Latent hypoxia

Last updated September 17, 2024

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 when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers.

The term latent hypoxia strictly refers to the situation while the potential victim is at depth, still conscious, and not yet hypoxic, but is also loosely applied to the consequential blackout, which is a form of hypoxic blackout also referred to as blackout of ascent or deep water blackout, though deep water blackout is also used to refer to the final stage of nitrogen narcosis.^[1]

Mechanism

The minimum tissue and vascular partial pressure of oxygen which will maintain consciousness is about 20 millimetres of mercury (27 mbar).^[2] This is equivalent to approximately 30 millimetres of mercury (40 mbar) in the lungs.^[3] Approximately 46 ml/min oxygen is required for brain function. This equates to a minimum arterial oxygen partial pressure (P_{a_O₂}) of 29 millimetres of mercury (39 mbar) at 868 ml/min cerebral flow.^[2]

An ascent blackout, or deep water blackout, is a loss of consciousness caused by cerebral hypoxia on ascending from a deep freedive (breath-hold dive), typically of ten metres or more when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it,^[4]^[5]^[6]^[7] or from a dive using underwater breathing apparatus while using a breathing gas which has too low an oxygen fraction to support consciousness at the surface. Breath-hold victims typically black out close to the surface, sometimes even as they break surface, and have been seen to approach the surface without apparent distress only to sink away. Breath-hold victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout by this mechanism may occur even after surfacing from depth and breathing has commenced if the inhaled oxygen has not yet reached the brain and may be referred to as a surface blackout.^[8] Divers ascending using breathing apparatus typically ascend at slower ascent rates to avoid decompression sickness, and the depth at which consciousness is lost tends to follow the oxygen partial pressure of the breathing gas.

The partial pressure of oxygen in the air or other breathing gas mixture in the lungs controls the oxygen loading of blood. A critical P_O₂ of 30 millimetres of mercury (40 mbar) in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a P_O₂ of 60 millimetres of mercury (80 mbar). At 10 msw (2 bar), for the same 2% oxygen, the P_O₂ would be 30 millimetres of mercury (40 mbar), i.e. marginal. At the surface the same 2% oxygen drops to 15 millimetres of mercury (20 mbar), ignoring metabolic use.^[3]

Consequences

The usual consequence, if the airway is not protected, is drowning. A breath-hold diver who has blacked out and has been promptly returned to the surface, will usually regain consciousness within seconds. While the diver is still unconscious underwater, they are at high risk of drowning. While unconscious the diver has lost voluntary bodily control, but still has protective reflexes that protect the airway. One of these is laryngospasm, which closes the larynx, to preventing water from entering the lungs. If the diver has reached the surface, and the divers face is kept above water, when the laryngospasm relaxes spontaneous breathing will often resume.^[9] The laryngospasm will eventually relax, and if the diver is still underwater then water will enter the airway and may reach the lungs which will cause complications if resuscitation is successful, and secondary drowning is possible.^[9] The time between loss of consciousness and death varies considerably depending on a number of factors but can be as little as two and a half minutes.^[10]

If the diver's airway is protected by a full-face mask or diving helmet, the immediate risk is death by asphyxiation, which can occur within a few minutes of cessation of breathing. If the diver sinks and the pressure increases sufficiently, the gas may become capable of supporting consciousness again, but the problem of latent hypoxia remains until a higher oxygen content gas is provided. If the diver is on surface supply, a prompt switch of gases may be sufficient to restore consciousness, and this may also apply to a scuba diver if immediate and appropriate action is taken by another diver. Immediate surfacing of a hypoxic diver using underwater breathing apparatus presents the risk of decompression illness from lung barotrauma or decompression sickness, and the risk depends on the pressure exposure history of the diver.

Scope of risk

Risk of latent hypoxia leading to blackout and further complications depends on the mode of diving in use.

