Nitrox - 1.40 or 1.60 PO2?

[Papa_Bear]

A link would be nice! I am not aware of any documented hits below 1.6 unless great amount of time or other factors including health of the diver..... It would be interesting to know all the facts before passing any judgment on the story!

 

[Tomeck]

At the hospital, I breathed 100% O2 during 12 hours, CNS was 237%.

 

[Thalassamania]

A link would be nice! I am not aware of any documented hits below 1.6 unless great amount of time or other factors including health of the diver..... It would be interesting to know all the facts before passing any judgment on the story!

I think Gene already answered your question and clarified your mistake, the references that he provided (in case you missed them) were:

Dick Vann's presentation. There were 5 cases of symptoms and/or fatalities 1.6 or below reviewed (2 of these at 1.4, 1 at 1.3, ). Drugs may have been a factor in some but we know a significant potion of the recreational diving community does take some type of medication. (RRR ID: 7684)

Leitch, DR. A study of unusual incidents in a well-documented series of dives. Aviat Space Environ Med. 1981 Oct;52(10):618-24. PubMed ID: 7295250

Arieli R, Arieli Y, Daskalovic Y, Eynan M, Abramovich A. CNS oxygen toxicity in closed-circuit diving: signs and symptoms before loss of consciousness. Aviat Space Environ Med. 2006 Nov;77(11):1153-7. PubMed ID: 17086769

 

[Anti-Hero]

There is really nothing to judge in this case. The woman tox'd at 1.4 PO2, that is a fact. Why is the question.

The Deco stop thread: The Deco Stop

It was initially thought to have been exacerbated by the use of Afrin nasal spray, but I think that's been sufficiently refuted in this case. Post #72 also discusses decreased tolerance to 02 during hyperbaric treatments.

Dan article discussing 1.3 ppO2 being the threshold for 02 Toxicity.
DAN Divers Alert Network : OXTOX: If You Dive Nitrox You Should Know About OXTOX

Article discussing C02 as another factor for lowering the tox threshold.
PCO2 threshold for CNS oxygen toxicity in rats in the low range of hyperbaric PO2 -- Arieli et al. 91 (4): 1582 -- Journal of Applied Physiology

I've read other anecdotal reports from other sources as well, but I cannot find any references at the moment. I'll post them if I do come across them.

 

[Thalassamania]

Aviat Space Environ Med. 2006 May;77(5):526-32.Links
CNS toxicity in closed-circuit oxygen diving: symptoms reported from 2527 dives.

Arieli R, Shochat T, Adir Y.
Israel Naval Medical Institute, IDF Medical Corps, Haifa, Israel. rarieli@netvision.net.il
INTRODUCTION: Oxygen toxicity is a problem in diving and can have fatal consequences in the water. Various aspects of oxygen diving have been studied in dry hyperbaric chambers, but there is a lack of information on in-water diving using closed-circuit oxygen apparatus. METHOD: We collected 2527 dive reports from 473 closed-circuit oxygen divers (a mean of 5.2 reports per diver), and analyzed the relationships between various symptoms and their dependence on depth and diving time. RESULTS: No CNS oxygen toxicity-related symptoms were reported at a depth of 2 m seawater (msw), but their proportion increased at depths from 3 to 6 msw. We found that CNS oxygen toxicity-related symptoms appeared in 2.5% of dives conducted at a Po2 of 119 kPa. The main symptoms and signs reported were headache: 4.5%; nausea: 2.6%; hyperventilation: 2.6%; heavy breathing: 2.4%; dizziness: 1.6%; hiccups: 1.5%; bloody sputum: 1.4%; cold shivering: 1.1%; tinnitus: 0.9%; difficulty maintaining a steady depth: 0.9%; disorientation: 0.6%; tiredness: 0.5%; tingling in the limbs: 0.4%; hearing disturbances: 0.4%; a choking sensation: 0.4%; extreme effort: 0.4%; and loss of consciousness: 0.3%. DISCUSSION: Environmental factors, light vs. dark and temperature, had no effect on symptoms. The number of symptoms increased with diving time. Divers who experienced amnesia, facial twitching, hearing disturbances (p < 0.001), and disorientation (p < 0.014) were prone to suffer loss of consciousness. It was found that some divers are more sensitive to oxygen than others (p < 0.0001).

