Thursday, August 13, 2015

A “shallow water blackout” is a silent killer

In Jacksonville, Florida, a 50-year-old woman was found at the bottom of her backyard swimming pool. She was an experienced scuba diver who “often stayed at the bottom of the 9-foot deep end without oxygen to increase [her] lung capacity for future dives.”

Despite receiving CPR from her son, she could not be revived.

The Associated Press story about this tragic incident did not explain why a swimmer with her background drowned.

It appears to be a classic case of “shallow water blackout.” This phenomenon occurs when people hyperventilate before diving.

An increasing level of carbon dioxide (CO2) is what triggers the urge to breathe. Hyperventilating causes hypocapnia, a reduced amount of CO2 in the blood. If a swimmer uses up enough oxygen to pass out before the CO2 trigger point for breathing is reached, drowning will occur without notice. Victims are usually found at the bottom of the pool.

Here’s what it looks like in a diagram from Wikipedia:

A physician who lost her son to this little-known phenomenon started a website to heighten awareness of the problem. The site contains more information and stories about other drownings caused by shallow water blackouts.

Here is a video of a woman swimming laps of a pool underwater. Advance to the 0:50 point and watch what happens as she begins to slow down. [Addendum 8/13/15 12:50 pm: Warning. The video is graphic. It shows the unconscious swimmer being pulled from the water.]

A shallow water blackout may have been responsible for the death of Natalia Molchanova, the world’s foremost freediver, who went missing a few days ago.

Hyperventilating prior to diving is not recommended. Tell your friends.


artiger said...

Wow. It makes perfect sense, but I've never really thought about it. Thanks for posting this.

Anonymous said...

Yes thank you. As a swimmer, many of us will try to stay underwater as long as possible because of the benefits. This is a good reminder.

William Reichert said...

Yes. As a pulmonologist, I found out over the years that breathing is good.Everyone should be encouraged to do this frequently every
minute. This requires surrounding oneself with air. Unless, of course,
you are a fish.

Skeptical Scalpel said...

Excellent advice from a lung doctor. You can only get information like this on Skeptical Scalpel.

Anonymous said...

I realize this is an old post, but I’m new to this blog and just came across it.
I just wanted to relate to a “game” my friends and I played when we were elementary school-age. Basically the first child takes lots and lots of deep breaths. Then he holds his breath while a second child gives him a bear hug around this chest. What happens is the first child falls to the ground (more of a slow kneeling) and faints, and recovers after maybe a minute or so. We played this game a number of times because some of us enjoyed the feeling it gave us. I know this is going to sound off the wall to some people, but it was my first experience of being “high,” or having something else going on in my consciousness than normal. Parents would scold us if they saw us doing it and tell us stories of children who wouldn’t wake up.
I just mention this, because the basics of our “game” and what happens to these underwater swimmers is probably very similar. The pressure from being underwater is probably equivalent to the bear hug the second child gave, and who knows if the swimmers just took one big breath or hyperventilated with many deep breaths. Of course, in our game we weren’t underwater, so we recovered.

Skeptical Scalpel said...

Thanks for the comment. You may be right that your game was similar to the shallow water blackout. Certainly the fact that you were not in the water and had friends in attendance kept you alive.

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