Causes and Prevention of Shallow Water Blackout
Sudden Death While Snorkelling and the Role of Hyperventilation
Oct 3, 2009
Every now and then reports flow through the media about the sudden death of a snorkeller who has mysteriously drowned on Australia's Great Barrier Reef. It can be a young, fit person with no known medical problems; and no obvious signs of why they died.
In many cases the reason is Shallow Water Blackout.
What is Shallow Water Blackout
Shallow Water Blackout occurs when a person loses consciousness underwater and then drowns. The victim usually has no other medical problems, or obvious reasons for sudden death. Shallow Water Blackout usually occurs in depths less than 15 feet or five metres. The victim is usually found floating on the sea floor or at the bottom of the pool.
Due to the mixture of gases in their blood, the victim of Shallow Water Blackout simply loses the urgent desire to breathe on a breath hold dive. The medical term for the cause of death is cerebral hypoxia, which means there is a reduced amount of oxygen available in the brain.
Human Breathing Mechanism and Shallow Water Blackout
Before understanding how Shallow Water Blackout occurs, an understanding of the human breathing mechanism is needed.
The urge to take a breath is triggered by the level of Carbon Dioxide (CO2) in the blood. As the oxygen (O2) in the blood is used by the body’s cells, CO2 is created. The level of CO2 rises until it reaches a certain level and then it triggers the body to breathe.
When a snorkeller holds their breath on a breath-hold dive this phenomenon can be observed. When ducking below the surface, after a small amount of time, the urge to breathe hits and becomes a desperate urge that forces the snorkeller to the surface to take in a lungful of air.
How does Shallow Water Blackout Occur
Shallow Water Blackout is usually associated with hyperventilation.
Hyperventilation is when a person takes a series of long, deep breaths that are faster and deeper than the body naturally requires.
This hyperventilation actually decreases the amount of CO2 in the blood, which in turn reduces the urge to breathe. Shallow Water Blackout occurs when the body doesn’t have this desperate urge to breathe near the end of the breath-hold dive.
Near the end of the dive the O2 levels are also dropping rapidly as the oxygen is being used.
In this deadly combination there is not enough O2 to maintain consciousness; and there is no urge to breathe. The result is the snorkeller losing consciousness and drowning.
Hyperventilation and Shallow Water Blackout
Snorkellers usually hyperventilate in the mistaken belief that this will increase the oxygen levels in their blood. However, this is not correct. Hyperventilation is a dangerous practice.
No Sign of Impending Shallow Water Blackout
The biggest problem with Shallow Water Blackout is that the snorkeller usually has no sign of impending Shallow Water Blackout. It occurs without warning.
With no urge to breathe, there is no desperate desire to race for the surface. The snorkeller doesn’t know what happens. And the snorkeller's buddy usually won’t know anything is wrong, except maybe when they see the still body lying on the sea bed.
Why Does Hyperventilation Lead to Longer Time Underwater?
Some snorkellers will attest that hyperventilation leads to a longer time underwater. This may be true, but the reason behind it is not based on getting more O2 into the blood.
Hyperventilation works in reducing the time needed to breathe by extending the time the urge to breathe kicks in.
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