Why is Hyperventilation Bad for Freediving?
I know I know, it’s been such a very long time since you heard from guru of freediving physiology and darling of riverbank critters, the world-famous Dr Otter. I’m sure that all you die-hard fans of Freedive-Earth’s favourite learned Lutrine have been… well… dying… for her next installment. Never fear dear freedivers, feeling your pain through cyberspace we’ve sent the intern down to her surgery where he’s been hammering on the door for the last 3 weeks and demanding a submission. Here, then, are the fruits of her (and his) labour with perhaps one of the most important articles she’s offered us so far.
If you’re new to Dr Otter’s physiological freestyle, have a look at some of her earlier articles here, or follow the links for more information about any terms that you don’t quite understand.
What is Hyperventillation?
Put simply, hyperventilation is any type of breathing that exceeds the need that the body has for oxygen, and for the removal of CO2. Typically we think of it as fast, rapid shallow breathing - the kind that tends to make your mates hand you a brown paper bag and tell you to calm the fuck down - but in fact, even deep, slow breathing of the kind that Freedivers typically incorporate into the breathe-up can also tick the same boxes.
For a long time, hyperventilation has been the shortcut to breath-holding success for new freedivers and untrained spearfishers alike, and it’s not difficult to see why: Although it has pretty much no impact on the level of oxygen in the blood (more on this just below), Hyperventilation of any sort causes a decrease in the arterial partial pressure of CO2 which, as all good Freedive-Earth fans will know, is the root cause of the urge to breathe. Less CO2, therefore, means less urge to breathe, which you might see as a good thing in the beginning. In the long term, though, you’d be horribly horribly wrong about this.
Why is Hyperventilation so Bad?
For a start, hyperventilation has almost no effect on the arterial partial pressure of oxygen. It’s true that there is a small gain to be had in this department (something like 1-2 KPa) but since the haemoglobin (where most of the oxygen is stored) is already 100% saturated with normal breathing, the actual volume of extra oxygen you get by doing this is really really small. It’s a big myth, therefore, that “hyperventilation oxygenates the blood more” or other similarly offensive nonsense. Please don’t spread it around any further!
Second - You might remember from our article on the alkaline diet, that CO2 is actually your friend. In the presence of CO2, haemoglobin is more inclined to give up oxygen to the tissues, which is where it’s needed. This means that when arterial PCO2 (PaCO2) is higher, the amount of available oxygen is also higher. This is explained neatly(ish) by the graph below:
You can see here that for a given PaO2 (e.g. 8KPa in this example), the saturation of haemoglobin is much lower in the high CO2 condition. Since the total amount of oxygen is the same in both examples, it must be the case that where the PaCO2 is high, the ‘missing’ oxygen has already moved out of the blood and into the tissues. This is exactly what we need as freedivers towards the end of a breath-hold. Hyperventilation, by contrast, has the opposite effect - dropping the PaCO2 and causing the haemoglobin to hold on to oxygen way beyond the point where it should be given up to the tissues:
The bottom line here is that hyperventilation actually decreases your maximum breath-hold time, all other things being equal.
But Hyperventilation Makes me Feel More Relaxed!
It’s true that in the beginning, your maximum breath-hold time may be increased by hyperventilation if the urge to breathe is the main cause of you coming to the surface, rather than low PaO2 and impending blackout. If this is the case then I suggest that you get stuck in to some CO2 tolerance training. This is the best way to learn to relax with high levels of CO2 so that you can get the maximum benefit from the Bohr shift. Check out our training suggestions here.
Is Hyperventilation Dangerous
Not only does hyperventilation decrease the amount of oxygen available to you during your breath-hold, it also decreases your awareness of the urge to breathe. If you’ve hyperventilated to any significant degree and find yourself at, say, 20m, experiencing the urge to breathe, the chances are you’re already at risk of blackout on the way to the surface because of the effect of decreasing pressure on the PaO2 as you ascend.
This, in short, is why the number 2 rule for freediving safety (after never dive alone), not to mention freediving performance, is “Never Hyperventilate”.
That’s it for this week, stay tuned for more from the physiologically fantastical Dr Otter just around the corner. And if you can’t wait that long, check out our ever growing archive of freediving physiology articles, more accurate, more advanced and more entertaining than anything else on this subject you’ll find on the internet… we reckon.