Hypoxia and Diving Equipment

Hypoxia secondary to inadequate inspired O2 results from the failure or improper use of the diving equipment. Apart from running out of air on open-circuit scuba, this disorder occurs mainly with the use of closed-circuit or semi-closed-circuit rebreathing apparatus.

Of the following six causes only the first two mechanisms are possible with open-circuit scuba:

1. Exhaustion of gas supply.
2. Low O2 concentration.
3. Inadequate flow rates.
4. Increased O2 consumption.
5. Dilution hypoxia.
6. Hypoxia of ascent.

Exhaustion of gas supply

The ‘out of air’ situation remains a major cause of diving accidents, despite contents gauges, reserve supplies and training (Case Report 16.1).


Two commercial divers were engaged in making a 110-metre mixed gas dive from a diving bell. The purpose of the dive was to tie in a 6-inch riser. While one diver was in the water at depth working on the riser, the diving bell operator excitedly informed topside that the bell was losing pressure and flooding. The surface operator, who was disconcerted by this information, opened valves to send gas to the bell. Communication with the bell operator was lost.

The diver who was in the water working on the riser was instructed to return to the bell, which he did. When the diver arrived at the bell, he found the bell operator unconscious and lying on the deck of the bell. The diver climbed out of the water into the bell, took off his Kirby-Morgan mask, and promptly collapsed. When topside personnel realized that they had completely lost communications with the bell, they made ready the standby divers. The first standby diver was dressed, put on his diving helmet and promptly collapsed unconscious on deck. At this point the bell with the diver and the bell operator was brought to the surface with the hatch open and without any decompression stops. The divers were extricated from the bell and recompressed in a deck decompression chamber. Both the diver and the bell operator died in the deck decompression chamber at 50 metres, of fulminating decompression sickness.

Examination of the rack (the collection of gas cylinders to be used during the dive) showed that the rack operator had mistakenly opened a cross-connect valve that should have been ‘tagged out’ (labeled to indicate that it should not be used). This valve permitted 100 per cent helium to be delivered to the diving bell and the standby divers, instead of the appropriate helium-oxygen breathing mixture.

Diagnosis: acute hypoxia and fulminant decompression sickness.

Low oxygen concentration in the gas supply

Accidental filling of an air cylinder with another gas, such as nitrogen, may result in unconsciousness. Low-percentage O2 mixtures (10 per cent O2 or less), designed for use in deep or saturation diving, would lead to hypoxia if breathed near the surface.

Rusting (oxidization) of scuba cylinders can reduce the O2 content, and It has led to at least one fatality and several ‘near misses’ (Case Report 16.2).


MB, a civilian diver, was asked to cut free a rope that was wrapped around the propeller of a diver’s charter boat. Because of the very shallow nature of the dive (3 metres maximum), he used a small steel cylinder not often used by divers. After he entered the water, his diving partner noticed that he was acting in a strange manner and swam to him. At this point the diver was lying on the bottom and was unconscious but still breathing through his single hose regulator. The diving partner rescued the unconscious diver and got him on deck. His fellow divers prised the mouthpiece from him and gave him cardiopulmonary resuscitation, and the diver promptly regained consciousness.

On analysis, the gas in the cylinder was found to be 98 per cent nitrogen and 2 per cent oxygen. There was sea water present in the interior of the cylinder, together with a considerable amount of rust.

Diagnosis: acute hypoxia resulting from low inspired oxygen(see Chapters 6 and 47) concentration.

Inadequate flow rates

Many rebreathing diving sets have a constant flow of gas into the counterlung (see Chapters 4 and 62 for an explanation of this equipment). A set designed to use various gas mixtures has a means of adjusting these flow rates. The flow rate should be set to supply enough O2 for the diver’s maximum requirements, in addition to that lost through the exhaust valve. The higher the O2 concentration of the gas, the lower the required flow rate will be, and vice versa.

If an inadequate flow rate is set for the O2 mixture used, then the inert gas (e.g. nitrogen) will accumulate in the counterlung. Low concentrations of O2 will then be inspired by the diver (Case Report 16.3). Other causes include blockage of the reducer by ice, particles among others.


