Does Exertion Affect N2 Absorption?

[Cave Diver]

This came up in another thread http://www.scubaboard.com/forums/accidents-incidents/320007-spiegel-incident.html. Dont want to derail that one any more so asking it here.

If I work hard on a dive do I absorb more nitrogen? To be precise,is the VOLUME of nitrogen that goes into my body greater? If it is greater what is the mechanism for that?

I totally agree that getting bent is more likely on a strenuous dive but I am not interested in that here.

I had a similar question during my BOW class. I wondered if a heavy breather, or "hoover" would absorb more nitrogen than a lighter breather, since they inspired a greater volume of nitrogen over a given time period.

The answer appears to be that regardless of the amount of nitrogen that you intake, your tissues only absorb it at a certain rate. So unless increased activity somehow increases the tissues ability to absorb inert gas, I'd have to say no to your question.

 

[ianr33]

Ian, My guess is that the volume change is unmeasurable but the increase in the chance of DCS while exercising is because the nitrogen diffuses further into the tissues. That is merely my opinion and guess, and really has nothing substantial to back it up..

Frank,that is exactly my take on it.

I had a similar question during my BOW class. I wondered if a heavy breather, or "hoover" would absorb more nitrogen than a lighter breather, since they inspired a greater volume of nitrogen over a given time period.

The answer appears to be that regardless of the amount of nitrogen that you intake, your tissues only absorb it at a certain rate. So unless increased activity somehow increases the tissues ability to absorb inert gas, I'd have to say no to your question.

A hoover will inspire a greater volume of N2 but maybe 99.999% of it is breathed straight back out again. Only a very small proportion of inspired nitrogen is actually absorbed, so for all intents and purposes the pN2 in the lungs is constant,therefore constant amount absorbed.

I asked on TheDecoStop a while back how much N2 is actually absorbed. Apparently not much. Around 1 litre per atmosphere at saturation! i.e go sit at 33 feet for a couple of days. After breathing thousands of cubic feet of gas only 1 litre of N2 will have been absorbed by the body!!! (and yes I know I'm mixing ft3 and litres)

 

[DCBC]

If I work hard on a dive do I absorb more nitrogen? To be precise,is the VOLUME of nitrogen that goes into my body greater? If it is greater what is the mechanism for that?

Yes; with increased workload the capillaries in the active tissues open up to modulate the blood flow. Perfusion rates vary significantly with added activity.

 

[Gene_Hobbs]

I think the paper that would make you most happy was published by Dick et al in 1984:

Dick, AP; Vann, RD; Mebane, GY; Feezor, MD. Decompression induced nitrogen elimination. Undersea Biomed Res. 1984 Dec;11(4):369-80. RRR ID: 2981

A method for measuring nitrogen elimination after air diving has been developed in which a subject breathes air instead of oxygen or helium-oxygen. Accuracy is improved with this method because only nitrogen absorbed during the dive is eliminated. Nitrogen stored in the lungs and tissues at sea level is unaffected. Measurements were made with a closed-circuit breathing apparatus using a spirometer as a counterlung. The oxygen partial pressure in the apparatus was controlled at 0.209 +/- 0.003 atm. The spirometer volume was recorded periodically with the subject holding his breath at functional residual capacity. Increases in spirometer volume were used to define a nitrogen elimination curve. Elimination measurements were made after resting and exercising dives to 60, 100, and 130 fsw (2.8, 4.0, and 4.9 atm) at the U.S. Navy no-decompression exposure limits. Exercise during a dive increased the volume of nitrogen eliminated after the dive, but results for both resting and exercising divers were variable. Possible causes of this variability include bubble formation and changes in blood flow.

Some others that might be of interest (from our collection anyway) can be found with the keywords "nitrogen elimination".

 

[AndyNZ]

CO2 is a by-product of exertion which is thought to accelerate the negative effects of nitrogen (DSC) and oxygen (CNS) issues.

CO2 is also a killer in it's own right, if you look at the Meyer-Overton results it would appear to have about the same narcotic effect as Nitrous Oxide, the gas dentists use whilst pulling your teeth out.....

 

[Herk_Man]

CO2 is also a killer in it's own right, if you look at the Meyer-Overton results it would appear to have about the same narcotic effect as Nitrous Oxide, the gas dentists use whilst pulling your teeth out.....

Ya, and as we all *cough* know it is melting the polar ice caps and causing the sea to swallow up coastal states. Which means all scuba divers should start paying a tax for every breath they exhale.

