How much hypoxia does it take to cause (lasting) brain damage? Do long-term freedivers run into problems, like old boxers and American footballers? I've always been leery of any sort of intense breathing exercise because of this concern.
Certainly not in any obvious way like people that suffer repeated blows to the head. There's some debate over whether loss of motor control (they call it "samba" because it's kinda like you start dancing involuntarily) can cause damage that makes it more likely to happen again in the future, but I haven't been able to find any evidence that there is any damage at all in normal training and even the former seems to be controversial.
Understand that the urge to breath is driven by the body’s desire to rid itself of carbon dioxide (CO2)--not (as some assume) your body's desire to take in oxygen (O2).
Interestingly enough, this isn't entirely true. If you get a pulse oximeter and a bottle of oxygen you can have some fun with it.
Because of the nonlinearity in the oxygen dissociation curve, oxygen saturation tends to hold pretty steady for a while and then really tank quickly, whereas CO2 discomfort builds more uniformly. In my experience, when I get that really "panicked" feeling and start breathing again, the pulse oximiter on my finger shows my saturation tank shortly after (there's a bit of a delay, which is useful here for knowing that it's not the numbers on the display causing the distress).
If it were just CO2 causing the urge to breathe, CO2 contractions and the urge to breathe should come on in the exact same way when breathing pure oxygen, and this is not the case. Instead of coming on at ~2-2.5min and being quite uncomfortable, they didn't start until four minutes and were very very mild. I've broken five minutes when I was training more, and it was psychologically quite difficult. Compartively speaking, 5 minutes on pure O2 was downright trivial, and at 7 minutes it wasn't any harder. The only reason I stopped the experiment then is that I started feeling narcosis from the CO2 and figured I should do some more research about hypercapnia (too much CO2) before pushing further.
Along those same lines, rebreather divers sometimes drown when they pass out due to hypercapnia, and while you'd think it'd be way too uncomfortable to miss, this doesn't seem (always) to be the case. In my own experiments, rebreathing a scrubberless bag of oxygen did get uncomfortable quickly, but when they did a blind study on it five out of twenty people failed to notice that there was no CO2 being removed in 5 minutes.
At the same time, a scrubbed bag with no oxygen replacement is completely comfortable even as the lights go out, so low O2 alone isn't enough to trigger that panic.
Surfing the Urge: A distress tolerance skill
I learned this meditation technique from my freediving coach. It's a technique we employ during a breath-hold to help us deal with an increasingly strong urge to breath.
Understand that the urge to breath is driven by the body’s desire to rid itself of carbon dioxide (CO2)--not (as some assume) your body's desire to take in oxygen (O2). The urge to breath is a warning, however. It can be safely ignore up to a certain limit. That limit, for our purposes, is the loss of consciousness (cerebral hypoxia); however, the urge to breath becomes quite strong well before your blood oxygen saturation low enough to cause a blackout.
Under normal conditions, the human body has a blood-oxygen saturation of around 98 to 100 percent (the higher number being the most oxygen that the blood could possibly contain). Physical stress or sickness can decrease oxygen saturation to about 95 percent. Few healthy people will ever go below this, but during dives, expert divers have registered oxygen-saturation levels as low as 50 percent, an extraordinarily low number. Oxygen saturations below 85 percent generally cause an increased heart rate and impaired vision; 65 percent and below greatly impairs basic brain functions; 55 percent results in unconsciousness. But well-trained divers have not only remained conscious with oxygen saturations of 50 percent but maintained muscle control and extremely low heart rates, reportedly as low as seven beats per minute.
Hold your breath too long, and you start to feel malaise. The urge to breath appears and it seems as if the malaise will only increase in intensity until you submit to releasing you breath and starting to breath again. But what happens if you don't give in and keep holding your breath? Ultimately static apnea (breath hold) is a matter of pure, mind-over-matter willpower.
After a while, along with the discomfort, you're start to experience involuntary contractions in your diagram. These is where the meditation technique comes in. When you feel a contraction, just observe it. As they increase in frequency, simple witness them as if you were a third-party.
At first, you may feel like these urges will rise up, stronger and stronger and become more and more unbearable, but if you simple observe and remain detached, you'll notice a pattern. The urges come in waves. They build up to a peak, and then they recede. As the intensity of an urge builds, it may feel alarming, but if you stay calm and occupy your mind by following the urge as it ebbs and flows. In the beginning, there are long pauses between each wave, and the wave is often little more than a twitch. Over time they come more frequently and with more intensity. But all the while you simple follow the urge as it rises, neutrally observing, in non-attached and non-judgemental manner, how it differed from the one before it. As the contractions increase in frequency and strength, you can try and soften them with by gently contracting your diaphragm to smooth them out. And knowing that it too shall pass.
When you experience an urge and then act on that impulse, your brain makes the connection that you can only make the urge go away by engaging in the behavior. This connection can create a habit or automatic behavior.
“Surfing the urge” is a mindfulness technique that can be used not only to help freedivers, by also to deal with pain management, anger management, and impulse control issues such as sex and eating addictions. Instead of automatically acting on destructive impulses, it can help people regain some control over their urges resisting until they pass. When you notice an urge, rather than fighting against it, imagine you are on a surfboard riding with it. Notice the shifting sensations, in a non-attached and non-judgmental manner, how they rise and fall, come and go.