A deep water blackout is a loss of consciousness caused by cerebral hypoxia

Cerebral hypoxia

Cerebral hypoxia refers to a reduced supply of oxygen to the brain. Cerebral anoxia refers to a complete lack of oxygen to the brain. There are four separate categories of cerebral hypoxia; in order of severity they are; diffuse cerebral hypoxia , focal cerebral ischemia, cerebral infarction, and...

 on ascending from a deep freedive or 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. Victims typically black out close to the surface, usually within the top three metres, sometimes even as they break surface and have often been seen to approach the surface without apparent distress only to sink away. It is quite rare for blackouts to occur while at the bottom or in the early stages of ascent; divers who drown in these stages are usually found to have inhaled water, indicating that they were conscious and succumbed to an uncontrollable urge to breathe rather than blacking out. Victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before. Survivors of deep water blackout are typically puzzled as to why they blacked out.
Breath-hold diving is often referred to elsewhere as dynamic apnoea diving or free-diving

Free-diving

Freediving is any of various aquatic activities that share the practice of breath-hold underwater diving. Examples include breathhold spear fishing, freedive photography, apnea competitions and, to a degree, snorkeling...

. Blackout may also be referred to as a syncope (medicine)

Syncope (medicine)

Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...

 or fainting.

Deep water blackout versus shallow water blackout

In some places, notably in US scuba diving open water curricula, a blackout from a deep freedive is sometimes referred to as a shallow water blackout

Shallow water blackout

A shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres , when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have...

. Where this happens there is usually little or no discussion of the phenomenon of blackouts not involving depressurisation and the cause may be variously attributed to either depressurisation or hypocapnia

Hypocapnia

Hypocapnia or hypocapnea also known as hypocarbia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation....

 or both. This problem probably stems from the original identification of the mechanism of latent hypoxia arising in the context of a string of fatal, shallow water accidents with early military, closed-circuit rebreather

Rebreather

A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...

 apparatus prior to the development of effective partial pressure oxygen meters

Electro-galvanic fuel cell

An electro-galvanic fuel cell is an electrical device, one form of which is commonly used to measure the concentration of oxygen gas in scuba diving and medical equipment....

. In the very different context of dynamic apnoea sports careful consideration of terms is needed to avoid potentially dangerous confusion between two phenomena having different characteristics, mechanisms and prevention. The application of the term shallow water blackout to deep dives and its subsequent association with extreme sports has tended to mislead many practitioners of static apneoa and dynamic apnoea distance diving into thinking that it does not apply to them even though true shallow water blackout

Shallow water blackout

A shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres , when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have...

 kills many swimmers every year, often in shallow swimming pools.

Some consider deep water blackout to be a special condition or subset of shallow water blackout, which is more accurate and may be acceptable. However, in the interests of accuracy, clarity, helpfulness and safety this article treats the two as separate phenomena with the following characteristics:

• Deep water blackout occurs as the surface is approached following a breathe-hold dive of over ten metres and typically involves deep, free-divers practicing dynamic apnoea depth diving usually at sea. The immediate cause of deep water blackout is the rapid drop in the partial pressure of oxygen in the lungs on ascent.

• Shallow water blackout only occurs where all phases of the dive have taken place in shallow water where depressurisation is not a factor and typically involves dynamic apnoea distance swimmers, usually in a swimming pool. The primary mechanism for shallow water blackout is hypocapnia
  Hypocapnia
  Hypocapnia or hypocapnea also known as hypocarbia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation....
  
   brought about by hyperventilation
  Hyperventilation
  Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
  
   prior to the dive.

This confusion is exacerbated by the fact that in the case of deep water blackout hypocapnia may be involved even if ascent is the actual precipitator.

Deep water blackout and nitrogen narcosis

Some scuba divers use the term deep water blackout loosely to refer to the final stage of nitrogen narcosis

Nitrogen narcosis

Narcosis while diving , is a reversible alteration in consciousness that occurs while scuba diving at depth. The Greek word ναρκωσις is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates...

. Nitrogen narcosis is a neurological impairment caused by dissolved blood nitrogen under pressure. The progressive signs and symptoms of nitrogen narcosis are:

• Impairment of reasoning, judgement, memory and concentration;
• Sense of well-being and levity;
• Anxiety;
• Loss of coordination and physical dexterity;
• Hallucination, terror and vertigo;
• Unconsciousness and death.