 

[sydney-diver]

My thinking (guess!) is that the total volume of absorbed gas would remain constant.Nitrogen absorption across the lung membrane into the blood is presumably a (relatively) simple process that depends on the N2 pressure in the lungs, the N2 "pressure" in the blood and the diffusion rate of N2.

That's what I would expect, as I understand it the lung -> blood half time is so small it's (assumed) essentially constant. Also the total volume of N2 absorbed in steady state (i.e. >> 6 half times) will be constant since it's determined by the ambient pressure. What will change is the tissue perfusion, i.e. the blood to tissue absorbtion rate - if you're breathing faster presumably your heart rate is elevated as well so your tissue is receiving more blood and that a major factor idetermining the tissue half time?

John Lippmann and Dr Simon Mitchell (Deeper into Diving) and Mark Powell (Deco for Divers) are excellent books and both mention that activity increases tissue on/off gassing.

Uwatec's ZH8-ADT algorithm isn't documented but annecodates indicate they modify the tissue half times to account for increased workload. Also from what I understand they use seperate on and off-gassing half times and increase the off-gassing half times in cold water.

 

[Bubbletrubble]

Perfusion rates vary significantly with activity. Presumably capillaries in the active tissues open up to modulate the blood flow. At that point there is diffusion of the inert gas in solution into the tissues. J. S. Haldane in his 1908 paper suggested that increased activity would increase offgasing during decompression. But more recently the suggestion has been that friction causes seed bubbles that then will grow in critically supersaturated tissues. So increased activity during decompression is not recommended. I would expect that the total volume of absorbed inert gas goes up with activity. But only active mid compartment tissues are impacted. The fast tissues are typically saturated anyway and cannot absorb any more gas into solution, and the slowest tissues do not vary much in perfusion rates with activity.

I'm fairly certain that no data exists to support this statement. It would entail first identifying which tissues are "slow" and then measuring perfusion rates with respect to activity level. That said, there are ways to investigate perfusion in skeletal muscle...and exercise physiologists do look at the effect of increased cardiac output on muscle perfusion. I suppose that BOLD functional MRI can be used to look at circulation in [-]internal organs[/-] the CNS. I'm quite certain that this methodology hasn't been used to investigate off-gassing in the context of deco burden.

That only works for tissues that are saturated. The solubility of the gas does not change with changes in activity levels. So for a saturation dive there would be no change, and there is no practical change for fast tissues which are saturated. But tissues with time constants greater than 120 minutes typically will not be saturated for a recreational dive. For those tissues increased activity can increase perfusion by as much as ~7x and shift the active tissue to a faster compartment increasing the gas load.

In discussing the concept of off-gassing, I think some people are trying to apply our understanding of perfusion (changes in cardiac output upon exercise, blood/fluid shifts due to ambient temperature/pressure) to the imaginary construct of fast/slow tissue compartments (as defined in deco theory). This kind of discussion is highly speculative.

Please understand that I'm not disputing the usefulness of utilizing the concept of fast/slow tissue compartments to tweak deco algorithms in order to come up with profiles that are less likely to lead to DCS. I do take issue, however, with saying "such-and-such tissue compartment increases/decreases perfusion X-fold due to an increase/decrease in activity."

I call BS until someone can cite references to the contrary.

FWIW, I found what Herk Man was discussing in Post #10 quite interesting.

 

[Mr Carcharodon]

So if you have an unsaturated tissue and pump more nitrogen into it by increasing the perfusion of that tissue what do you suppose that does to the time constant of that tissue? The idea that tissue time constants are dependent on perfusion goes back to Haldane’s paper in 1908. If you want a more quantitative approach I would suggest Hennessy’s paper (Bulletin of Mathematical Biology 36, 5, October 1974). Or there is an overview of the effects of blood flow on gas loading in chapter XIII of Mike Powell's Decompression Physiology syllabus.

Also take a look at Gene's post #14 above.

 

[Puffer Fish]

Interesting issue... one of the ways to look at this is to ask:

1. How would a locallized change in temperature effect gas absorption?

2. Would increased circulation rate effect gas absorption?

Well raising the temperature at depth would actually reduce the saturation point... which would be a good thing (and bad at the surface)...if and only if one were "saturated".

On the other hand, if one is not saturated, then increased circulation would increase absorption. One can think of this in terms of heat, where a convection oven heats quicker than a static air oven...has to do with boundry layer concentration differences.

So if one measured increased Nitrogen off gassing after exersize, the best assumption I can come up with is that part or all the person was not at saturation..

While I play as a chemical engineer during the day, my real education is in Biochemistry...where membranes and gas pressure differentials are very common topics.