PO2 Max - Why 1.4/1.6?

[CWK]

NOAA does not publish limits above 1.6 where the single dive limit is 45 mins. According to Shearwater, at 1.65, it is 1% every 4 seconds, which works out to 100% in 6.66 minutes. See

Shearwater and the CNS Oxygen Clock - Shearwater Research

 

[CuriousRambler]

Not a direct answer to my original question, but here's an interesting Alert Diver article on oxtox: Alert Diver. Some discussion of erectile dysfunction meds lowering tolerance and increasing risk, and some suggesting psuedoephedrin may also increase risk at high doses.

 

[hroark2112]

Chamber dives >2.0 may cause you to suffer from oxygen toxicity, but convulsions are not fatal in a chamber like they would be underwater.

 

[NothingClever]

Cheetah,

Not a direct answer to your original question of how the dive medicine community arrived at determining the 1.4 benchmark but portions of Chapter I, History of Diving, and Chapter 3, Underwater Physiology and Diving Disorders, of the US Navy Diving Manual (Revision 7, 2016) may be of interest on a long flight.

 

[NAM001]

I would have to look but there is a formula used to calculate either otu or cns. I think it is otu but in general the problem gets more severe as PPO2 increases. it is linear pretty much until you get to about 1.65 then it skyrockets The effects are by 2:1 most will start feeling the life threatenting effects to mobility and othe rbody functions. by 3 most will be knocking at deaths door and 4 you are dead., I will try to find that formula and make it available. The only reason I remember the formula is because it has some goofy exponent on a value in it. Illsee if I can find it post it and you can use it i n Excel adn make a plot of the values to see the curve.

I think this i it

Pulmonary Oxygen Toxicity Calculations Constant depth profile: For a constant depth profile (in which the PO2 remains constant), the cumulative pulmonary toxic dose (CPTD), expressed as oxygen toxicity units (OTU), is calculated by Equation 1: (Eq. 1) where tx is the time of exposure, PO2 is constant, and is an exponent 0.8333. -5/6 power is the .8333



OTU = tx * (.05 / (PO2-.5) to the -5/6 power



here is a link that may help hope it is the right one..

https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf

 

[jeff h]

Historically, I believe it was the US Navy that originally came up with the limits, but I'm sure there was other research going on throughout the world. If you can find an old US Navy Dive Manual or look through some history books you can find more of the background. If I recall the US Navy determined that very physically fit Navy divers could tolerate exposures up to PPO2 of 2.0 for short duration of time during military operations. These limits were then accepted by commercial divers along with recommendations for OTU limits. Later Dr. Bill Hamilton and Dick Rutkowski developed the first "recreational" nitrox limits and made them more conservative. This might have been in combination with NOAA at the time?? Here's a descent write up on the topic of setting the limits: http://www.swiss-cave-diving.ch/PDF-dateien/Oxygen-Hamilton.pdf

There's lots of good books that cover some of the history of this too. "The Terrible Hours" about the Sqaulus submarine rescue that took place back in the 1930's is good read. It talks about how US Navy Dive Officer Charles "Swede" Momsen came up with some of the limits for deep diving and mix gas to rescue the trapped crew members. I don't recall if covers O2 specifically. There's also lots of good books written about Dr. Hamilton and Rutkowski and early days of tech diving. I think the old NAUI and IANTD manuals covered some of this, but maybe it is glossed over in more the modern nitrox training.

In the end, this is still similar to asking someone how many beers they can handle. Everyone is different and their physiology changes from day to day. Unfortunately, you can't test everyone for their individual susceptibility or account for the environmental stresses they will experience on each unique dive. So the best that is currently available is to recommend conservatism since O2 toxicity and convulsions are very often fatal under water. When I started cave diving it was very common to run 1.4 PPO2 as standard bottom mix. Unfortunately, good people have been lost using the limits and now it is much more accepted to run a 1.2 PPO2. I strongly recommend using 1.2 or below for working or longer duration dives.

 

[Lorenzoid]

. . . I have no interest in debating the choice of these numbers, and I'm certainly not trying to suggest there's a better choice. Intellectually, I just want to know more. Were these numbers arrived at simply through mass trial and error, were they backed out of decompression algorithms, or are they based on existing physiological research independent of the rec/tech dive world? I can imagine plenty of scenarios where human oxygen toxicity limits might be of interest to other sectors like aerospace, medicine, commercial diving, etc. . . .

