What IsUnderwater Hypoxic Blackout?

Underwater Hypoxic Blackout (also known as shallow water blackout) happens when low oxygen levels suddenly cause a person to faint underwater. It often occurs after prolonged breath-holding, especially if (intentionally or unintentionally) hyperventilating beforehand.

Swimmers of all abilities are susceptible to UHB, including the most physically fit and experienced swimmers. Anyone who engages in breath-holding activities is particularly at risk, like freedivers, spearfishers, competitive swimmers, children playing games involving breath-holding, and fitness enthusiasts.

UHB occurs without warning, and due to oxygen deprivation, brain damage and/or death can occur rapidly. There’s also a lack of education and awareness surrounding UHB; even expert swimmers sometimes engage in dangerous hyperventilation that can increase the risk of death.

Hyperventilation, or over-breathing, lowers the CO₂ levels in the body. This delays the body’s natural urge to breathe and lets oxygen hit dangerously low levels.

Some swimmers mistakenly hyperventilate to stay underwater longer, unaware of the risks.

UHB has been formally known as Shallow Water Blackout for many years, but UHB is a more inclusive term for the condition as UHB can occur at any depth of water.

A “shallow water blackout” typically refers to losing consciousness underwater at a depth of less than 15 feet, usually occurring due to prolonged breath-holding, often preceded by hyperventilation; however, the term “shallow water” can be misleading as the blackout itself is caused by oxygen deprivation and can happen at any depth, especially if hyperventilation is present.

“Shallow water blackout” is also a freediving term referring to blackouts upon ascent from deeper water.

• Sudden stillness. The swimmer may suddenly stop moving or appear to be floating motionless.
• Lack of struggle. Unlike typical drowning, there may be no struggle or signs of distress.
• Unresponsiveness. The swimmer may not respond to calls or taps.
• Eyes closed or glazed. The swimmer’s eyes may be closed or have a glazed, unfocused look.
• Body position. The swimmer might be face down or in an unusual, limp position in the water.
• Bubbles. Look for small bubbles coming from the swimmer’s nose or mouth as they lose consciousness.

Always swim with a buddy who knows about UHB. Practice safe breathing techniques together, check in regularly, and take breaks to make sure you both feel healthy and comfortable. If breath-holding under water, keep your buddy nearby so you can tap on their shoulder every 10-15 seconds to signal consciousness. Your buddy should never be breath-holding with you and should keep their focus on you.

Spearfishers should have a designated watcher for each fisher.

UHB is a life-threatening emergency and it’s important to act fast.

• Get them out of the water, keeping their head above the surface.
• Call for help and dial emergency services.
• Check for breathing and responsiveness. If they’re not breathing, start rescue breathing followed by CPR if needed.
• Monitor their condition until help arrives. If conscious, keep them still and calm.

It’s an important component of CPR used to assist someone who has stopped breathing or is not breathing adequately. The goal is to get oxygen to the person’s lungs, helping sustain vital functions until normal breathing resumes or medical help arrives.

Where you swim plays a big part in what safety precautions may be in place. Remember that where you swim could have an impact on:

• Supervision: Lifeguards and buddy systems provide safety protocols.
• Water conditions: Calm, shallow water reduces risk (but remember that UHB often happens in shallow water toward the end of a swim); open water increases danger due to currents, waves, and reduced visibility.
• Temperature: Cold water can accelerate blackout risk, amplifying the body’s shock response and causing hypoxia faster.
• Safety rules: Clear warnings and rules against breath-holding minimize risky behavior, and prohibiting hyperventilation can prevent unnecessary risks.
• Safety gear: Access to flotation devices and oxygen aids in emergencies can aid in preventing UHB-related fatalities.

• Urgent need to breathe. A strong, uncontrollable urge to take a breath is a key indicator that you need to come up for air immediately.
• Dizziness or lightheadedness. This is a sign that your brain is not getting enough oxygen.
• Tunnel vision. If your vision narrows or darkens at the edges, it’s a clear warning to surface and breathe.
• Confusion or disorientation. Feeling confused or unable to think clearly means your oxygen levels are critically low.
• Weakness or fatigue. Sudden muscle weakness or a heavy, tired feeling can indicate that you’re running out of oxygen.
• Tingling or numbness. Sensations like tingling or numbness in your limbs or face are warning signs of hypoxia (low oxygen).
• Euphoria. Feeling a sudden, inexplicable sense of euphoria or overconfidence can be a dangerous sign that your body is not recognizing its need for oxygen.

Using proper breathing techniques will help your body maintain healthy levels of CO₂ and oxygen. Remember to:

• Surface for air regularly. Establish a rhythm of breathing and surfacing to keep oxygen levels sufficient.
• Control your breathing. Take deep, slow breaths before getting in the water to help calm your body, preventing overexertion.
• Avoid hyperventilation. Breathing too much before swimming stops CO₂ levels from rising naturally, preventing your body from telling you it’s time to breathe.

• Stress or anxiety can elevate your heart rate, cause erratic breathing, and contribute to your panic responses. Listen to your body and take breaks to breathe and relax when needed.
• Alcohol consumption can increase the risk of UHB by 90% when breath-holding.
• Being ill or recovering from a recent illness, including colds, the flu, or COVID-19, which can strain your respiratory system and make strenuous activity more risky.