[Mike]

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

[Nick uk.]

I would say absolutely you should be more carefull CCR, and even more so any of you guy's SCR. The potential for a higher CO2 build up is far greater than in OC by design, so be carefull!

[EBT]

Quote: (Originally Posted by Mike)

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?

Yes, since it has to rely on scrubbing exhaled gas. OC has the advantage that every breath WILL be fresh and free of CO2.

[LS_DK]

Hey,

Yes, this fits very well with my observations on my body.
My nose-breathing goes away when I start breathing [very] deeply.
I thought flushing my loop did the trick, but it was simply the deeper breathing that did the trick (You are forced to breathe very deeply when you flush the loop).. I guess the relative dead volume get less when you breathe deeply, and that clears out the excess Co2 in the lungs and body..
When you use your snorkel, you also increase the dead space, just like when not breathing deeply enough on the loop. I think it makes sense..

Cheers,
Lasse

[Ken]

You conducted a very interesting experiment. I also think there is a great deal of physiological variability when people undergo extended levels of muscular exertion/CV stress. Personally, while swimming hard I have not noticed an urge to suck into the mask. Why, I don't know. I do tend to get cross-eyed though. But you've got me thinking that perhaps there is some brain-spatial relationship here?

With regards to the rebreather maintaining a rhythm of breathing is very, very important. These days I pace myself as much as possible. If I breathe hard on the loop I try to stop before a cycle of hard breathing begins. It is all too easy to get into. Pushing a human body loaded with heavy gear, draggy suit through a dense fluid medium is asking a lot from the muscles and heart. Add current and stress - Whoa! Additionally, human CO2 retention (large variable in my opinion) as well as the scrubbers ability to remove CO2 would be factors in addition to age of sorb, scrubber design, depth etc. so best to take it easy.

A smooth bore snorkel would most likely have less WOB than a loop where you physically have to move the gas around around paths and hose corrugations. However, if you had one of those wacky snorkels with a ball it would arguably be like breathing through a straw.

[Genesis]

The big deal with a snorkel is the dead space - its huge compared to a loop (or an OC reg)

The entire length of the tube is "rebreathed" (without scrubbing!) every time you inhale. If you get "over the edge" on the CO2 retention side, I can see where you'd potentially be in trouble there, because the short, rapid respiration pattern isn't going to evacuate that dead space effectively at all......

[Mike]

Quote: (Originally Posted by Ken)

A smooth bore snorkel would most likely have less WOB than a loop where you physically have to move the gas around around paths and hose corrugations.

I disagree actually - the hydrostatic pressure difference created by using a snorkle seems to result in a significantly higher WOB on inhale than a over shoulder lung rebreather. Personally I think that snorkles breath like shit (but the swim really well )

Mike

[mortenkjerulff]

Quote: (Originally Posted by Mike)

I disagree actually - the hydrostatic pressure difference created by using a snorkle seems to result in a significantly higher WOB on inhale than a over shoulder lung rebreather. Personally I think that snorkles breath like shit (but the swim really well )

Mike

I am with Mike here. I am quite sure that high WOB can trigger nose-breathing.

I tried this myself on a converted Dolphin (the Dolphin has back mounted CL), just after I got a backplate adaptor. I put the adaptor too close to the Dolphin case, so that there was just barely enough CL-volume. The following dives, I would start to nose breathe 10-15 minutes into the dive. It scared the # out of me until I figured out what was wrong, because I could not control my urge to suck my mask in through my nose :-).
Readjusting the backplate adaptor for more CL volume and thereby lower WOB helped.

On a later dive I experience the problem again after diving with my upper body slightly lower than the lower body (thus the WOB was increased with my back mounted CLs).

Morten

[Ken]

Quote: (Originally Posted by Mike)

I disagree actually - the hydrostatic pressure difference created by using a snorkle seems to result in a significantly higher WOB on inhale than a over shoulder lung rebreather. Personally I think that snorkles breath like shit (but the swim really well )

Mike

That may very well be the case. The last time I snorkeled was a year ago - I'll have to give it a shot again. When I used to swim distance in the ocean I used a particular, large bore Scubapro snorkel that felt comfortable on both phases of the breathing cycle. I'll keep this on my mind as an experiment for the summer.

[caveseeker7]

Intresting experiment, Mike.

CO2 buildup being the trigger for your nasal breathing seems like a reasonable conclusion.
Both deadspace and WOB should be factors, as well as your excertion of course.
Next time you snorkel exhale through your nose to eliminate rebreathing.
Would be intresting to see how much of a difference that will make.

For some odd reason I've always done that when snorkeling, before I ever heard the term hypercapnia. Probably because bubbles seemed more like diving when I was a kid. Don't remember ever having a CO2 problem or hycap symptoms, though in the last 20 years I've only snorkeled once or twice.