How conservative is the PPO2 Limit

[Foxfish]

Besides, she took Oxymetazoline regularly as well before
the dive, which might have affected her CNS:
Oxymetazoline - Wikipedia, the free encyclopedia

What was your reference for the drug she was taking. I saw somewhere she had used a nasal spray but cannot find the reference now.

Mixed feelings about screening of oxygen toxicity. What happens one day is not necessarily what happens on the next. It could give a false sense of security. In this case, a full face screen would have helped.

 

[tamas970]

"For medication, Elizabeth used birth control and used one spray of Afrin containing oxymetazoline in each nostril first thing in the morning of a dive day." - "The Diver, from the link you quoted.

Agreed, screening is not good for negative control (=if you feel fine in the chamber it doesn't mean anything for dives or once you dive to ppO2 2bar doesn't mean that 1.5 won't kill you the other day). However, if someone has repeated issues it's a good reason for additional precaution.

What was your reference for the drug she was taking. I saw somewhere she had used a nasal spray but cannot find the reference now.

Mixed feelings about screening of oxygen toxicity. What happens one day is not necessarily what happens on the next. It could give a false sense of security. In this case, a full face screen would have helped.

 

[SailNaked]

some of this seems to be measuring the mass of an elephant with a laser.
we can all agree that we are all physiologically some what different and since we do not know what is actually going on we have to make guesses based on the results.


one bad dive does not make a limit invalid, 100s of dives do not make one dive safe.
your body will do what it does and if you eat meat, or take asperine, or drive a foreign car. the dive you are doing now is different from the dive someone else did last week.

limits are guidelines, you pays your money and you takes your chances. we could be discussing if 60 ft for 60 minutes is safe or if 55 minutes is better, and it will not make any difference to anything because we still do not measure anything in our blood or bodies, we measure the average results of many dives.

 

[Foxfish]

some of this seems to be measuring the mass of an elephant with a laser.
we can all agree that we are all physiologically some what different and since we do not know what is actually going on we have to make guesses based on the results.


one bad dive does not make a limit invalid, 100s of dives do not make one dive safe.
your body will do what it does and if you eat meat, or take asperine, or drive a foreign car. the dive you are doing now is different from the dive someone else did last week.

limits are guidelines, you pays your money and you takes your chances. we could be discussing if 60 ft for 60 minutes is safe or if 55 minutes is better, and it will not make any difference to anything because we still do not measure anything in our blood or bodies, we measure the average results of many dives.

I don't think anyone is suggesting that one death at a given limit makes the limit invalid. But what about three potential fatalities due to divers losing consciousness in 1000 dives. Is that acceptable?

 

[SailNaked]

I would not say it was acceptable, I would say it happened. and we do not know why. I tend to go with the idea that if a theory has exceptions that it is not the correct theory. that only means that we do not know yet.

you take whatever risk you accept. If I tox at 1.7 on my next dive it will not mean that I was reckless or ignorant or wrong, it will mean that my body had a problem that day at that depth with that PPO2.
it is pointless to generalize from individual points.

there are shark attacks and yet we dive with sharks all the time. I hear it all the time, "You dive with sharks? I could never do that" well if a shark kills me some day I will say the same thing, it does not make swimming with sharks dangerous.

with all that said I would be interested in knowing what else was going on with the people that had problems within the limits?
like nose spray, cocaine use? liver issues? I dont know what all could be relavant, but the limits are safe even when they are not, the same way bicycle riding is safe except when you hit a tree without a helmet.

 

[Doppler]

I am dismayed once again to have read through a few dozen postings about oxygen toxicity and find no specific mention of time as it relates to oxygen limits.

A PO2 of 1.6 is no safer and no more dangerous than one of 1.4 or 1.2 bar without reference to time. I have no hesitation breathing a gas that delivers 1.6 bar PO2 for several minutes, but would baulk if asked to spend three hours at depth breathing one delivering 1.4 bar.

MOREOVER, when considering TIME as a factor in one's likelihood to tox while underwater, readers might be well-advised to revisit NOAA's DAILY-LIMIT CNS TABLES. I have two friends who are now dead thanks to episodes of CNS toxicity, and the evidence points to their exceeding 24-hour limits as the main culprit that triggered the cascade of events leading to them being dead.

