Why doesn't oxygen cause decompression sickness?

[Puppeteer]

That's not quite accurate. Hemoglobin has 4 units of heme, each of which contains an iron atom surrounded and stabalized by a porphyrin ring, that helps prevent the iron from oxidizing into the ferric state. The whole oxygen molecule binds to the ferrous atom in a reversible manner forming an octohedral iron complex, and the physical structure of the hemoglobin makes a physical pocket into which the oxygen will selectively fit (though other similar sized atoms will also fit, which is why carbon monoxide is such a problem). When oxygen binds, the hemoglobin shape changes (conformation), which makes is more likely to bind additional oxygen molecules until it is full.

When bound, the oxygen molecules are complexed within the hemoglobin, it is in a solution state and therefore not compressible. Hemoglobin binding to oxygen is affected by the partial pressure of oxygen (the more oxygen available, the more likely it is to complex with hemoglobin, but once bound, the oxygen is deemed dissolved, and no longer gaseous.

Without hemoglobin, the amount of oxygen that can be dissolved in solution is very low (about 7ppm). Hemoglobin will increase the oxygen dissolved in the blood to around 275ppm

 

[Smache]

One speculation is that O2 is recognized as "good" and does not trigger immune response and inflammation, unlike inert gases.

That's really interesting. I've wondered whether DCS was from more than the physical effects of bubbles.

 

[lowwall]

Think of hemoglobin like rust.

It does. I don't have the link but there is a study from the 50's where they managed to bend goats on pure O2. It should be findable on PubMed. It was probably the first time I saw the word "miraculous" in a scientific paper: the symptoms spontaneously resolved within minutes.

One speculation is that O2 is recognized as "good" and does not trigger immune response and inflammation, unlike inert gases.

You'd expect gross symptoms to go away as local metabolic activity and deoxygenated hemoglobin nibble away at any trapped O2 bubbles.

I found your goat study which I've attached: "Oxygen bends." KW Donald, J Appl Physiol. 1955 May;7(6):639-44.

I got the citation from "The Physiology of Nitrox Diving" by Richard Vann (also attached) which included a lengthy section that directly addressed the OP's question. The TLDNR summary of the article and followup discussion is that something detrimental appears to be happening when you hold inert gas loadings constant at different pressure levels by varying the percentage of O2 (at least beyond 3 atmospheres absolute). But it wasn't clear if it was truly oxygen bends, some other effect of high ppO2 concentrations, statistical noise or some combination of the three.

Note that the Vann article was part of a 1989 workshop hosted by NOAA which was an important early step on the path that led to Nitrox acceptance in recreational diving. You can find the full thing at

https://library.oarcloud.noaa.gov/noaa_documents.lib/OAR/Undersea_Research/NURP_research_report/RR_89-1.pdf

 

[Jai Bar]

I think it was mentioned in quoted paper above: although gas laws (Henri, Boil etc.) apply to all gases, naturally, therefore Oxygen may (and probably does) create bubbles upon pressure changes.

But it depends not only on each of the gasses' partial pressure but also on its solubility constant.

By the way: even when breathing 100% O2 there is still Nitrogen dissolvedin all tissues.

 

[duke dive medicine]

We all know decompression sickness is caused by nitrogen (or helium) absorbed by tissues in the body. If a diver ascends too quickly, the gas doesn't have time to leave the body safely, and forms bubbles. I'm wondering why oxygen doesn't have the same problem.

One explanation I've gotten is that oxygen doesn't have a chance to get absorbed because it's metabolized. But any rebreather diver can confirm that most of the oxygen in each breath is not metabolized, and gets exhaled along with the nitrogen.

I've also heard that it's because oxygen is just not soluble in body tissues. That also seems unlikely, since there's at least enough oxygen dissolved in sea water for marine animals to survive.

Anyone have an explanation?

As noted above, most oxygen is carried by hemoglobin in red blood cells at sea level, where the inspired partial pressure of oxygen is .21 ATA. Still, there is a small amount of oxygen dissolved in the plasma at this pressure; a normal arterial pO2 value is 85-100 mmHg.

Divers breathe O2 at higher partial pressures - up to 1.4 ATA on the bottom and 1.6 ATA while decompressing (equivalent to 100% O2 at roughly 13 and 20 feet, respectively) - any higher and the risk of CNS O2 toxicity increases to unacceptable levels . This will fully saturate the hemoglobin and also cause more oxygen to dissolve in the plasma in accordance with the alveolar gas equation (calculator linked here for those interested), but not to a level significant enough to cause bubbles to form on decompression. Also as noted above, this dissolved O2 is being continuously metabolized, unlike N2 and He.

You could theoretically experience symptomatic DCS on O2 but the partial pressure you'd have to expose yourself to in order to do so would cause bigger problems than DCS.

Best regards,
DDM

 

[arkstorm]

Question: If you experienced DCS on O2, would the body's metabolic processing of O2 consume the offending bubble(s), thus, alleviating the condition without the need to recompression therapy?

 

[duke dive medicine]

Question: If you experienced DCS on O2, would the body's metabolic processing of O2 consume the offending bubble(s), thus, alleviating the condition without the need to recompression therapy?

That's a big if as noted above. A more likely scenario might be a combat diver on a 100% O2 CCR who experiences a gas embolism. An oxygen bubble would resolve much faster, but depending on size and duration, could also cause an inflammatory reaction that could in turn lead to neurological symptoms, which hyperbaric oxygen could help alleviate.

Best regards,
DDM

 

[SpeedyJ]

As noted above, most oxygen is carried by hemoglobin in red blood cells at sea level, where the inspired partial pressure of oxygen is .21 ATA. Still, there is a small amount of oxygen dissolved in the plasma at this pressure; a normal arterial pO2 value is 85-100 mmHg.

Divers breathe O2 at higher partial pressures - up to 1.4 ATA on the bottom and 1.6 ATA while decompressing (equivalent to 100% O2 at roughly 13 and 20 feet, respectively) - any higher and the risk of CNS O2 toxicity increases to unacceptable levels . This will fully saturate the hemoglobin and also cause more oxygen to dissolve in the plasma in accordance with the alveolar gas equation (calculator linked here for those interested), but not to a level significant enough to cause bubbles to form on decompression. Also as noted above, this dissolved O2 is being continuously metabolized, unlike N2 and He.

You could theoretically experience symptomatic DCS on O2 but the partial pressure you'd have to expose yourself to in order to do so would cause bigger problems than DCS.

Best regards,
DDM

Out of curiosity what is the tipping point where partial pressure (moles) of O2 is greater than the metabolic capacity?

 

[RobPNW]

Isn't it mostly because O2 is removed by the metabolic process as well as offgassing and N2 is not?

 

[dmaziuk]

Isn't it mostly because O2 is removed by the metabolic process as well as offgassing and N2 is not?

Look up VO2 max: the "mostly" is pretty small even if you're a trained athlete swimming a 5-minute mile.