C02 Elimination

[windapp]

I've read a few articals stating that CO2 elimination goes down as you go deeper due to the higher partial pressures of CO2 in your breathing gas. Curious to see, I worked out the calculations, and was surprised to find out that your CO2 elimination would only be reduced by about 5% at 200 ft. Is this really enough to cause problems?

 

[dumpsterDiver]

I don't know if the PP of the tank air has anything to do with it. I think that the overall work of breathing and the thickness of the air at 200 ft is probably more of an issue. Regardless of the cause, I can assure you that it is much harder to "catch your breath" after over-exertion at 200 than 30 ft (on air)...

 

[Scott L]

I've read a few articals stating that CO2 elimination goes down as you go deeper due to the higher partial pressures of CO2 in your breathing gas. Curious to see, I worked out the calculations, and was surprised to find out that your CO2 elimination would only be reduced by about 5% at 200 ft. Is this really enough to cause problems?

Just curious, how did you calaculate a compicated question such as that?

 

[windapp]

Just curious, how did you calaculate a complicated question such as that?

Here it is... only I have extended on it a little bit:

The rate of CO2 elimination is proportional to the difference between the partial pressure of CO2 in the bloodstream, and the partial pressure of CO2 in our lungs.

Partial Pressure of CO2 in the bloodstream at its lowest level (elimination is faster at higher levels of CO2) is 0.046385 atm. PPCO2 in air is .000396 atm. At 200 ft, it is .00272 atm. The difference on the surface is .045989 atm, and the difference at 200 ft is .043613. The ratio of the two is 95%.

Now here is where it gets interesting. The real difference in elimination is going to ocur as the air in your lungs starts to gain PP from the CO2 you are eliminating. Since the elimination rates are pretty close if your lungs are full of fresh air, we will call them the same. Whether you are at depth, or on the surface, the rate of elimination by mass will be roughly the same. At depth, the increase of mass fraction of the air in your lungs will be 1/7 what it is at the surface because you have 7X the air in your lungs. However, the partial pressure increase will be 7X what it would be at the surface because the pressure is 7X what it is at the surface. 1/7 X 7 = 1.

Conclusion: Other than a relatively negligible difference, at the begining of a breath, the rate of elimination of CO2 should be the same at depth as it is at the surface.

This of course assumes that you are able to breath the gas at depth as easily as at the surface. Since I am only talking about the relative partial pressures of CO2 in air, this assumption is valid.

 

[boulderjohn]

This of course assumes that you are able to breath the gas at depth as easily as at the surface. Since I am only talking about the relative partial pressures of CO2 in air, this assumption is valid.

That's the kicker.

At the depths you are talking about, it really doesn't end up being that much of a factor. There are some who believe that at very great depths, the work of breathing (WOB) may create more CO2 than is eliminated during the act of breathing. There is some belief that something akin to that led to the demise of David Shaw. If you want to avoid that, stay well above 900 feet.

 

[TSandM]

The question is not entirely clear to me, but reminds me of one I asked as a new diver.

There is a certain partial pressure of CO2 in inspired air on land, although it is very low. It might become significant when compressed and breathed under pressure, if it weren't for the fact that the filter systems on dive shop compressors scrub all the CO2 out of the air that's pumped into your tank.

CO2 eliminate, in the absence of inspired CO2, is completely determined by total minute ventilation (volume, not mass) put through the lungs. As you observe, if the same volume is breathed at 100 feet as at sea level, CO2 ought to remain normal.

However, three things change at depth. One is that the air you are breathing is denser. This creates turbulent flow in the smaller airways, which results in more resistance and less EFFECTIVE ventilation of the alveoli, or air sacs, where gas exchange occurs. Secondly, dense gas is harder to pull through a regulator, resulting in increased work of breathing. We are not used to this -- breathing on land is all but effortless -- and increased work of breathing quickly results in ineffective CO2 elimination. Which brings us to the final point, which is that, for some unknown reason, some people seem to tolerate elevated CO2 levels when diving, whereas almost no one with normal lungs tolerates them on land. (In fact, the inability to normalize CO2 generally makes people VERY anxious.) The bottom line is that elevated CO2 levels are easy to achieve and frequently reasonably tolerated while diving -- but it doesn't take much of an elevation above where you are tolerant to cause a high anxiety state.

 

[dumpsterDiver]

This is the first that I have heard that a compressor removes Co2.... is that from a charcoal filter or something?

 

[TSandM]

I don't know enough about compressors to know which filter does it, but I posed the question of increased pC02s when I was an OW student, and the instructor didn't know the answer -- but the shop owner did, because he told me his compressor setup had to scrub the ambient CO2 out of the compressed gas. I guess I have no authority for saying that, other than that someone who ran a shop compressor told me so.

 

[dumpsterDiver]

For some reason, I think that the media is supposed to pick up hydrocarbons, but I am curious about CO2.

 

[Carl_F]

CO2 isn't removed by filter media. And I haven't seen a breathing air compressor with a CO2 chemical scrubber. Submarines use a chemical reaction to remove CO2 (using mono ethanolamine.) LiOH will remove CO2 also. I doubt that dive shops use either method to remove CO2 but perhaps a dive shop owner can attest one way or another.