Breath-holding and shallow-water blackout

Breath-holding is part of mermaiding and underwater dance. Done calmly, within your limits and with someone watching, it is a skill like any other. Done wrongly, it can make you lose consciousness underwater with no warning at all. This is usually called shallow-water blackout.

This guide explains how it happens in plain words, who it happens to, and the rules that keep it from happening to you.

What shallow-water blackout is

It is a loss of consciousness during or just after a breath-hold, caused by too little oxygen reaching the brain. The name comes from where it is often reported. Dr Joanna Chan of Singapore General Hospital, writing for ActiveSG, notes that many cases happen in water less than 5 m deep. She also stresses that it can happen at any depth. A 2 m pool, or water you could stand in, is not safe from it.

You may also see it called hypoxic blackout or breath-hold blackout. They describe the same thing.

How your body tells you to breathe

Most people assume the urge to breathe comes from running out of oxygen. It mostly does not.

Your body uses oxygen and makes carbon dioxide all the time. When you hold your breath, carbon dioxide builds up in your blood. Divers Alert Network (DAN) explains that this rising carbon dioxide is the main trigger for the urge to breathe. Oxygen falling is not what you feel.

In a normal breath-hold, that works in your favour. The carbon dioxide alarm goes off while you still have plenty of oxygen, so you come up in good time.

What over-breathing does

Over-breathing means breathing faster or deeper than your body needs before going under. People call it hyperventilating, "power breaths", "cleansing breaths" or "work-up breathing". DAN is clear that these are all the same thing, with the same risk.

Over-breathing blows off carbon dioxide. Your blood starts the breath-hold with less of it, so the alarm goes off much later. Meanwhile, your oxygen falls at the normal rate.

• The extra oxygen you gain is tiny. DAN describes it as fairly trivial.
• The delay to the warning is large. That is why the breath-hold feels easier.
• Oxygen can run out first. You can black out before you ever feel you need air.

In short, over-breathing does not give you more air. It switches off your early warning.

Why the end of a breath-hold is the riskiest part

On the way up

When you dive down, water pressure squeezes your lungs slightly, which helps the oxygen in them pass into your blood. As you rise, that pressure falls and the help disappears. DAN explains that this is why blackouts are most likely in the final part of the ascent.

Just after you surface

Your first breath does not reach your brain instantly. For a few seconds after surfacing, oxygen in the brain can keep falling. A swimmer can break the surface, look fine, and black out moments later. DAN advises close watch for at least 30 seconds after surfacing.

After hard effort

Swimming hard, kicking down or holding a pose all use oxygen faster. DAN warns that people easily overestimate how long they can safely hold their breath while working. A breath-hold after a sprint, or during a demanding move, will be shorter than one at rest.

What happens after a blackout

A blackout is silent. The swimmer stops moving. There is no struggle, which is why it can look like deliberate breath-holding.

Once unconscious, the body eventually tries to breathe again by reflex. Dr Chan explains that water can then enter the lungs, or the airway can go into spasm, and drowning begins. An older study she cites suggested that the time from blackout to death may be as short as two and a half minutes.

This is why a watcher matters so much. Someone who sees a blackout and gets the swimmer's face out of the water quickly gives them the best chance.

Warning signs a watcher can see

The swimmer often cannot feel the danger coming, but a trained watcher may see it. DAN lists these signs:

• Technique falling apart, kicking that changes or becomes jerky
• Tightening shoulders and neck, trembling fingers
• A change in facial expression, or blue lips
• A large burst of bubbles from the mouth, a classic sign that a blackout is happening

Freedivers call it a "loss of motor control" when a breath-holder briefly loses control of their body. DAN treats it as a serious warning: stop for the day, and a DAN article by Capps advises staying out of the water for at least 24 hours.

Who is at risk

Not only beginners. Confidence is part of the danger.

In 2015 the CDC's journal MMWR published a study by New York City's health department of 16 people in New York State who drowned or nearly drowned after dangerous breath-holding. Four died. All four were aged 17 to 22, were advanced or expert swimmers, and had deliberately over-breathed first.

Dr Chan lists the groups most often affected:

• Competitive and advanced swimmers and divers
• Highly trained people who misjudge their own limits
• Anyone breath-holding in play, training, sport or work
• Swimmers doing repeated long breath-holds, such as underwater lengths without surfacing between them

DAN describes how one diver's earlier blackout put him at greater risk of another.

The rules

1. Never over-breathe before a breath-hold. Breathe normally and calmly.
2. Never breath-hold alone. Someone watches you the whole time, and for at least 30 seconds after you surface. See Never alone.
3. Answer the urge to breathe. When you feel it, come up. Do not push past it.
4. No breath-holding games or contests, in or near water. "Who can stay under longest?" is how many accidents begin.
5. One length, one breath. Surface and breathe between underwater lengths; do not chain them.
6. Rest between breath-holds. Give your body time to recover before the next one.
7. Allow for effort. After hard swimming, keep breath-holds short.
8. Stop when you are tired, cold or unwell.

The safety guidance Dr Chan quotes from Shallow Water Blackout Prevention, a US charity, makes the same points: never hyperventilate, never ignore the urge to breathe, never swim alone, never play breath-holding games, and "one lap, breathe".

Myths

"Breathing hard first gives me more air." It barely changes your oxygen. It delays the warning.

"I'll feel it coming." Often you will not. That is the whole problem.

"It only happens in deep water." Many cases happen in less than 5 m of water, including pools.

"It only happens to beginners." In the New York study, everyone who died was an advanced or expert swimmer.

"Breathing exercises online are safe in the pool." Breathing methods that combine over-breathing with breath-holds have been linked to deaths in water. Dr Chan notes that the best-known of them warns people not to practise near water. Keep them on dry land, if at all.

If you see someone black out

Act at once. Get their face out of the water, call for help, and get them out of the pool. Start CPR if they are not breathing normally. If you are trained, begin with two rescue breaths. If not, the 995 call taker will talk you through it, and chest compressions alone are far better than nothing. Even if they recover quickly, they need to be checked by a doctor. StatPearls, a medical reference, advises hospital assessment for anyone who loses consciousness in the water. See Never alone for more.

In our lessons

Breath-holding is taught step by step, starting from calm, normal breathing, never over-breathing. Holds are kept well within your comfort and lengthened gradually as you grow more relaxed in the water.

Lessons are mostly one-to-one and never more than four, and every breath-hold is watched, one student at a time, in the shallow end. Shallow water is no protection on its own, which is why every hold is watched. In lessons, a separate photographer takes the photos, so her eyes stay on the water, not on a camera. On the MFI path, at Advanced (M1) and Master (M2) level, MFI tests rescue skills, including a rescue tow, alongside a written exam passed at 75% or above, so you learn to look after a partner as well as yourself.

The goal is never a record. It is a calm, beautiful hold that you finish with air to spare.