Whether it's studies by NOAA, the Navy, or just thousands of divers having over the decades that Nitrox has been in use related "what has worked for them," I don't think it would be incorrect to say that the PPO2 thresholds that are now widely accepted as standards by the dive training agencies for purposes of teaching how to use Nitrox are more or less the result of what you refer to as "mass trial and error" in the diving world. While oxygen toxicity is relevant in other fields, scuba diving involves different circumstances--mainly, a desire for more conservatism because of the risk of drowning if a seizure occurs--so the PPO2 thresholds that are deemed acceptable in other fields aren't all that meaningful to divers.

 

[ccrprospect]

Tec diving luminaries back during the dawn of tec diving in the 90s routinely flouted these limits. I recall reading a part of Bret Gilliam’s book Deep Diving where there was an implicit undercurrent of disdain towards the arbitrariness of the 1.4 /1.6 limits. The founder of TDI used to hold the record for deepest dive on air at 452 fsw/137 msw (PO2 of 3.0!). Similarly Rob Palmer, part of tec diving’s royalty back in the day, stated that a PO2 of 3.0 was acceptable. As recounted by scubaboard’s @John Bantin:

“As Rob repeated his deep dives throughout the latter part of the week, our exchanges in his cabin became more and more heated. When he expressed the mind-boggling opinion that a PO2 of 3 bar was safe, he did so in private”.

He paid his life for it.

Get it wrong and you're dead

 

[Dr Simon Mitchell]

Hello,

Sorry, I had not seen this. I know its an oldish thread Dick Vann's article in the DAN technical diving workshop proceedings (p 38) is an excellent resource in this regard. I suspect you will get all your answers there. That document seems very hard to find on line at the moment so I have uploaded it to this post.

Note that there is a paper published last year in which the authors describe a case of what they believed was cerebral oxygen toxicity in a diver breathing a PO2 of 1.0 but who was also very hypercapnic (high CO2) at the time.

Symptoms of central nervous system oxygen toxicity during 100% oxygen breathing at normobaric pressure with increasing inspired levels of carbon dioxide: a case report - PubMed

Simon M

 

[Angelo Farina]

Here in Italy training of new divers was done using CC pure-oxygen rebreathers since 1948 and until around 1980. These units were called ARO, and were developed during WW2 by Italian Submarine Incursors (COMSUBIN), which managed to use them in a number of successful attacks to UK vessels.
The problem of oxygen toxicity was deeply studied by military doctors in the forties, but these results were not made available to the public.
I was trained in 1975, so I was using this ARO equipment for several months (an OW course was 6-months long at the time). At that time the military information had just been released, so our diving manual did contain a "safety curve" table, defining the maximum exposure time at each ppO2, based on those medical studies.
This did allow to go down to 18m, but for just a few minutes.
My OW certification enables me explicitly to dive with an ARO down to 10m (where ppO2 is almost 2.0 bar) for 30 minutes, with a surface interval breathing air for no less than 15 minutes before diving again.
But there was a second point very relevant: when using an ARO in open water (not in the pool) the usage of a full-face mask was mandatory. This was very important, as in case of convulsions a full face mask avoids to loose the mouthpiece and to drawn. After a couple of minutes of convulsions, the diver comes back to control, and if he has not drowned he can usually ascend safely.
So these "safety limits" were not "safe enough" for avoiding convulsions at all, they were set for military operations with CC rebreathers, and allowing that under some circumstances the diver will be hit by CNS toxicity.
When the problem of oxygen toxicity was moved from CC pure-oxygen military-grade rebreathers to recreational OC Nitrox systems, it was necessary to avoid entirely the risk of convulsions (also because the usage of a full face mask is generally not practiced for recreational diving).
Hence the max ppO2 limits were lowered initially to 1.6 bar, and more recently to 1.4 bar.
I think that a 1.6 bar limit is reasonable for using a CC rebreather or a deco stage tank at small depth, where
the mixture has an high percentage of oxygen. And instead, when doing a deep dive around 40m, better to stay with a safer 1.4 bar limit, as there are other adverse factors which can exacerbate the problem, such as CO2 retention, narcosis, effect of high nitrogen pressure, etc.
Personally I avoid Nitrox when there is chance to extend the dive below 30m depth. In those cases I prefer to use plain air with a shorter dive time, or, even better, to plan for a deco dive (with the required equipment, training and logistic setup), which I consider generally safer than a dive executed "on the edge of the NDL", without being equipped for a deco dive.