An inhalational anaesthetic is a chemical compound possessing general anaesthetic properties that can be delivered via inhalation. They are administered by anaesthetists (a term which includes anaesthesiologists, nurse anaesthetists, and anaesthesiologist assistants) through an anaesthesia mask, laryngeal mask airway or tracheal tube connected to some type of anaesthetic vaporiser and an anaesthetic delivery system. Agents of significant contemporary clinical interest include volatile anaesthetic agents such as isoflurane, sevoflurane and desflurane, as well as certain anaesthetic gases such as nitrous oxide and xenon.
List of inhalational anaesthetic agents 
- Currently used agents
- Previously used agents
Although some of these are still used in clinical practice and in research, the following anaesthetic agents are primarily of historical interest in developed countries:
- diethyl ether
- vinyl ether
- Future agents
Volatile anaesthetics 
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Other gases or vapours which produce general anaesthesia by inhalation include nitrous oxide, cyclopropane and xenon. These are stored in gas cylinders and administered using flowmeters, rather than vaporisers. Cyclopropane is explosive and is no longer used for safety reasons, although otherwise it was found to be an excellent anaesthetic. Xenon is odourless and rapid in onset, but is expensive and requires specialised equipment to administer and monitor. Nitrous oxide, even at 80% concentration, does not quite produce surgical level anaesthesia in most persons at standard atmospheric pressure, so it must be used as an adjunct anaesthetic, along with other agents.
Hyperbaric anaesthesia 
Under hyperbaric conditions (pressures above normal atmospheric pressure), other gases such as nitrogen, and noble gases such as argon, krypton, and xenon become anaesthetics. When inhaled at high partial pressures (more than about 4 bar, encountered at depths below about 30 metres in scuba diving), nitrogen begins to act as an anaesthetic agent, causing nitrogen narcosis. However, the minimum alveolar concentration (MAC) for nitrogen is not achieved until pressures of about 20 to 30 atm (bar) are attained. Argon is slightly more than twice as anaesthetic as nitrogen per unit of partial pressure (see argox). Xenon however is a usable anaesthetic at 80% concentration and normal atmospheric pressure.
Neurological theories of action 
The full mechanism of action of volatile anaesthetic agents is unknown and has been the subject of intense debate. "Anesthetics have been used for 160 years, and how they work is one of the great mysteries of neuroscience," says anaesthesiologist James Sonner of the University of California, San Francisco. Anaesthesia research "has been for a long time a science of untestable hypotheses," notes Neil L. Harrison of Cornell University.
"Most of the injectable anesthetics appear to act on a single molecular target," says Sonner. "It looks like inhaled anesthetics act on multiple molecular targets. That makes it a more difficult problem to pick apart."
The possibility of anaesthesia by the inert gas argon in particular (even at 10 to 15 bar) suggests that the mechanism of action of volatile anaesthetics is an effect best described by physical chemistry, and not a chemical bonding action. However, the agent may bind to a receptor with a weak interaction . A physical interaction such as swelling of nerve cell membranes from gas solution in the lipid bilayer may be operative. Notably, the gases hydrogen, helium, and neon have not been found to have anaesthetic properties at any pressure. Helium at high pressures produces nervous irritation ("anti-anaesthesia"), suggesting that the anaesthetic mechanism(s) may be operated in reverse by this gas (i.e., nerve membrane compression). Also, some halogenated ethers (such as flurothyl) also possess this "anti-anaesthetic" effect, providing further evidence for this theory.
See also 
- Fowler, B; Ackles, KN; Porlier, G (1985). "Effects of inert gas narcosis on behavior—a critical review.". Undersea Biomed. Res. 12 (4): 369–402. PMID 4082343. Retrieved 2008-09-21.
- Rogers, W. H.; Moeller, G. (1989). "Effect of brief, repeated hyperbaric exposures on susceptibility to nitrogen narcosis". Undersea Biomed. Res. 16 (3): 227–32. ISSN 0093-5387. OCLC 2068005. PMID 2741255. Retrieved 2008-09-21.
- Burov, NE; Kornienko, Liu; Makeev, GN; Potapov, VN (November–December 1999). "Clinical and experimental study of xenon anesthesia". Anesteziol Reanimatol (6): 56–60. Retrieved 2008-11-03.
- John Travis, "Comfortably Numb, Anesthetics are slowly giving up the secrets of how they work," Science News. (July 3rd 2004). .