Any reported cases of Ox Tox between 1.4 and 1.6?

[tursiops]

It seems like both of you are trying to say that the physiology outlined in these papers doesn’t exist?

I can’t find any newer studies saying that hey this effect we’ve measured in vivo doesn’t exist can you guys point them out.

It sounds like you are unwilling to accept anything that contradicts your beliefs.

 

[crofrog]

It sounds like you are unwilling to accept anything that contradicts your beliefs.

Not at all, nothing has been presented contrary to the research that outlines the physiology that was established back in the 1950s.

Just trying to be evidence based.

 

[PfcAJ]

This sounds like the opposite of what we are trying to do in decompression, which is to let dissolved gas exit the body.

“ Breathing oxygen at a deeper depth has the advantage of a greater hydrostatic pressure to hold dissolved gas in solution.”

I think you’re misinterpreting that sentence.

Getting on oxygen deeper gets you on the *most* efficient deco gas. There are no inerts. The deeper you are, the less chance of bubble formation.

If you could get on oxygen deeper than 20’, you would. But you can’t.

The deeper you are, the less bubble formation you have. Getting (and staying) on oxygen as deep as you can prevents as much bubble formation as possible. We know that even after you’ve “cleared” you decompression, bubble formation still happens during ascent.

But all of this is a balancing act. To work against CNS toxicity, it has been demonstrated that air breaks work. Moving up to 15 or 10 feet probably works too. To what degree, I certainly don’t know. In practice, I do the extreme bulk of my deco at 20’. YMMV.

 

[nadwidny]

Pretty much this. Which is why it shouldn't be referred to anymore. It belongs in the trash heap of misunderstandings of yesteryear along with stuff like acclimatizing to narcosis.

Old habits die hard. I still sometimes spike my O2 to 1.6 at 70' because that's what I did to take advantage of the "oxygen window" 20 years ago.

 

[inquis]

Old habits die hard. I still sometimes spike my O2 to 1.6 at 70' because that's what I did to take advantage of the "oxygen window" 20 years ago.

The reason at the time may have been "oxygen window", but I'd say lowering the inspired inert gas pressure is a better justification. Going from a PO2 of 1.2 to 1.6 at 70 ft adjusts the inspired inert gas pressure by a factor of about 0.8x, resulting in a higher off-gassing rate (CNS & pulmonary effects notwithstanding).

 

[nadwidny]

The reason at the time may have been "oxygen window", but I'd say lowering the inspired inert gas pressure is a better justification. Going from a PO2 of 1.2 to 1.6 at 70 ft adjusts the inspired inert gas pressure by a factor of about 0.8x, resulting in a higher off-gassing rate (CNS & pulmonary effects notwithstanding).

I suppose but unless I keep my setpoint at 1.6 for the entire deco, the advantage on my Shearwater is lost as soon as I go back to a setpoint of 1.3. Maybe it's a Shearwater thing algorithm thing, I don't know.

Going back to old school I let my PO2 degrade from the 1.6 at 70' back to 1.3 as i ascend. Just a habit. I was told on my Meg crossover to just let the unit handle things, but as I said, old habits die hard.

 

[justinthedeeps]

I think we can all agree with @PfcAJ

The effect of staying under higher pressure, while breathing in none of the gas(es) we want to eliminate, does sound like a worthwhile attempt to reduce or avoid bubble size/formation (particularly for the bubble model kind of folks.) And that's probably consistent with recompression/decompression chamber treatments.

If that's what we mean by 'oxygen window,' then I think the way it is described and often presented (e.g. giant bar charts emphasizing arterial ppO2 as a causative factor) causes confusion to the point of being misleading.

The high ppO2 seems more like an indirect side effect of staying deeper on a gas that doesn't contain nitrogen (or helium, for helium dives), rather than being magically better for its own sake.

So back to: do we need to use 100% at 6m/20ft to decompress helium from a heliox (no nitrogen) dive, or is 80% or 50% nitrox just as good for that, but with less CNS/OTU (lower ppO2)? Does the nitrogen in the nitrox prevent helium off-gassing? If so, are we sure that excess unmetabolized oxygen gas partial pressure in the 100% doesn't also do that?

 

[inquis]

So back to: do we need to use 100% at 6m/20ft to decompress helium from a heliox (no nitrogen) dive, or is 80% or 50% nitrox just as good for that, but with less CNS/OTU (lower ppO2)?

The rate of off-gassing of He from a heliox dive will be the same on O2, Nx80, or Nx50 at a given depth. The helium gradient is as large as it can be. However, getting on Nx before 6m will get you out of the water sooner.

And yes, I'd agree with you: there is zero need to invoke the oxygen window to understand any of this. It's not hard to see why that's fallen by the wayside.

 

[diverdoug1]

The accident report seems a little mangled or incomplete. It states, "Tanks were filled with 24/25 trimix". Was that based on a direct gas analysis or tank labels or dive computer gas setting? There have been other incidents where the gas blender made an error and the diver trusted the sticker without analyzing. Assuming the mix information is accurate then pO2 at maximum depth of 160ft would be 1.4atm, although the average depth and pO2 was presumably less.

The description of the victim's symptoms seems consistent with a seizure. But assuming it was a seizure, was it caused by CNS oxygen toxicity or something else? Other medical conditions and medications such as Wellbutrin (bupropion) can increase seizure risk. Do we have the victim's medical history?

Was a postmortem toxicology report ever produced for the victim? There is some evidence that certain medications such as the decongestant pseudoephedrine could increase the risk of CNS oxygen toxicity but this isn't well understood. I'm not trying to blame the victim here or minimize the risk of oxygen exposure but this incident raises more questions than answers.

As a meta comment it's frustrating to reach these incident reports. The data is usually so inconsistent and fragmentary that it leaves us unable to analyze what really happened. I don't mean this as a criticism of the recovery divers; they're usually volunteers just trying to help, often at great personal risk and expense. But as a community we lack the data to determine root causes or make any systematic safety improvements. Like I think I know now what practices are safe or unsafe but when challenged I can't really back it up with hard data or even reliable anecdotes.

At one time the reports were posted on the IUCRR website. The suspected cause of death was drowning secondary to oxygen toxicity induced seizure. The pO2 at time of seizure was less than 1.4 atm and greater than 1.2 atm. There was also a discussion regarding risk factors such as genetic predisposition and the female sex.

 

[Nick_Radov]

At one time the reports were posted on the IUCRR website. The suspected cause of death was drowning secondary to oxygen toxicity induced seizure. The pO2 at time of seizure was less than 1.4 atm and greater than 1.2 atm. There was also a discussion regarding risk factors such as genetic predisposition and the female sex.

Huh? I'm pretty sure we don't have any really reliable scientific data on genetics or sex as oxygen toxicity risk factors.