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AD_ward9 · 5th May 2006, 12:02

Quote: (Originally Posted by Drmike)

Hi Alex,

What validation has ever been done on these CNS limits?

I ask because it seems to me that many divers exceed them regularly and some divers (myself included) routinely do dives with huge CNS% with no ill effect.
I know its hardly subjective but when you have divers doing dives with huge% CNS it does make a bit of a mockery

There are a lot of papers measuring CNS on humans, rats and pigs. There is an accepted ratio between incidence in rats and humans, as there is also for DCS (it is about 4:1 if I remember, rats being 4 times more tolerant). These give the incidence of O2 tox: the 75% level causing O2 tox symptoms in 1% of the study group. Unfortunately this gives reasonable data on risk with high CNS loadings, but not how the curve reduces with lower loadings, because the sample group is small in each study.

There is concensus that there is a wide range in susceptibility. There is only very weak statistical correlation between specific individuals in a study group and O2 tox susceptibility. This means you can get away with it one day, and be hit the next.

I would point to the fact that 6 incidents of apparent CNS convulsions on this forum would indicate a risk factor of between 1 in 10,000 and 1 in 100,000 hours. Most divers do not run CNS loadings above 50% on many dives, so anecdotal evidence you mention needs to be treated very carefully indeed.

There are quite a few papers looking at the risk of He or CO2 in O2 tox, and these seem to indicate a down rating is needed of around 0.15 in PPO2 levels. Moving the risk from 1% of a group at 75% to 1 in 10^-9 at 100% requires a further reduction in PPO2. The most optimistic figure I get to, is 0.1: this is simply because the different studies give such a spread. Add the two together and we get 0.25 as the PPO2 downrating. This issue is dealt with quite nicely by Genesis above in principle, but the risk does seem to reduce sharply with reducing PPO2. Once the risk is below the 1 in a billion hour figure, it can be ignored for practical purposes.

There is a lot of experience in commercial saturation diving, where a PPO2 of 0.7 is used, and a lot of research has been carried out on behalf of different companies to arrive at this figure.

Cheers,

Alex

5th May 2006, 12:02	#14 (permalink)
AD_ward9 So much more to learn Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jun 2005 Location: Scotland Posts: 1,862	Re: CNS Calculation and Verification Model Quote: (Originally Posted by Drmike) Hi Alex, What validation has ever been done on these CNS limits? I ask because it seems to me that many divers exceed them regularly and some divers (myself included) routinely do dives with huge CNS% with no ill effect. I know its hardly subjective but when you have divers doing dives with huge% CNS it does make a bit of a mockery There are a lot of papers measuring CNS on humans, rats and pigs. There is an accepted ratio between incidence in rats and humans, as there is also for DCS (it is about 4:1 if I remember, rats being 4 times more tolerant). These give the incidence of O2 tox: the 75% level causing O2 tox symptoms in 1% of the study group. Unfortunately this gives reasonable data on risk with high CNS loadings, but not how the curve reduces with lower loadings, because the sample group is small in each study. There is concensus that there is a wide range in susceptibility. There is only very weak statistical correlation between specific individuals in a study group and O2 tox susceptibility. This means you can get away with it one day, and be hit the next. I would point to the fact that 6 incidents of apparent CNS convulsions on this forum would indicate a risk factor of between 1 in 10,000 and 1 in 100,000 hours. Most divers do not run CNS loadings above 50% on many dives, so anecdotal evidence you mention needs to be treated very carefully indeed. There are quite a few papers looking at the risk of He or CO2 in O2 tox, and these seem to indicate a down rating is needed of around 0.15 in PPO2 levels. Moving the risk from 1% of a group at 75% to 1 in 10^-9 at 100% requires a further reduction in PPO2. The most optimistic figure I get to, is 0.1: this is simply because the different studies give such a spread. Add the two together and we get 0.25 as the PPO2 downrating. This issue is dealt with quite nicely by Genesis above in principle, but the risk does seem to reduce sharply with reducing PPO2. Once the risk is below the 1 in a billion hour figure, it can be ignored for practical purposes. There is a lot of experience in commercial saturation diving, where a PPO2 of 0.7 is used, and a lot of research has been carried out on behalf of different companies to arrive at this figure. Cheers, Alex Last edited by AD_ward9 : 5th May 2006 at 12:06.
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