Posted on 05/05/2017, by Jeremy Edsall
Dangers of Hypoxic Blackout
Here's a swimming danger you might not be aware of or have heard of before, it's called hypoxic blackout or shallow water blackout and its reputation is that of a silent killer among swimmers.
If you've trained for and raced in competitive swimming events, you may be aware of this condition, as it occurs most often with experienced swimmers. The danger exists for those unaware of hypoxic blackout or among novice swimmers or children who may play breath holding games in the pool. I know I wasn't aware of this until researching National Water Safety Month.
The cause of shallow water blackouts is due to low levels of both oxygen and carbon dioxide from hyperventilation before submerging. I guess a similar imbalance might occur if you were asked to blow up a bunch of balloons. Inhaling first to gather enough air then expelling it into the balloon over and over might make you feel like you are going to pass out.
In the water when your carbon dioxide levels fall your brain doesn't receive the signal to return to the surface, you wind up fainting under water from low oxygen levels, and you drown. You can see why this is of concern. Children often like to test their limits of how long they can hold their breath underwater, and it can become a competition among friends. They also like to play games of retrieving items from the bottom of the pool and might do this over and over while in the pool.
I recall doing these things in the pool as a child but was never mindful of the fact that, aside from getting to the surface before needing air, it could pose a drowning danger. I was never aware that my body and brain might cause me to pass out on the bottom of the pool or that it could happen to a friend. That's why hypoxic blackout is the silent killer. Unlike drowning where a victim might struggle in the water before going under these swimmers just fail to surface while under the water.
There have probably been many shallow water blackouts classified as drownings, especially among people swimming alone. To learn more about hypoxic blackout or shallow water blackouts you can watch this video or read more here.