 

[Gene_Hobbs]

I think Gene already answered your question and clarified your mistake, the references that he provided (in case you missed them) were:

I am not really sure I did answer it given all the qualifications listed. And there is no real answer to the quest... Just more fun physiology... But it has been a great discussion to follow. Thanks to all!

Aviat Space Environ Med. 2006 May;77(5):526-32.Links
CNS toxicity in closed-circuit oxygen diving: symptoms reported from 2527 dives.

Full text for Aviation, Space, and Environmental Medicine articles from 2003 on are available online.

Anti-Hero, I have seen no real evidence of a decrease for CNS nor a mechanism that would make since for that. Donald showed the differences in day to day better than anyone has or will be able to. Some of those variations were hours different. I would be willing to put my guess there.

 

[Bismark]

The article referring to the relationship between higher PC02's and P02's is particularly interesting. In a recent discussion in a technical diving course, the point was made that we don't pay enough attention to the problems associated with C02 build up and concentrate on high P02's etc. The relationship between C02, narcosis and lowering the CNS threshhold should make any technical diver pay attention. A nasty combination that is often overlooked but very relevant and part of the reason agencies advocate a lower P02 for technical diving. Very interesting. This one goes in the files.

 

[Gene_Hobbs]

The article referring to the relationship between higher PC02's and P02's is particularly interesting.

A good article that reviews the history pretty well is Mike Natoli's Master's Thesis:

Natoli, M; Vann, RD. (1992) Factors Affecting CNS Oxygen Toxicity in Humans. MS Thesis Duke University. RRR ID: 21

 

[Anti-Hero]

The article referring to the relationship between higher PC02's and P02's is particularly interesting. In a recent discussion in a technical diving course, the point was made that we don't pay enough attention to the problems associated with C02 build up and concentrate on high P02's etc. The relationship between C02, narcosis and lowering the CNS threshhold should make any technical diver pay attention. A nasty combination that is often overlooked but very relevant and part of the reason agencies advocate a lower P02 for technical diving. Very interesting. This one goes in the files.

I have also heard, and it's only heresy at this point; that the narcotic properties of N2, act as a seizure suppressant. By removing N2 with larger fractions of He, we're increasing the susceptibility to high P02 seizures.

I don't have any data to back that up, but I've seen it discussed as a "maybe."

 

[garyfotodiver]

I disagree with your approach. You are making a black and white distinction between technical diving and recreational diving in respect to CNS limits where such a distinction does not exist. You have also chosen to simply repeat what one diving manual suggests which does nothing more than tell us what SSI's position on the matter is. This stuff is not rocket science and suggesting that recreational divers do not need to understand the issue further that what is suggested in your manual is doing a disservice to divers. If you are smart enough to pass your open water exam you can understand this topic much more than just "trust me on the limits" as presented by SSI, or many other agencies for that matter. I believe that is important that we treat divers as intelligent thinking individuals and do no "dumb down" topics such as this. (With the exception of Nereas of course......) Telling someone just to keep their PP02 within the 1.1 to 1.4 limits is a "trust me" approach and not one to which I subscribe in my learning and teaching.

My approach is consistent and conservative; it has served me well for almost forty (40) years of diving.

I do not suggest that I will not learn and understand more about tis or any other subject. However, in my first post (which apparently you did not read) I stated that I am making my first two dives tomorrow using EAN and NOT following air tables or computer set to 21% O2.

We are taught to dive within our limits and this is what I am doing. When I gain an appropriate amount of experience diving EAN, I will be able to set new limits. If you teach new divers other than to dive within their limits, it is you who are doing the disservice to divers. I would not let you teach my children to dive.

And yes, I am a recreational diver and very PROUD of it!