Lack of oxygen during a free ascent?

[Akimbo]

Just to clarify, CO2 is the primary stimulus for our normal automatic breathing. It is also the primary cause of the extreme urgency to breath when CO2 levels are high -- holding our breath or breathing in an enclosed space. When CO2 in the lung is reduced below a minimum threshold, all stimulus to breathe ceases -- along with consciousness.​

CO2 tells the brain but only to a point....I think...

Hyperventilating on pure oxygen is interesting. It doesn't inhibit the normal respiration reflex, but it sure extends the time and reduces urgency of high CO2 symptoms. I noticed the same moderating effect you describe.

The subject came up during a freediving course I took last year. My take on it was there are some competing theories, but there is no real motivation to $tudy the phenomenon to prove them. Deep apneists don't care that much since there is no competitive category for pure oxygen, recreational/commercial/military divers aren't that interested, and general medicine has a lot more important stuff to figure out first. There is a similar lack of funding to study high PPCO2. Just avoiding high levels pretty much solves the whole problem for the relatively few people exposed to hyperbaric environments.

The prevailing explanation of the physiology I have been exposed to explains the reduced urgency to breathe when a Scuba diver performs a free ascent, compared to a freediver, is due to blood hyper-oxygenation and lower CO2 concentrations due to expelling expanding gas on ascent. That may be all there is to it, or the effect you described may also contribute to the reduced urgency that divers experience on free ascents.

I should point out that most divers don't notice the relative lower urgency to breathe until after practicing several free ascents. I suppose there is so much distraction from controlling ascent rate, exhalation rate, and keeping an open airway that it is hard to separate from a desire to breath stimulated from CO2. But once they get "over the hump", everyone I have ever discussed it with agreed they were surprised how easy it is compared to swimming a similar distance horizontally on one breath.

 

[Thalassamania]

Free ascents are routinely done in the submarine escape training towers and they're usually (I believe) 100 feet tall. In the old days we were required to do a free ascent from our endorsement depth (30, 60, 100, 130, 150, 190 progressively) and lots of us did, without any miss-steps. Most all of what has been written about CO2 and O2 is spot on, and TS&Ms comment about how a little care goes a long way in preventing such problems is key. Where I disagree, however, is with the view that a free ascent (even on an empty lung) is particularly difficult to execute or physically taxing. Those problems are mainly in your head and when you get your head straight they go away, along with the panic reaction at the thought that you might have to do a free ascent and the feeling of dread when you have to breath hold on an empty set of lungs. Practice, practice and more practice can make these things rather ho-hum, every day happenings.