02 at depth ??

[DiveBandit]

Ok, this may be obvious, but I don't know the answer and I'm asking. Call me stupid if you must.

When doing decompression, I use 100% at 20' and above. I've been taught and told numerous times that breathing pure 02 deeper than 20' is a death sentence. SO, how do they get away with giving someone in a recompression chamber 100% at chamber depths of 66' ?

Thanks,

Dave.

 

[Walter]

Oxtox doesn't kill you, but it can cause you to drown. If you go into convulsions in a chamber, you aren't going to drown.

 

[Jimmer]

Well I'm no expert by any stretch of the imagination, but I would guess that the fact that they are stationary, no exertion, warm, dry, etc. helps counteract the ox tox effects.

 

[diver 85]

lots of malpractice insurance backing up the letters after their name......

 

[DeepSeaExplorer]

And... breathing O2 deeper than 20ft is not a death sentence. It happens. You don't plan to do it, but it happens.

While it's under different conditions, I dare say most divers that are put on O2 at 60ft in a chamber, do not go into convulsions. Yes, there is a possibility. That's why a medic will goes in with you.

 

[SteveAD]

Breathing ppo2 in excess won't directly kill you, but it does have a significant probability of causing seizures. Probability of seizures will increase as ppo2 increases as well as time of exposure. Having a seizure while under water on scuba gear will very likely result in death by drowning.

In a recompression it is usually an air environment and the paitient is administered O2 by mask, with exposure time closely monitored, and the patient given regular air breaks. In a multi place chamber there will be a medic in the camber who can remove the mask in the event of a seizure. In a single place chamber, the staff outside the chamber would have access to controls to switch the mask from o2 to air.

In a dry environment the patient can have a seizure without risk of drowning.

 

[ClayJar]

Immersion (dive reflex) and cold stress (more likely in the water than in a warm, dry hyperbaric chamber) have both been included in lists of factors increasing the risk of oxygen toxicity, and lying down is about as resting as you can conveniently be.

Trading a higher risk of seizures (which will not result in drowning in a chamber) for the substantial benefits of higher pressure (to reduce bubble sizes and force gases back into solution) and breathing pure oxygen (for the greatest nitrogen gradient to promote offgassing) seems to be easily justifiable.

 

[DA Aquamaster]

A couple of thoughts...

100% O2 at 50 or 60 feet will not kill you immediately. The deeper you go, the higher the partial pressure, the faster you will tox. In addition other factors come into play such as temp, activity level, CO2 levels, etc.

So a short exposure at 30 feet may not kill you right away but your risks increase the deeper you go and the elss support you have.

It was not uncommon in the past for commercial divers to be brought up from their 40 ft stop and then be blown back down to 40' in a recompression chamber while breathing 100% o2 through a demand mask. The thing is air breaks were used to prevent ox tox and in the event symptoms of an incipient ox tox were felt (tingling lips, mouth, visual distortion, etc) the diver would remove the O2 mask and breath air from the chamber. In the event he convulsed with no warning symptoms, the hand held mask would fall off anyway and the PPO2 would again immediately drop.

In the water it is a different story as the reg gets spit out and you drown when you convulse. Some in water recompression protocols call for O2 at a 30' stop, but as such they require a bosun's chair to assist the diver in maintaining depth, a full face mask and a monitoring diver in attendance to deal with any ox tox issues that may arise.

Deep air record dives by definition involve short exposures at extreme PPo2's and the US Navy has even developed a chart of acceptable (and more or less safe) exposore times at high PPO2's, but they are very short and in general get much shorter as the PPO2 increases.

So it is not so much that a PPO2 of 1.6 is safe while 1.7 will kill you but rather that anything over 1.6 is not considered prudent due to the shorter exposure times allowed and the greater varaibility found in individual diver suceptibility at those higher PPO2 levels.

As an aside I read a wonderful account recently by a diver who participated as a virtual lab rat in some rebreather experiements in England during WWII - when they had no real idea of oxygen toxicity and how it worked. Generally they found that it varied but in a specific case, when placed in a pressure tank under water divers toxed after about 10 minutes at 50 feet on 100% O2. Scary stuff. Scarier still when you consider they kept doing it until they figured out what the problem was.

 

[String]

During a chamber dive we got to talk to the hyperbaric consultant and asked similar things.

He said routinely people were put at ppO2 of 2.1 or so in the chamber and he'd not yet in his career seen a convulsion.
However the reason 1.6 is chosen for underwater is due to cold, more exertion and as others have said, if they DO oxtox in a chamber its really not a serious problem and they'd just come off the O2. Also peoples tolerance varies so underwater partial pressures are chosen to err on the very conservative side to be "safe" for just about all members of the group not just most.

One of our group actually asked could he test a 2.0 or higher in the chamber just to see if he had issues or not but was told no.

 

[ianr33]

In the past divers would run a ppO2 of 2.0 pretty often. Read Caverns Measureless to Man. Exley was doing all sorts of stuff that would be considered crazy today.

Toxing at 2.0 is probably not that likely for a short(ish) exposure ,but the consequences if it does happen are not so good.