Elimination of body N2 using Nitrox

[ScubaJorgen]

Hold on tight, I've got a few questions.

I am working myself through a 'Recreational Nitrox Diving' by Robert N. Rossier. On page 16 on the advantages of Nitrox:

...the increased fraction of oxygen in the breathing gas helps eliminate N2 from the body. Since CO2 is much more soluble in blood than oxygen, an equivalent number of CO2 molecules can be transorted at a lower partial pressure or tension. As we increase the arterial oxygen partial pressure, the CO2 tension of the venous blood increase by a lower percentage. So, increasing arterial oxygen tension results in a decreased venous gas tension, thus allowing more nitrogen to dissolve into the venous blood and be transported to the lungs. For this reasong, the use fo nitrox speeds the elimination of N2 from the body during ascents, and during safety/decompression stops

I've read it 5 times. It still is not clear to me...:wazzup:

My questions:
- How do O2 and CO2 partial pressures influence each other?
- Do O2 molecules bound to hemoglobin contribute to the tissue tension (normally resulting from dissolving gas)?
- How do O2 and/or CO2 partial pressures (tension) influence N2 partial pressure in tissue (tension) (guess no influence) or the amount of dissolved N2?
- Does the same number of molecules per volume of a gas always result in the same partial pressure (tension) at constant temperature? Or does the presence of other gasses influence this?
- Basically: the book suggests Nitrox's higher O2 level enhances the amount of N2 that is being transported (off-gassing), given a fixed gradient between N2 partial pressure in body and air. Is this true? How is this possible? Basically this means Nitrox lowers the half-time of the tissue.....:surprised:
- If true, does it hold for on-gassing?

 

[rmediver2002]

Hold on tight, I've got a few questions.

I am working myself through a 'Recreational Nitrox Diving' by Robert N. Rossier. On page 16 on the advantages of Nitrox:

...the increased fraction of oxygen in the breathing gas helps eliminate N2 from the body. Since CO2 is much more soluble in blood than oxygen, an equivalent number of CO2 molecules can be transorted at a lower partial pressure or tension. As we increase the arterial oxygen partial pressure, the CO2 tension of the venous blood increase by a lower percentage. So, increasing arterial oxygen tension results in a decreased venous gas tension, thus allowing more nitrogen to dissolve into the venous blood and be transported to the lungs. For this reasong, the use fo nitrox speeds the elimination of N2 from the body during ascents, and during safety/decompression stops

My questions:
- How do O2 and CO2 partial pressures influence each other?

The greatest influence is going to be amounts absorbed due to partial pressure within the inspired gas, little direct influence.

- Do O2 molecules bound to hemoglobin contribute to the tissue tension (normally resulting from dissolving gas)?

Yes if you consider the blood as a single fluid, it is disolved gas.

- How do O2 and/or CO2 partial pressures (tension) influence N2 partial pressure in tissue (tension) (guess no influence) or the amount of dissolved N2?

Less specific information on this interaction but it has been noted and referenced in several publications (NOAA manual has a paragraph on this) that the higher levels of O2 (NITROX) have been shown to increase dissolved levels of Co2 and the symptoms of excess CO2 (hypercapnia)

The relationship between excess CO2 and nitrogen narcosis (although narcosis is not currently tied directly to gas absorbsion rate) has also been documented, makes the symptoms appear earlier and present as more signifigant...

- Does the same number of molecules per volume of a gas always result in the same partial pressure (tension) at constant temperature? Or does the presence of other gasses influence this?

This is a more difficult question than it appears to be... although partial pressure in a given fluid would not be influenced we are talking about a more complex system within the body.

O2 transport is a function of the ability of and the number of available hemoglobin to transport O2, not just the pp O2.

CO presence is an example of how this O2 transport system can be disrupted even though adequate ppO2 is present... (so it is going to depend greatly on the gasses present)


- Basically: the book suggests Nitrox's higher O2 level enhances the amount of N2 that is being transported (off-gassing), given a fixed gradient between N2 partial pressure in body and air. Is this true? How is this possible? Basically this means Nitrox lowers the half-time of the tissue.....:surprised:

Don't think of tissue half-time, the percentage manipulation of the inspired gas gives the result...

The ability of NITROX to decrease on-gassing and increase off-gasing is influenced by the percentage of each gas in the inspired gas. Less N2 in the lungs allows more of-gassing from the blood, higher O2 content in the inspired gas (lower N2) would allow less N2 ongassing at a given depth. (this statement is all relating to a specific pressure or depth...)



- If true, does it hold for on-gassing?

Yes at a given depth when compared to air...

This is also the reason NITROX is being used as a decompression and treatment gas (although at higher percentage of O2 than normally dove...)


