Hypoxia and brain function
Your hypoxic brain

Hypoxia and brain function

In this post we are going to take a closer look at how your judgement changes due to hypoxia. Being hypoxic means having too little oxygen to support your body. Hypoxia manifests itself as fatigue, lightheadedness, tunnel vision, altered colour perception, and most importantly, impaired judgement.

How do we recognize hypoxia?

The body has no receptors that tell us we are hypoxic at all. Instead what you feel when you are holding your breath is the increase in CO2. This leads to a buildup of carbonic acid in the blood, and thus increased acidity. If we do not build up any CO2 and have gas in our lungs (any gas), there will be no alarm bells going off. Most freedivers notice the uncomfortable feeling associated with hypercapnia (elevated CO2 levels), but unfortunately have no idea about hypoxia. For obvious reasons, this can be problematic.

Luckily we can have a peek at what happens at low levels of O2 because of pilots’ altitude training. It is revealing, and really, a bit scary:

What do we learn from this? By the time we reach PaO2 = 60%, our judgment is so impaired that we are unable to make any sensible decisions. This carries the implication that as a freediver you need to be on your way to the surface at this point, and hopefully you can complete your surface protocol by force of habit. During a breathhold the drop of oxygen saturation tends to stall for a bit at PaO2 = 70% before dropping further. At this level you should be experiencing tunnel vision and other funny effects, although this will differ for everyone personally.

Lucky breaks at depth

We do get some lucky breaks at depth, thanks to the pressure. Oxygen reacts at higher rates at depth. Because of that, your oxygen saturation is unlikely to drop very low until you come closer to the surface and the pressure decreases. This is the reason that most blackouts occur at, or close to the surface. Let’s say you are at 40 meters and you have 5% total O2 in your lungs, this will react as if you have 5 x 5% =  25% O2 in your lungs because of the pressure. However, if you now go back up and by doing so you drain the lungs to 3% total oxygen at 20 meters, the result of the pressure at this depth will be that the O2 reacts as if you have 9% in your lungs. Oxygen will move back from the blood into the lungs and you are now in the low O2 zone, where you are prone to blacking out (more info on this can be found in this article on shallow water blackout). The point: once you are on your way back up make sure you go back to the surface fast.

Is identifying hypoxia useful?

Knowing this, is it still helpful to know when we are hypoxic? I think so, but you also need to realize when you are going to notice it. This is probably in the last 10 – 20 m of your ascent (depending on how deep you dive). If you have dipped below 70% or 60% PaO2 you should notice this at the surface as some type of lightheadedness or tunnel vision. The depth and duration of that dive should probably be your maximum for the day unless you are still warming up. It will vary daily and between dives, depend on what you have eaten, rest, hydration, and so forth. Doing a 2 minute dive to 30 meters on one day is no guarantee that you can do a 1 minute dive to 20 meters on another day. Even in one dive session you may not always get the same results, so be careful. You can use an oximeter and exhale statics if you want to know what hypoxia feels like. However, note also that in some cases (if your mind wanders at the wrong time?) you may not sense it at all.



Jaap is a geologist by trade and a freediver by passion. Jaap wrote the book Longer and Deeper in 2018. His book teaches how to train for freediving and spearfishing on land.

This Post Has 2 Comments

  1. Luca Malaguti

    Interesting stuff!! You could also hyperventilate like crazy (in a safe environment like your couch) and hold your breath until you reach the hypoxia zone right?

    1. Jaap

      You can, with the caveat that other funny things might happen if you hyperventilate like crazy. You can try an exhale static with an oximeter to see if you can reach PaO2 = 70% without hyperventilation.

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