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Shallow Water Blackout vs. Hypoxic Blackout
![Child swimming in shallow water at pool.]()
Date Published: July 2, 2024
Hypoxic and shallow water blackouts happen for different reasons and under different circumstances – one is due to a lack of carbon dioxide, the other is due to dilute oxygen levels that can occur while freediving. Watch as Dr. Joshua M. Tobin explains the difference and find out how to prevent some underwater blackouts.
Hypoxic blackout is a phenomenon in which a patient passes out for many hypoxic reasons underwater. There are two different concepts here that are sometimes confused or used interchangeably.
Hypoxic Underwater Blackout
In one type of hypoxic underwater blackout, a diver may go down to a great depth, say 60 feet or something like that. So, they're doing whatever work they're doing at depth on a breath hold, and as they come back to the surface, the oxygen in their system is now going to become, in a manner of speaking, more diluted. Because as you rise in the water column, pressure decreases and so the oxygen molecules become further spaced out. And so, it's kind of like the oxygen's more diluted. And so sometimes, divers will blackout as they're approaching the surface. Hence shallow water blackout.
Shallow Water Blackout vs. Hypoxic Blackout
Shallow Water Blackout: When you free dive in deep water and head to the surface, oxygen in your body gets diluted because you held your breath. As you come to the surface, you blackout.
Hypoxic Blackout: Hyperventilation occurs and you blackout. When you hold your breath and ignore the urge to breathe, you drive down the carbon dioxide in your blood.
Underwater Swimming Competitions
Another thing that can occur is people, often very good swimmers, will have underwater swimming competitions to see who can go the furthest. And they'll hyperventilate before they do this and then they'll swim underwater to see who can go the furthest and then that's the contest. Well, what you do when you hyperventilate is drive down the carbon dioxide level in your blood. Now this is important because the carbon dioxide level in your blood is what makes you want to breathe. That's what gives you the sensation of needing to take a deep breath. It's not the low oxygen level but the high carbon dioxide level.
So, when you hyperventilate, when you breathe off all that CO2, you drop that carbon dioxide level so you don't have that feeling of air hunger like you would if you were just breathing normally. Now, when you're doing these underwater swimming competitions, you don't have that urge to breathe, and so you can blackout. And that's what the hypoxic underwater blackout is. The reason you blackout is you've used up all your oxygen and you just didn't know it because you've blown off all your CO2.
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