Freediving blackout

Freediving blackout, also known as apnea blackout, is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that would cause a blackout. This condition is particularly associated with freediving, but can also occur in any form of breath-hold diving or swimming.

Causes

The primary cause of a freediving blackout is hypoxia, or insufficient oxygen to maintain consciousness. During a breath-hold dive, the body continues to consume oxygen while carbon dioxide (CO2) builds up. A diver may lose consciousness when the oxygen level falls below the threshold necessary to sustain brain function, even if they do not feel the urge to breathe. This lack of urge is due to the body's CO2 tolerance, which can mask the low oxygen levels, leading to a sudden blackout without warning.

Types

Freediving blackouts can be classified into different types based on when they occur:

• Shallow water blackout: Occurs during ascent, typically within the last 10 meters (33 feet) due to the rapid decrease in pressure and consequently partial pressure of oxygen in the lungs, leading to a critical drop in oxygen delivery to the brain.
• Deep water blackout: Happens at depth and is often related to the narcotic effects of nitrogen under pressure (nitrogen narcosis), combined with hypoxia.
• Surface blackout: Occurs at the surface, immediately after surfacing, often because the diver hyperventilates before the dive, lowering CO2 levels without increasing oxygen stores, leading to a delayed response to hypoxia.

Prevention

Prevention of freediving blackout involves several strategies:

• Adequate training and education on the risks and signs of hypoxia.
• Avoiding hyperventilation before dives, as it can lower CO2 levels and delay the urge to breathe without increasing oxygen saturation.
• Diving with a buddy who can monitor and assist in case of a blackout.
• Listening to one's body and respecting personal limits to avoid pushing beyond safe breath-hold times.

Treatment

Immediate treatment for a freediving blackout is essential and follows the basic principles of rescue for any drowning victim:

• Bringing the victim to the surface (if not already) and ensuring the airway is clear.
• Administering rescue breaths as soon as possible.
• Calling for emergency medical help.
• Continuous monitoring and support of vital functions until professional help arrives.

See also

• Hypoxia (medical)
• Drowning
• Breath-hold diving
• Spearfishing
• Underwater diving safety

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• Oxygen-Haemoglobin dissociation curves
  Oxygen-Haemoglobin dissociation curves
• Shallow Water Blackout
  Shallow Water Blackout
• Shallow water blackout diagram 1
  Shallow water blackout diagram 1
• Shallow water blackout diagram 2
  Shallow water blackout diagram 2
• Diver about to surface (seen from below)
  Diver about to surface (seen from below)