Shallow-water blackout is loss of consciousness at a shallow depth due to hypoxia during a

dive Diving most often refers to: * Diving (sport), the sport of jumping into deep water * Underwater diving, human activity underwater for recreational or occupational purposes Diving or Dive may also refer to: Sports * Dive (American football), a ...

, which could be the result of any one of significantly differing causative circumstances. The term is ambiguous, and the depth range in which it may occur is generally shallow relative to the preceding part of the dive, but also occurring when the entire dive takes place at an almost constant depth within a few metres of the surface. Various situations may be referred to as shallow water blackout but differ in how the hypoxia is induced: Some occur in a context of

freediving Freediving, free-diving, free diving, breath-hold diving, or skin diving, is a mode of underwater diving that relies on breath-holding until resurfacing rather than the use of breathing apparatus such as scuba gear. Besides the limits of breat ...

, others occur during ascent while

scuba diving Scuba diving is a Diving mode, mode of underwater diving whereby divers use Scuba set, breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The word ''scub ...

, usually when using a

rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantial unused oxygen content, and unused inert content when present, of each breath. Oxygen is a ...

, and occasionally while

surface-supplied diving Surface-supplied diving is a mode of underwater diving using equipment supplied with breathing gas through a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. ...

Freediving

Two very different breathhold dive profiles can lead to hypoxic blackout at shallow depth.

At constant depth

Blackout may occur when all phases of a breathhold dive have taken place in shallow water, where depressurisation during ascent is not a significant factor, and the blackout may occur without warning before the diver attempts to surface. The mechanism for this type of shallow water blackout is lack of arterial oxygen expedited by low carbon dioxide levels, as a consequence of voluntary hyperventilation before the dive. Blackouts which occur in swimming pools are probably driven only by excessive hyperventilation, with no significant influence from pressure change. There is broad agreement among diving physiologists to call this shallow water blackout or constant pressure blackout.

During ascent

Blackout can occur during ascent from a deep freedive or immediately after surfacing. This is due to a relatively rapidly lowered oxygen partial pressure caused by a reduction in ambient pressure after much of the available arterial oxygen has been used up at the higher partial pressures induced by depth, leaving the diver in a state of

latent hypoxia Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs wh ...

, with actual cerebral hypoxia inevitable during ascent. Blackout in the shallow stage of ascent from deep free-dives is less ambiguously called " ascent blackout", or unambiguously " freediving blackout of ascent", and has also sometimes been called "deep water blackout", which is also an ambiguous term, being used by some authors for loss of consciousness that occurs at depths of greater than about when breathing air, hypothetically as a consequence of

nitrogen narcosis Nitrogen narcosis (also known as narcosis while diving, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gas ...

oxygen toxicity Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lung ...

, or both.

Scuba and surface-supplied diving

One of the hazards of rebreather diving is a hypoxic loss of consciousness while ascending because of a sudden uncompensated drop of oxygen partial pressure in the

breathing loop A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantial unused oxygen content, and unused inert content when present, of each breath. Oxygen is ad ...

. This occurs as a result of the pressure reduction during ascent, usually associated with

manually controlled closed circuit rebreather A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of ea ...

s and

semi-closed circuit rebreather A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of ea ...

s, (also known as

gas extender A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when pres ...

s), which do not use automatic

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...

from the measured oxygen partial pressure to control the mixture in the loop. A similar effect can occur in open circuit scuba and surface-supplied dicing if a diver continues to breathe a hypoxic gas intended for avoiding oxygen toxicity in the deep sector, at a depth shallower than the

minimum operating depth A breathing gas is a mixture of gaseous chemical elements and compounds used for breathing, respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and ...

for the gas, but this is usually just called hypoxia.

References

{{reflist, refs= {{cite book , title=Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. , last1=Brubakk , first1=A. O. , first2=T. S. , last2=Neuman , year=2003 , publisher=Saunders Ltd. , location=United States , isbn=978-0-7020-2571-6 , page=800 {{cite web , url=http://www.scuba-doc.com/latenthypoxia.html , title=Free Diving and Shallow Water Blackout , last=Campbell , first=Ernest , year=1996 , work=Diving Medicine Online , publisher=scuba-doc.com , access-date=24 January 2017 {{cite book , title=Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. , editor1-last=Lindholm , editor1-first=P , editor2-last=Pollock , editor2-first=N.W. , editor3-last=Lundgren , editor3-first=C.E.G. , year=2006 , publisher=Divers Alert Network , location=Durham, NC , isbn=978-1-930536-36-4 {{cite journal , last1=Elliott , first1=D. , title=Deep Water Blackout. , journal=South Pacific Underwater Medicine Society Journal , volume=26 , issue=3 , year=1996 , issn=0813-1988 , oclc=16986801 {{cite journal , vauthors=Lindholm P, Lundgren CE , title=Alveolar gas composition before and after maximal breath-holds in competitive divers , journal=Undersea Hyperb Med , volume=33 , issue=6 , pages=463–7 , year=2006 , pmid=17274316 {{cite report , last1=Edmonds , first1=C. , title=Shallow Water Blackout. , publisher=Royal Australian Navy, School of Underwater Medicine. , work=RANSUM-8-68 , year=1968 {{cite web , url=http://www.xray-mag.com/content/insidious-threat-hypoxic-blackout-rebreather-diving , title=The Insidious Threat of Hypoxic Blackout in Rebreather Diving , last=Pridmore , first=Simon , date=22 April 2012 , website=X-Ray Mag , publisher=AquaScope Media ApS , access-date=21 March 2018 , archive-date=22 March 2018 , archive-url=https://web.archive.org/web/20180322020537/http://www.xray-mag.com/content/insidious-threat-hypoxic-blackout-rebreather-diving , url-status=live Underwater diving physiology Underwater diving safety