Partial Pressures and Depth Question

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pelagic_one

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Hi all, I got nitrox certified but still study and review the subject. I know coming up from depth reduces pressure and Nitrogen will work its way out of your system. Ascending too fast will make a greater Nitrogen pressure gradient, possibly leading to the bends. Now the question is, if you switch to Nitrox as the gas on ascent, are you making a greater partial pressure gradient, as you would by ascending too fast, leading to the bends?

When administering oxygen, is not the pressure gradient the greatest?

It would seem the rate of Nitrogen expulsion is the most important factor, not expell the Nitrogen as quickly as possible.

Thanks for any comments.
 
The bends are due to insufficient ambient pressure, not the partial pressure gradient or difference. As you ascend (regardless of what you're breathing), the ambient pressure drops, and if it drops more than about 70% of a tissue's inert gas content, bad things start happening. (The other way to look at it is the max tissues pressure the tissues can tolerate about 1.5x ambient.)

ETA: and yes, oxygen yields the highest off-gassing gradient. That is why technical divers with mandatory stops use it starting at 20 ft. There it's safe enough that you won't have oxygen toxicity issues while also making the "hang time" the shortest it can be (for the tissues to reach a given proportion of tolerated pressure at the surface).
 
this confused the heck out of me as a new diver, too.

The pressure gradient you are worried about on ascent is the gradient between your internal gas pressure and the pressure around you (ambient). The lesser ambient pressure as you ascend is your concern, just as opening a bottle of soda at altitude is more likely to create a bibble explosion than opening it at sea level. The greater the gradient between what your tissues and the gas you are breathing, the better.
 
If I am understanding your question, you are asking about the magnitude of the gradient between dissolved nitrogen in your blood and the nitrogen in your breathing gas.

I think what you are missing is that n2 offgassing happens in your lungs and dcs happens in your “tissues”. The magnitude of the gradient in your tissues isn’t changed by switching to a higher o2 / lower n2 so dcs isn’t increased. Magnitude of gradient in the lungs is increased and faster off gassing can occur.

EDIT: as always I reserve the right to be FoS and will delete the above if needed
 
I conceptualize it (probably over simplified): that the ascent (i.e., decreasing ambient pressure) is what stimulates bubbles, but once they form, you want the highest partial pressure differential practical to start eliminating them.
 
OP, this is a great question that demonstrates you are really thinking about the dynamics.

I understand your question to be: for a given ambient pressure, with a given tissue saturation, if you vary the fraction of nitrogen in the inspired gas, does that change the pressure gradient (supersaturation) of the tissues, and therefore lead to an increase risk of decompression illness?

The answer is, to the best of my limited knowledge, no. In the Buhlmann models we use for calculating this, the supersaturation of the tissues is related to the ambient pressure, not to the difference in pressure between the partial pressure of the inspired gas and the supersaturation of the tissues. Bubbles form in the tissues directly, as a direct consequence (detail below) of the ambient pressure combined with how much gas is dissolved into the tissues (saturation) (and other factors). The contents of the inspired/breathed gas has no impact on the pressure of the gas you are breathing, or on the immediate likelihood of bubbles forming anywhere in the body, but it will change the dynamics of the diffusion of gas into and out of the blood. As an example of a diver descending, for a greater partial pressure of nitrogen in the inspired gas, then there is a greater rate of diffusion of that gas into the blood as you breath. Alternatively, there is an associated decrease in nitrogen diffusion rate given an increase in the fraction of oxygen in an inspired gas. This is why you can stay for longer at a given depth given a higher fraction of oxygen, as the differential pressure between the inspired partial pressure of nitrogen and the tissue diffused gas pressure (saturation) is lower than had there been comparatively more nitrogen in the inspired gas. A bad analogy is like drinking a cup of 5% alcohol (beer) vs. drinking a cup of straight 50% alcohol (hard liquor). You would need fewer of the 50% cups to reach the same level of intoxication, sort of how you need less time at a given depth to reach a similar saturation of nitrogen if the fraction of nitrogen inspired is greater. You ask specifically about ascending, and as above, the tissue saturation (amount of diffused gas at a given pressure) is dependent upon the ambient pressure, so breathing a higher fraction of oxygen doesn't lead to greater tissue saturation, but does lead to greater rate of gas diffusion out of the blood through the lungs and into the expired gas, commonly called off-gassing.

Good comments, and I believe that lostsheep has a nice answer which addresses your specific question. I apparently can’t help but digress way too far into the weeds.

