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Diving and Hyperventilation

Posted Apr 13 2009 12:40am

I recently learned about the dangers of attempting to increase the length of time you can spend underwater by hyperventilation (breathing too fast) before holding your breath.

We breathe in Oxygen from the air which contains about 21% Oxygen, this then enters the lungs where it diffuses into the bloodstream. The amount that diffuses into the blood is measured by what is called Pulse Oximetry.

Pulse oximetry measures the number of Oxygen atoms attached to Hemoglobin in the blood and is usually measured via a fingertip sensor (Pulse Oximeter).

Normal pulse oxygen saturation levels can range from about 95% to 99% in healthy people.

The relevance of all this to diving may not yet be clear, but please hang on, I’m getting there.

At the other end of the spectrum is pulse carbon dioxide levels. These are pretty much the same, except the levels of these in the blood stream would be far less then oxygen.

Carbon dioxide turns the blood acidic as it builds up in our systems  before it is exhaled. It basically lets us know when we need to breathe.

So having set the scene…

Some people believe that in a swimming pool, before they take a deep breath and hold it so they can dive, if they breathe really fast for a bit (hyperventilate) then they will be able to hold their breath for longer and swim further.

This is not strictly true and can have dire consequences.

I have attached a couple of inaccurate graphs that I have attempted to draw.. and failed. Although they may not be accurate, they show the correct trend. I will explain below…

Pulse oxygen of a normal swimmer holding breath

Pulse oxygen of a normal swimmer holding breath

As you can see on the chart above, there is a point in time when the Pulse Oxygen level crosses the Pulse Carbon Dioxide level.

At the point that these lines intersect, if you were swimming, you would be feeling like you could not hold your breath any longer and that you are about to “explode”. Also note that the point at which the lines intersect is above the point at which you are likely to pass out.

Pulse oxygen level of a swimmer underwater and holding breath after hyperventilating.

Pulse oxygen level of a swimmer underwater and holding breath after hyperventilating.

As you can see from the chart of the swimmer that hyperventilated, the point at which the lines representing Oxygen and Carbon Dioxide intersect after the swimmer is likely to have passed out.

So basically, they could be swimming along, underwater, see the end of the pool getting closer, and then just black out without warning. Really, they better hope that the life guard on duty is particularly on the ball.

Hope I haven’t confused you too much, comments or corrections welcome! Please post a comment!

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