Freediving blackout
Divers and swimmers who black out or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time.
[5] Freediving blackout has a high fatality rate, and mostly involves males younger than 40 years, but is generally avoidable.
[9] This problem may stem from the origin of the term latent hypoxia in the context of a string of fatal, shallow water accidents with early military, closed-circuit rebreather apparatus prior to the development of effective oxygen partial pressure measurement.
[1] Other terms generally associated with freediving blackout include: The minimum tissue and venous partial pressure of oxygen which will maintain consciousness is about 20 millimetres of mercury (27 mbar).
[2][3][4][25] 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 saturation.
[10] The mechanism is as follows: The primary urge to breathe is triggered by rising carbon dioxide (CO2) levels in the bloodstream.
The body detects carbon dioxide levels very accurately and relies on this as the primary trigger to control breathing.
Hypocapnia reduces the reflexive respiratory drive, allowing the delay of breathing and leaving the diver susceptible to loss of consciousness from hypoxia.
For most healthy people, the first sign of low oxygen levels is a greyout or unconsciousness: there is no bodily sensation that warns a diver of an impending blackout.
[10] 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 above.
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 normally balanced prior to diving and that appropriate safety measures are in place.
[12] Note that the urge to breathe is triggered by rising carbon dioxide levels in the blood and not by the reduction of oxygen.
[10] Persistently elevated levels of carbon dioxide in the blood, hypercapnia (the opposite to hypocapnia), tend to desensitise the body to carbon dioxide, in which case the body may come to rely on the oxygen level in the blood to maintain respiratory drive.
[12] An ascent blackout, or 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 caused it.
Victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before.
A critical pO2 of 30 millimetres of mercury (40 mbar) in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive.
The drop in intrathoracic pressure may also reduce cardiac output for this period and thereby further compromise the cerebral oxygen supply.
[30] If the diver is still underwater when the laryngospasm relaxes, then water will enter the airway and may reach the lungs, which will cause complications even if resuscitation is successful.
[30] The sudden and unexpected death of a swimmer, with no involuntary drowning sequence, can be difficult to ascribe to a specific cause.
[31] Experienced free-divers are at particular risk because of their practiced ability to suppress the carbon dioxide induced urge to breathe.
Many drownings unattributed to any other cause are assumed to result from shallow water blackout, and could be avoided if this mechanism was properly understood and the practice controlled or eliminated.
Increased advocacy to improve public awareness of the risk is one of the few available ways to attempt to reduce the incidence of this problem.
[6] Shallow water blackout can be avoided by ensuring that carbon dioxide levels in the body are normally balanced prior to diving and that appropriate safety measures are in place.
The following precautions are recommended by several organizations:[10][33][34] A high level of hypocapnia is readily recognized as it causes dizziness and tingling of the fingers.
The mask is adequate protection of the nasal passages if in place, and a hand can be used to cover the mouth and hold it closed.
Thereafter a sequence of two breaths and 30 chest compressions is recommended, repeated until vital signs are re-established, the rescuers are unable to continue, or advanced life support is available.
[35] Attempts to actively expel water from the airway by abdominal thrusts or positioning head downwards should be avoided as they delay the start of ventilation and increase the risk of vomiting, with a significantly increased risk of death, as aspiration of stomach contents is a common complication of resuscitation efforts.