AS was diving to 20 metres using a 60/40 oxygen-nitrogen mixture in a semi-closed-circuit rebreathing system. After 15 minutes he noted difficulty in obtaining enough gas. He stopped to try and adjust his relief valve and then suddenly lost consciousness. Another diver noticed him lying face down on the bottom. The second diver flushed the unconscious diver’s counterlung with gas and took him to the surface, after which the set was turned to atmosphere, so that the diver was breathing air. The diver started to regain consciousness but was initially still cyanosed. He became aware of his surroundings and did not require further resuscitation. Equipment investigation revealed that carbon dioxide absorbent activity was normal, but reducer flow was set at 2 lpm instead of the required 6 lpm. This would supply inadequate oxygen for the diver’s expected rate of utilization.

Diagnosis: hypoxia resulting from inadequate gas flow rate.

Increased oxygen consumption

Most rebreathing sets are designed for maximum O2 consumption between 1 and 2.5 lpm depending on the anticipated exertion. Commonly, the maximum O2 uptake is assumed to be 1.5 lpm. Several studies have shown that divers can consume O2 at higher rates than these. Values of more than 2.5 lpm for 30 minutes and more than 3 lpm for 10 to 15 minutes have been recorded without excessive fatigue underwater.

This increased exertion may be tolerated because of the cooling effect of the environment and/or greater tissue utilization with increased amount of O2 physically dissolved in the plasma. This indicates that it is possible for a diver to consume O2 at a greater than the often quoted rate under certain conditions. In rebreathing sets, a hypoxic mixture could then develop in the counterlung in response to accumulation of nitrogen (i.e. dilution hypoxia).

Dilution hypoxia

This term applies mainly to O2 rebreathing sets. Dilution hypoxia is caused by dilution of the O2 in the counterlung by inert gas, usually nitrogen. The unwanted nitrogen may enter the system by three methods:

  1. From the gas supply.
  2. From failure to clear the counterlung of air before use, thus leaving a litre or more nitrogen in it.
  3. Failure to clear the lungs before using the equipment; e.g. if a diver breathes into the set after a full inhalation, he or she may add up to 3 litres of nitrogen to the counterlung. This may also occur if the diver surfaces and breathes from the atmosphere, to report activities or for some other reason.

Dilution hypoxia is more likely if O2 is supplied only ‘on demand’ (i.e. when the counterlung is empty), rather than having a constant flow of gas into the bag. As the diver continues to use up the O2, the nitrogen remains in the counterlung. CO2 will continue to be removed by the absorbent, thereby avoiding dyspnoea. Thus, the percentage of O2 in the inspired gas falls as it is consumed. There is approximately 1 litre of nitrogen dissolved in the body, but the amount that would diffuse out into the counterlung to cause dilution hypoxia would be a small contribution.

Hypoxia of ascent

By one of the foregoing mechanisms, the percentage of O2 being inspired may drop to well below 20 per cent. An inspired O2 concentration of 10 per cent can be breathed quite safely at 10 metres because the partial pressure would still be adequate (approximately 140 mm Hg).

Hypoxia develops when the diver ascends sufficiently to reduce this PO2 to a critical level (Case Report 16.4). The disorder is therefore most likely to develop at or near the surface.


RAB was diving to 22 metres while using a semi-closed-circuit rebreathing set with a 40/60 oxygen-nitrogen mixture. After 36 minutes he was instructed to ascend slowly. At approximately 3 to 4 metres he noted some difficulty in breathing but continued to ascend and then started to climb on board, but he appeared to have some difficulty with this. When asked whether he was well he did not answer. He was cyanosed around the lips, and his teeth were firmly clenched on the mouthpiece. On removal of his set and administration of oxygen, he recovered rapidly but remained totally amnesic for 10 minutes. Examination of his diving equipment revealed that both main cylinders were empty and the emergency supply had not been used.

Diagnosis: hypoxia of ascent.