The onset of symptoms may start around 30 metres and unconsciousness may occur at around 90 metres.
Because free-divers start and finish the dive with only a single lungful of nitrogen to absorb, nitrogen narcosis has long been assumed to be an issue only for divers on compressed air. However, this issue may be more complex than that and needs to be resolved.
There is a lot more to nitrogen narcosis than blackout and it is imprecise and probably not helpful to use the term deep water blackout to refer to a single aspect of it. Therefore, for the purposes of this article deep water blackout refers to latent hypoxia at depth and is unrelated to nitrogen narcosis.

Possible mechanisms for deep water blackout

Three causes have been put forward, possibly acting simultaneously:

1. Voluntary suppression of breathing;
2. Rapid depressurisation;
3. Self induced hypocapnia.

1. Voluntary suppression of breathing. Deep water blackout is sometimes attributed to the practiced diver’s ability through training to suppress the urge to breathe. If surviving divers are aware that they have heavily suppressed the urge to breathe towards the end of the dive there is a tendency to look no further for an explanation. However, there are two problems with this as an explanation:

• a). Even with a high level of training the urge to breathe is almost impossible to overcome; swimmers typically suffer an uncontrollable, violent, deep inhalation of water even when, intellectually, they know that to do so is fatal. In this case the drowned swimmer will be found with water deep in the alveoli of the lungs consistent with inhalation and is a simple case of running out of air and drowning
  Drowning
  Drowning is death from asphyxia due to suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia....
  
  . Victims of deep water blackout, if they have any water in the lungs at all will have a limited amount in the bronchi consistent with natural ingress after death.

• b). Victims of deep water blackout closely observed from both below and above water often do not exhibit the signs of distress associated with an uncontrollable urge to breathe and those that have survived a blackout often report no such distress. Many blackout events have been closely observed and even filmed because deep dynamic apnoea dives are a competitive event and very deep dives require a considerable support crew both above and below water. Anecdotal accounts of healthy divers holding their breath to the point of unconsciousness are difficult to substantiate and the ability, if it exists, is certainly extremely rare.

For these two reasons voluntary suppression of breathing might be considered a contributory factor in that it helps push other factors past the threshold limit but it is not the primary factor.

2. Rapid depressurisation. Because deep water blackout occurs as the diver approaches the surface without necessarily showing signs of distress, depressurisation is clearly implicated in the cause.
Consciousness depends on a minimum partial pressure

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 of oxygen in the brain not on the absolute quantity of the gas in the system. (See pulmonary alveolus

Pulmonary alveolus

An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...

 for details of gas pressure in the lung.) At the surface, the air in the lungs is under 1 atmosphere of pressure; at 10 metres, the water pressure doubles the pressure of air in the lungs to 2 atmospheres. Recreational deep breath-hold dives can often go to 20 metres, competitive deep dives can go much deeper, and the current free-dive record

Free-diving

Freediving is any of various aquatic activities that share the practice of breath-hold underwater diving. Examples include breathhold spear fishing, freedive photography, apnea competitions and, to a degree, snorkeling...

 exceeds 200 metres.
Ten metres is easily achievable by a reasonably fit and competent swimmer.
Most people go unconscious when the partial pressure of oxygen (ppO₂) in their lungs, normally 105 mmHg falls below ~30 mmHg. Ten metres of water effectively doubles this to 60 mmHg putting the diver above the blackout threshold until they hit the 30 mmHg limit on ascent. A ppO₂ of 45mmHg at ten metres will pretty much ensure a blackout between four metres and the surface. S.Miles termed this latent hypoxia. Although quite comfortable on the bottom the diver may actually be trapped and unaware that it is now no longer possible to ascend safely through the shallow layers above and a diver pushing the limits may expect to go unconscious without warning just as he or she approaches the surface.
Rapid depressurisation certainly answers the question as to why victims black out so close to the surface and without distress but it is still unclear why it happens at all. A physiological explanation dependent on pressurisation of the body is required. Two theories have been put forward as to how this might work:

