Does High PO2 = High CO2?

[Mr. Bubble]

Just curious, after taking notrox afew years ago, I never remember them mentioning anything about carbon dioxide. If you increase the PO2, would you not also have a higher concentration of CO2 to dispell after each breath?

Thanks

 

[bleeb]

Not necessarily. I'm not an expert, nor have I stayed in a Holiday Inn Express recently, but here's how I understand it:

The *absolute* amount of CO2 you output is not a function of PO2, but depends on how fast your metabolism is running. To first order, it doesn't vary with absolute pressure. (There may be second order effects.)

The partial pressure of CO2 in your exhalation depends on the absolute amount you're generating and how fast you're moving it from blood stream to lungs to outside world. Generally, your exhalation doesn't vary much (IIRC 0.5-2%) since the reflex to breathe faster or slower is driven by our body sensing blood CO2 levels.

However, some people have been found have been found to slow down their breathing at higher than atmospheric PO2 levels, which increases the absolute amount of CO2 that must be removed with each breath i.e. increasing the ppCO2.

Is that what you meant, or were you thinking of something else?

 

[gt3073b]

My understanding is that your body will only use so much oxygen regardless of depth or P02. It depends only on your physiology and level of exertion. If you were to breath 100% O2 at the surface, you would still exhale the same amount of CO2 as if you were breathing air, and a lot more oxygen.

If you go deeper, the fraction of O2 is the same but there is more oygen (and everything else) per breath. Since your body can only use so much, you exhale the same amount of CO2. Since that CO2 is under twice the pressure, the fraction would be half.

For example (making up numbers here...), lets say at the surface the oxygen you use makes up 8% of the air you inhale since you don't deplete the oxygen in each breath. You would then bind that oxygen to carbon and exhale CO2. If you went down to 33fsw, you would only metabolize 4% of the air, since you actually took twice as much oxygen (and everything else) into your lungs but only used the same amount. The CO2 you exhale would be the same as at the surface, but diluted through twice as much nitrogen and unused oxygen.


Bryan.

 

[Mr. Bubble]

Yeah, that explains it. I was just wondering that if you increased your PO2 level, would'nt it increase your consumption of O2 and thus, increase the amount of CO2 created? You aanswered my questions. I was wondering if Nitrox had any effect on the breathing reflex due to increased CO2 build up..I guess not.

Thanks

 

[z3r0c00l]

However, and I would ask the Docs to comment on this to see if it is right or wrong, but I was under the impression that high PPO2s in the blood might reduce the efficiency of the Haldane - Bohr feedback systems. Essentially the lack of or increase in one gas, alters the affinity of the blood to the dissolution of each. This is also similar to what I was told in my EAN class, where they said CO2 problems at depth are more likely with higher PPO2s than with EAN20.95 (lol). Is there any truth to this or is this more like the "EAN makes me feel better" issue?

 

[Mr. Bubble]

Good to see this one still going. I have been reading more into it lately myself. As with the Haldane theory, the body would be saturated at 21% on the surface. He states that it takes 30 min to reach saturation of the 5min tissue, so, at depth on a recreational dive, your body would not have time to become saturated at the new EAN level. I still have not found any mention in any book that addresses the issue of my initial post topic. So , I guess I am getting to the crux of it myself. I wanted to know if there was any reason to believe that the higher level of EAN would cause higher CO2 output if exherting at depths below 100ft, causing the panic cycle (SOB, panic.etc..)

There appears to be a gray area on this subject.....I would like to get some clarification as I noticed another person asked a similar question on another thread. They were harassed for asking because they were a "Dive Master" and "experienced"
diver. They were expected to "Do their homework" before asking a question.....

Man, some of the 'tudes on this board floor me.......:no

 

[Mr. Bubble]

12345

 

[TSandM]

VO2 (consumption of oxygen) is determined by metabolic rate, which is related to activity level. Providing additional oxygen does not increase oxygen usage; it just raises the oxygen content of the venous blood. Oxygen is poorly soluble in water, and is carried primarily on hemoglobin molecules. Hemoglobin in normal people is about 99% saturated with oxygen when one is breathing air at the surface. The increased pp02 at depth does cause a small amount of additional oxygen to dissolve in plasma, but compared with the oxygen content of the hemoglobin, it's trivial.

CO2 production is related to VO2, or in other words, to the amount of metabolic activity taking place. The pCO2 in the blood is very closely regulated, because it determines the blood pH, and many body processes are very dependent on maintaining a precise pH. The pCO2 is directly related to the volume of air that is moved through the lungs in a minute. Whether you are breathing at the surface or at depth, what you are breathing is, to all intents and purposes, devoid of CO2. What else is in it really doesn't matter; what matters is that the gradient for offgassing CO2 is maximal at all depths. Therefore, your minute ventilation (amount of air moved to remove the CO2) will not vary with depth.

Some people apparently acclimatize to elevated CO2s when they are diving. This is what happens with skip breathing. It is not a very safe procedure. Elevated CO2 levels are both narcotic and implicated in increased oxygen toxicity.

 

[knotical]

Interesting read here:
http://scuba-doc.com/CO2acclim.pdf
including:

(4) Some individuals are much more likely to develop high carbon dioxide tensions than others, but all individuals show a tendency in this direction especially when breathing a nitrogen-oxygen mixture.

 

[z3r0c00l]

Basically what I was saying is that you have feedback systems in your blood that alter the rate of O2 or CO2 dissolution (and perhaps the max concentrations), and as I was told the extra oxygen (surplus just hanging around) makes your blood less able to carry CO2 away. Your metabolism doesnt change, I understand that fact, but you blood may be so saturated with O2 that there isnt any room for CO2 - and the conditions (pH etc) are less than ideal because of all the oxygen. Does the blood get locked in O2 mode basically? The haldane Bohr systems are like forward and reverse gears in your blood that shift to better remove co2 and take in O2, what if they get shifted towards the O2 too much Could they?

That is the idea anyway, I suspect that it really isnt such a big deal, else it would be a problem in the chamber when guys dive to 1.6+ PPO2s.