Open circuit scuba and surface supplied diving using hypoxic breathing mixtures to avoid oxygen toxicity: Breathing mixtures for diving must limit partial pressure of oxygen to avoid the risk of acute oxygen toxicity, Recreational technical divers generally limit partial pressure of oxygen at the maximum planned depth of a dive to approximately 1.4 bar. When diving to depths below 57 m this requires the use of breathing gases with less than 21% oxygen. Gases with less than about 0.16 bar partial pressure of oxygen are considered insufficient to reliably maintain consciousness, so for depths below about 77 m the breathing gas which is safe to breathe at depth is not considered safe to breathe at the surface, and this effect increases with depth. At 130 m the richest gas mix acceptable would be about 10% oxygen. The depth at which this provides 0.16 bar P_O₂ is about 6 m, so there is a high probability of loss of consciousness if this gas is used shallower than 6 m To reduce this risk, divers will use a travel gas which is suitable for the first part of descent, and switch to bottom gas when it is convenient and safe to breathe it. On the ascent, oxygen rich gases are valuable for accelerating decompression, so there is an additional reason to switch. Unless there is insufficient overlap in safe depth range for decompression gas and bottom gas, the same gas can be used for travel and decompression, which reduces the number of cylinders that must be carried. Failure to switch gas at the required depth can lead to blackout. Surface supplied divers may be required to work at maximum depth for longer and the partial pressure of oxygen may be limited to reduce pulmonary oxygen toxicity, so the discrepancy in oxygen content when surfacing may be greater, however in this case gas switching is controlled by the surface personnel, and the diver's airway is protected by the full-face mask or helmet, and the surface personnel can monitor the status of the diver on the voice communications system, so the overall risk is reduced in comparison with scuba.
Rebreather diving: During ascent at a rate where oxygen addition to the loop does not adequately compensate for partial pressure reduction due to decreasing ambient pressure, the oxygen concentration in the breathing loop can drop below the level required to support consciousness.
Freediving: During breath-hold diving there is no additional breathing gas available during the ascent. If the diver stays down long enough to use up the available oxygen to the extent where tissue concentration has dropped below a level sufficient to support consciousness at surface pressure, there is a very high risk of blackout before the surface can be reached.

Management

Avoidance

In open circuit diving using underwater breathing apparatus, the diver must switch to a breathing mixture with sufficient oxygen fraction to maintain an acceptable partial pressure during the next stage of ascent. This adjustment may be done as many times as is necessary and should be scheduled in the dive plan.
Semi-closed circuit rebreathers may be flushed to increase oxygen fraction in the loop before and during ascent. This will ensure that the gas in the loop has not been partially deoxygenated by rebreathing, and has the highest available oxygen content. This is particularly relevant in passive addition rebreathers, as the expansion of loop gas during the ascent may overwhelm the capacity of the system to dump the appropriate amount of rebreathed gas from the loop.
Closed circuit rebreathers can have oxygen added to the loop either manually or automatically, to maintain the desired oxygen partial pressure set-point during ascent.
Freedivers can only avoid getting into a situation where latent hypoxia occurs. Once oxygen levels reach this point they will probably lose consciousness during the ascent and are at a high risk of drowning. Ensuring that they are positively buoyant in the event of blackout will give them a chance of recovery if they reach the surface.

Rescue

The diver who has lost consciousness due to latent hypoxia is already hypoxic, and should be brought to the surface as soon as possible, while protecting the airway. There are no contraindications to immediate surfacing for a breath-hold diver, and the mouth and nose may be blocked to prevent involuntary aspiration of water.^{[ citation needed ]}

First aid and medical treatment

The first priority is ventilation at the earliest possible opportunity, and in many cases this may be sufficient. If ventilation with sufficient oxygenation is achieved promptly, and there has been no aspiration of water it is quite possible that no further treatment will be needed. If there is a delay the prognosis deteriorates rapidly. CPR may be necessary. Aspiration of water may require hospitalisation for observation, and if necessary, treatment for complications due to water in the lungs. Treatment is generally as for drowning. Supplementary oxygen is generally recommended.^{[ citation needed ]}

Related Research Articles

Technical diving is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.