• a). Pressurisation delays the urge to breathe by interfering with the ability of the body's carotid body
  Carotid body
  The carotid body is a small cluster of chemoreceptors and supporting cells located near the fork of the carotid artery ....
  
   to detect CO₂ levels in the blood or its ability to act appropriately on what it does detect, the system being calibrated for sea level only. The body relies on the carotid body to accurately measure the build up of CO₂ in the blood to determine the trigger for the urge to breathe; it cannot detect a lack of oxygen prior to a syncope so anything that disrupts the process is dangerous. Although interesting this theory remains to be proved. Intuitively it might be supposed that increasing the partial pressure of CO₂ at depth would, if anything, bring forward the urge to breathe rather than delay it but in practice pressure would appear to have no effect on the urge to breathe. Furthermore, if pressure did delay the urge to breathe then the ascent itself should bring on this urge, this also does not seem to be the case and a characteristic of deep water blackout is that the urge is absent or not acute. At the moment this theory appears unconvincing.

• b). There is some discussion that the act of rapid depressurisation through at least one atmosphere when the body is close to but has not yet crossed the sea-level threshold level for cerebral hypoxia is enough in itself to cause syncope but this too remains unproven and the physiology unclear.

If neither of these theories can fully account for blackout it is necessary to seek another cause.

3. Self induced hypocapnia. Hyperventilation leading to hypocapnia and subsequent loss of an appropriate urge to breathe is the mechanism behind shallow water blackout; refer to the shallow water blackout

Shallow water blackout

A shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres , when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have...

 article for a detailed description of how this commonly misunderstood mechanism works. Many practitioners of deep water breath-hold diving use hyperventilation as a misguided method of extending the duration of their bottom time and so this mechanism is almost certainly relevant to deep water blackouts also.
If the diver has hyperventilated, the mechanism is essentially that for shallow water blackout but hypoxia is delayed by pressure at depth and sets in only when the pressure drops to 1.25 to 1 atmospheres. This explains why divers who die like this do so very close to the surface on their way up and why they may not have felt any urgency to breathe at all; fit, free-divers ascending from deep dives can go unconscious without any warning.

In conclusion, depressurisation on ascent is an explanation for the consistent depth location of deep water blackouts but is currently an insufficient explanation of cause unless accompanied by an underlying suppression of the urge to breathe through self induced hypocapnia via hyperventilation. Experienced free-divers are put at special risk by their practiced ability to suppress the carbon dioxide induced urge to breathe.
Unfit and inexperienced swimmers rarely have the ability to descend deep enough to induce deep water blackouts and are unable to suppress the urge to break surface at the very onset of distress, if they drown it is much more likely to be from other causes. The strong and the fit are at most risk.
Practitioners of deep breath-hold diving should be counselled on the role of depressurisation regarding the timing of deep water blackouts but should be given the same advice for dive preparation as suggested for shallow water blackout, which centres on relaxation and avoids hyperventilation. Where deep breath-hold divers are observed to use hyperventilation timely and informed advice may save their lives but experience suggests that divers are reluctant to change their practice unless they have a very clear understanding of the mechanics of the process.

See also

• Shallow water blackout
  Shallow water blackout
  A shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres , when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have...
  
  , for the mechanism behind self induced hypocapnia.
• Free-diving
  Free-diving
  Freediving is any of various aquatic activities that share the practice of breath-hold underwater diving. Examples include breathhold spear fishing, freedive photography, apnea competitions and, to a degree, snorkeling...
  
  , for more on the practice of breath-hold diving as a sport.
• Drowning
  Drowning
  Drowning is death from asphyxia due to suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia....
  
  , for more on the mechanism and physiology of drowning and deaths from drowning.
• Pulmonary alveolus
  Pulmonary alveolus
  An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...
  
   for a discussion of gas pressure in the lung.
• Apnoea, for more about breath-holding.

External links

• Aquatic Safety Research Group - excellent article and further links
• U.S. Naval Safety Center - interesting SWB example
• FREEDIVE CENTRAL - all about freedivers and competitions
• Apnea Academy- for resources on underwater apnea
• Apnea Mania - extensive apnea information
• Deeper Blue - for freedive medical articles
• FREE Organization - for freedive regulations and education