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 websiteto 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.

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