Ox Tox

PeteS

Oxygen Toxicity. Are the problems caused by breathing O2 caused by:-

A/ The amount of time that we breathe a given PPO2 for? - Time versus PPO2.

or

B/ The volume of O2 that the body processes? - Litres of O2 consumed by a diver.

So far in all my training, I have been given time based tables, but if B is true, then someone with a high O2 consumption rate should Ox tox before someone with a low rate of consumption. It may even be a combination of the two.

As they say in all the best exam papers, "discuss"......

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Pete S.

http://www.steggle.co.nz

Every silver lining has a cloud

nigelh

I archived this as interesting.

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nigelh

PeteS

Yes, very interesting. Thanks Nigel.

But it didn't really address the PPO2 versus Metabolised O2 question. Or did it and I just didn't read it right!

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Pete S.

http://www.steggle.co.nz

Every silver lining has a cloud

dteubner

The relationship between time to oxygen toxicity and PPO2 is not linear. If the average diver fits in 4 minutes at 6.0 ATA that doesn't mean that s/he will fit in 24 minutes at 1.0 ATA.

CNS oxygen toxicity is about the bodies ability to deal with toxic free radicals. The higher the PPO2 the more are produced. The longer the exposure to relatively high PPO2's the more likely the mechanisms are to be exhausted.

Dave T

jasondrake

I spoke to a diving anaesthetist about this once. Short answer is no-one knows. Ox-tox is a mystery to medical science, the mechanisms are barely understood. And it's obviously tricky to do research.
Rebreather divers are at the forefront of medical research.
Cool.

PeteS

I seem to recall that it is like an exponential relationship.

The point I am questioning is different divers having different metabolic rates consume O2 at different rates. Does this alter the timing of Ox Tox? As you said above, "the average diver". Does looking at actual O2 consumption alter the Ox Tox calculations to remove the "average" factors?

After all, if you just look at CNS, this is more remote from inspired PPO2 as the nervous system is only exposed to metabolised O2. Or is the blood PPO2 based upon inspired PPO2 and that is what is causing CNS symptoms?

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Pete S.

http://www.steggle.co.nz

Every silver lining has a cloud

dteubner

Almost certainly not. CNS oxygen toxicity is caused by the level of O2 to which the CNS is exposed - ie the arterial concentration of O2. This is essentially the same as the alveolar concentration of O2 which is essentially the same as the isnpired concentration. Only a small part of O2 is consumed by the CNS, the majority by muscles etc. The CNS consumption is essentailly constant and doesnt change with different total body O2 consumptions.

This is not true at all
Indeed.

CNS O2 toxicity is idiosyncratic, in that it will occur at different levels/times in different divers or the same diver on different days. The CNS O2 clock is based on the NOAA recommendations which have a 10 fold conservatism built in to actual observations of toxicity. What is very interesting is that, despite this, CNS O2 toxicity does occur in recreational divers.

Whislt I certainly don't agree that we understand nothing about it, there is a lot more to learn.

Dave T

ScubaDadMiami

Check out Donald, Oxygen and the Scuba Diver (The SPA Ltd 1992). Although some of the studies are old, they are still valid as far as the end result:

"The most striking finding was the enormous variation in oxygen tolerance in a group of human beings. . . . The tolerance of each subject was unpredicatable." (Page 29).

"It is apparent that an attempt to plot a curve giving times of saftey when breathing oxygen at various depths, for even a single diver, is quite impossible owing to his variation of tolerance from day to day." (Page 44).

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Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

UWSojourner

So basically,

you run more risk at a setpoint of 1.4 than 1.3 and

you run more risk at a setpoint of 1.3 than 1.2 and

you run more risk at a setpoint of 1.2 than 1.1 and

you run more risk at a setpoint of 1.1 than 1.0 etc.

Seems like we should just be asking, "Do I need a setpoint that high?" If not, lower it a bit and ask the same question.

nigelh

This is a downward ratchet and until you know there is a risk you shouldn't go that way.
If I thought like that I wouldn't have spent a lot of my life on racing motorbikes.
I would have stayed safely at home and had no fun.

The risk involved in running 1.3ppO2 is, to my mind, minimal. Perhaps, statistically, somebody will get a hit at that but I'm probably more at risk crossing the road and yet I'm still going to work today not hiding at home.
If I increase my unproductive time in the water, time I am dangling on a string not having fun, I am increasing the chance of all sorts of other nasty things happening to me and I suspect that balancing known risks against unknown ones the higher setpoints, well only 1.3, are statistically safer.

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nigelh