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Swim Risk: What Is Shallow-Water Blackout?


WRITTEN BY: Sports Medicine
Wednesday, July 13th, 2016

Unless you’re wading through shark-infested waters, going for a swim hardly seems dangerous — especially for skilled swimmers.

But a common practice, holding your breath, can put you at risk for a dangerous condition called shallow-water blackout. This potentially deadly condition can happen to any swimmer, though it’s most common in competitive swimmers, free divers, Navy SEALS, and spear fishers.

About Shallow-Water Blackout

Simply put, shallow-water blackout occurs when someone faints underwater. This condition takes hold during a two-step process.

  1. The risk begins when you hyperventilate or “over-breathe,” usually because of exertion. (Think of how you might feel during serious exercise or when you get nervous at a meet) This lowers your body’s level of carbon dioxide (CO2).
  2. The risk continues when you hold your breath — an act common for those who swim underwater, whether because they’re competing, free diving, or just playing around with friends. As a result, your oxygen levels drop and your CO2 levels rise. Eventually, your body becomes starved of oxygen, and you lose consciousness beneath the surface.

When that happens, your body will reflexively take a breath — underwater. Unless a swimmer suffering shallow-water blackout receives immediate medical attention, he or she will drown. The condition can also lead to brain damage.

Despite the multi-step process, shallow-water blackout usually occurs without warning. It can occur in any body of water, from the ocean to your bathtub.

With Shallow-Water Blackout, Prevention Is Key

You’re at greatest risk for shallow-water blackout if you make repetitive laps with prolonged breath-holding and very little rest in between. That’s why this problem is most common in competitive swimmers. However, anyone who swims or plays in water can help avoid shallow-water blackout with the following tips:

  • Never hyperventilate
  • Never ignore the urge to breathe
  • Never swim alone
  • Never play breath-holding games
  • Never take repetitive underwater laps; breathe after every lap

Learn more at the UPMC Sports Medicine webpage.

Sports Medicine

UPMC Sports Medicine is the region’s largest and most experienced center dedicated to caring for athletes of all levels. For more than 25 years of world-class care, we’re proud to offer more services, have more physicians, and treat more student athletes than any other sports medicine provider in the region. Read More