Nitrox Question

[diverdoug1]

I made this comment on another active posting

Oxygen induced seizures are sometimes induced (atmittedly rarely) at relatively low partial pressures of oxygen. The lowest I have personally seen is 1.5 and 1.55 (both in female patients), but last year we had a diver fatality in Florida, attributed to oxygen toxicity and her ppo2 was only 1.4. I would love to see all the agencies go to 1.4 as their max. ppo2

 

[TSandM]

DUIDiver, one of the things your Nitrox class should have covered is that oxygen toxicity is very unpredictable. If you look at the actual studies, the variability from person to person, or even with the same person on different days/dives is absolutely frightening. The reason that agencies have chosen 1.4 as an acceptable ppO2 is because very few cases of oxygen toxicity have been reported at that level -- but there have been some. We lost an experienced female cave diver a couple of years ago to seizures at 1.4. Nonetheless, it's rare enough for most agencies to choose 1.4 as a safe working ppO2.

If you dip to 1.6, is it going to hurt you? Nobody can tell you that. It's a probability thing, with data so bad that nobody can predict what that probability -- for YOU, on THAT day -- is going to be. So your question about whether it's "safe" to go to the deeper depth is unanswerable.

But the answer is easy. If you plan a dive to a depth that might be excessive for your mix, either do a different dive or use a different mix.

 

[diverdoug1]

The fatality I mentioned is the same that TSandM references, and I stand corrected. It was a couple of years ago. Don't want to give the impression that this has happened more times than it really has.

 

[PfcAJ]

Another thing to consider is gas density and co2 retention. Co2 can decrease a person's tolerance to hyberbaric oxygen, resulting in a convulsion.

At 130ft, the density of the gas you're breathing is getting pretty high, and efficient transfer of co2 can be compromised. Keep it shallow and you're good to go.

Personally, I'm a fan of the 1.2ish for the working/bottom portion of the dive, and no more than 1.6 for decompression.

 

[Guy Alcala]

The fatality I mentioned is the same that TSandM references, and I stand corrected. It was a couple of years ago. Don't want to give the impression that this has happened more times than it really has.

According to the account I read in Alert Diver, while their (3 person team) planned max. PO2 was just under 1.4 (bottom gas 24/26, MOD 164 ffw) IIRR, and their max. depth 156' for ~ 20 minutes, she toxed after they'd returned to where their deco gas was hung at 146ffw, while she was clipping the deco bottle back on. So she toxed at something under a PO2 of 1.3 (~1.27), although the time spent at a higher PO2 on that dive and the previous dive they'd done that day presumably contributed. Her husband did state that this was the first time they'd planned for a PO2 max. of 1.4, as they'd previously used 1.2.

AFAIA, there are no instances of divers toxing at 1.1 or less, but trying to pin down an exact number from sources is difficult. I will mention that NOAA still (as of the 4th ed. NOAA Diving manual) allows a working PO2 max. of 1.6, as does IANTD unless they've changed recently, but both recommend backing off that for heavy work, cold water or a few other factors.

Guy

 

[ianr33]

. Her husband did state that this was the first time they'd planned for a PO2 max. of 1.4, as they'd previously used 1.2.

I believe although this was the first time they had planned on 1.4 with trimix they had used 1.4 with nitrox previously.And presumably used 1.6 on deco with O2.

Is it generally accepted this was indeed an O2 tox incident?

 

[Guy Alcala]

I believe although this was the first time they had planned on 1.4 with trimix they had used 1.4 with nitrox previously.And presumably used 1.6 on deco with O2.

Is it generally accepted this was indeed an O2 tox incident?

I don't know about 'generally accepted', but IIRR from her husband's account she started convulsing, and continued to do so for another 15-20 minutes while her husband and their other buddy ascended with her and tried to ventilate her with her reg, but by the time they finished their deco she was unconscious/dead.

Guy

 

[Van Isle]

Right. And while these are valuable lessons for technical divers on trimix, I still stand by the panel of experts I quoted earlier:

"The conclusion then is that there are no documented oxygen seizures
from recreational nitrox diving at 1.6 atm or less on single exposures." (emphasis my own)

Lang, M.A. (ed.). 2001. Proceedings of the DANNitrox Workshop, November 3-4, 2000.
Divers Alert Network, Durham, NC.

Can anyone cite a source documenting a CNS oxtox within recreational, non-decompression diving within accepted limits at or below a PPO2 of 1.6?

As of the above reference none of the experts from DAN, IANTD, PADI, TDI, SSI, ANDI, NASA, NOAA, AAUS, DSAT, Smithsonian Institution, UCSD Medical Centre, et cetera could think of one. Not one. In millions of recreational nitrox dives. Compare that to the risks of diving over 40years of age. Or the risks associated with flying to the dive destination.

SCUBA risk, oxtox risk, like all risk, is to be managed. Risk management should be done armed with information.

 

[Guy Alcala]

Right. And while these are valuable lessons for technical divers on trimix, I still stand by the panel of experts I quoted earlier:

"The conclusion then is that there are no documented oxygen seizures
from recreational nitrox diving at 1.6 atm or less on single exposures." (emphasis my own)

Lang, M.A. (ed.). 2001. Proceedings of the DANNitrox Workshop, November 3-4, 2000.
Divers Alert Network, Durham, NC.

Can anyone cite a source documenting a CNS oxtox within recreational, non-decompression diving within accepted limits at or below a PPO2 of 1.6?

As of the above reference none of the experts from DAN, IANTD, PADI, TDI, SSI, ANDI, NASA, NOAA, AAUS, DSAT, Smithsonian Institution, UCSD Medical Centre, et cetera could think of one. Not one. In millions of recreational nitrox dives. Compare that to the risks of diving over 40years of age. Or the risks associated with flying to the dive destination.

SCUBA risk, oxtox risk, like all risk, is to be managed. Risk management should be done armed with information.

It's that *single exposures* qualification that worries me. Not many recreational divers around here do only one dive a day, and given our cold temps, often heavy surge and rough beach entry/exits, I don't want to push the edge. Sure, I know many of the graybeard gods used to go to 1.8, 2.0 or even more repeatedly with no trouble, but I see no reason to take the chance, especially if it might interfere with a Table 6 treatment (unlikely to impossible though that is for a reasonable day's diving). Sure would be nice to use 32% down to 132', though.

Guy

 

[bell47]

You have about 45 minutes at 1.6. You shouldn't have a problem with a single exposure. I also have a Suunto Vitec ds. I won't get another suunto. I finally got a VR-3 and use the suunto as a bottom timer for back up. The VR-3 is much easier to use. I know they are expensive, it stung me a little buying mine but well worth it now.