Prevent Shallow Water Blackout – don’t ignore your body signals!

As part of my research on swim safety and drowning prevention, I have spent a considerable amount of time studying Shallow Water Blackout (SWB), a nightmare for elite swimmers. As more and more swimmers are pushing the envelope of physical endurance, I feel that swimmers need to pay more attention to SWB drowning. Death of two NAVY Seals last year in a Virginia Beach training pool was attributed to SWB. Even a recent death of a Dartmouth varsity swimmer in Florida was initially suspected to SWB drowning and later attributed to a rare heart condition per the autopsy report.

Chances of SWB happening is higher for professional and well-trained swimmers -who do rigorous underwater training or underwater breath holding for increasing their endurance limit. Underwater training by itself is not the cause; it happens when the swimmer stretches the ‘boundary’, let the competitive spirit take over without being aware that it may lead to SWB. Statistics show drowning among the well-trained swimmers is mostly due to SWB.

Under normal circumstance, as oxygen is metabolized in our lungs, the O2 level goes down and CO2 level goes up. The brain ‘monitors’ the CO2 and when the level rises to a threshold value, it sends the signal and urges the body to breathe. A professional swimmer (when the competitive pride and sense of invincibility takes over) can suppress the urge to breathe and deprive the brain of Oxygen. Alternatively (done more often), swimmer can trick the brain by lowering the CO2 level by hyperventilation before diving into the water. The starting CO2 level is so low that even if it rises while the swimmer is underwater, it never rises to the threshold before the brain is deprived of O2. Due to O2 deprivation to the brain, the swimmer passes out. Eventually, the CO2 level reaches the threshold and the body gets the urge to breathe and that results in lungs filled with water and eventual drowning.

Despite the concept of Shallow Water Blackout is being understood, there is a general lack of awareness among the swimmers and coaches. Even though I am not a professional swimmer, I have never been told by any coach of the implications of rigorous exercise (that can cause hyperventilation) prior to underwater swimming or breath holding can cause blackout.


Sense of invincibility is important to winning, but stay tuned and respond to your body signals.


I am glad to see that recently some famous swimming personalities, including Michael Phelps and his coach Bob Bowman, have taken upon themselves to bring awareness to other coaches and swimmers. We definitely need more Phelps and Bowmans! Some local governments, and swimming facilities are taking steps in the right direction to implement policies to prevent SWB. A good start, but a long way to go!

– Meghna Sil

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