Breathing rate - too fast or slow?

Kaltim Diver · Dec 12, 2008

During the deco portion of the few tech dives I've done I slow my breathing rate right down such that I often take only four breaths per minute. I use good, deep breathing techniques and I use four breaths per minute to help time my deco stops (still rely on the watch and computer, obviously). As a result of this I sometimes have quite a low SAC for this part of the dive.

A number of students I've taught (generally small, female) have had such low breathing rates that one in particular used to use one single 80 ft tank for two 45 minute dives.

My question is - what impact does a slow (or fast) breathing rate have on off gassing? Would we speed up nitrogen removal if we took more breaths, or is a slower rate preferred? I worry that if I slowed down too much I wouldn't be off gassing as quickly or as efficiently as I should.

Not a crucial problem, more out of curiosity but would be useful to know for future dives.

KT Diver

dschonbrun · Dec 12, 2008

Moderator, please moved this to Dive Medicine

texdiveguy · Dec 12, 2008

You should maintain 'your' normal rate of breathing through the dive profile....recreational or technical.....deep or shallow. We each have some differing rates of breathing, but breath deep and exhale fully. As to your rate of breathing, this has no bearing on the rate of off gassing that is occurring in your bodies tissues during decompression of inert gases.

TSandM · Dec 13, 2008

As to your rate of breathing, this has no bearing on the rate of off gassing that is occurring in your bodies tissues during decompression of inert gases.

This is actually not entirely true. The rate of transport of gas across the alveolar membrane in the lungs (from the blood to the alveolar gas, when you are offgassing) is going to depend in part on the relative partial pressures of that gas in the blood and in the alveolus. If you stop breathing, the two pressures will equalize, and no further transport will occur. As you breathe, you refresh the alveolar gas with gas which has a lower partial pressure of inert gas than you have in your blood (assuming you are ascending, or sitting on deco), and thus gas transport will continue to occur. Therefore, if you drop your breathing rate too much, you WILL decrease inert gas removal from your bloodstream. However, in most cases, you will also begin to retain CO2 at that point, and most people don't tolerate that very well without becoming short of breath and anxious. There are some people who DO tolerate high CO2s while diving, though, and the OP may be one of them, in which case he may be impeding his effective offgassing by his respiratory rate reduction.

Theunis · Dec 13, 2008

texdiveguy:
You should maintain 'your' normal rate of breathing through the dive profile....recreational or technical.....deep or shallow. We each have some differing rates of breathing, but breath deep and exhale fully.

Good point. From experience the emphasis should be on exhale fully.

dumpsterDiver · Dec 13, 2008

During a Deco of more than a few minutes I do the following: rest and really try to relax and get the pulse rate down and the breathing rate slow. Then in a minute or two after that, I will try to gently swim in circles, move my shoulders, flex my hips, pump my arms in and out like push-ups and even flex my fingers for maybe two minutes, or what ever it takes to make my respiration increase a little. This moderate, non-stenuous activity promotes increased circulation and will increase the respiration rate a little. Then I will rest motionless for 3 minutes and then resume another brief activity cycle.

I had a friend who took an elbow hit because (we think) he kept his one arm bent the entire stop.

It does not make sense to me that on the deco phase of a dive, it is desirable to go into a passive statis and attempt to reduce respiration to an absolute minimum.

texdiveguy · Dec 13, 2008

TSandM:
This is actually not entirely true. The rate of transport of gas across the alveolar membrane in the lungs (from the blood to the alveolar gas, when you are offgassing) is going to depend in part on the relative partial pressures of that gas in the blood and in the alveolus. If you stop breathing, the two pressures will equalize, and no further transport will occur. As you breathe, you refresh the alveolar gas with gas which has a lower partial pressure of inert gas than you have in your blood (assuming you are ascending, or sitting on deco), and thus gas transport will continue to occur. Therefore, if you drop your breathing rate too much, you WILL decrease inert gas removal from your bloodstream. However, in most cases, you will also begin to retain CO2 at that point, and most people don't tolerate that very well without becoming short of breath and anxious. There are some people who DO tolerate high CO2s while diving, though, and the OP may be one of them, in which case he may be impeding his effective offgassing by his respiratory rate reduction.

Your description is true ... 'you do have to maintain some rate of breathing', but even during the act of stopped breathing (not recommended!) their is a time frame were off gassing is still occurring. My point was was that many divers have a misconception that tissue off gassing and rate off breathing are strongly driven by proportion. The key's are proper ascending and partial pressure, and of course breathing at a normal rate. I believe we are in fact talking about the same thing!

TSandM · Dec 13, 2008

Yes, that's true, gas transport would occur at a progressively slower rate until it stopped, if you quit breathing altogether.

Gas elimination from the body requires the step of diffusion across the alveolar membrane, which is rate-limited by the differences in partial pressure on either side, and the diffusion constant of the gas in question. The gas, once in the alveolus, has then to be washed out by dilution with inhaled gas containing a smaller partial pressure of the gas in question. It's at this step that nitrogen is different from CO2 -- In effect, the gas you inhale has NO carbon dioxide in it, whereas it does have significant nitrogen, so the gradient between the blood and the alveolus, and the alveolus and the inspired gas, is smaller for nitrogen than for CO2. Still, increasing your respiratory rate and increasing the washout of nitrogen probably slightly increases the rate of offgassing. It also drops your CO2, though, and that has other, undesirable effects.

The bottom line is that normal respiration should be maintained on deco.

Kaltim Diver · Dec 17, 2008

Thanks all, some good comments here and has given me some ideas.

"It does not make sense to me that on the deco phase of a dive, it is desirable to go into a passive statis and attempt to reduce respiration to an absolute minimum." - absolutely agree, but just wanted to know if there was a best case scenario - slow and deep as you should be diving normally, or would there be improved gas exchange and hence reduced nitrogen in the blood by breathing faster and exposing more breathing gas to the lungs for increased nitrogen washout.

Guess I'll just stick with my normal breathing rate as suggested.

Thanks again, KTD

texdiveguy · Dec 17, 2008

Kaltim Diver:
Guess I'll just stick with my normal breathing rate as suggested.

Thanks again, KTD

Best idea.

Breathing rate - too fast or slow?

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