (e.g. chloroform)
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