respiratory questions

[scuba andy]

Hi,
Ive been reading about the narcotic effects of gases at depth. I cant seem to find answers to some questions after doing some online searching, so here goes...


1. Is it only a minimum partial pressure of O2 in a gas that enables successful external and internal respiration of O2?

e.g. could a mix of 3.5% O2 be breathed as successfully at 6 ATM (nitrogen narcosis aside) as normal air can be breathed at 1 ATM?

2. Why does the pulmonary exchange of CO2 appear to be less efficient than that of O2? - I understand that the blood PO2 increases (150%) from 40mmHg to 100mmHg as it passes through the alveolar capillaries, but that blood PCO2 decreases (only 11.1%) from 45mmHg to 40mmHg. Is it just that the O2 molecule has been made bigger having become CO2 after metabolism?

3. Is the safe (non-narcotic) arterial PCO2 range actually narrower at depth?

4. How quickly canarterial PCO2 increase above safe levels with exertion or deficient breathing? And how quickly can safe homeostasis be re-established after PCO2 spiking into dangerous levels?


Hope the questions arent phrased too poorly to make sense.

thanks,

andy

 

[TSandM]

The answer to #1 is yes -- that's the basis of hypoxic mixes for technical diving.

#2 -- CO2 actually diffuses more easily than oxygen does, but oxygen isn't dependent on solubility in plasma. As quickly as the oxygen passes through the alveolar membrane, it's picked up on hemoglobin, therefore driving the equilibrium toward more oxygen diffusion. And the pO2 in alveolar air is high, where the ppO2 in venous blood is low, therefore creating a strong gradient for diffusion. CO2 levels are regulated by minute ventilation, and minute ventilation is set to keep pCO2 at 40.

#3 -- not sure what you are asking here. pCO2 has to be kept at 40, plus or minus very little, because CO2-HCO3 equilibrium is the primary buffer to keep blood pH constant. Minute ventilation is adjusted to control this. If pCO2 is not constant, pH falls, the person feels acute air hunger, and it will add to narcosis. Some people are CO2-tolerant while diving, and do allow their pCO2 to rise without distress, but the narcotic potential of the gas does not change just because the person is tolerating it.

#4 -- pCO2 begins to rise the minute that minute ventilation is inadequate to clear it. If you hold your breath for 30 seconds, your pCO2 has risen. If it has reached significantly elevated levels, it will take several minutes (and a lot of heavy breathing) to bring it back down to normal. But we are talking a handful of minutes here, not hours.

 

[scuba andy]

The answer to #1 is yes -- that's the basis of hypoxic mixes for technical diving. .

Great

#2 -- CO2 actually diffuses more easily than oxygen does, but oxygen isn't dependent on solubility in plasma. As quickly as the oxygen passes through the alveolar membrane, it's picked up on haemoglobin, therefore driving the equilibrium toward more oxygen diffusion. And the pO2 in alveolar air is high, where the ppO2 in venous blood is low, therefore creating a strong gradient for diffusion. CO2 levels are regulated by minute ventilation, and minute ventilation is set to keep pCO2 at 40.

Yeah, this one is harder for me to understand.
The gradients of PO2 and PCO2 at the membrane seem comparable. V/Q ratio is more or less 1:1. I assume the CO2 bonded to haemoglobin in venous blood must at this point be fully released to free it up for oxygenation.
If CO2 doesn’t find it harder to diffuse and CO2 is regulated by minute respiration, is it then that there is much more CO2 storage potential in the 4 litres of blood than the 5 litres of inspired air? Does the chemical storage of CO2 as bicarbonate and carbonic acid act as a huge reservoir that ‘tops up’ the PCO2 levels during and after external ventilation to the necessary 40mmHg to maintain the critical blood PH levels? Ahh, thats it, right? That’s why as you said CO2 levels are regulated by minute ventilation. Perfusion/ventilation probably immediately reduces PCO2 very low but the level is immediately increased by that enzyme releasing more CO2 from bicarbonate reserves. So breathing faster to reduce CO2 isnt just clearing CO2, its depleting the bicarbonate stores. Please tell me thats it!! If its not, I may stop reading stuff and just dive the body without understanding how the engine works.

#3 -- not sure what you are asking here. pCO2 has to be kept at 40, plus or minus very little, because CO2-HCO3 equilibrium is the primary buffer to keep blood pH constant. Minute ventilation is adjusted to control this. If pCO2 is not constant, pH falls, the person feels acute air hunger, and it will add to narcosis. Some people are CO2-tolerant while diving, and do allow their pCO2 to rise without distress, but the narcotic potential of the gas does not change just because the person is tolerating it.

I really wanted to know the tolerances (the plus or minus) before narcosis occurs at a detrimental level. If those tolerance levels are reduced at depth and if so, why? And if not, why is CO2 more of a problem at depth than on the surface? -or is it? Could we have reduced mental performance when exercising due to CO2 retention but just don’t particularly notice it?

#4 -- pCO2 begins to rise the minute that minute ventilation is inadequate to clear it. If you hold your breath for 30 seconds, your pCO2 has risen. If it has reached significantly elevated levels, it will take several minutes (and a lot of heavy breathing) to bring it back down to normal. But we are talking a handful of minutes here, not hours.

-thanks.

 

[Duke Dive Medicine]

Andy, you seem to have a pretty good understanding of this already. CO2 regulation more complex because of the mechanisms you've talked about and TSandM has explained. In addition to simple diffusion, there's also biochemistry involved in the regulation of blood CO2 level. At depth, the tolerance window does not get narrower, but depth definitely has an influence on arterial pCO2. Increasing depth results in increased gas density, which causes arterial pCO2 to increase. Here's a link to an article I wrote for DiveAssure on diving and CO2: http://www.diveassure.com/new/documents/Diving_and_Carbon_Dioxide.pdf

Best regards,
DDM

 

[TSandM]

You are entirely correct -- blowing off CO2 also depletes bicarbonate stores. Similarly, retaining CO2 will result in higher bicarb levels, although some of this is a relatively slow process, because it's in part done by the kidneys. The body has a very strong desire to keep the pH very close to 7.4.

Eric, what a great article! I need to bookmark that for folks who ask these sorts of questions.

 

[scuba andy]

DDM: I started looking at this subject a couple of days ago after someone posted a link to a 1978 article "Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis" in one of the SB threads. Fairly fascinating subject, so Im trying to lean as best I can into the learning curve. So, cheers for the reply and your article. Jolly interesting and just the sort of stuff I'm interested in.

TS&M: So anyway, Im getting the idea that these arent discrete biological mechanisms but overlap with all sorts of other stuff. Lol, much to learn.