Nitrox - 1.40 or 1.60 PO2?

[Bismark]

I did not know Bismark claims to be an instructor. I keep him muted to save bandwidth. So I saw it in your quote.

The best solution to anything upsetting is the mute feature, besides saving bandwidth as well.

In logic, everything is either fact or opinion.

Fact: there is water all over the place;

Opinion: it rained.

Unfortunately for us, your overzealous approach to saving bandwidth has not extended to Scubaboard. Just think how much faster you could get information from Google......

 

[Thalassamania]

Diving knowledge is not the sole repository of diving instructors. ...

Some of the most knowledgeable divers I know are not instructors, and some of the instructors I know are the least knowledgeable divers I've ever come across.

I really enjoy these types of discussions, thanks to everyone who is trying to contribute! I always learn something even if it because I am researching my own point of view!

The last post is interesting in what effect skip breathing might have with Co2 build up delaying toxicity? One of the reasons I like Nitrox is as an old time diver and a Photographer I tend to skip breath and don't get the associated head ache with the enriched air and I have always assumed it was the higher O2 levels that are washing out the Co2 evening all things out in my blood.....???? Nitrox is invaluable in cave and survey diving that I have done and it is the only way I can get in 5 or 6 dives a day with my camera!

Nitrox has absolutely no effect on the rate at which you produce CO₂. I have seen no data to support your concept of the slightly higher _ppO₂ washing out the produced CO₂.

 

[Papa_Bear]

Well, going on what has been said about retention of Co2 and the use of O2 does effect Co2..... So because nothing is readily available does not really mean there is no cause and effect! Possibly no meaningful or useful relationship, but I find that hard to believe that increased O2 doesn't effect the content of the waste gas and the physiology and the up take based on breathing rate and retention? My evidence is antidotel at best I understand that, but something is going on......

 

[Thalassamania]

Well, going on what has been said about retention of Co2 and the use of O2 does effect Co2..... So because nothing is readily available does not really mean there is no cause and effect! Possibly no meaningful or useful relationship, but I find that hard to believe that increased O2 doesn't effect the content of the waste gas and the physiology and the up take based on breathing rate and retention? My evidence is antidotel at best I understand that, but something is going on......

Something is always going on, and just because you, or I, find something hard to believe doesn't make it so. I've also heard it said that since oxygen is a vasoconstrictor, EAN results in increased CO2 retention and all of the concomitant issues. So which one is right? Or are they both flights of fancy?

When we state opinions, or our own observations, it is usually considered good form to soft pedal it a bit. Not state it as though it was absolute fact. If is quite possible that someone following your lead below could get themselves badly hurt.

The last post is interesting in what effect skip breathing might have with Co2 build up delaying toxicity? One of the reasons I like Nitrox is as an old time diver and a Photographer I tend to skip breath and don't get the associated head ache with the enriched air and I have always assumed it was the higher O2 levels that are washing out the Co2 evening all things out in my blood.....????

 

[vladimir]

My evidence is antidotel at best I understand that, but something is going on......

I guess your antidotal evidence counteracts the effects of science and education? Making it a lot like anecdotal evidence in that regard?

 

[Scuba-Stu]

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.

To be fair it is not fact. The cause of death was unknown. Oxtox was the educated guess based on the circumstances.

 

[Scuba-Stu]

Preface: I'm not picking on you but you, Papa_Bear, are talking in absolutes so it makes quoting you easier than some of the others... In fact I agree with your acceptable level of risk and reasons why. It amazes me how many miss the time component in this.



At least we all keep evaluating our position and checking all the available information. Oh wait, someone didn't download the Vann 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)



Maybe we should all do more reading/ watching to make sure we know what is actually in the literature.

More reading that includes large numbers of dives and symptoms below 1.6 ATA (reviewed by Vann in the presentation BTW):

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

For the record, the USN has a reason they use 1.3 as a rebreather set point...

There has to be better articles than these.

 

[Gene_Hobbs]

There has to be better articles than these.

Well, Donald's work is by far the best but I don't see many people having that one.

The Bennett and Elliott Chapter is the probably the best review of the topic available but experience tells me that most divers don't own that either (or most of the physiology threads on boards like this would not exist).

So, I pulled from the more recent reviews with large numbers... The references used there are also great for finding other reviews but there are not many studies available with a series as large as those.

I do like the wikipedia article for pointing out where literature can be found. So that's not a bad starting point either...


OFF TOPIC, well sort of...

Funding for diving research from the military has remained the same since the early 1980's. Most commercial (oil) groups have their questions answered so no new funding from there anymore either. I hate to think that we may know as much as we ever will on many of the diving medical topics we all care about.

The lack of researchers is also becoming a problem:

In 2002, The Office of Naval Research tasked the Undersea and Hyperbaric Medical Society (UHMS) with performing a comprehensive analysis of the Navy's research and development program in Undersea Medicine (1). The panel found that 60% of the young researchers in the field left within less than ten (10) years. Additionally, it was discovered that many of the senior scientists in this arena would retire in the near future (52% retiring in less than 10 years and 96% retiring in less than twenty years). Further, the Navy had not trained any investigators in the previous ten (10) years. This turnover in researchers and the loss of senior scientists potentially signifies a great loss in human knowledge that can be transitioned to new researchers.

Sad state of where diving medicine is...

 

[Scuba-Stu]

Sad state of where diving medicine is...

Indeed.

 

[Anti-Hero]

To be fair it is not fact. The cause of death was unknown. Oxtox was the educated guess based on the circumstances.

The cause was unknown? The woman had a seizure underwater and drowned. That is both fact, and quite easy to understand. The coroner determined oxygen toxicity as the cause of the seizure. ALL the divers that participated said it was at 1.4 pp02.

So the only "maybe" here is the cause of the seizure. And that is pretty much conclusive itself.