A while ago there was some discussion about involuntary nose breathing on rebreathers, and what the cause was. I'd experienced it once, on one of my first rebreather dives, and put it down to unfamilliar gear. Now I've changed my mind.
I've just returned from a 'relaxing beach resort holiday' ie the kind of thing that bores me to tears within 30 minutes. So, in an attempt to alleviate the boredom, I grabbed mask and snorkle and went for a swim. 500m out to a reef, not much to see so I started doing a swim training exercise - 100m sprint, 100 easy, 100 sprint etc. An exercise that doesn't give you time to recover between sprints so you gradually get more and more out of breath
Now I've swum all my life, doing both sprint and longer distances at various times. And one thing I've learnt is that there is a line where the energy use exceeds your ability to maintain adequate respiration. Once you cross that line, you are going into oxygen debt / CO2 build up - you can do it for a while, but you are getting more and more out of breath as you do so, and will need to slow down and breath to recover. Same in other sports, but swimming forces you to concentrate on breathing paterns a bit more, so it's more noticible (to me).
Anyway, I'm pushing along, on the wrong side of the line, and I start to suck in through my nose. Drop back to a cruise, breathing relaxes, and I stop nose breathing. Push on again, and the involuntary nose breathing starts again, no matter how much I concentrate, I can't stop doing it while I'm swimming hard, but again I slow down and manage to stop it.
So I'm thinking that this is cool, nose breathing seems to be triggered when CO2 production exceeds my ability to get rid of it, nice to know.
So I push on for a while to see how bad it gets. (Note that this is still a comfortable swimming pace for me - only thing different is that I'm using a snorkle, so I can breath all the time, rather than every three strokes as normal). Getting to be that every breath I'm sucking in through the nose, can't control it - breathing at a rate that is on the high side of comfortable exercising rate.
So I decide to stop and catch my breath.
I can't.
Breathing through the snorkle, I'm unable to control my breathing - it keeps getting worse until after about 2 minutes I'm forced to go head up and breath that nice big free air supply above the water.
Now a snorkle is smaller diameter than a rebreather loop, and arguably has worse WOB, however this was still disconcerting. I'd been breathing hard, but not at a rate which I would have been hugely worried about underwater - I've breathed that hard open circuit a couple of times and been OK.
So, after that lengthy ramble, I have a thought, and a question.
The thought is that, at least for me, any involuntary nose breathing is a sign to stop and relax NOW! It probably indicates some CO2 buildup, and will get worse quickly if not dealt with.
The question is; is a rebreather (like a snorkle) LESS tolerant of being over breathed than OC? Should I be more careful of exersion on CCR than on OC?
Thoughts?
Mike