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Nitrogen narcosis
Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect), is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high pressure. The Greek word ''ναρκωσις'' (narcosis) is derived from ''narke'', "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness (alcohol intoxication), or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than .
Except for helium and probably neon, all gases that can be breathed have a narcotic effect, although widely varying in degree. The effect is consistently greater for gases with a higher lipid solubility, and there is good evidence that the two properties are mechanistically related. As depth increases, the mental impairment may become hazardous. Divers can learn to cope with some of the effects of narcosis, but it is impossible to develop a tolerance. Narcosis affects all divers, although susceptibility varies widely from dive to dive, and between individuals.
Narcosis may be completely reversed in a few minutes by ascending to a shallower depth, with no long-term effects. Thus narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms, and are able to ascend to manage it. Diving beyond is generally considered outside the scope of recreational diving. Below these depths, as narcosis and oxygen toxicity become critical risk factors, specialist training is required in the use of various helium-containing gas mixtures such as trimix or heliox. These mixtures prevent narcosis by replacing some of the breathing gas with non-narcotic helium.
== Classification ==
Narcosis results from breathing gases under elevated pressure, and may be classified by the principal gas involved. The noble gases, except helium and probably neon, as well as nitrogen, oxygen and hydrogen cause a decrement in mental function, but their effect on psychomotor function (processes affecting the coordination of sensory or cognitive processes and motor activity) varies widely. The effects of carbon dioxide consistently result in a diminution of mental and psychomotor function. The noble gases argon, krypton, and xenon are more narcotic than nitrogen at a given pressure, and xenon has so much anesthetic activity that it is a usable anesthetic at 80% concentration and normal atmospheric pressure. Xenon has historically been too expensive to be used very much in practice, but it has been successfully used for surgical operations, and xenon anesthesia systems are still being proposed and designed.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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