How fast is O2 tox?

[Web Monkey]

Does anybody know how long it takes for O2 toxicity to cause seizures for any particular PO2? Is it an immediate effect, as in any given person has a threshold and will have a seizure when it has been reached, or does it take a significant amount of time to build up?

For example, say someone is diving EAN 36@ 95', which gives a PO2 of 1.4, then sees a diver in distress (OOA, etc.) at 130', which would be a PO2 of 1.78, if the first diver went down to 130' to assist the second diver for a couple of minutes, and then do a normal ascent to the surface with both divers, how probable would it be, or how long would it take for an O2 hit to occur?

What about the same situation, but going down to 150'? (PO2 of 2.0)

The real question is: Would going significantly beyond your maximum PO2 for a short time be likely to create 2 victims, or a successful rescue?

On a related note, suppose the rescuer had an additional cylinder of a non-toxic mix (like air). Would a switch to air, then an immediate descent to 130 or 150 be likely produce any O2 symptoms (IE. how long does it take for the gas change to become effective in your body?)

Terry

 

[PerroneFord]

Didn't the US Navy used to use a PP02 of 2.0 in general?

 

[amascuba]

Does anybody know how long it takes for O2 toxicity to cause seizures for any particular PO2? Is it an immediate effect, as in any given person has a threshold and will have a seizure when it has been reached, or does it take a significant amount of time to build up?

The studies are inconclusive. The studies that I read talked about a person who started having seizures within minutes of breathing o2 under pressure one day. The next day lasted 100+ minutes under the same environment, and then again on another day lasted just a few minutes again.

It doesn't look like something that you can build a tolerence for. It most likely occurs faster on people with medical conditions and people who are stressed more.

Based on that information the diver may go down to handle the situation and come up unscathed, or the diver could go down and immediately start having convulsions.

 

[divingjd]

Remember that there are two kinds of O2 toxicity: central nervous system (CNS) toxicity and pulmonary toxicity. My understanding (I am sure I will be corrected if I'm off) is that CNS toxicity is a function of ppO2 and not a function of time. If you can get a diver who is convulsing from CNS toxicity into shallower water (reducing ppO2), the CNS toxicity and convulsions should stop. Pulmonary toxicity has a time element, but it's a lot longer than a couple of minutes.

I'm not sure about the possibility of switching to a different gas. Be aware, however, that air is not, strictly speaking, non-toxic. CNS O2 toxicity is also possible on air. It just takes a much greater depth to get to 1.4 or 1.6, so it is not an issue within recreational limits.

 

[Diver Dennis]

Excellent question WM. I've thought about that too.

 

[TheRedHead]

I would opt not to dive a mix whose MOD exceeds the hard bottom. We always dive air on walls in Cozumel where there is a chance of getting caught in a downcurrent. PO2 of 1.6 is used for deco mixes, which are used "at rest." 1.4 is used for the working part of the dive. I never spend much time at 1.6 because you only have 45 minutes per day at that pressure.

 

[amascuba]

Remember that there are two kinds of O2 toxicity: central nervous system (CNS) toxicity and pulmonary toxicity. My understanding (I am sure I will be corrected if I'm off) is that CNS toxicity is a function of ppO2 and not a function of time. If you can get a diver who is convulsing from CNS toxicity into shallower water (reducing ppO2), the CNS toxicity and convulsions should stop. Pulmonary toxicity has a time element, but it's a lot longer than a couple of minutes.

I'm not sure about the possibility of switching to a different gas. Be aware, however, that air is not, strictly speaking, non-toxic. CNS O2 toxicity is also possible on air. It just takes a much greater depth to get to 1.4 or 1.6, so it is not an issue within recreational limits.

The studies that I was referring to are chamber dives in controlled environments at higher levels PPO2 specifically to study the effects of higher partial pressures of O2. The studies showed that a diver can be subjected to higher PPO2's on any given day and may have a different reaction to them each time. The specific individual that it referred to only lasted a few minutes the first time around, the second time lasted for 100+ minutes, and the third time only lasted a few minutes again.

The studies were inconclusive to provide specific information on individual limits of higher levels PPO2. Just because you go down to to a deth that brings your PPO2 to 1.6 doesn't mean you are going to convulse. You might be able to stay at that pressure for over an hour and be totally fine, but the risk is higher. You might do another dive on a different day and start convulsing as soon as you hit 1.5 PPO2.

There is allot of unknowns when it comes to elevated PPO2's. That's why pretty much everybody say's to limit you MOD to 1.4 PPO2 for working portions of the dive and 1.6 PPO2 for deco portions of the dive. Allot of technical divers are now even advocating 1.2 PPO2 for working dives.

 

[Diver Dennis]

Is there a quick onset though? Web Monkey had a great question, is it possible to help a diver in trouble that is over your MOD for a few minutes or is it too dangerous?

 

[Web Monkey]

I would opt not to dive a mix whose MOD exceeds the hard bottom. We always dive air on walls in Cozumel where there is a chance of getting caught in a downcurrent. PO2 of 1.6 is used for deco mixes, which are used "at rest." 1.4 is used for the working part of the dive. I never spend much time at 1.6 because you only have 45 minutes per day at that pressure.

No matter what mix you're diving, unless you have a hard bottom at or above your MOD, this question would always be pertinent.

Terry

 

[rigdiver]

Studies are inconclusive. Some could handle high exposures for days and then convluse. Others that could not handle high ppO2 exposures had some "good" days where they could handle highppO2's. The commercial dive schools in the past did an O2 tolerance test. Candidates were put in a chamber and pressurized to 50fsw and breathed pured O2. This test has been done away with because it really proved nothing. There is a history channel documentary about diving caves in Cozumel. There was some old USN footage of a guy actually convulsing in a chamber. I wonder if he was a volunteer or candidate. He was actually wearing a diaper. Real convulsions cause loss of control.