Volatile anaesthetic

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The volatile anaesthetics are a class of general anaesthetic drugs. They all share the property of being liquid at room temperature but evaporating easily.

The ideal volatile anaesthetic agent offers smooth and reliable induction and maintenance of general anaesthesia with minimal effects on other organ systems. In addition it is odourless or pleasant to inhale; safe for all ages and in pregnancy; not metabolised to toxic products; rapid in onset and offset; potent; and safe for exposure to operating room staff. It is also cheap to manufacture; easy to transport and store, with a long shelf life; easy to administer and monitor with existing equipment; stable to light, plastics, metals and soda lime; non-flammable and environmentally safe.

None of the agents currently in use is ideal, although many have some of the desirable characteristics. For example, sevoflurane is pleasant to inhale and is rapid in onset and offset. It is also safe for all ages. However, it is expensive (approximately 15 times more expensive than isoflurane), and approximately half as potent as isoflurane.

Anaesthetists adminster these agents using an anaesthetic vaporiser attached to an anaesthetic machine.

They include:

diethyl ether - pungent smelling and extremely flammable; still used in the Third World
chloroform - now abandoned for clinical use
trichloroethylene - now abandoned for clinical use
halothane (N01[AB01]) - sweet smelling, slow onset and offset, potent, risk of hepatitis with repeated use
methoxyflurane (N01[AB03]) - analgesic, slow onset and offset, still used by paramedics as an emergency analgesic in trauma cases, but abandoned for hospital use as it is metabolised to toxic fluoride ions
enflurane (N01[AB04]) - pungent smelling, lowers seizure threshold, slowish onset and offset
isoflurane (N01[AB06]) - pungent smelling
sevoflurane (N01[AB08]) - sweet smelling and quite rapid onset and offset, expensive
desflurane (N01[AB07]) - pungent smelling but very rapid onset and offset, not very potent, expensive, requires special vaporiser

Other gases or vapours which produce general anaesthesia by inhalation include nitrous oxide, cyclopropane and xenon. These are stored in cylinders and administered using flowmeters, rather than vaporisers. Cyclopropane is explosive and is no longer used for safety reasons. Xenon is odourless and extremely rapid in onset, but is extremely expensive and requires specialised equipment to administer and monitor.

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