Nitrogen narcosis (also known as narcosis while diving, 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 partial pressure. The Greek word (narkōsis), "the act of making numb", is derived from (narkē), "numbness, torpor", 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 presumably 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 although the mechanism of this phenomenon is still not fully clear, 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 can affect all ambient pressure divers, although susceptibility varies widely among individuals and from dive to dive. The main modes of underwater diving that deal with its prevention and management are scuba diving and surface-supplied diving at depths greater than .
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 much beyond is generally considered outside the scope of recreational diving. To dive at greater depths, as narcosis and oxygen toxicity become critical risk factors, gas mixtures such as trimix or heliox are used. These mixtures prevent or reduce narcosis by replacing some or all of the inert fraction of the breathing gas with non-narcotic helium. There is a synergy between carbon dioxide toxicity and inert gas narcosis which is recognised but not fully understood. Conditions where high work of breathing due to gas density occur tend to exacerbate this effect.
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. At its most benign, narcosis results in relief of anxiety – a feeling of tranquillity 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 and the familiar benzodiazepine drugs such as diazepam and alprazolam. Such effects are not harmful unless they cause some immediate danger to go unrecognized and unaddressed. 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 and the diver's medical or personal history. When more serious, the diver may feel overconfident, disregarding normal safe diving practices. Slowed mental activity, as indicated by increased reaction time and increased errors in cognitive function, are effects which increase the risk of a diver mismanaging an incident. Narcosis reduces both the perception of cold discomfort and shivering and thereby affects the production of body heat and consequently allows a faster drop in the core temperature in cold water, with reduced awareness of the developing problem.
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 below depth. This is a rough guide to give new divers a comparison with a situation they may be more familiar with.
Reported signs and symptoms are summarized against typical depths in meters and feet of sea water in the following table, closely adapted from Deeper into Diving by Lippman and Mitchell:
+ Signs and symptoms of narcosis, breathing air Pressure (bar) Depth (m) Depth (ft) Comments 1–2 0–10 0–33 2–4 10–30 33–100 4–6 30–50 100–165 6–8 50–70 165–230 8–10 70–90 230–300 10+ 90+ 300+
Causes +Some components of breathing gases and their relative narcotic potencies Gas Relative narcotic potency He 0.045 Ne 0.3 H2 0.6 N2 1.0 O2 1.7 Ar 2.3 Kr 7.1 CO2 20.0 Xe 25.6
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). It has been 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 of narcosis. The breathing gas mix entering the diver's lungs will have the same pressure as the surrounding water, known as the ambient pressure. After a change of depth, the partial pressure of inert gases in the blood passing through the brain catches up with ambient pressure within a minute or two, which results in a delayed change in narcotic effect after descending to a new depth. Rapid compression potentiates narcosis owing to carbon dioxide retention.
A divers' cognition may be affected on dives as shallow as , but the changes are not usually noticeable. There is no reliable method to predict the depth at which narcosis becomes noticeable, or the severity of the effect on an individual diver, as it may vary from dive to dive even on the same day.
Significant impairment due to narcosis is an increasing risk below depths of about , corresponding to an ambient pressure of about . Most sport scuba training organizations recommend depths of no more than because of the risk of narcosis. When breathing air at depths of – an ambient pressure of about – narcosis in most divers leads to hallucinations, loss of memory, and unconsciousness. A number of divers have died in attempts to set air depth records below . Because of these incidents, Guinness World Records no longer reports on this figure.
Narcosis has been compared with altitude sickness regarding its variability of onset (though not its symptoms); its effects depend on many factors, with variations between individuals. 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. Increased risk of narcosis results from increasing the amount of carbon dioxide retained through heavy exercise, shallow or skip breathing, high work of breathing, or because of poor gas exchange in the lungs.
Narcosis is known to be additive to even minimal alcohol intoxication. Other sedative and analgesic drugs, such as opiate narcotics and benzodiazepines, add to narcosis.
Mechanism
The precise mechanism is not well understood, but it appears to be the direct effect of gas dissolving into nerve membranes and causing temporary disruption in nerve transmissions. While the effect was first observed with air, other gases including argon, krypton and hydrogen cause very similar effects at higher than atmospheric pressure. Some of these effects may be due to antagonism at NMDA receptors and potentiation of GABAA receptors, similar to the mechanism of nonpolar anesthetics such diethyl ether or ethylene. However, their reproduction by the very chemically inactive gas argon makes them unlikely to be a strictly chemical bonding to receptors in the usual sense of a chemical bond. An indirect physical effect – such as a change in membrane volume – would therefore be needed to affect the ligand-gated ion channels of nerve cells. Trudell et al. have suggested non-chemical binding due to the attractive van der Waals force between proteins and inert gases.
Similar to the mechanism of ethanol's effect, the increase of gas dissolved in nerve cell membranes may cause altered ion permeability properties of the neural cells' lipid bilayers. 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 brain's 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.
Diagnosis and management 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; carbon dioxide toxicity caused by rebreather scrubber malfunction, excessive work of breathing, or inappropriate breathing pattern, or hypothermia causing rapid breathing and shivering. Nevertheless, the presence of any of these symptoms can 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. In the event of misdiagnosis when another condition is causing the symptoms, the initial management – ascending to a shallower depth – is still beneficial in most cases, as it is also the appropriate response for most of the alternative causes for the symptoms.
The management of inert gas narcosis is usually simply to ascend to shallower depths, where much of the effect disappears within minutes. Divers carrying multiple gas mixtures will usually switch to a mixture with more helium before significant narcosis is noticeable during descent. In the event of complications or other conditions being present, ascending remains the correct initial response unless it would violate decompression obligations. Should problems persist, it may be necessary to abort the dive. The decompression schedule can and should still be followed unless other conditions require emergency assistance.