Recompression & O2 Toxicity

[beezwax]

I just finished taking a Med Diver course through Dive Rescue Int'l. When the class turned to recompression treatment, I learned something that startled me and I'm wondering if anyone can offer an explanation. In several of the navy treatment tables, people are put on 100% O2 when the chamber is pressurized to SIXTY FEET. That's coming up on 3 ata, and since it's pure oxygen of course your PO2 is your ata. How can that be? The NOAA nitrox tables are based on never exceeding a PO2 of 1.6, and I believe recreational nitrox tables even drop that to 1.4, due to the threat of CNS O2 toxicity. But here we're sticking people in a chamber at 2.8 or so. I know people have differing tolerances to O2, but isn't this at a level when most folks would convulse and have seizures? Also, I know the navy tables incorporate 5-minute normoxic air breaks as a preventative, but how can that be enough when you're at 100% for 20-minute intervals, for a total pure O2 time at 60 feet of 40-60 minutes? I don't know any diver who would intentionally go down to 60 feet and breathe off pure O2, even for a minute.

What say you, scubaboard? I asked the instructor of course, and he had to call up a dive doc in Cleveland (that's where he was from) who runs one of their hyperbaric chambers. After some discussion with them and other divers I have a few theories, but I'd like to hear from all the experience on this board before I go speculating. Neither the instructor nor the dive doc could say for sure why these navy tables work-- they'd never thought about it and had never been asked this question before....

 

[H2Andy]

my guess is that the benefits of 2.8 PPO O2 outweight the dangers of an
ox tox hit.

basically, the danger of an ox tox hit is that you'll drop your reg and drown, which
can't happen if you're in the chamber

i don't think there's any lasting damage from an ox tox hit otherwise

on the other hand 100% 02 can really make a difference at depth

i'm just thinking out loud

 

[do it easy]

I'm certainly not an expert, and I'm really only posting to increase my post count-

I thought that, for whatever reason, humans tolerate higher pp02 when dry than when wet. It's bizarre, and I don't have the reasons for it, but I'm under the impression that the probability of toxing is lower when dry.

Also, the probability of death is much lower when toxing in a dry environment than when wet. This might be an acceptable risk to balance the pressure to alleviate the symptoms of DCS with the benefit of pure O2.

Incidentally, the chamber tech said that skin bends were more common when dry (in a chamber) than when diving.

 

[beezwax]

my guess is that the benefits of 2.8 PPO O2 outweight the dangers of an
ox tox hit.

basically, the danger of an ox tox hit is that you'll drop your reg and drown, which
can't happen if you're in the chamber

i don't think there's any lasting damage from an ox tox hit otherwise

on the other hand 100% 02 can really make a difference at depth

i'm just thinking out loud

I was wondering this too-- of course if you convulse in the chamber at least you won't drown, so the risk is less, but:

1) is there any other danger or lasting damage from an O2 hit
2) sometimes people do convulse in the chambers, and sometimes docs actually give anti-seizure meds to the recompression patients to prevent this (I was told), but neither of these situations seem to be the norm, so why aren't more people convulsing even if there is no lasting harm from it?

 

[beezwax]

I thought that, for whatever reason, humans tolerate higher pp02 when dry than when wet. It's bizarre, and I don't have the reasons for it, but I'm under the impression that the probability of toxing is lower when dry.

This could explain it, but I can't for the life of me think how it could be true. The gas transfer takes place in the lungs, and what difference could it make to the physiological results if your skin is wet or dry? The pressure on the skin (body) is the same...

(I'm just thinking out loud here too, I appreciate everyone's thoughts and comments)

 

[Darnold9999]

I have been told that the reason is simple. Everyone's reaction to O2 toxicity is different. I may get hit at 1.5 you might not get hit until 2.2. The risk of dying from a convulsion underwater is extremely high, so to be safe we pick a number where all but those at the very extreme end of the bell curve will be safe, and they probably shouldn't be diving anyway.

Above water there is virtually no chance that you will die from a convulsion, so we can use higher numbers and you get the benefit of the 100% O2 at 60 feet where most people are safe and those that are not can be dealt with.

That was the explanation I was given - makes sense, but I know absolutley nothing about medical matters.

 

[beezwax]

I have been told that the reason is simple. Everyone's reaction to O2 toxicity is different. I may get hit at 1.5 you might not get hit until 2.2. The risk of dying from a convulsion underwater is extremely high, so to be safe we pick a number where all but those at the very extreme end of the bell curve will be safe, and they probably shouldn't be diving anyway.

Above water there is virtually no chance that you will die from a convulsion, so we can use higher numbers and you get the benefit of the 100% O2 at 60 feet where most people are safe and those that are not can be dealt with.

That was the explanation I was given - makes sense, but I know absolutley nothing about medical matters.