Informed by this and a little experience doing deepish, longish dives, I AM VERY CONSERVATIVE WITH REGARDS 24-HOUR LIMITS... less so with some other, and in my opinion, lesser evils.

 

[Nirvana]

I am dismayed once again to have read through a few dozen postings about oxygen toxicity and find no specific mention of time as it relates to oxygen limits.

A PO2 of 1.6 is no safer and no more dangerous than one of 1.4 or 1.2 bar without reference to time. I have no hesitation breathing a gas that delivers 1.6 bar PO2 for several minutes, but would baulk if asked to spend three hours at depth breathing one delivering 1.4 bar.

MOREOVER, when considering TIME as a factor in once's likelihood to tox while underwater, readers might be well-advised to revisit NOAA's DAILY CNS TABLES. I have two friends who are now dead thanks to episodes of CNS toxicity, and the evidence points to their exceeding 24-hour limits as the culprit.

Steve,

On the topic of 24-hour exposure limits, there is a question that comes to my mind. When planning a dive with a software such as V-Planner, we get information on OTUs and CNS%. I have always assumed that this CNS% refers to single exposure CNS toxicity limits. The same happens with the dive computers I have seen. They provide information on OTUs and (as far as I know) single exposure limits percentages. So, if I understand it correctly, we must rely on tables to complement the information we get from planning software and dive computers to adequately monitor CNS oxygen toxicity, specifically the 24-hour limit.

Am I getting it right? If so, do you know why they do not incorporate 24-hour tracking into existing planning and monitoring tools?

Thank you for your help.


ps: I have read your "Six Skills" book and always check your blog. Very nice work. Please write more!


ps2: I think some people did not write sooner on time limits of O2 exposure because they assumed the topic had been reasonably well covered on this recent thread by the OP.

 

[Doppler]

Steve,

On the topic of 24-hour exposure limits, there is a question that comes to my mind. When planning a dive with a software such as V-Planner, we get information on OTUs and CNS%. I have always assumed that this CNS% refers to single exposure CNS toxicity limits. The same happens with the dive computers I have seen. They provide information on OTUs and (as far as I know) single exposure limits percentages. So, if I understand it correctly, we must rely on tables to complement the information we get from planning software and dive computers to adequately monitor CNS oxygen toxicity, specifically the 24-hour limit.

Am I getting it right? If so, do you know why then do not incorporate 24-hour tracking into existing planning and monitoring tools?

Thank you for your help.


ps: I have read your "Six Skills" book and always check your blog. Very nice work. Please write more!


ps2: I think some people did not write sooner on time limits of O2 exposure because they assumed the topic had been reasonably well covered on this recent thread by the OP.

V-planner does not track daily limits and I have brought the topic up with Ross. Also, I am unaware of any PDC that actually tracks 24-hour CNS limits. As you suggest, the tracking they display is for single-dive limits, and therefore the NOAA tables are pulled out when planning multiple dives.

THIS IS A MAJOR F-UP in my opinion... and I am talking to a manufacturer about it right now. Not sure it'll do any good but I teach students to be very wary of daily CNS limits. I am a wimp about them.

There's a surprisingly high ignorance level of these limits and I constantly read posts from people who mix-up this type of CNS toxicity up with OTUs and whole-body or pulmonary tox... which for most divers can be ignored. The oversight is an obvious problem.

There are a couple of things about the original work published by NOAA. One is the 150 minute daily limit for 1.6 bar and the suggestion that loading for at this level, may decay... the suggestion was a 90-minute half-life.

The second and what fascinates and disturbs me is that many divers... technical divers no less... ascribe similar behavior to CNS loading for all PO levels, and I would suggest this cannot be the case.

Much safer to simply track your exposure using the NOAA table... http://decodoppler.files.wordpress.com/2009/02/noaa-tables.jpg?w=235&h=300

1.6 = 45 mins. single / 150 mins. daily
1.5 = 120 mins. single /180 mins. daily
1.4 = 150 mins. single /180 mins. daily
1.3 = 180 mins. single / 210 mins. daily
1.2 = 210 mins. single / 240 mins. daily
1.1 = 240 mins. single / 270 mins. daily
1.0 = 300 mins. single / 300 mins. daily

and so on.

Thanks for your support! Appreciate it and glad you enjoyed the book... have another one due out in a couple of months. And I would write more in the blog but work get's in the way!

 

[Foxfish]

I am dismayed once again to have read through a few dozen postings about oxygen toxicity and find no specific mention of time as it relates to oxygen limits.

As pointed out by others, time limitation at MOD was recently discussed here: ScubaBoard - Scuba Diving Forum - Diving Social Network - Daily Limits for CNS Oxygen Toxicity.

The question I raised on that thread was along the lines that if hyperoxic exposure is time dependent, why don't divers simply monitor their oxygen time exposure on the computer and ignore MOD. If you look at the PPO2 versus time graph, it would seem a logical approach. As one diver suggested, the current approach taught on Nitrox courses is that if you exceed the MOD limit then it is as if your time exposure immediately becomes 100 %. That contradicts the PPO2 versus time graph. I'm not advocating this and I do respect the MOD limit, but I'd like more insight into why that is the case.

Anyhow, the focus of this thread has been to provide some empirical evidence to demonstrate that the PPO2 limits set by agencies is not overly conservative and should be respected.

---------- Post added September 12th, 2013 at 07:45 PM ----------

MOREOVER, when considering TIME as a factor in one's likelihood to tox while underwater, readers might be well-advised to revisit NOAA's DAILY-LIMIT CNS TABLES. I have two friends who are now dead thanks to episodes of CNS toxicity, and the evidence points to their exceeding 24-hour limits as the main culprit that triggered the cascade of events leading to them being dead.

Informed by this and a little experience doing deepish, longish dives, I AM VERY CONSERVATIVE WITH REGARDS 24-HOUR LIMITS... less so with some other, and in my opinion, lesser evils.

From what I can gather you do not consider the single and 24 hour time limits (45 minutes and 150 minutes respectively) without further qualification to be adequate for recreational diving, especially when doing multiple dives a day for a number of days which often occurs on a liveaboard.

There are a couple of things about the original work published by NOAA. One is the 150 minute daily limit for 1.6 bar and the suggestion that loading for at this level, may decay... the suggestion was a 90-minute half-life.

Do you have a reference for the 90 minute half life? Is is something you take into consideration in planning dives with Nitrox?

There's a surprisingly high ignorance level of these limits and I constantly read posts from people who mix-up this type of CNS toxicity up with OTUs and whole-body or pulmonary tox... which for most divers can be ignored. The oversight is an obvious problem.

My Suunto Cobra computer calculates exposure to both CNS and pulmonary toxicity, determines which is the governing scenario and displays the result as a percent ie. OTU's. I'd think that on a recreational dive monitoring this OTU while staying within the prescribed MOD/exposure time limits would be an adequate safeguard against oxygen toxicity. What do you think?

 

[mathauck0814]

As pointed out by others, time limitation at MOD was recently discussed here: ScubaBoard - Scuba Diving Forum - Diving Social Network - Daily Limits for CNS Oxygen Toxicity.

The question I raised on that thread was along the lines that if hyperoxic exposure is time dependent, why don't divers simply monitor their oxygen time exposure on the computer and ignore MOD. If you look at the PPO2 versus time graph, it would seem a logical approach. As one diver suggested, the current approach taught on Nitrox courses is that if you exceed the MOD limit then it is as if your time exposure immediately becomes 100 %. That contradicts the PPO2 versus time graph. I'm not advocating this and I do respect the MOD limit, but I'd like more insight into why that is the case.

Anyhow, the focus of this thread has been to provide some empirical evidence to demonstrate that the PPO2 limits set by agencies is not overly conservative and should be respected.

---------- Post added September 12th, 2013 at 07:45 PM ----------



From what I can gather you do not consider the single and 24 hour time limits (45 minutes and 150 minutes respectively) without further qualification to be adequate for recreational diving, especially when doing multiple dives a day for a number of days which often occurs on a liveaboard.



Do you have a reference for the 90 minute half life? Is is something you take into consideration in planning dives with Nitrox?



My Suunto Cobra computer calculates exposure to both CNS and pulmonary toxicity, determines which is the governing scenario and displays the result as a percent ie. OTU's. I'd think that on a recreational dive monitoring this OTU while staying within the prescribed MOD/exposure time limits would be an adequate safeguard against oxygen toxicity. What do you think?

Do you have any idea who Doppler is? Are you seriously questioning his understanding of the subject?