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 much beyond 40 m (130 ft) is generally considered outside the scope of recreational diving. In order to dive at greater 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 or all of the inert fraction 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 effect of carbon dioxide is a consistent 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. Signs and symptoms
Due to its perception-altering effects, the onset of narcosis may be hard to recognize.
The partial pressure of a gas required to cause a measured degree of impairment correlates well with the lipid solubility of the gas: the greater the solubility, the less partial pressure is needed.An early theory, the Meyer-Overton hypothesis, suggested that narcosis happens when the gas penetrates the lipids of the brains nerve cells, causing direct mechanical interference with the transmission of signals from one nerve cell to another. More recently, specific types of chemically gated receptors in nerve cells have been identified as being involved with anesthesia and narcosis. However, the basic and most general underlying idea, that nerve transmission is altered in many diffuse areas of the brain as a result of gas molecules dissolved in the nerve cells fatty membranes, remains largely unchallenged. Management and diagnosis
The management of narcosis is simply to ascend to shallower depths; the effects then disappear within minutes. In the event of complications or other conditions being present, ascending is always the correct initial response. Should problems remain, then it is necessary to abort the dive. The decompression schedule can still be followed unless other conditions require emergency assistance.The symptoms of narcosis may be caused by other factors during a dive: ear problems causing disorientation or nausea; early signs of oxygen toxicity causing visual disturbances; or hypothermia causing rapid breathing and shivering. Nevertheless, the presence of any of these symptoms should imply narcosis. Alleviation of the effects upon ascending to a shallower depth will confirm the diagnosis. Given the setting, other likely conditions do not produce reversible effects.