The term altitude diving refers to diving at an altitude of 300 metres or more above sea level. Non-diving disorders should be considered, such as the dyspnoea and hypoxia induced by high altitude and the altitude sickness that frequently develops above 3000 metres. Diving at altitudes higher than this is strongly discouraged.
The following numerical examples do not represent actual diving conditions and are used to explain the problems as simply as possible, thus avoiding complicated mathematics. The conventional idea of diving is that a diver descends with the sea surface (1 ATA) as the reference point and returns there when he or she has finished the dive. A diver may have to dive at altitude, in a mountain lake or dam, where the pressure on the surface is less than 1 ATA. Problems stem from the physics at this altitude.
For simplicity’s sake, the following description is based on the useful, but not strictly correct, traditional theory that the ratio between the pressure reached during the dive and the final pressure determines the decompression required. If this ratio is less than 2:1, then a diver can ascend safely without pausing during ascent. This means that a diver from the sea surface (1 ATA) can dive to 10 metres (2 ATA) and ascend safely, as regards decompression requirements. A diver operating in a high mountain lake, with a surface pressure of 0.5 ATA, could dive only to 5 metres (1 ATA) before he or she had to worry about decompression. This statement ignores the minor correction required with fresh water. Fresh water is less dense than salt water.
Another pressure problem occurs when a diver, who dives at sea level, then flies or ascends into the mountains after the dive. For example, a 5-metre dive (1.5 ATA) from sea level could be followed by an immediate ascent to a pressure (altitude) of 0.75 ATA, with little theoretical risk. Deeper dives or greater ascents may require the diver to pause at sea level if the diver is to avoid decompression sickness. If the diver ascends, in a motor vehicle or an airplane, the reduced pressure will expand ‘silent’ bubbles or increase the gas gradient to produce larger bubbles, thereby aggravating the diseases of pulmonary barotrauma and decompression sickness.
Thus, exposure to altitude after diving, or diving at altitude, increases the danger of decompression sickness, compared with identical dives and exposures at sea level. It influences the decompression obligations, the depths and durations of decompression stops, the nitrogen load in tissues afterward, the safe durations before flying or repetitive diving, the ascent rates recommended during diving and so forth. Formulae are available to convert the equivalent altitude decompressions to sea level decompressions.
Another problem of diving in a high-altitude lake is the rate at which a diver may have to exhale during ascent. A diver who ascends from 10 metres (2 ATA) to the ocean surface (1 ATA) would find that the volume of gas in the lungs has doubled. Most divers realize this and exhale at a controlled rate during ascent. They may not realize that an equivalent doubling in gas volume occurs in only 5 metres of ascent to the surface, if the dive was carried out at an altitude (pressure) of 0.5 ATA. Equivalent effects are encountered with buoyancy, which can more rapidly get out of control at altitude.
The diver’s equipment can also be affected or damaged by high-altitude exposure. Some pressure gauges start to register only when the pressure is greater than 1 ATA. These gauges (oil-filled, analogue and mechanical types) may try to indicate a negative depth, perhaps bending the needle, until the diver reaches 1 ATA pressure. Thus, the dive depth would have to reach more than 5 metres before it even started measuring, if the dive had commenced at an altitude of 0.5 ATA.
The other common depth gauge, a capillary tube, indicates the depth by an air-water boundary. It automatically adjusts to the extent that it always reads zero depth on the surface. The volume of gas trapped in the capillary decreases with depth (Boyle’s Law). For a diver starting from 0.5 ATA altitude, this gauge would read zero, but it would show that the diver had reached 10 metres when he or she was only at 5 metres depth. Theoretically, the diver could plan the dive and decompression according to this ‘gauge’ depth, but only if he or she was very courageous.
Many electronic dive computers do permit correction for altitude, and some need to be ‘re-zoned’ at the dive site. Other decompression meters are damaged by exposure to altitude (e.g. as in aircraft travel), and the applicability of other dive computers to altitude diving or saturation excursions is questionable.
Divers who fly from sea level to dive at altitude, as in high mountain lakes, may commence the dive with an already existing nitrogen load in excess of that of the local divers, who have equilibrated at the lower pressures. Thus, the ‘sea level’ divers are in effect doing a repetitive dive, and ‘residual nitrogen’ tables must be employed.
Decompression tables that supply acceptable modifications for altitude exposure include the Buhlmann and Canadian Defence and Civil Institute of Environmental Medicine (DCIEM) tables (see Appendix A).
Altitude exposure and altitude diving are more hazardous extensions of conventional diving. They are not as well researched, and the greater the altitude, the more applicable is this statement. It includes not only the problems already mentioned, but also the complication of diving in fresh, often very cold, water. This water may contain debris that has not decomposed as it would in the ocean and may therefore threaten entrapment. The sites are often distant from diving medical facilities. Undertaking a specialized course in altitude diving is a basic prerequisite.