Shallow water blackout
Encyclopedia
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 (16 feet), 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 caused by taking several very deep breaths, or hyperventilating, just before a dive. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers, and have not experienced problems before.
Many drowning
and near drowning events occur among swimmers who black out underwater while free-diving
or doing breath-hold pool laps. Blacking out, or greying out, near the end of a breath-hold dive is common. Although the mechanism is well understood, it is not common knowledge among breath-hold divers.
Shallow water blackout is related to, but differs from deep water blackout in its characteristics, mechanism and prevention; deep water blackout
is precipitated by depressurisation on ascent from depth. Blackout may also be referred to as a syncope or fainting.
. Survivors of shallow water blackouts often report using hyperventilation as a technique to increase the time they can spend underwater. Hyperventilation, or over-breathing, involves breathing faster and/or deeper than the body naturally demands and is often used by divers in the mistaken belief that this will increase oxygen
(O2) saturation. Although this appears true intuitively, under normal circumstances the breathing rate dictated by the body alone already leads to 98-99% oxygen saturation of the arterial blood
and the effect of over-breathing on the oxygen intake is minor. What is really happening differs from divers' understanding; these divers are extending their dive by closing down the body's natural breathing mechanism, not by increasing oxygen load. The mechanism is as follows:
The primary urge to breathe (more precisely: to exhale) is triggered by rising carbon dioxide
(CO2) levels in the bloodstream. CO2 builds up in the bloodstream when O2 is metabolized and it needs to be expelled as a waste product. The body detects CO2 levels very accurately and relies on this to control breathing. Hyperventilation artificially depletes this (CO2) causing a low blood carbon dioxide condition called hypocapnia
. Hypocapnia reduces the reflexive respiratory drive, allows the delay of breathing and leaves the diver susceptible to loss of consciousness from hypoxia
. For most healthy people the first sign of low O2 is a brownout
or unconsciousness; there is no bodily sensation that warns a diver of an impending blackout.
Significantly, victims drown quietly underwater without alerting anyone to the fact that there is a problem and are typically found on the bottom as shown in the staged image at the right. Pool lifesavers are trained to scan the bottom for the situation shown.
Breath-hold divers who hyperventilate before a dive are at risk of drowning. Many drownings unattributed to any other cause result from shallow water blackout and could be avoided if this mechanism was properly understood and the practice eliminated.
Shallow water blackout can be avoided by ensuring that carbon dioxide levels in the body are properly calibrated prior to diving and that appropriate safety measures are in place; this can be achieved if divers do the following:
Excessive hypocapnia is readily identifiable as it causes dizziness and tingling of the fingers, refer to hyperventilation
for details. Conservative breathe-hold divers who hyperventilate but stop doing so before the onset of these symptoms are likely already hypocapnic without knowing it. These extreme symptoms are caused by the increase of blood pH (alkalosis
) following the reduction of CO2, which is required to maintain the acidity of the blood. The absence of any symptoms of hypocapnia is not an indication that the diver’s CO2 is properly calibrated and cannot be taken as an indication that it is therefore safe to dive.
Note that the body can actually detect low levels of oxygen but that this is not normal. Persistently elevated levels of carbon dioxide in the blood, hypercapnia
(the opposite to hypocapnia
), tend to desensitise the body to CO2, in which case the body may come to rely on the oxygen level in the blood to maintain respiratory drive. This is illustrated in the scenario of type II respiratory failure
. However, in a normal healthy person there is no subjective awareness of low oxygen levels.
Shallow water blackout should be considered alongside deep water blackout.
Unconsciousness
Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a...
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...
towards the end of a breath-hold
Apnea
Apnea, apnoea, or apnœa is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged...
dive in water typically shallower than five metres (16 feet), 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 caused by taking several very deep breaths, or hyperventilating, just before a dive. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers, and have not experienced problems before.
Many 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....
and near drowning events occur among swimmers who black out underwater while 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...
or doing breath-hold pool laps. Blacking out, or greying out, near the end of a breath-hold dive is common. Although the mechanism is well understood, it is not common knowledge among breath-hold divers.
Shallow water blackout is related to, but differs from deep water blackout in its characteristics, mechanism and prevention; deep water blackout
Deep water blackout
A deep water blackout is a loss of consciousness caused by cerebral hypoxia 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...
is precipitated by depressurisation on ascent from depth. Blackout may also be referred to as a syncope or fainting.
The role of hyperventilation
Otherwise unexplained blackouts underwater have been associated with the practice of hyperventilationHyperventilation
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...
. Survivors of shallow water blackouts often report using hyperventilation as a technique to increase the time they can spend underwater. Hyperventilation, or over-breathing, involves breathing faster and/or deeper than the body naturally demands and is often used by divers in the mistaken belief that this will increase oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) saturation. Although this appears true intuitively, under normal circumstances the breathing rate dictated by the body alone already leads to 98-99% oxygen saturation of the arterial blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
and the effect of over-breathing on the oxygen intake is minor. What is really happening differs from divers' understanding; these divers are extending their dive by closing down the body's natural breathing mechanism, not by increasing oxygen load. The mechanism is as follows:
The primary urge to breathe (more precisely: to exhale) is triggered by rising carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) levels in the bloodstream. CO2 builds up in the bloodstream when O2 is metabolized and it needs to be expelled as a waste product. The body detects CO2 levels very accurately and relies on this to control breathing. Hyperventilation artificially depletes this (CO2) causing a low blood carbon dioxide condition called 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....
