|
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. ==Hyperventilation== Otherwise unexplained blackouts underwater have been associated with the practice of hyperventilation.〔〔〔 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 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 greyout 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: # 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; epinephrine (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 resuscitation (CPR) current practice. Excessive hypocapnia is readily identifiable as it causes dizziness and tingling of the fingers; refer to hyperventilation for details. Conservative breath-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 surface and deep water blackout. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「shallow water blackout」の詳細全文を読む スポンサード リンク
|