In its simplest terms, prevention comes by avoiding exposure to partial pressures of inert gas known to produce intoxication. In practice, safe diving on compressed air requires an awareness of the condition and its effect on performance and judgement at depths greater than 30 metres. The maximum depth limit for an air dive should be between 30 and 50 metres, depending on the diver’s experience and the task to be performed. Safe diving at a greater depth requires the substitution of a less narcotic agent to dilute the oxygen, such as helium, neon or hydrogen (one form of ‘technical’ diving).
There is a firm belief among divers that adaptation to IGN can develop over repeated daily exposures and that one can therefore ‘work up’ to deep dives. Several studies have shown that, although subjective adaptation can occur, measurement of standing steadiness or reaction time showed no improvement with repeated exposure3. As with alcohol, confidence is not matched by performance, thus possibly compromising safety.
Saturation at depths between 30 and 40 metres is said to allow the development of adaptation. Excursion dives to greater depths can then be made with more safety and improved work performance. A conventional working dive to 100 metres would be inconceivable using air as the breathing medium. However, operational dives may be performed to 100 metres if the excursion is from a saturated depth of 40 metres. At that depth, the diver becomes acclimatized to the nitrogen narcosis, with a progressive improvement of job performance, approaching ‘surface’ efficiency.
For most contemporary deep diving, the effect of IGN is avoided by substituting helium, or helium-nitrogen, as the diluent gases for oxygen. Oxygen cannot, of course, be used alone because of its toxicity at high pressure (see Chapter 17), but it can partially replace nitrogen in various nitrox mixtures. Hydrogen is also being used as a substitute for nitrogen and would be ideal except for the formation of an explosive mixture with oxygen.
Evidence that helium also has some narcotic effect arises from the observation that HPNS is not the same under hydrostatic pressure as it is under helium pressure. It has been postulated that both helium and oxygen need to be considered when calculating the narcotic effects of respired gases under great pressure.
Although amphetamines ameliorate narcotic slowing of reaction time, the use of drugs to reduce narcosis has, as yet, no place in diving. Conversely, divers should be warned of the risks of taking central nervous system depressant drugs, which, in the diver, may include alcohol, sedating antihistamines (in cold and sinus preparations) and anti–motion sickness drugs. These drugs may act synergistically with nitrogen in impairing performance and judgment, although this has been clearly shown only with alcohol.