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Shallow Water Blackout Syndrome

Shallow Water Blackout syndrome, an unfamiliar term that most of us have never heard before, but medical experts are all too familiar with. Shallow Water Blackout, abbreviated as SWB, is what it seems. It is basically passing out/blacking out in water that is 12 feet or less in depth.

SWB is also a great example how even the best of swimmers are not drown proof, though they may think they are.

First, let’s look at the more technical, medical side of SWB and how exactly it happens. When you pass out, it is a result of a lack of oxygen to the brain, increasing the carbon dioxide level in the brain/blood. There are two common situations that lead to SWB:

  1. Low CO2 level: CO2 may be lowered with hyperventilation from rapid, deep breathing. Blackout from low O2 occurs before the trigger level of CO2 to breathe.

  2. Low O2 level: O2 levels may be lowered with repetitive breath-holding and exercise leading to exertion and exhaustion.

Now that we understand the most common medical reasonings, how does it happen? The answer is simple: It is often seen in competitive swimmers who are holding their breaths for extended periods of time underwater (like doing “underwaters” for example), or simply kids at the pool playing breath holding games at a local pool.


The above are the more common ways that SWB occurs, but there are certainly many more possible ways. Now we get onto the arguably most important part: Prevention. One important thing to understand is that hyperventilating is involved in almost all SWB cases. It raises CO2 and ultimately decreases oxygen to the brain. But now, here are some crucial steps to avoid SWB (from shallowwaterblackoutprevention.org)

  1. Never hyperventilate, take deep breaths.

  2. NEVER ignore your body’s urge to breathe because if you do it could be fatal.

  3. DO NOT EVER swim alone.

  4. Do not play breath holding games.

  5. Never swim laps underwater, especially repetitively.

SWB is also a great example how even the best of swimmers are not drown proof, though they may think they are. Olympic Gold Medalist Michael Phelps urges coaches to end the risky swim team tradition of marathon breath-holding workouts. “Most competitive swimmers believe they are invincible. If they can survive a 10,000-meter workout during the holiday break or do a 2,000-meter butterfly set and ask for more, then nothing can stop them," said Jeff Commings of Swimming World Magazine.

SWB is caused when oxygen levels in the brain get drastically low, and carbon dioxide in the body is even lower, often due to hyperventilation before submerging. The low level of carbon dioxide is the catalyst for shallow water blackout, as it’s often too low to signal the brain to tell the body to surface. Once oxygen is depleted, the swimmer faints underwater and drowns.

Unfortunately, it is not rare for SWB to occur in experienced swimmers. Swimming World Magazine even named it, “The silent killer of completive swimmers”. In 2012, this tragically happened to a swimmer of the renowned North Baltimore Aquatic Club (the club of Michael Phelps for many years), where one of their top swimmers became a victim of SWB. “Though lifeguards were on the deck, it’s understandable to think that they didn’t need to watch him closely, as he was a very accomplished swimmer. A few minutes later, a lap swimmer noticed the swimmer motionless at the bottom of the pool. He was pronounced dead later that day, the victim of shallow water blackout. It’s caused when oxygen levels in the brain get drastically low, and carbon dioxide in the body is even lower, often due to hyperventilation before submerging. The low level of carbon dioxide is the catalyst for shallow water blackout, as it’s often too low to signal the brain to tell the body to surface. Once oxygen is depleted, the swimmer faints underwater and drowns.” (swimmingworldmagazine.com). The quote above is from Swimming World Magazines article titled “Shallow Water Blackout, the Killer of Completive Swimmers” by Jeff Commings.


Shallow water blackout, aka SWB, is when someone passes out in water that is less than 12 feet in depth. This can happen to absolutely anyone including competitive swimmers. Always be wary of your breathing in water and most importantly, no matter your skill level, never swim alone. Should it ever occur that you need to take that breath when under water remember to surface and take that breath and not risk SWB by pushing yourself.