Great questions ScubaJorgen, it is obvious your going to get the most out of every training oppertunity, striving to understand the mechanics of something instead of memorizing what is written or stated by the instructor is one factor in developing a safe and competent diver...

Jeff Lane

 

[pt40fathoms]

If you were to breath air at a depth of 10m of fresh water or 2 ata, the air would have a partial pressure of 2.0. Air is commonly referred to 21 percent Oxygen and 79 percent Nitrogen, and at the surface the partial pressure of the gases would be measured as .21 O2 and .79 N2. At the depth shown above, the partial pressure of the gasses in air would be .42 O2 and 1.58 N2.

Simple so far....

At the depth we are using for this discussion, when breathing air your blood and tissues will eventually reach equilibrium at the partial pressures of the gas that you are breathing. That is to say that the N2 PP will be 1.58 and the O2 PP will be .42.

If you were to switch to 36% Nitrox at this depth, the following would happen....

First off 36% Nitrox is 36% O2 and 64% N2, so at the depth we are using the partial pressures would be .72 O2 instead of .42 with air, and 1.28 N2 instead of 1.58 again with air. The lowered partial pressure of Nitrogen you are now breathing and the higher partial pressures of Oxygen will equalize in your body. This will have the effect of you off gassing N2 until equilibrium is achieved, as well as charging your body with more O2 until equilibrium with the O2 is also achieved.

This doesn't answer all your questions, but I hope it gives and insight into the remainder of the questions you had posted. If you would like to discuss the relationship of O2/CO2 in hemoglobin and how the body gets rid of CO2 let us know.

Have fun and Dive safe.

 

[lamont]

what the original poster is actually referring to is the oxygen window.

(i'll take a shot at explaining it, but i'm just learning too, so i'd appreciate a critique):

the oxygen window occurs because CO2 is more soluble in tissue (blood?) than O2 is. so as you metabolize O2 into CO2, you are getting more atoms dissolved into the blood and you will have a lower ppC02 of the metabolized gas than you had ppO2 of the metabolized gas. this leads to a decrease in the overall tissue tension which help in offloading nitrogen.

in fit divers, you will be metabolizing O2 more efficiently and you will have a larger oxygen window (fitness is good, helps prevent DCS).

if you have a higher ppO2 (nitrox) you will also have a higher oxygen window.

i believe i a;sp read that to get the benefits of the oxygen window you need to have super-saturated your oxygen-hemoglobin binding so that you have dissolved oxygen in your tissues? (and parenthetically high pp02 that saturate tissues with dissolved oxygen can be used to treat CO poisoning which interferes with oxygen-hemoglobin binding).

 

[rmediver2002]

what the original poster is actually referring to is the oxygen window.

(i'll take a shot at explaining it, but i'm just learning too, so i'd appreciate a critique):

the oxygen window occurs because CO2 is more soluble in tissue (blood?) than O2 is. so as you metabolize O2 into CO2, you are getting more atoms dissolved into the blood and you will have a lower ppC02 of the metabolized gas than you had ppO2 of the metabolized gas. this leads to a decrease in the overall tissue tension which help in offloading nitrogen.

in fit divers, you will be metabolizing O2 more efficiently and you will have a larger oxygen window (fitness is good, helps prevent DCS).

if you have a higher ppO2 (nitrox) you will also have a higher oxygen window.

i believe i a;sp read that to get the benefits of the oxygen window you need to have super-saturated your oxygen-hemoglobin binding so that you have dissolved oxygen in your tissues? (and parenthetically high pp02 that saturate tissues with dissolved oxygen can be used to treat CO poisoning which interferes with oxygen-hemoglobin binding).

I am not sure this was what the original question was asking about but if so...

Here is some good information on the oxygen window:
http://www.sandiegoscuba.com/Studen...of hyperbaric oxygen on the oxygen window.doc

Do not focus on the ppCO2, it is not having a signifigant affect on the disolved nitrogen.

To saturate the hemoglobin with O2 your would need around 3 ATA, which is taught in hyberbaric medicine classes not basic NITROX.

Jeff Lane

 

[lamont]

I am working myself through a 'Recreational Nitrox Diving' by Robert N. Rossier. On page 16 on the advantages of Nitrox:

...the increased fraction of oxygen in the breathing gas helps eliminate N2 from the body. Since CO2 is much more soluble in blood than oxygen, an equivalent nIumber of CO2 molecules can be transorted at a lower partial pressure or tension. As we increase the arterial oxygen partial pressure, the CO2 tension of the venous blood increase by a lower percentage. So, increasing arterial oxygen tension results in a decreased venous gas tension, thus allowing more nitrogen to dissolve into the venous blood and be transported to the lungs. For this reasong, the use fo nitrox speeds the elimination of N2 from the body during ascents, and during safety/decompression stops

That quote sounds like its talking about the oxygen window to me... Correct me if I'm wrong...

