Most diving is based on the use of compressed air and other oxygen–nitrogen mixtures as a breathing gas. Commercial, military, technical and experimental diving may involve the use of other gas mixtures. For this reason, it is desirable to give the reader some salient points on the gases mentioned in this text and related literature.
Oxygen (atomic weight 16, molecular weight 32) is the essential constituent of all breathing mixtures. At high altitude people survive with less than 0.1 ATA in their inspired air. However, for diving, oxygen should be present at a partial pressure of at least 0.2 ATA to avoid hypoxia. At higher partial pressures oxygen causes oxygen toxicity. Prolonged exposure to more than 0.55 ATA causes pulmonary oxygen toxicity, and shorter exposure to more than about 1.5 ATA results in central nervous system effects. The risk of these problems may be acceptable in a recompression chamber, where oxygen may be used at partial pressures of up to 2.8 ATA. Oxygen toxicity is discussed in Chapter 17.
In the range 0.2 to 2.8 ATA, oxygen has little effect on the respiratory centre and minute volume will remain close to normal. Oxygen is vasoactive; high oxygen tensions cause vasoconstriction.
Nitrogen (atomic weight 14, molecular weight 28) is the major component of air – about 79 per cent. Nitrogen is often considered to be physiologically inert. Bubbles, composed mainly of nitrogen, can cause DCS if a diver who has been breathing air or an oxygen–nitrogen mixture ascends too rapidly. In solution, it may cause nitrogen narcosis at depth (see Chapter 15). At partial pressures of nitrogen greater than about 3 ATA, there is a demonstrable decrement in the diver’s performance. At higher partial pressures, the effect is likely to cause the diver to make mistakes. The other problem that restricts the use of nitrogen is that its density at increased pressure increases the work of breathing.
Despite these disadvantages, nitrogen is of major importance in diving, at depths less than 50 metres and as a part of more complex mixtures at greater depths.
Helium (atomic weight 4) is a light, inert gas. It is found in natural gas wells in several countries. Helium is used to dilute oxygen for dives to depths greater than 50 metres, where nitrogen should not be used alone. The two major advantages of helium are that it does not cause narcosis and, because of its lightness, helium-oxygen mixtures are easier to breathe than most alternatives. Helium-oxygen mixtures can allow a shorter decompression time (albeit often with a different profile) than an equivalent saturation dive with the diver breathing air because helium diffuses more rapidly than nitrogen.
The use of helium can cause several problems. The speech of a diver at depth may need electronic processing to make it understandable because of the distortion. A diver in a helium atmosphere is more susceptible to heat and cold because the high thermal conductivity speeds the transfer of heat to and from the diver. The other problem with the use of helium is that it is associated with a disorder called the high-pressure neurological syndrome (HPNS) (see Chapter 20).
Hydrogen (atomic weight 1, molecular weight 2) has the advantage of being readily available at low cost. Because of its lightness it is the easiest gas to breathe. These factors may lead to its use as a replacement for helium. The reluctance to use stems from fears of explosion. Explosions can be prevented if the oxygen level does not exceed 4 per cent, and such a mixture is breathable at depths in excess of 30 metres. Hypoxia can be prevented by changing to another gas near the surface. Hydrogen causes thermal and speech distortion problems similar to those encountered with helium.