Shallow water blackout

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A shallow water blackout is a loss of consciousness caused by cerebral hypoxiatowards 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 caused it. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers, and have not experienced problems before. Survivors of shallow water blackout are typically puzzled as to why they blacked out. Many drowningand near drowning events occur among fit, strong swimmers who black out underwater while free-diving or doing breath-hold pool laps. Blacking out, or browning out, near the end of a breath-hold dive is common and although the mechanism is very well understood breath-hold divers are at risk because few have ever heard of it and do not understand how it may be caused by their own actions. The mechanism involved is quite subtle and perhaps for this reason is rarely taught in swimming clubs and is usually absent from scuba diving or snorkelling course curricula. Shallow water blackout is related to but differs from deep water blackout in its characteristics, mechanism and prevention; deep water blackout is precipitated by depessurisation on ascent from depth, refer to deep water blackoutfor a detailed account. Blackout may also be referred to as a syncopeor fainting.

The role of hyperventilation in shallow water blackout

Otherwise unexplained blackouts underwater have been associated with the practice of hyperventilation. When questioned afterwards, many of the survivors of shallow water blackout report that they had hyperventilated before the dive and many of the drowned divers were known to use hyperventilation as a technique for extending underwater time. Hyperventilation, or over-breathing, involves breathing faster and deeper than the body naturally demands. Free divers use it in a bid to increase their blood oxygenation believing that they can stay underwater longer after hyperventilation because their bodies have loaded up with more oxygen (O₂) than usual. This is not true. Provided that they are breathing normally, a healthy body is saturated with as much O₂ as it is ever going to hold. Even after a long breath-hold dive the O₂ levels recover to 100% very quickly so hyperventilation does little good. 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 urge to breathe is triggered by high carbon dioxide (CO₂) levels in the bloodstream not low O₂ levels; the body has no way of detecting O₂ levels other than blacking out or browning out. The first sign of low O₂ is a brown-out if lucky or unconsciousness if unlucky; there is no other sensation whatever to inform the diver, he just fades out. What gives the diver that irresistible urge to breathe is triggered by the rising CO₂ levels in the bloodstream. CO₂ builds up in the bloodstream when O₂ is metabolised, it is a waste product and needs to be expelled. However, the body can detect CO₂ levels very accurately and it relies on this to control breathing. In fact, the CO₂ level is the only trigger to breathe. Washing away the CO₂ too fast by hyperventilating leads to hypocapniaand there is no urge to breathe whatever as the diver slides gently into unconsciousness, underwater, with no warning.

The diagram below shows the O₂ and CO₂ levels in the blood over the duration of a safe dive. Prior to the dive the green area shows the stabilisation of O₂ and CO₂ through normal breathing. The dive ends safely when the diver has an uncontrollable urge to breathe. She holds on for a while but cannot resist it.

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In the next diagram hyperventilation prior to the dive has artificially depressed CO₂ levels without elevating the O₂ level. This unnatural pre-dive state is now a candidate for shallow water blackout. Now note how the O₂ level drops into the diver's blackout zone before the CO₂can rise enough to force the diver to breathe. The dive is extended a little, but this diver may not survive.

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Breathe-hold divers who hyperventilate before a dive are setting themselves up to drown. Many drownings unattributed to any other cause could be avoided if this mechanism were properly understood and practitioners of hyperventilation were warned. Hyperventilation is unlikely to extend dive time significantly anyway. Shallow water blackout can be avoided by ensuring that carbon dioxide levels in the body are properly calibrated prior to diving. Divers should be given the given the following advice:

• 1. Take a moment on the edge of the water to relax; half a minute is generally enough to replace all the oxygen needed.
• 2. Breathe absolutely normally; allow the body to dictate the rate of breathing to make sure the carbon dioxide levels are properly calibrated.
• 3. If you are excited or anxious about the dive take extra care to calm yourself and breathe naturally; adrenalinalso causes hyperventilation without you knowing.
• 4. When the urge to breathe comes on near the end of the dive, believe your body, you need to breathe.
• 5. Never pre-ventilate using blended gas mixtures such as Nitrox, Trimixor Heliairthat might purge carbon dioxide in a bid to extend the dive.
• 6. Don't dive alone. Find a buddy, have them watch your dive like a hawk, and make sure they know their CPR.

Shallow water blackout should be considered alongside deep water blackout.

Deep water blackout

The mechanism for deep water blackout differs to 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 blackoutfor more detail.

See also

• Deep water blackout, for more on this closely related topic.
• Drowning, for more on the mechanism and physiology of drowning.
• Cheyne-Stokes respiration, another condition involving oxygen / carbon dioxide imbalance and which can affect healthy mountaineers.
• Free-diving, for more on the practice of breath-hold diving as a sport.

External links

• Aquatic Safety Research Group- excellent article and further links
• U.S. Naval Safety Center- interesting SWB example
• 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

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Shallow+water+blackout Wikipedia article Shallow water blackout.