<span class="mw-page-title-main">Scuba diving</span> Swimming underwater, breathing gas carried by the diver

Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The name scuba is an anacronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their own source of breathing gas, usually compressed air, affording them greater independence and movement than surface-supplied divers, and more time underwater than free divers. Although the use of compressed air is common, a gas blend with a higher oxygen content, known as enriched air or nitrox, has become popular due to the reduced nitrogen intake during long or repetitive dives. Also, breathing gas diluted with helium may be used to reduce the effects of nitrogen narcosis during deeper dives.

Diving disorders, or diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

Freediving blackout, breath-hold blackout, or apnea blackout is a class of hypoxic blackout, a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be provoked by hyperventilating just before a dive, or as a consequence of the pressure reduction on ascent, or a combination of these. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout may also be referred to as a syncope or fainting.

A bailout bottle (BoB) or, more formally, bailout cylinder is a scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure. A bailout cylinder may be carried by a scuba diver in addition to the primary scuba set, or by a surface supplied diver using either free-flow or demand systems. The bailout gas is not intended for use during the dive except in an emergency, and would be considered a fully redundant breathing gas supply if used correctly. The term may refer to just the cylinder, or the bailout set or emergency gas supply (EGS), which is the cylinder with the gas delivery system attached. The bailout set or bailout system is the combination of the emergency gas cylinder with the gas delivery system to the diver, which includes a diving regulator with either a demand valve, a bailout block, or a bailout valve (BOV).

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

An emergency ascent is an ascent to the surface by a diver in an emergency. More specifically, it refers to any of several procedures for reaching the surface in the event of an out-of-gas emergency, generally while scuba diving.

Shallow-water blackout is loss of consciousness at a shallow depth due to hypoxia during a dive, 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, others occur during ascent while scuba diving, usually when using a rebreather, and occasionally while surface-supplied diving.

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic product. Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, low noise levels, and lack of bubbles.

Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

The following outline is provided as an overview of and topical guide to underwater diving:

The science of underwater diving includes those concepts which are useful for understanding the underwater environment in which diving takes place, and its influence on the diver. It includes aspects of physics, physiology and oceanography. The practice of scientific work while diving is known as Scientific diving. These topics are covered to a greater or lesser extent in diver training programs, on the principle that understanding the concepts may allow the diver to avoid problems and deal with them more effectively when they cannot be avoided.

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 each breath. Oxygen is added to replenish the amount metabolised by the diver. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, to eliminate the bubbles produced by an open circuit system. A diving rebreather is generally understood to be a portable unit carried by the user, and is therefore a type of self-contained underwater breathing apparatus (scuba). A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible or surface installation is more likely to be referred to as a life-support system.

References

↑ Pollock, Neal W. (25 April 2014). "Loss of Consciousness in Breath-Holding Swimmers". Fact Sheets, Water Safety. National Drowning Prevention Alliance (NDPA.org). Archived from the original on 2 February 2017. Retrieved 17 January 2017.
1 2 Stec, A.A.; Hull, T.R., eds. (2010). "4.2 Asphyxia, hypoxia and asphyxiant fire gases". Fire Toxicity. Woodhead Publishing in materials. Vol. Part II: Harmful effects of fire effluents. Elsevier. pp. 123–124. ISBN 9781845698072 . Retrieved 27 January 2017.
1 2 Lindholm, Peter (2006). Lindholm, P.; Pollock, N. W.; Lundgren, C.E.G. (eds.). Physiological mechanisms involved in the risk of loss of consciousness during breath-hold diving (PDF). Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. Durham, NC: Divers Alert Network. p. 26. ISBN 978-1-930536-36-4 . Retrieved 24 January 2017.
↑ Brubakk, A. O.; Neuman, T. S. (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN 0-7020-2571-2.
↑ Lindholm, P.; Pollock, N.W.; Lundgren, C.E.G., eds. (2006). Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. Durham, NC: Divers Alert Network. ISBN 978-1-930536-36-4. Archived from the original on October 7, 2008. Retrieved 2008-07-21.{{cite book}}: CS1 maint: unfit URL (link)
↑ Elliott, D. (1996). "Deep Water Blackout". South Pacific Underwater Medicine Society Journal. 26 (3). ISSN 0813-1988. OCLC 16986801. Archived from the original on April 15, 2013. Retrieved 2008-07-21.{{cite journal}}: CS1 maint: unfit URL (link)
↑ Campbell, Ernest (1996). "Free Diving and Shallow Water Blackout". Diving Medicine Online. scuba-doc.com. Retrieved 24 January 2017.
↑ Lane, Jordan D. (2017). "Drowning Deaths From Unsupervised Breath Holding: Separating Necessary Training From Unwarranted Risk". Military Medicine. 182 (January/February). Association of Military Surgeons of the U.S.: 1471–. doi: 10.7205/MILMED-D-16-00246 . PMID 28051962.
1 2 Etzel, Cliff (18 October 2001). "Rescue procedures for Freediver Blackout". Freediving. DeeperBlue. Retrieved 24 January 2017.
↑ Craig, AB Jr. (1976). "Summary of 58 cases of loss of consciousness during underwater swimming and diving". Med Sci Sports. 8 (3): 171–175. doi: 10.1249/00005768-197600830-00007 . PMID 979564.

