Nitrogen narcosis [nahr-koh-sis] (also known as ‘raptures of the deep’ and the ‘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 ‘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 intoxication caused by drinking alcohol or inhaling nitrous oxide. It can occur during shallow dives, but usually becomes noticeable at depths greater than 30 meters (100 ft).
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 (the ability to diffuse directly through the fatty part of a cell membrane), 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 40 m (130 ft) 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.
Narcosis resulting from breathing gases under elevated pressure 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 (the coordination of cognitive, sensory, and motor processes) 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 a practical anesthetic, but it has been successfully used for surgical operations, and xenon anesthesia systems are still being proposed and designed.
Due to its perception-altering effects, the onset of narcosis may be hard to recognize. At its most benign, narcosis results in relief of anxiety – a feeling of tranquility and mastery of the environment. These effects are essentially identical to various concentrations of nitrous oxide. They also resemble (though not as closely) the effects of alcohol or marijuana or benzodiazepine drugs (e.g. Valium). Such effects are not harmful unless they cause some immediate danger not to be recognized and addressed. Once stabilized, the effects generally remain the same at a given depth, only worsening if the diver ventures deeper.
The most dangerous aspects of narcosis are the impairment of judgement, multi-tasking and coordination, and the loss of decision-making ability and focus. Other effects include vertigo and visual or auditory disturbances. The syndrome may cause exhilaration, giddiness, extreme anxiety, depression, or paranoia, depending on the individual diver’s medical or personal history. When more serious, the diver may feel overconfident, disregarding normal safe diving practices. The relation of depth to narcosis is sometimes informally known as ‘Martini’s law,’ the idea that narcosis results in the feeling of one martini for every 10 m (33 ft) below 20 m (66 ft) depth. Professional divers use such a calculation only as a rough guide to give new divers a metaphor, comparing a situation they may be more familiar with. Most sport scuba training organizations recommend depths of no more than 40 m (130 ft) because of the risk of narcosis. A number of divers have died in attempts to set air depth records below 120 m (400 ft). Because of these incidents, Guinness World Records no longer reports on this figure.
The cause of narcosis is related to the increased solubility of gases in body tissues, as a result of the elevated pressures at depth (Henry’s law). Modern theories have suggested that inert gases dissolving in the lipid bilayer of cell membranes cause narcosis. More recently, researchers have been looking at neurotransmitter receptor protein mechanisms as a possible cause. The breathing gas mix entering the diver’s lungs will have the same pressure as the surrounding water, known as the ambient pressure. After any change of depth, the pressure of gases in the blood passing through the brain catches up with ambient pressure within a minute or two, which results in a delayed narcotic effect after descending to a new depth. Rapid compression potentiates narcosis owing to carbon dioxide retention.
Narcosis has been compared with altitude sickness insofar as its variability (though not its symptoms); its effects depend on many factors, with variations between individuals, and even from dive to dive in the same individual. Thermal cold, stress, heavy work, fatigue, and carbon dioxide retention all increase the risk and severity of narcosis. Carbon dioxide has a high narcotic potential and also causes increased blood flow to the brain, increasing the effects of other gases. Heavy exercise, shallow or skip breathing, or poor gas exchange in the lungs can lead to elevated carbon dioxide retention and risk of narcosis.
Because of similar and additive effects, divers should avoid sedating medications and drugs, such as marijuana and alcohol before any dive. A hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely. Experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs. Abstinence time needed for marijuana is unknown, but owing to the much longer half-life of the active agent of this drug in the body, it is likely to be longer than for alcohol.
The Daily Omnivore 