. Hypocapnia reduces the reflexive respiratory drive, allows the delay of breathing and leaves the diver susceptible to loss of consciousness from hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
. For most healthy people the first sign of low O2 is a brownout
Brownout (medical)
A greyout is a transient loss of vision characterized by a perceived dimming of light and color, sometimes accompanied a loss of peripheral vision...
or unconsciousness; there is no bodily sensation that warns a diver of an impending blackout.
Significantly, victims drown quietly underwater without alerting anyone to the fact that there is a problem and are typically found on the bottom as shown in the staged image at the right. Pool lifesavers are trained to scan the bottom for the situation shown.
| The diagram above shows the O2 and CO2 levels in the blood over the duration of a safe dive. Stabilisation of O2 and CO2 levels through normal breathing are shown on the left. The dive ends safely when the diver is forced to the surface by an urgent need to breathe. | In the diagram above hyperventilation prior to the dive has artificially depressed CO2 levels without elevating the O2 level. This pre-dive state is likely to result in shallow water blackout. The O2 level drops into the diver's blackout zone before the CO2 can rise enough to force the diver to resurface to breathe. The dive length is extended but the diver may not survive. |
Breath-hold divers who hyperventilate before a dive are at risk of drowning. Many drownings unattributed to any other cause result from shallow water blackout and could be avoided if this mechanism was properly understood and the practice eliminated.
Shallow water blackout can be avoided by ensuring that carbon dioxide levels in the body are properly calibrated prior to diving and that appropriate safety measures are in place; this can be achieved if divers do the following:
- Take a moment on the edge of the water to relax and allow blood oxygen and carbon dioxide to reach equilibrium.
- Breathe absolutely normally; allow the body to dictate the rate of breathing to make sure the carbon dioxide levels are properly calibrated.
- If excited or anxious about the dive take extra care to remain calm and breathe naturally; epinephrineEpinephrineEpinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
(adrenaline) also causes hyperventilation without the diver knowing. - When the urge to breathe comes on near the end of the dive immediately seek access to air.
- Never dive alone. Dive in buddy pairs, one to observe, one to dive.
- Buddy pairs must both know cardiopulmonary resuscitationCardiopulmonary resuscitationCardiopulmonary resuscitation is an emergency procedure which is performed in an effort to manually preserve intact brain function until further measures are taken to restore spontaneous blood circulation and breathing in a person in cardiac arrest. It is indicated in those who are unresponsive...
(CPR) current practice.
Excessive hypocapnia is readily identifiable as it causes dizziness and tingling of the fingers, refer to 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...
for details. Conservative breathe-hold divers who hyperventilate but stop doing so before the onset of these symptoms are likely already hypocapnic without knowing it. These extreme symptoms are caused by the increase of blood pH (alkalosis
Alkalosis
Alkalosis refers to a condition reducing hydrogen ion concentration of arterial blood plasma . Generally, alkalosis is said to occur when pH of the blood exceeds 7.45. The opposite condition is acidosis .-Causes:...
) following the reduction of CO2, which is required to maintain the acidity of the blood. The absence of any symptoms of hypocapnia is not an indication that the diver’s CO2 is properly calibrated and cannot be taken as an indication that it is therefore safe to dive.
Note that the body can actually detect low levels of oxygen but that this is not normal. Persistently elevated levels of carbon dioxide in the blood, hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
(the opposite to 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....
), tend to desensitise the body to CO2, in which case the body may come to rely on the oxygen level in the blood to maintain respiratory drive. This is illustrated in the scenario of type II respiratory failure
Respiratory failure
The term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
. However, in a normal healthy person there is no subjective awareness of low oxygen levels.
Shallow water blackout should be considered alongside deep water blackout.
Deep water blackout
The mechanism for deep water blackout differs from that for shallow water blackouts and does not necessarily follow hyperventilation. However, hyperventilation will exacerbate it and the two should be considered together. Shallow water blackouts can happen in extremely shallow water; brownouts can be induced even on dry land following hyperventilation and apnoea. However, the effect becomes much more dangerous in the ascent stage of a deep free dive. Refer to deep water blackout for more detail. There is considerable confusion surrounding the terms shallow and deep water blackout and they are made to refer to different things, or used interchangeably, in different water sports circles. For the purposes of this article the two are separate phenomena with the following characteristics:- Deep water blackout occurs as the surface is approached following a breath-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 depressurization 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 brought about by hyperventilation prior to the dive.
See also
- Cheyne-Stokes respirationCheyne-Stokes respirationCheyne-Stokes respiration is an abnormal pattern of breathing characterized by progressively deeper and sometimes faster breathing, followed by a gradual decrease that results in a temporary stop in breathing called an apnea. The pattern repeats, with each cycle usually taking 30 seconds to 2...
, another condition involving oxygen / carbon dioxide imbalance and which can affect healthy mountaineers. - Buteyko methodButeyko methodThe Buteyko method or Buteyko Breathing Technique is a form of complementary or alternative physical therapy that proposes chronic "breathing retraining" as a treatment for asthma as well as other conditions. The method takes its name from the late Ukrainian doctor Konstantin Pavlovich Buteyko ,...
External links
- Dying for Air - example by Walter Griffiths, M.D. and Tom Griffiths, Ed.D. in Aquatics International 2005
- Shallow-Water Blackout is No Joke - Lt. Douglas Chandler, Naval Safety Centre
- Shallow Water Blackout - Naval Safety Centre
- Swimmer Discovers Dangers of Water Blackout - Lifesaving Resources Inc. for interesting personal perspective
- Shallow Water Death - Maj. James Law in the Combat Edge 2003
- Shallow Water Blackout - Dr. Scott Duke in YMCA SCUBA Currents
- The Dangers of Underwater Swimming Are Real - Bruce Wigo in ASCA Online
- Scubadoc's Diving Medicine Online: Latent Hypoxia