Does Exertion Affect N2 Absorption?

[ianr33]

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.

 

[Mr Carcharodon]

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.

 

[ianr33]

I would expect that the total volume of absorbed inert gas goes up with activity.

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.

 

[TSandM]

So increased activity during decompression is not recommended.

I believe there are actually studies that indicate that remaining in quiet motion during decompression is advantageous. Which makes sense, if you think about the perfusion issue; sitting still, especially in cold water, is going to reduce blood flow to the extremities, therefore slowing offgassing there.

It is generally recommended, when planning dives on tables, to plan a dive with significant exertion as though it were ten feet deeper than it is. On what basis that recommendation is made, I don't know. Whether it's increased N2 absorption from increased perfusion, or increased microbubbles, somebody seems to have thought that exertion correlated with a greater DCS risk.

 

[Scott L]

CO2 is a by-product of exertion which is thought to accelerate the negative effects of nitrogen (DSC) and oxygen (CNS) issues. I was told somewhere along my training journey that being in the horizonal postion allows ones cardiovascular system to work more efficiently and thus off gass more efficiently although I defer to TSandM on all such physiological discussions...

 

[ianr33]

Thanks for the replies but once again I am primarily interested in the VOLUME of nitrogen absorbed.

I totally agree that exertion at depth is a bad thing,but believe that is because it alters offgassing of the absorbed nitrogen,rather than altering the amount of nitrogen being absorbed in the first place.

 

[Mr Carcharodon]

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.

You would think that increased activity would be helpful on decompression as well. And it probably is as long as the inert gas stays in solution. But we do know that increased activity leads to an increased population of seed bubbles through cavitation. Michael Powell has been pretty adamant about avoiding increased physical activity during decompression for this reason.

 

[gcbryan]

It's recommended to not go from a cold dive to a warm hot tub and Dr. Deco has suggested that conditions during off gassing should be kept fairly similar to conditions during on gassing so temp and movement does seem to affect on and off gassing. I do think that movement to be avoided regarding micro nuclei formation is excessive exercise. I think slow movement conducive to good circulation is not what they were talking about.

 

[Wookie]

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.

Edit. Uhhh, yeah. What Mr Carcharodon said.

 

[Herk_Man]

So in your question, you are assuming that two physiologically identical divers dive the exact same profile and breath the exact same volume of air. But one diver exerts himself physically while the other stays relatively still. Which one has the higher amount of nitrogen in his tissue when the profile is completed?

I'm no expert diver and I'm not a physiologist so everything I say is wild conjecture. But the first thing I would assert is that the above scenario is not possible, simply because the diver exerting is going to consume gas faster and be at depth a shorter time. But let's assume that he is somehow able to breathe at the same rate (which defies the assumption that they are physiologically identical). So now the question is whether this exertion somehow allows more nitrogen into the tissues than the non-exerting diver. Well, there would have to be an increase in blood flow due to a higher heart rate (while somehow breathing the same???) In my mind, the higher flow rate would make it more difficult for the nitrogen to exit the blood and enter the tissues.

But since we know the above situation is not realistic, then we have to go to the following. The exerting diver will have a shorter time at depth. And due to the higher rate of breathing, the air will spend less time in the lungs and less time in contact with the tissues that transfer the nitrogen into the blood stream. In my mind, this means that more nitrogen is being exhaled by the exerting diver where the non-exerting diver is allowing that nitrogen to be absorbed and has a higher CO2 exhalation level. Add to this that the exerting diver is not under pressure for as long and you have another reason to believe nitrogen absorption is lower in the exerting diver.

So does this imply that there is a counterintuitive answer to this question? I dunno. But it's interesting.

So now let's say they dive the same profile and consume different levels of air with the exerting diver obviously using more. In this case, despite the previous argument, I think the exerting diver is going to have more nitrogen absorption just because a higher amount of nitrogen is introduced to the tissue and they are at depth for the same amount of time.

Any real physiologists want to show me how stupid I am? :mooner: