Oxygen toxicity question

[Foxfish]

The reason you don't remember pulmonary oxygen toxicity in your basic nitrox course is because it probably was barely mentioned--if at all.

When I took nitrox back in the day, it was a big deal, and I had to do a lot of problems tracking it. What I learned from those problems was that I would have to do some really exceptional diving to make it an issue.

Today's courses don't make a big deal of it because of that. If you are going to do the kind of diving that will make that come into play, which is incredibly unlikely, you will also probably have taken the more advanced courses that will explain it to you.

Same in the SDI manual. Just a paragraph saying you don't need to worry about it. Still think that a little more explanation wouldn't go astray. Especially given the confusion that exists over the various NOAA limits.

 

[Doppler]

For some reason I figured you must have taken it out of a book but it seems quite strange that they would make such quotes when people assuredly have got CNS O2 Tox from lesser exposure.

Be very interested for you to cite the instances of this, since I served on the training board of a major tech agency, have researched NOAA's original data on the topic of CNS toxicity, had conversations with both Dick Rutkowski and the late Bill Hamilton about various aspects of oxygen toxicity, have written manuals on nitrox, edited manuals on nitrox, and written numerous articles on the topic, and I cannot find anywhere in the literature documentation that supports the assertion that lesser exposures than those contained in NOAA's work have resulted in tonic-clonic episodes.

In all so-called "undeserved" incidents of suspected CNS, of which I am aware at any rate, exceeding the 24-hour limits seemed to be the culprit: and that limit is certainly identified in NOAA's work.

 

[Lorenzoid]

For some reason I figured you must have taken it out of a book but it seems quite strange that they would make such quotes when people assuredly have got CNS O2 Tox from lesser exposure.

Maybe the author whom Teller is quoting got his abbreviations confused and really meant "e.g." or "for example" instead of "i.e." or "that is." Adjusting the quote this way might reflect the reality of uncertainty better:

"CNS O2 TOX equals high exposure to O2 over short time, e.g., single exposure PPO2 1.6 for over 45 minutes or a single exposure to PPO2 1.4 for over 150 minutes."

 

[DCBC]

Outside hyperbaric treatment for DCS, CNS oxygen toxicity is the main form seen in diving. Pulmonary oxygen toxicity is not of concern except in very long technical dives lasting over 6 hours.

 

[Doppler]

Maybe the author whom Teller is quoting got his abbreviations confused and really meant "e.g." or "for example" instead of "i.e." or "that is." Adjusting the quote this way might reflect the reality of uncertainty better:

"CNS O2 TOX equals high exposure to O2 over short time, e.g., single exposure PPO2 1.6 for over 45 minutes or a single exposure to PPO2 1.4 for over 150 minutes."

Maybe, but I am particularly interested to understand what Crass means by "exposure."

He/She seems to be working under the incorrect assumption that CNS loading at 1.6 bar is somehow different to loading at 1.4 bar, or 1.3, 1.2 et al. Of course it is not. If one has "consumed" x-percent of the allowable time at 1.6, the CNS loading is precisely the same as x-percent of the allowable time at 1.4. Of course the actual number of minutes to reach that percentage will be different -- for example 50 percent of 45 minutes is not the same as 50 percent of 150 minutes -- but a 50 percent CNS loading is 50 percent CNS loading.

There seems to be a general misunderstanding of the work (and scrupulously analyzed data) behind NOAA's tables and how to interpret them. Oxygen dose is a function of oxygen partial pressure and time, and not the partial pressure alone.

 

[Teller]

No one was critical about it because its straight from what NOAA recommends and seeing how this isnt a Nitrox class I didnt feel the need to go in depth about symptoms.

For some reason I figured you must have taken it out of a book but it seems quite strange that they would make such quotes when people assuredly have got CNS O2 Tox from lesser exposure.

Many things in diving are based on theory. Clearly 45 minutes at 1.6 is not an exact science but in my experience most instructors go by what NOAA recommends which is what I posted earlier.

 

[Subcooled]

Oxygen dose is a function of oxygen partial pressure and time, and not the partial pressure alone.

What effect does exertion have? It affects breathing rates. The amount of dissolved oxygen in the plasma may be ~independent of this but I wonder what increased CO2 does to oxygen tolerance.

 

[Crass3000]

Maybe, but I am particularly interested to understand what Crass means by "exposure."

I was just trying to say that at 1.4 I assume one could tox in less than 150 minutes and that at 1.6 people could tox at less than 45 minutes. My point was that while there may be typical exposure times at certain PO2 levels that are generally safe, others who have thought so are sure to have got convulsions in less time at said levels.

How conservative are those times for their respective PO2 levels? I don't doubt you as you seem to be respected as one of the smartest technical trainers on the board. The levels do seem familiar. I was reading the TDI Advanced Nitrox book and seem to remember a table that those values were cited.

 

[Doppler]

What effect does exertion have? It affects breathing rates. The amount of dissolved oxygen in the plasma may be ~independent of this but I wonder what increased CO2 does to oxygen tolerance.

The only research work I have been made aware of is a study in rats. An engineer at one of the larger CCR manufacturers mentioned it to me at DEMA last year suggesting that ALL CCR divers work at effectively managing CO2 build-up as part of their strategy to lessen the potential of CNS poisoning. I also read a very well-done article by Petar Denoble for DAN where he stated that increased carbon dioxide partial pressure shortened CNS latency.

The bottom line is that CNS oxygen toxicity is a physiological manifestation, and biology is about Latin and not Maths so all bets are off. Be conservative! LOL.

My issue with the posting by Crass was that it made some vague reference to something that, as far as I know, is pure BS... classroom speculation. He/She seemed to to not have a clear grasp of how oxygen exposure is measured and also seemed to be suggesting that NOAA's tables are not reliable (read conservative perhaps) and their use has resulted in several CNS episodes. This is simply not the case. NOAA knew their ****. Well at least, Rutkowski, Hamilton et al, did.

Anyhow, back to the unanswered portion of your question. I believe that under dive conditions (hyperoxia) levels of O2 in blood plasma would be independent of respiration rate. Once again, I'm more aware of studies done on hypoxic hypoxia in climbers than in divers, so perhaps someone to whom physiology is less of a mystery than is it to me can give us an authoritative answer....

---------- Post added September 20th, 2013 at 01:51 PM ----------

I was just trying to say that at 1.4 I assume one could tox in less than 150 minutes and that at 1.6 people could tox at less than 45 minutes... How conservative are those times for their respective PO2 levels?

Perhaps I misunderstood... but it's important to understand completely what oxygen exposure is. It seemed you thought 1.6 was "worse" somehow than lesser ppO2s. It is not.

As mentioned previously, anything to do with biology and physiology is more alchemy than science because there are simply too many variables in the equation. However, that said, NOAA's work (and everything in TDI's manuals stands on the shoulders of NOAA's original research), is pretty solid and can be taken to the bank.

As with ANY limit, approaching it is always dangerous... an F1 driver may get away with brushing the wall on an out-turn a dozen times before the wall on his rear tire dissolves. The way that TDI approaches it is to suggest that divers plan only to consume 80 percent of the allowable single-dive time.

But I am still interested in the examples you may have of people staying within NOAA oxygen exposure limits and having challenges with CNS. I simply do not know of any and, perhaps more to the point, have not heard of any "failure to warn" law suits based on NOAA's work.