Nitrogen Build-up and Breathing?

[Dave Zimmerly]

Simple question from a simple mind.

So on our way back to the surface, we do the standard 3 min safety stop at 15 fsw.

Question: Does the rate of breathing have any affect on decreasing your nitrogen load? If I were to breathe more deeply would I be helping in dispersing nitrogen build-up?

Thanks,
Dave (aka "Squirt")

 

[knotical]

Excellent question! A variant of this was discussed some time back on a different bulletin board. Since the blood flowing through your lungs quickly equilibrates to the partial pressures of the gases in your lungs, how fast you breathe has very little to do with how much nitrogen you absorb (or give off). It’s primarily a function of partial pressure which does not change with breathing rate. There can be a difference if your heart rate also changes significantly, causing the amount of blood flowing through the alveoli to increase or decrease. But generally, the short answer would be: “no”.

 

[TSandM]

Well, this is actually an interesting question.

You can, for example, lower the pCO2 in your blood if you move more liters of air through your lungs in a minute. CO2 concentrations in the blood are far higher than in the inspired air, and each time you exchange your alveolar gas for inspired air, you have increased the diffusion gradient from the capillaries into the alveolus. It would similar, it seems to me, if you were breathing 100% O2 on deco -- each time you exchanged alveolar gas, you'd be replacing it with gas containing no nitrogen, and therefore increasing the diffusion gradient. CO2 is quite soluble and diffuses extremely easily, and nitrogen, to my knowledge, is fairly similar, so I would think you could actually drop your blood pN2 by increasing your ventilation. (This is purely thinking through the physiology, and not anything I know.)

However, the downside of attempting to do so would be blowing your CO2 down, which can cause dizziness, muscle cramps, and loss of consciousness, so it's not advisable.

Plus, most of us on SB are "decoing out" on air (or Nitrox) which has a significant N2 content, and therefore the gradient is not as great, and the effect of increasing ventilation might very well be minimal.

Fun to think about!

 

[MookieMoose]

Like TSandM said above, disolved nitrogen crosses thealveolar membrane very rapidly, so theoretically, higher minute ventilation (the total amount of gas you move in and out of your lungs in one minute) would result in greater off-gassing. But there are a couple of wrenches thrown into the works. For 1, CO2 also gets off-gassed quickly. Not only does this cause the symptoms listed above (headache, diziness, etc.) but it also increases what is called ventilation-perfusion mismatch. Basically this means that the further CO2 levels get from normal, in either direction, the more the blood moving through your lungs gets shunted away from parts of the lung with the most gas moving through them. This would cause, in this case, CO2 and nitrogen to be retained, but also hampers oxygen getting in.

To further complicate things, a more important factor is not how much gas moves through your lungs, but how much gas is removed from tissues into blood and can get back to your lungs. this can be affected by a lot of things, especially skin temperature and muscle activity - getting cold reduced blood flow, and activity increases blood flow. Since we are usually holding still and can get a little chilly (becuase we are holding still) on our safty stops, these effects reduce the speed of nitrogen off-gassing far more than increased minute ventilation could correct for.

I guess this all boils down to "Yes, increased breathing can increase off-gassing, in theory, but not enough to bother goven some of the risks."

Cam

 

[miketsp]

Simple question from a simple mind.

So on our way back to the surface, we do the standard 3 min safety stop at 15 fsw.

Question: Does the rate of breathing have any affect on decreasing your nitrogen load? If I were to breathe more deeply would I be helping in dispersing nitrogen build-up?

Thanks,
Dave (aka "Squirt")

As knotical already stated, the short answer is "no" if you're breathing air.
It all comes down to partial pressures at the interface and the N2 PP doesn't change during the breathing cycle.
If you run a search on this board you'll find quite a few discussions on the inverse situation: "Is a hoover subject to faster on-gassing and higher N2 loading?"
Again the answer is "no".

 

[Charlie99]

Well, this is actually an interesting question.

You can, for example, lower the pCO2 in your blood if you move more liters of air through your lungs in a minute. CO2 concentrations in the blood are far higher than in the inspired air, and each time you exchange your alveolar gas for inspired air, you have increased the diffusion gradient from the capillaries into the alveolus. It would similar, it seems to me, if you were breathing 100% O2 on deco -- each time you exchanged alveolar gas, you'd be replacing it with gas containing no nitrogen, and therefore increasing the diffusion gradient. CO2 is quite soluble and diffuses extremely easily, and nitrogen, to my knowledge, is fairly similar, so I would think you could actually drop your blood pN2 by increasing your ventilation. (This is purely thinking through the physiology, and not anything I know.)

The body generates a lot more CO2 than it will ever offgas in N2. I'm too lazy to look up solubilities of all the different tissues in the body and figure out total dissolved N2, but somewhere I recall seeing a number on the order of 1 liter or less of dissolved N2 per ata.

OTOH, your body uses 1 liter of O2 and generates 1 liter of CO2 in a relatively short time.

The net effect is that the offgassing N2 doesn't materially affect the aveolar N2 levels since there is so little being offgassed.

 

[RICoder]

Short answer is no...much like the short answer to build-up is no (which is why SAC isn't used in the tissue compartment calculations on computers).

I tried to be nice and search for you...all I could come up with was this archive - http://www.scubaboard.com/archive/index.php/t-7164.html