 

[lamont]

sorry to double post, but here's a related question i've got which could clear something up -- what is the difference between partial pressure and tissue tension?

 

[rcohn]

sorry to double post, but here's a related question i've got which could clear something up -- what is the difference between partial pressure and tissue tension?

Partial pressures only exists in a gas phase. Gas pressure is created from the kinetic energy of the molecules striking other molecules or the walls of the container. This is clearly not the case in the liquid where the liquid molecules dominate. Gases disolved in tissues and blood are said to have the analagous quanity to gas partial pressure, which is called tension.

Concentration = (Solubility)(Tension)

If a liquid is allowed to come into equilibrium with a gas phase, the gas molecules will be disolved in the liquid until the tension in the liquid equals the partial pressue in the gas. Then the concentration of the gas molecules in the liquid can be determined from the product of the solubility and the tension.

Ralph

 

[rmediver2002]

...the increased fraction of oxygen in the breathing gas helps eliminate N2 from the body.

(cut information)

For this reasong, the use fo nitrox speeds the elimination of N2 from the body during ascents, and during safety/decompression stops

The original statement seems to try and discuss the oxygen window but the factors discussed are effects of modification of the pp of inert gas to gain the mentioned benefits.

Here is some more information on the oxygen window, again it is talking specifically about very high partial pressures of O2 and the link is talking about decompression procedures not NITROX diving.

http://www.wkpp.org/articles/Decompression/oxygen_window.htm


On another note I have not read the book mentioned so can not reference and ensure the statement is not in context with a larger section on O2 window or decompression. The statement viewed alone would appear to be misleading but depending on other factors to include target audience of the text, year authored (information available), reference material used, etc. I would certainly not eviscerate the author for this alone. I would like to read the entire publication before making an assessment.



rcohn - good explaination of the difference between partial pressure and gas in solution.

Jeff Lane

 

[DepartureDiver]

Okay Jorgen ... time for some more two cents. I think the term oxygen window can become confusing. Other terms are inherent unsaturation and partial pressure vacancy. They all are the same thing. Many think that increasing oxygen to a maximum increases the oxygen window. This is not necessarily true. In case it helps to understand, let’s use the term partial pressure vacancy. Due to oxygen being metabolized, if divers saturate themselves at a depth, the diver is not “saturated” since the pressure inside the tissues is less than the surrounding ambient pressure. This is because oxygen has been metabolized which results in less pressure/tension. In other words, there is a partial pressure vacancy, or the diver is inherently unsaturated. Increasing the oxygen content in a gas will decrease the percentage of the other inert gases thus resulting in a greater partial pressure vacancy. But this is a result of the decreased fraction of inert gas and not the increased fraction of oxygen. The issue that now needs to be considered is not to increase the oxygen fraction “too much” since this can result in other problems. Oxygen is the “evil” that is necessary to decrease the other gases, but it has it’s own issues. If there is too much oxygen, it is now not all being carried on the red blood cell. It is now being freely dissolved in plasma. This can allow oxygen to now be in the tissue as a gas possibly adding to decompression issues. This effect could be minimal and is what opens up the discussion of oxygen bends. Decompression illness is not caused by “inert” gases, but by any gas that is present in a tissue and if oxygen is there then it can add to this tension (especially considering that oxygen is more soluble than nitrogen). But the theory is that any excess oxygen will be metabolized during the ascent, provided this opportunity is available. There is still another consideration. When oxygen has the RBC tied up, it loses the ability for CO2 to bind to it and be eliminated through that process. While CO2 was mentioned in the article from the book, it was just ignored starting in the next sentence. While the ability to lose nitrogen may increase, the inability to lose CO2 via the RBC is a consideration. I personally hate Co2 headaches not to mention other effects. And let’s not forget vasoconstriction from increased oxygen. I’ve never seen it in print, but the way I like to think about it as follows. Capillaries open and shut at different times in the body. There is not enough blood to supply the entire body at once. Everyone knows the sensation of shock when they were a little kid and got caught stealing a cookie from the cookie jar. Tissues get oxygen when they ask for it … a capillary opens up and then oxygen is removed from the RBC as needed. The capillary will then close again. However, if the RBC is full of oxygen (you can see this in the article by Dr. Brian referred to in a previous post), then oxygen is simply flowing freely into a cell without it asking for it. This causes the capillary to close from this hyper-oxygenation. Now with the capillary closed, the other gases in the cell cannot leave due to there being no blood flow. So the math states that switching to a high ppO2 decompression gas is good and will require less time at a stop, especially at the depth of the switch, in reality more time may be required since a short stop such as one minute could cause a vasoconstrictive effect and actually lessen off-gassing.

So be cautious of simple mathematics that say the more oxygen the better. But any ppO2 in the range of 1.3 to 1.4 should be satisfactory.