Recompression & O2 Toxicity

[vertaqua]

Were there any air breaks during the 40fsw surface deco?

ON Sur D O2 dives you have a certain ammount of decompression in the chamber depending on depth and time at that depth(these tables can be found in the USN dive manual). Every 30min you get an air break, I forget the length of each air break. But say the deco obligation is 45 min once you reach 40fsw you'll have a 30min O2 breathing period then an air break and then 15 min of O2 breathing. There are additional rules if your diver has a seizure in the chamber.

I work in clinical hyperbarics and the incidence of O2 tox is about 1 in 10,000 pt's, and a lot of times if we know the patient has a low threshold for seizures than the doc might prescribe anti convulsant medications suc as atavan.

 

[pipedope]

I am suprised that now one has mentioned CO2.
The higher your CO2 buildup the greater the risk of O2 Tox.

A diver in the water cannot be completely at rest but must do tasks that require muscle action increasing the CO2 and also increasing the risk of O2 Tox.

In the chamber you are sitting or lying down and at rest so there is likely to be very little chance of O2 buildup.

This is in addition to the other factors discussed.

 

[Saturation]

I am suprised that now one has mentioned CO2.
The higher your CO2 buildup the greater the risk of O2 Tox.

A diver in the water cannot be completely at rest but must do tasks that require muscle action increasing the CO2 and also increasing the risk of O2 Tox.

In the chamber you are sitting or lying down and at rest so there is likely to be very little chance of O2 buildup.

This is in addition to the other factors discussed.

You're quite right PD, C02 is very likely a co-factor that makes a differences between one who convulses and not, during exertion.

However, even at rest, once the P02 of 3.0 is exceeded, is just a matter of waiting for it to happen, usually anytime within 60min or so after exposure. It may not be just c02, but epinephrine and hormones associated with stress since the mechanism is that these drugs or C02 vasodilate cerebral arteries and increase the net volume of blood into the brain circulation. Deep air diving was possible because the high PN2 reduced the threshold for convulsions and added sedation induced by nitrogen narcosis. You were more likely to black out on deep air than convulse from 02, despite very high P02. Typical deep air divers dove to 300' [P02 = 2.1] and the records are in the 400-500' range [max P02 of 3.4].

http://www.forcefin.com/FF_PAGES/stores/hwfl03.htm

 

[Rick Murchison]

Interesting. The above would seem to indicate that a diver that accidentally switches to pure O2 at 70 feet (rather than 50% ) actually stands a pretty good chance of getting away with it ( for a modest amount of deco anyway) Not something I plan on ever finding out though.

Yes... a pretty good chance with a short exposure. But we won't hear about that one will we? We will hear about the one who toxes and dies. And if you look at the variability chart you see that a tox event in as little as five minutes is possible - not a bet I'd want to make.
Rick

 

[Rick Murchison]

I am suprised that now one has mentioned CO2.

Good point. I second you and Saturation.
Rick

 

[beezwax]

You're quite right PD, C02 is very likely a co-factor that makes a differences between one who convulses and not, during exertion.

Could someone explain the relationship between CO2 toxicity and O2 toxicity to me? I know that it's rising CO2 in the body that stimulates us to breathe, and that the body maintains a relatively stable CO2 level, even during exertion, by simply increasing the breathing rate. I also know that when CO2 does rise to toxic levels it can itself lead to unconsciousness and death, and that this buildup generally happens to divers in one of two ways: either an equipment issue (scrubber fails in a rebreather, or too much dead space in a helmet or ffm) or when, as we are discussing here, a diver is breathing elevated PO2 mixes so does not experience any oxygen-deprivation distress and therefore fails to ventilate adequately. That could lead to rising CO2 and perhaps CO2 tox, but how does that contribute to the likelyhood of O2 tox?

 

[H2Andy]

well... again, i'm on the edge of my knowledge here, but apparently
CO2 build up causes cerebral vasodilation (the blood vessels get bigger)
and leads to an increase in blood flow and thus more O2 being present in the bloodstream hitting the brain = higher chance of a CNS O2 tox hit

btw, CO2 build up in the blood is by far the greatest trigger of ox tox... tons
of tests in animals back this up

but why... well... nobody really knows

 

[beezwax]

well... again, i'm on the edge of my knowledge here, but apparently
CO2 build up causes cerebral vasodilation (the blood vessels get bigger)
and leads to an increase in blood flow and thus more O2 being present in the bloodstream hitting the brain = higher chance of a CNS O2 tox hit

btw, CO2 build up in the blood is by far the greatest trigger of ox tox... tons
of tests in animals back this up

but why... well... nobody really knows

so remembering to breathe deeply at deco would actually decrease the chances of O2 tox by decreasing the onset of CO2 trigger (?)

 

[Thalassamania]

In the old days, don't know about now, it was standard to have to pass an oxygen tollerance test at 60 fsw in a chamber. Very few folks failed it. oxygen was delivered through an overboard dump system (BIBS) and if a diver convulsed the tender just pulled the diver's mask off.

Oxygen treatments are much the same, delivered through a mask of a hood, either of which may be removed at the first sign of trouble.

BEEZWAX is correct. For those who'd like to stump thier instructors: Acute Oxygen Toxicity is know as the Paul Bert Effect, while Chronic Oxygen Toxicity is the Lorraine Smith Effect.

In any case, remember John Crea's words of wisdom, "Oxygen ... a potent hazard if poorly managed."

 

[H2Andy]

by acute you mean CNS toxicity? and i guess chronic is pulmunar oxygen
toxicity?


BEEZWAX: anything that can relieve CO2 build-up in your body is a good thing
to do against ox tox