1/2 CNS % after 90 min. SI?

[Guy Alcala]

I do not know where GUE came up with their values. But exponential recovery seems to be the prevalent view in the literature.

This reference by Arieli seems to be what you are looking for: Eur J Appl Physiol Occup Physiol. 1997;75(2):182-7, “Recovery time constant in central nervous system O2 toxicity in the rat”, Arieli R, Gutterman A.

Thanks for that. I think this earlier study by the same author:

Modeling pulmonary and CNS O2 toxicity and estimation of parameters for humans -- Arieli et al. 92 (1): 248 -- Journal of Applied Physiology

may be even more what we're looking for as it includes data from humans, although If I'm understanding it correctly they calculate the 95% recovery time from CNS toxicity as only 39 minutes. Either I've got it wrong, or someone decided to apply a large measure of conservatism and decided on 90 minutes.

Guy

 

[limeyx]

may be even more what we're looking for as it includes data from humans, although If I'm understanding it correctly they calculate the 95% recovery time from CNS toxicity as only 39 minutes. Either I've got it wrong, or someone decided to apply a large measure of conservatism and decided on 90 minutes.

Guy

Well, then consider guys like the WKPP that regularly run several hundred percent or even in excess of 1000% of CNS with appropriate low PPO2 gas breaks (12 mins on O2 and 6 mins off, repeat for multiple hours)...

Generally I never give CNS a huge amount of concern in any of my dive planning, except taking appropriate low PPO2 gas breaks after spiking the PPO2 on decompression.

 

[lowviz]

Well, then consider guys like the WKPP that regularly run several hundred percent or even in excess of 1000% of CNS with appropriate low PPO2 gas breaks (12 mins on O2 and 6 mins off, repeat for multiple hours)...

Generally I never give CNS a huge amount of concern in any of my dive planning, except taking appropriate low PPO2 gas breaks after spiking the PPO2 on decompression.

You risk pulmonary irritation with cumulative OTU's in excess of the limit. Not much of a concern unless you are doing multiple day deco diving.

Your risk a CNS O2 hit (convlusions) with CNS over 1.6. I presume that you mean that you ignore the CNS oxygen clock rather than the CNS PPO2 limit?

 

[Mr Carcharodon]

Thanks for that. I think this earlier study by the same author: although If I'm understanding it correctly they calculate the 95% recovery time from CNS toxicity as only 39 minutes. Either I've got it wrong, or someone decided to apply a large measure of conservatism and decided on 90 minutes.

Guy

Well it appears there is not much in the way of data for human subjects -- while diving. So that is likely why the values are as conservative as they are. I guess it could be debated if current guidelines are excesively conservative, but that comes down to risk tolernace which is pretty subjective. Just like nitogen limits it makes you realize how fuzzy the line is.

Arieli does seem to be "the man" on ox tox. Lots of good stuff in J. Appl. Phys. by him on ox tox, and air breaks.

 

[PfcAJ]

You might get the answer you're looking for if you post at forums.gue.com

The people who wrote it will answer, most likely.

 

[limeyx]

You risk pulmonary irritation with cumulative OTU's in excess of the limit. Not much of a concern unless you are doing multiple day deco diving.

Your risk a CNS O2 hit (convlusions) with CNS over 1.6. I presume that you mean that you ignore the CNS oxygen clock rather than the CNS PPO2 limit?

For all but the biggest of expeditions, you (I) will never come even close to Pulmonary OTU limits. They are huge.

For CNS it's more that the techniques I use make it mostly a non-issue.
As you say, keep max PPO2 1.6 or less.
I Keep bottom PPO2 1.2 or less (even less for longer dives)

with "Air breaks" then the 45 mins at 1.6 PPO2 is not really a "limit" (this is NOT diving advice by the way

People doing big cave and deco dives can do multiple hours on O2 on a single dive (way past the 45 min limit) by taking regular breaks on a low PPO2 gas.

So now, think about the "Oxygen clock"

If doing a schedule of 12 mins on O2, 6 mins off, or 10 on 5 off means you can do multiple hours on Oxygen. What happens when you spend 60-90 mins on the surface breathing a low PPO2 gas (air) ?

Not advising, just posing some questions

 

[Couv]

It's been a while since I studied treatment tables, but if I recall correctly some of the tables have the patient at near FO2 of 3.0 for 20 min before an air break, then back on.

I'll have to look through some old diving manuals I've put away; the half time after x number of minutes SI rings a bell from commercial diving school.

 

[rjack321]

It's been a while since I studied treatment tables, but if I recall correctly some of the tables have the patient at near FO2 of 3.0 for 20 min before an air break, then back on.

Some of those folks do tox or get CNS symptoms though (was puking at min18 on O2 at 30ft). And its alot easier to tox in the water than in a dry chamber too.

Otherwise I essentially ignore 24hr CNS limits with backgas breaks like Limeyx. Not agonizing over the 90min surface half life goes along with this pragmatic approach.

 

[Jarrod]

A subject near and dear to my own heart;-). I have invested considerable energy into this subject and long considered it the most nebulous and challenging risk we face in very long exposures at depth. What we know (or don’t know) about CNS toxicity is startling. What we don’t know about CNS toxicity as it relates to a tracking, multi-exposure et al is even more breathtaking. The 90min is largely derived from work done by Bill Hamilton for the release of the Bridge Nitrox computer (if I remember right sometime in the late 1990s). I pressed Bill pretty hard a couple times because I thought “recovery” was much faster and because I was equally curious about numbers that just “appeared” and had been inaccessible to me despite significant research on the subject. My best assessment is they are a WAG based upon a conservative (probably quite conservative) assessment of what will insulate most people (the overwhelming majority) from CNS toxicity. Like the “clock” this is mostly useless for aggressive technical diving. But it is not hard for most divers to use (average tech dives stay close) and because it has some measure of industry support/precedent we felt it was necessary to introduce. Just like decompression tables which may or may not be derived from an algorithm that is really telling us what happens, precedent and/or frequency of use creates some measure of reliability. Because CNS toxicity is so hopelessly divergent with respect to risk (inter and intra dependency is massive) and because there are precious few studies (and ever fewer that are reliable) we (GUE, industry etc) had to start somewhere. Regrettably we are not likely to see any meaningful or useful studies on this subject so we had to build our own assessments and procedures to find the best compromise. The lack of knowledge on this subject is why GUE procedures are cautious with respect to oxygen exposure, breaks etc.
Best Wishes,
Jarrod

 

[Blackwood]

Thanks for your insight, Jarrod.