That's quite logical, and maybe it is that simple. Certainly the risk/benefit analysis seems to be that obvious. But I guess I was under the impression that at levels approaching 3.0, even with varying individual tolerance, the great majority of people would be seizing. And that doesn't seem to happen in practice in these chambers, and that's what I can't figure...

 

[dilligaf368]

After just re-reading the US. Navy Diving Manule....no anti-seizure meds. We want to see any changes to the Diver/Patient. O2 tox. differs from person to person and Hour to Hour! That's why the Navy doesn't give O2 tolerance tests anymore.
IF the Diver doesn't do well on O2 then it's strictly Air.
The Diver wears a Half mask in the Chamber and is removed very quickly if the Diver gets a multitude of different effects Usally before Convulsions. Tunnel Vision, Flush complextion ect.
After over 30yrs of diving I've never heard about being able to control O2 tox. by staying DRY! That's a new one!

 

[beezwax]

After just re-reading the US. Navy Diving Manule....no anti-seizure meds. We want to see any changes to the Diver/Patient. O2 tox. differs from person to person and Hour to Hour! That's why the Navy doesn't give O2 tolerance tests anymore.
IF the Diver doesn't do well on O2 then it's strictly Air.
The Diver wears a Half mask in the Chamber and is removed very quickly if the Diver gets a multitude of different effects Usally before Convulsions. Tunnel Vision, Flush complextion ect.
After over 30yrs of diving I've never heard about being able to control O2 tox. by staying DRY! That's a new one!

The navy is quite standardized, compared to all the non-military hospitals and chambers everywhere else. It may not be navy policy to use meds, but apparently that does happen sometimes (according to one doc in Cleveland). But as I mentioned before, I don't think it's the norm to administer anti-seizure meds. So then that leaves me still wondering (yes I know individual tolerance varies) why don't more people convulse in chambers at this level? I just pulled the following from the US Navy Diving Manual (don't know why I didn't read this first):

Oxygen Toxicity. Breathing oxygen at high partial pressures may have toxic
effects in the body. Relatively brief exposure to elevated oxygen partial pressure,
when it occurs at depth or in a pressurized chamber, can result in CNS oxygen
toxicity causing CNS-related symptoms. High partial pressures of oxygen are
associated with many biochemical changes in the brain, but which of the changes
are responsible for the signs and symptoms of CNS oxygen toxicity is presently
unknown.

Off-Effect. The off-effect, a hazard associated with CNS oxygen toxicity, may
occur several minutes after the diver comes off gas or experiences a reduction of
oxygen partial pressure. The off-effect is manifested by the onset or worsening of
CNS oxygen toxicity symptoms. Whether this paradoxical effect is truly caused by
the reduction in partial pressure or whether the association is coincidental is
unknown.

Pulmonary Oxygen Toxicity. Pulmonary oxygen toxicity, causing lung irritation
with coughing and painful breathing, can result from prolonged exposure to
elevated oxygen partial pressure. This form of oxygen toxicity produces symptoms
of chest pain, cough, and pain on inspiration that develop slowly and become
increasingly worse as long as the elevated level of oxygen is breathed. Although
hyperbaric oxygen may cause serious lung damage, if the oxygen exposure is
discontinued before the symptoms become too severe, the symptoms will slowly
abate. This form of oxygen toxicity is generally seen during oxygen recompression
treatment and saturation diving, and on long, shallow, in-water oxygen
exposures.

So that tells me:
1) it happens after relatively brief exposure to elevated PP
2) there ARE other possible and permanent harms (including, the manual later states, arterial gas embolism)

So, why aren't more people toxing in chambers at 2.8 for 40-60 min?

 

[rmediver2002]

The recreational (1.4 ppo2) and scientific (NOAA 1.6 ppo2) for oxygen exposure are conservative levels to decrease the chance of O2 tox because of the reasons listed above (underwater / drowning / ect.)

In a chamber O2 tox can occur, but is pretty rare. I have seen more O2 hits at 40 FSW during surface decompression using oxygen than during treatment tables.

During treatments the patient is at rest, is being observed continuously, and are taking periodic air breaks.

As you stated above, why aren't more people toxing at these levels?


LAR-V - 100% O2 rebreather, standard for military combat diving operations / insertions. Unit is used at transit depth shallower than 30 FSW with limited exposure to deeper depths 50 FSW.

treatment tables (5/6) - treatment depth 60 FSW breathing 100% O2

Surface decompressions using O2 - 40 FSW 100% O2

The recreational standard is one that minimizes the incidence of CNS O2 tox. to an insignifigant amount.

The NOAA standard 1.6 is also very conservative. NOAA also has a table to compare time of exposure and level of exposure. (higher PPO2 / lower exposure rate)

The evidence supports us being able to tolerate higher levels of O2.