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[Pollock_2014-1] Pollock, Neal W. (25 April 2014). "Loss of Consciousness in Breath-Holding Swimmers". Fact Sheets, Water Safety. National Drowning Prevention Alliance (NDPA.org). Archived from the original on 2 February 2017. Retrieved 17 January 2017.

[Stec_and_Hull_2010-2] 1 2 Stec, A.A.; Hull, T.R., eds. (2010). "4.2 Asphyxia, hypoxia and asphyxiant fire gases". Fire Toxicity. Woodhead Publishing in materials. Vol. Part II: Harmful effects of fire effluents. Elsevier. pp. 123–124. ISBN 9781845698072 . Retrieved 27 January 2017.

[Lindholm_2006-3] 1 2 Lindholm, Peter (2006). Lindholm, P.; Pollock, N. W.; Lundgren, C.E.G. (eds.). Physiological mechanisms involved in the risk of loss of consciousness during breath-hold diving (PDF). Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. Durham, NC: Divers Alert Network. p. 26. ISBN 978-1-930536-36-4 . Retrieved 24 January 2017.

[Brubakk_and_Neuman_2003-4] Brubakk, A. O.; Neuman, T. S. (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN 0-7020-2571-2.

[Lindholm_Pollock_and_Lundgren_2006-5] Lindholm, P.; Pollock, N.W.; Lundgren, C.E.G., eds. (2006). Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. Durham, NC: Divers Alert Network. ISBN 978-1-930536-36-4. Archived from the original on October 7, 2008. Retrieved 2008-07-21.{{cite book}}: CS1 maint: unfit URL (link)

[Elliott_1996-6] Elliott, D. (1996). "Deep Water Blackout". South Pacific Underwater Medicine Society Journal. 26 (3). ISSN 0813-1988. OCLC 16986801. Archived from the original on April 15, 2013. Retrieved 2008-07-21.{{cite journal}}: CS1 maint: unfit URL (link)

[Campbell_1996-7] Campbell, Ernest (1996). "Free Diving and Shallow Water Blackout". Diving Medicine Online. scuba-doc.com. Retrieved 24 January 2017.

[Lane_2017-8] Lane, Jordan D. (2017). "Drowning Deaths From Unsupervised Breath Holding: Separating Necessary Training From Unwarranted Risk". Military Medicine. 182 (January/February). Association of Military Surgeons of the U.S.: 1471–. doi: 10.7205/MILMED-D-16-00246 . PMID 28051962.

[Etzel_2001-9] 1 2 Etzel, Cliff (18 October 2001). "Rescue procedures for Freediver Blackout". Freediving. DeeperBlue. Retrieved 24 January 2017.

[Craig_1976-10] Craig, AB Jr. (1976). "Summary of 58 cases of loss of consciousness during underwater swimming and diving". Med Sci Sports. 8 (3): 171–175. doi: 10.1249/00005768-197600830-00007 . PMID 979564.

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