Nitrox theoretical question

[Rhone Man]

I am going to tell a story which doesn't reflect especially well on me as a diver, but I think might be useful in the context of the theoretical question posed.

In January of last year, we were planning to dive a wreck at a max depth of approximately 180' (specifically, it was the San Francisco Maru in Truk Lagoon). The dive operator left everyone free to choose their own gases and plan their own dives, but his suggestion was that we plan for for a ppO2 of 1.6 ATA, as he asked everyone to keep their bottom time below 20 minutes. Accordingly, even on the NOAA tables, that left plenty of headroom. I followed the recommendation and asked for backgas of EAN24 (which gave a max ppO2 of about 1.55 ATA).

During the dive I accidentally went below 180' for a short period of time (maxed out at 189', so technically a ppO2 of around 1.62 ATA or so). No magic down current or evasion of sharks - just a healthy dose of narcosis and too much time enjoying the view (insert comment about wisdom of diving that deep with helium). When I realised my error I ascended and continued the dive. Most of the dive was at an everage depth of around 165-170'.

I ascended and made my gas switches to decompress on the way up, both at 1.6 ATA. Deco was uneventful, and then I surfaced. Total run time on the dive was around 45 minutes.

Now I know it is never a good idea to say: "my computer said it was safe, so it's alright", but after the dive it did occur to me that having breached 1.6 ATA and spent the majority of my deco near 1.6 ATA, what did my computer calculate my OLF percentage to be? 38%. Why so low? I am not expert, but I assume that like nitrogen loading, the periods when you are at lower partial pressures (so when I am swimming around at 165', or each stop except the first one after a gas switch) your oxygen loading is much lower.

There are various lessons one can try take from this, but the one I would mention in particular is that even when you are hovering at a ppO2 at or above 1.6 ATA, the increase of the OLF percentage is still a gradual process. You have time to sort things out.

What happens to the rate of increase of your OLF percentage if you find yourself at a ppO2 of (say) 2.0 ATA? No idea. Hoping never to find out.

 

[bleeb]

I have this vague memory that somewhere here on SB in the last few months, someone posted some link or recent research or something suggesting or showing the biochemical mechanism where CO2 makes ox tox much more likely (where I was under the impression that previously it the recommendation about exertion had been more an empirical thing). Anyone else here remember where that might have been or what the details were?

 

[Agility]

There are various lessons one can try take from this, but the one I would mention in particular is that even when you are hovering at a ppO2 at or above 1.6 ATA, the increase of the OLF percentage is still a gradual process. You have time to sort things out.

What happens to the rate of increase of your OLF percentage if you find yourself at a ppO2 of (say) 2.0 ATA? No idea. Hoping never to find out.

Oxygen toxicity comes in different flavors.
Oxygen toxicity - Wikipedia, the free encyclopedia
The most dangerous is the central nervous system toxicity
CNS oxygen toxicity.
It does not hit you the moment you cross 1.6 ATA ppO2 but a day to day individual variability in tolerance to hyperoxic seizures has been documented.
IMHO a OLF-percentage in the log does not represent the danger of convulsions in the logged dive.

I have this vague memory that somewhere here on SB in the last few months, someone posted some link or recent research or something suggesting or showing the biochemical mechanism where CO2 makes ox tox much more likely (where I was under the impression that previously it the recommendation about exertion had been more an empirical thing). Anyone else here remember where that might have been or what the details were?

quoted from CNS oxygen toxicity. :
"The exposure to hyperbaric oxygen in a wet environment increases sensitivity to CNS oxygen toxicity compared to exposure in a dry hyperbaric chamber (1, 16). Elevated concentrations of carbon dioxide (1, 14, 16, 22-25) and physical activity (exercise) dramatically decrease the duration of latent period for hyperoxia-induced seizures (1, 16, 23). The latent period for the appearance of electrical discharges in the EEG is significantly shorter in darkness than in light (26), suggesting the importance of visual input in the modulation of sensitivity to CNS oxygen toxicity. The risk for CNS oxygen toxicity is not determined solely by the partial pressure of the inspired oxygen, and even relatively low partial pressures of inert gases may contribute to hyperbaric-induced seizures (27). The increased sensitivity caused by inert gases could be explained by the involvement of free radical production (28). Circadian rhythm (29), various drugs, age (30), sex (31), interspecies differences and individual day to day variability (16) may contribute to the sensitivity to CNS oxygen toxicity (14)."

 

[soltari675]

Interesting responses. Thanks a bunch guys. And no, myself I don't get that close. I stay a healthy distance away from my max depth on nitrox. Call be paranoid, but I feel better that way. It was more just a curiosity of mine.

 

[Doppler]

As nitrox divers we have to understand that CNS loading is a function of oxygen partial pressure and TIME (and several other factors including workload and CO2 retention).

To believe that 1.6 is more risky than 1.4 is to misunderstand the issue.

For example, which is more likely to cause a CNS episode: breathing a gas delivering a PPO2 of 1.6 bar for one minute or one delivering a PPO2 of 1.4 bar for 150 minutes?

 

[markmantei]

Id stick with the EAN mixture but im going to explain the problems that i think that could arise by switching over to the air tank.

OK here are the issues that i see with making the switch from the nitrox mixture to the air mixture.

1. If you are using a computer unless you have a computer that can switch gases it will screw up the nitrogen loading if unable to tell the computer that you are now diving air. This can cause issues as what the computer thinks you are at and where you are at are two different spots.

2. If table diving did you calculate the switch into your dive plan.

Now I know this is a hypothetical question but im gonna nit pick a little bit. If when making your dive plan why did you not choose a lower mixture say of 32, 33, 34, 35 percent of oxygen, this way you do not have to worry about coming close to your MOD.

Next question, why if you know your MOD are you venturing as close to it as possible if you are worried about an o2 hit.

These are just the hypotheticals that i throw out there.

If its a choice between potential DCS on surfacing or potential convulsions at depth, I'd take my chances with the DCS, not to mention if your aware your computer is not tracking nitrogen loading correctly, it should be fairly easy to complete the dive conservativly (slow your ascent/lengthen stops) or end the dive early.

All of this does not discount the previous comments that a short stay below your MOD *probably* wont result in seizures.