Air consumption - nitrogen absorption

[CameronMartz]

The readers give a good response here. So little nitrogen is removed by the blood that the amount in the lungs is unchanged. It is essentially independent of breathing rate.

At the blood/lung barrier, this might be the case, but what about the rest of the body, which is really where on-gassing and off-gassing matters? If you are assuming a diver is a big blob of gel, it might not matter, but physiology and the laws of thermodynamics tie breathing rate to metabolism. Perfusion changes with changes in metabolism, as does pressure gradients at various tissues in the body. In other words, changes in breathing rate indicate changes in metabolism indicate changes in perfusion indicate changes in rates of on-gassing/off-gassing.

Cameron

 

[Dr Deco]

Hello Cameron:

In my answer, I was only considering individuals who have differing resting breathing rates.

Without question, if the breathing rates are different because of musculoskeletal activity, that would definitely indicate that blood perfusion will change because of the working muscles. You did indeed mention this in your initial reply.

Dr Deco :doctor:

 

[tulambi]

Thank you all, I'll try to accumulate all this info before I ask anything further.

 

[H2Andy]

In other words, changes in breathing rate indicate changes in metabolism indicate changes in perfusion indicate changes in rates of on-gassing/off-gassing.

i think i follow you

but, if you have two people with the same metabolism, but one breathes 1.5 times faster than the other (for whatever reason), their N2 on-gassing will still be the same, right?

 

[Charlie99]

but, if you have two people with the same metabolism, but one breathes 1.5 times faster than the other (for whatever reason), their N2 on-gassing will still be the same, right?

I think the more relevant thing is that when a person is exercising harder, then the increased blood flow effectively increases N2 absorption rates. Then if you are relatively idle during the decompression phase, the N2 won't offgas as fast as it went in. Another way to look at it is that changes in blood flow changes the effective halftimes of compartments/tissues such that offgassing and ongassing of dissolved gas is no longer symettrical.

Dr. Deco has previously posted some info about tests he ran at NASA where astronauts pedaled an exercise bicycle while prebreathing O2 before going to the reduced pressure of a spacesuit. The exercise helped to promote offgassing.

Charlie Allen

 

[Kendall Raine]

I think where Cameron is going is that overall fitness, leaving things like comparative fat content aside for a moment, leads to lower breathing rates from improved cardio vascular efficiency. Improved tissue perfusion results along with improved vascularity. Better cardio vascular fitness helps inert gas elimination. Similarly, improved fitness implies lower CO2 produced per unit of work. Now back to fat. The lower the body's fractional fat content the lower the amount of nitrogen dissolved in fat and the less inert gas to be eliminated. It's not the pulmonary, or venous, inert gas that gets you-unless it shunts-it's the dissolved tissue inert gas which feeds troublesome bubble seeds that cause the problems.

Back to the original question. I think the issue is not SAC per se, but the reasons one person's SAC is high while another's is low that drives the question of relative susceptability to DCI.

 

[H2Andy]

The exercise helped to promote offgassing.

but only under pressure, correct?

excercise at the surface after deco is asking for bubble formation, as far as i know

 

[CameronMartz]

i think i follow you

but, if you have two people with the same metabolism, but one breathes 1.5 times faster than the other (for whatever reason), their N2 on-gassing will still be the same, right?

While there is a voluntary portion to breathing rate, this is not something that is without cost. In other words, while you might be able to consciously slow or increase your breathing rate separately from changes in your metabolism, you cannot sustain this difference very long without causing problems.

In this example, if one person is breathing 1.5 times faster than the other for any time significant to decompression, for whatever reason, underlying that is a higher metabolism with a higher rate of gas exchange. Otherwise, either one person is hypervenilating, reducing blood CO2 levels below that which is necessary, and will eventually pass out, or the other person is hypoventilating, allowing CO2 levels to rise above what is sustainable, and will eventually pass out. Interestingly enough, the end result is the same because this is the only way the body can forcibly take over breathing rate and match it back to metabolic need.

Even differences in resting breathing rate are directly attributed to differences in resting metabolism- for a healthy person, these are practically synonymous. Our metabolisms are largely aerobic, meaning that they are fueled by a partial pressure differential of oxygen, and regulated by a partial pressure differential of CO2. This is at the blood tissue barrier, which can be a little harder to conceptualize than just thinking about the partial pressure of inspired gasses. If one person has a higher resting breathing rate than another, they have a higher resting metabolism and hence, a higher resting rate of gas exchange.

Complicating the answer even futher, an increased metabolism also allows faster off-gassing, so there is somewhat of a wash here when we think of the metabolic contribution to decompression obligation. Really, though, we're not interested in counting molecules- we're only concerned with managing tissue partial pressures relative to ambient pressures. It comes down to capacity versus rate of input versus rate of output. This is where the ability to think in terms of calculus really helps, but how many of us have ever really been able to do that. <g>

So, I can see why the rote answer to the question "does breathing rate matter?" has been "no," but that is not really true. Breathing rate will not affect what the dive table tells you to do, but it does represent the underlying rates of gas exchange occuring in a diver's body, resting or active, and may indicate how much of the safety factor built into the tables you are taking advantage of.

Cameron

 

[Charlie99]

but only under pressure, correct?

excercise at the surface after deco is asking for bubble formation, as far as i know

My understanding is that mild exercise that increases circulation is good. Heavy exertion, such as climbing a boat ladder in heavy gear is the sort of thing that promotes microbubble formation / tribonucleation / whatever this week's buzzword is.


Another example is that Dr. Deco had astronauts pedalling a bicycle. Good aerobic exercise, but not much stress on joints. OTOH, weightlifting is very bad, even though you are exerting yourself for just a few seconds.

I don't think whether you are at 1ata or 1.5ata makes much difference. Getting yanked and jerked around by a bouncing mooring line has been implicated in DCS cases. What makes that scenario particularly troublesome is that the first initial assumption is that you just have a sore elbow or shoulder from being banged around.

Disclaimer: the above is my understanding and summary of several different articles. Some say "exercise is bad", others say "exercise promotes offgassing". Reading between the lines as to what sort of exercise was involved leads me to what I posted above. Momentary heavy exertions such as weightlifting and climbing up a ladder are also known to tend to open up PFO shunts, so they can be a double whammy.

Charlie Allen

 

[ScubaMilo]

Cameron or DR. Deco
So let me see if what I'm thinking is true according to the statements above.
A meditational state during a dive will incur the least amount of on gassing due to slowing blood flow to out lying tissues.

Now on the same note couldn't this process be reversed?
After a mostly meditative dive.
Wouldn't a small amount of excercise say flexing and unflexing of large muscle groups like the quads or the biceps and shoulders during Deco promote blood flow and up respritory rate enough to accelerate off gassing of tissues to the blood an so on to the lungs where a slightly elevated respritory rate would help flush inert gasses?
Or would this raise the risk of bubble formation to an unacceptable level?
Hope I don't sound completely off base just trying to get another point of view.
This has been a theory that my team and I have been discussing.
I am for light excercise during Deco, but hope you can give a more educated view for me to take a look at.
Thanks,
Milo