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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 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 drowning and 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 or snorkelling courses. It is sometimes not stressed in scuba diving certification programs. 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, refer to deep water blackout for a detailed account. Blackout may also be referred to as a syncope or 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 brownout if lucky or unconsciousness if unlucky; there is no other sensation whatsoever to inform the diver, he or she 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 hypocapnia and there is no urge to breathe whatsoever 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. 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 CO2 can rise enough to force the diver to breathe. The dive is extended a little, but this diver may not survive. Breath-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 and that appropriate safety measures are in place; this can be achieved if divers do the following: 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 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 brought about by hyperventilation prior to the dive. See also | ||||||||
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