The conditions previously described occur because of differences in pressure that damage body structures in a direct, mechanical manner. The indirect or secondary effects of pressure are the result of changes in the partial pressure of individual gases in the diver's breathing medium. The mechanisms of these effects include saturation and desaturation of body tissues with dissolved gas and the modification of body functions by abnormal gas partial pressures.
3-10.1 Nitrogen Narcosis. In diving, inert gas narcosis impairs the diver's ability to think clearly. The most common form, nitrogen narcosis, is caused by breathing compressed air at depth.
3-10.1.1 Symptoms of Narcosis. The signs of narcosis are:
■ Loss of judgment or skill
■ A false feeling of well-being Lack of concern for job or safety Apparent stupidity Inappropriate laughter
Tingling and vague numbness of the lips, gums, and legs
Disregard for personal safety is the greatest hazard of nitrogen narcosis. Divers may display abnormal behavior such as removing the regulator mouthpiece or swimming to unsafe depths without regard to decompression sickness or air supply. There is no specific treatment for nitrogen narcosis; the diver must be brought to shallower depths where the effects are not felt.
3-10.1.2 Susceptibility to Narcosis. Inert gases vary in their narcotic potency. The effects from nitrogen may first become noticeable at depths exceeding 100 fsw, but become more pronounced at depths greater than 150 fsw. There is a wide range of individual susceptibility and some divers, particularly those experienced in deep operations with air, can often work as deep as 200 fsw without serious difficulty.
Experienced and stable divers may be reasonably productive and safe at depths where others fail. They are familiar with the extent to which nitrogen narcosis
impairs performance. They know that a strong conscious effort to continue the dive requires unusual care, time, and effort to make even the simplest observations and decisions. Any relaxation of conscious effort can lead to failure or a fatal blunder.
Experience, frequent exposure to deep diving, and training may enable divers to perform air dives as deep as 180-200 fsw, but novices and susceptible individuals should remain at shallower depths. The performance or efficiency of divers breathing compressed air is impaired at depths greater than 180 fsw. At 300 fsw or deeper, the signs and symptoms are severe and the diver may hallucinate, exhibit bizarre behavior, or lose consciousness. Furthermore, the associated increase in oxygen partial pressure at such depths may produce oxygen convulsions. (Helium is widely used in mixed-gas diving as a substitute for nitrogen to prevent narcosis. Helium has not demonstrated narcotic effects at any depth tested by the U.S. Navy.) Figure 3-16 shows the narcotic effects of compressed air diving.
3-10.2 Oxygen Toxicity. Partial pressure of oxygen in excess of that encountered at normal atmospheric conditions may be toxic to the body. Oxygen toxicity is dependent upon both partial pressure and exposure time. The two types of oxygen toxicity experienced by divers are pulmonary oxygen toxicity and central nervous system (CNS) oxygen toxicity.
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