First, characterizing the mathematical models that we use to calculate risk of decompression illness is one thing; and characterizing what exactly is happening in the body is another thing. The goal is of course to use mathematical models that are as close to reality as possible. While we know a lot, and are able to use that knowledge to dive safely, scientifically speaking, I tentatively believe as a layperson that the exact dynamics of the relationship between bubbles in the blood and minor decompression illness aren’t entirely understood. In simpler terms: decompression theory is complex and imperfectly understood, and one can dive deep into the science.

Bubbles. It’s important to note that I don’t think the Buhlmann model mathematically predicts bubbles specifically; it defines gas diffusion into and out of several “compartments” which are theoretically analogous to different types of tissues in the body. For example blood, and fat, which have different rates at which gas diffuses into or out. My haphazard and tenuous understanding is that bubbles are not perfectly correlated with symptoms of minor decompression illness, though they are indicators of decompression stress, and are always present in cases of serious decompression illness. Additionally, gas dissolves into and out of your body regardless of whether or not there are bubbles present. When the supersaturation of a tissue is large enough, i.e. the ambient pressure is way lower than the dissolved gas pressure in that tissue, bubbles will form, if the difference is small, bubbles will not form, and if the difference is in a medium range, they may or may not form. Keeping the supersaturation of a tissue compartment low enough is the primary method used to prevent decompression illness, which also hopefully prevents bubbles from forming. A huge part of dive training at every level is to give divers the ability to do this. The mechanics of bubble formation is complex, and there have been attempts to write decompression algorithms/models that incorporate this complex physics, though using those models is not popular today, as they seemed to be less capable of predicting decompression illness than the Buhlmann tissue gas diffusion compartment model. (If interested look up RGBM or deep stops, which I don’t use to plan dives)

In short, while preventing bubbles is a good way to think about and introduce the idea of decompression illness, but don’t focus too much on bubbles, focus more on compartment supersaturation if your understanding is at that level. Decompression illness is really what we want to prevent, and using more oxygen in the gas mix is a good way to do that, of course while keeping the risks of the oxygen low. Bubbles will be reduced in the process.

I can’t help but also bring up that there are other factors that influence all of these dynamics, from temperature, to probably physiology, to rate of pressure change, the list goes on… Also nitrogen isn’t the only inert gas out there. Good luck!

Please excuse my lack of brevity, and don’t trust what I’ve said without someone knowledgeable agreeing; please criticize! :)
 
The bends are due to insufficient ambient pressure, not the partial pressure gradient or difference. As you ascend (regardless of what you're breathing), the ambient pressure drops, and if it drops more than about 70% of a tissue's inert gas content, bad things start happening. (The other way to look at it is the max tissues pressure the tissues can tolerate about 1.5x ambient.)

ETA: and yes, oxygen yields the highest off-gassing gradient. That is why technical divers with mandatory stops use it starting at 20 ft. There it's safe enough that you won't have oxygen toxicity issues while also making the "hang time" the shortest it can be (for the tissues to reach a given proportion of tolerated pressure at the surface).

A lot of people find this counter intuitive.

According to dissolved gas models, how fast you off-gas is determined by the difference between the partial pressure of nitrogen dissolved in your body and the partial pressure of the same gas in your lungs. Your safety is not directly related to how fast you off-gas so you can make this difference as big as possible by having zero nitrogen in your breathing gas (pure oxyen typically)

Your safety is determined by the difference between the partial pressure of nitrogen dissolved in your body and the absolute ambient pressure (so only the depth) which you ideally want to keep as low as possible, certainly below the model's "M value"
 
Thanks to all of the responders. I am reading them very slowly to not miss anything. I am now thinking the ambient pressure at depth maintains gases in solution / dissolved in the tissues and bloodstream. As long as a gas is dissolved, the molecules migrate safely from tissues to bloodstream. At the blood/air interface, the breathing gas composition can affect the transfer rate of the various gases. There is a change of state, from dissolved gas to gaseous, at the alveoli. Are the lungs capable of rapid degassing? I guess so? Is there a study about this? Now as the better de-gassed blood travels around the body, it presents a greater pressure gradient to the various tissues, yet if everything is under sufficient ambient pressure, the gasses stay dissolved as they travel across various membranes.

Side bar: I've seen the demostration of a pop bottle being filled with air at depth, then it expands to bulge the bottle as it is brought up from depth. A better demonstration could be a carbonated beverage at depth has no bubbles but bubbles appear as it is brought up from depth.
 

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