Does Oxygen REALLY not contribute to DCS

[dlwalke]

So I just took an EANx course and learned that each dive contributes some to the total allowable oxygen exposure for a 24 hr period (i.e., the not quite accurately named oxygen clock) and that you need to keep track of this so as to lower the risk of oxygen toxicity on subsequent dives. So this implies that you do accumulate some oxygen from one dive to the next. I had previously been taught, at least I thought I was, that nitrogen contributes to DCS because it is metabolically inert, but that oxygen does not because it is metabolized to other stuff (mostly CO2 I presume) and maybe you just breathe this stuff out so quickly that it is not an issue. Perhaps this is stuff I assumed but didn't really read though. Anyhow, it seems to me that if you have to keep track of your percent allowable oxygen load from one dive to the next because it is cumulative across a 24 hr period and you need to avoid oxygen toxicity, then oxygen would also contribute to the risk of DCS, even if it is a minor contributant vis-a-vis nitrogen. Is this right or am I missing something?

Thanks,
Dave

 

[lamont]

its not the build up of oxygen which is the issue, but there's secondary effects (somewhat poorly understood) which are cumulative based on exposure...

 

[sea2summit]

CNS is too much O2 storing in your central nervous system which causes you to sieze up on occasion. DCS caused by Nitrogen is bubbles forming that settle in places and cause pain or wose. Two seperate monsters.

 

[dlwalke]

CNS is too much O2 storing in your central nervous system which causes you to sieze up on occasion. DCS caused by Nitrogen is bubbles forming that settle in places and cause pain or wose. Two seperate monsters.

Well, you're restating what I have learned, but it seems that if O2 is also a gas, which it is, then as you ascend it must also increase in volume and therefore also form expanding bubbles that would do the same thing as expanding nitrogen bubbles.

 

[sea2summit]

Not really, the O2 works more like a poison where your body reacts to the amount of O2 it is recieving.

 

[dsteding]

Well, you're restating what I have learned, but it seems that if O2 is also a gas, which it is, then as you ascend it must also increase in volume and therefore also form expanding bubbles that would do the same thing as expanding nitrogen bubbles.

The difference is that oxygen is metabolized in your tissues, while nitrogen is not. I've always wondered about whether oxygen off-gases, I can certainly envision scenarios where your tissues became oxygen super-saturated, and I guess that the determinative factor is whether tissues metabolize oxygen at a high enough rate to keep concentrations lower than the threshold at which tissue would both off gas oxygen and that off-gassing would be at a high enough rate to trigger bubble formation in the blood. My guess is that, since N2 is inert, and since N2 composes much more of the total fraction in (non-mix) breathing gas, that for all intents and purposes the oxygen can be ignored in the decompression models.

 

[TSandM]

Free oxygen tensions in tissue are relatively low, because oxygen is metabolized rapidly. This is in contrast to nitrogen, which equilibrates between blood and tissues and stays there until affected by pressure gradients.

Cumulative oxygen units have to do with the oxidative damage done to tissues by exposure to high oxygen concentrations, not with the actual oxygen persisting in the tissues.

 

[dlwalke]

OK, I think I understand...maybe. Is it the case then that oxygen toxicity is not due to the buildup of O2 bubbles, but instead of non-gaseous oxygen. In other words, is the purpose of the "oxygen clock" NOT to let O2 bubbles offgas, but to let the levels of oxygen in some other form approach basal values or, alternatively, to let the physiological consequences of previous oxygen exposure dissipate?

 

[dsteding]

Free oxygen tensions in tissue are relatively low, because oxygen is metabolized rapidly.

Lynne-

What is the rate limiting step in metabolism of oxygen? I vaguely remember something about mitochondria and stuff, but I am a geochemist (err, lawyer) not a biologist.

It seems to me that oxygen use is related to energy demand, and that the body has an upper limit on how fast it can metabolize oxygen. This is a total guess, but I'm wondering if that upper limit isn't set somewhere slightly above the oxygen partial pressures the body is subjected to at 1 ATA. At high partial pressures, is it possible to overload tissues, namely overwhelm a cell's ability to metabolize oxygen, causing super-saturation?

Given that oxygen partial pressures approaching 2 cause siezures, maybe the body siezes before that happens . . .

Anyways, some musings. I find this stuff fascinating, even though I know little about it . . .

 

[rjack321]

OK, I think I understand...maybe. Is it the case then that oxygen toxicity is not due to the buildup of O2 bubbles, but instead of non-gaseous oxygen. In other words, is the purpose of the "oxygen clock" NOT to let O2 bubbles offgas, but to let the levels of oxygen in some other form approach basal values or, alternatively, to let the physiological consequences of previous oxygen exposure dissipate?

OTUs are limited to let the cellular damage caused by the O2 heal. The O2 itself is long gone.

Its very hard to get close to the OTU limits unless you're diving 5-10 times a day on high ppO2 for several days in a row.