Oxygen tolerance

[The Iceni]

Hi all,

I have just joined the group and have a real poser for you.

I am trying to get my head around the Oxygen CNS clock. While I can understand that whole body toxicity is time dependent, from my reading it seems that CNS Oxygen toxicity, in theory at least, is dependent only the Oxygen partial pressure (all other variables being equal). I gather from chapter 18 of Edmunds, Pennefather & Lowry, (Diving and Subaquatic Medicine) it is toxic for most divers after moderately prolonged exposures of 1.8 bar but I find no mention of the CNS clock.

In England, we are discussing the relative merits of accelerated decompression for sports divers using 80% or 100% oxygen at partial pressures of 1.6 bar with or without air beaks. In recompression therapy and chambers in the commercial sector in the North Sea air breaks are regularly used to prevent CNS toxicity but these exposures are prolonged and/or exceed 1.8 bar. This is quite clearly not the case in sports diving.

For example a dive for 1 hour at forty metres on 28% Nitrox will require deco of 14 minutes from 6 M if 100% oxygen is used, giving a "CNS exposure" of about 56% while if 80% Nitrox is used the total stop time from 6M is 19 minutes with "CNS exposure" of about 46% (from Proplanner deco software). If a five minute air break is needed when using 100% oxygen there seems very little point in using it since the total effective stop time is not reduced.

I find this particularly confusing because the Oxygen-off effect also leads to convulsions, so any change from in-water Oxygen deco to an in-water air break could be just as hazardous, if not more hazardous, than continuous exposure to 100% at 1.6 bar.

The well recognised Oxygen-off effect seems to be quite counter-intuitive if the cause of CNS toxicity is simply a change in ppO2 or the accumulation of toxic metabolites.

I get the impression British sports divers are much more adventurous than their foreign counterparts and many will dive in these depths in The North Sea in the most appalling conditions, such as a force 5 wind and with underwater visibility of no more than 3 metres. Many, of course, will use Trimix recreationally, all without onboard chamber facilities so there are a lot of British divers out there who will be using 100% oxygen for deco, regardless of any risks, simply because it shortens their stop times.

We need the evidence to ensure that at least they can know what they are doing.

Anyone got the scientific answer?

kind regards,

Paul

 

[Dr Deco]

Dear Paul:

If I understand this question correctly, part of it involves the time dependence of CNS oxygen toxicity. If I have not gotten the meaning of your message, please ask again, and I will see what I can do.

I believe that the reason that time limits are not given with respect to CNS problems is because of the extreme variability of time to onset. This really renders any “safe time” on high partial pressures of oxygen to be rather useless. We are looking at the old concept of the Oxygen Tolerance Test. This was abandoned first in the Royal Navy and then in the US Navy because the data was of little value. The toxic response to oxygen is just too variable from day to day in an individual. :boom:

In light of this, it is therefore necessary to breathe oxygen under a given partial pressure to insure that a convulsion will not suddenly appear. Regrettably, a convulsion is not the first sign or symptom (e.g., ringing in the ears, twitching of facial muscles), but it follows very shortly thereafter. Fortunately for pulmonary oxygen toxicity (after low oxygen partial pressures and hours of exposure time), it develops slowly and in a progressive type of fashion. One feels a pain and fullness in the chest long before one develops pulmonary edema.

One is not afforded the luxury of known and predictable time dependence (the oxygen clock) for CNS toxicity that is present with pulmonary forms. If one is in the region where CNS toxicity may arise, it is best to be in a dry chamber where drowning is not possible. The risk of CNS toxicity is reduced by air breaks. These prevent most cases of CNS toxicity. The “off effect” appears with considerable less frequency than an oxygen convulsion, so it is not an “either/or” proposition.

In-water oxygen at high partial pressure of O2 is a dangerous situation. Recreational diving should be recreational – not life threatening.

Dr Deco

 

[The Iceni]

Wow Doc!

Thanks for getting back to me so quickly.

You say.

"It is therefore necessary to breathe oxygen under (below?) a given partial pressure to insure that a convulsion will not suddenly appear. Regrettably, a convulsion is not the first sign or symptom (e.g., ringing in the ears, twitching of facial muscles), but it follows very shortly thereafter.

I suppose some warning is better than none. IANTD teach the acnorym VENTID

"One is not afforded the luxury of known and predictable time dependence (the oxygen clock) for CNS toxicity that is present with pulmonary forms."

It seems we agree and that there is no validated time-dependent prediction for the onset of CNS toxicity which brings me back to the supposed purpose of air breaks when breathing Oxygen with a partial pressure of 1.6 bar.

Sounds like you would consider air breaks to be essential in water at that pp O2.

Is there any evidence?

Thanks again,

Paul

 

[Dr Deco]

Dear Paul:

I would really doubt that an air break would be needed at 1.6 atm of oxygen in a recreational situation because the exposure time is too short. For the air breaks to be of necessity, I generally think of people in a hyperbaric chamber on a treatment schedule. The exposures here are at 60 fsw on 100% oxygen (2.8 atm) and of several hours duration, and this is considerably above the levels used for the short intervals by recreational, in-water decompression.

It is my understanding that technical divers would use mixes for decompression and work up to a depth of 20 fsw. At this point, they would switch to 100% oxygen.

I would guess that air breaks under the conditions described (1.6 atm for several minutes) are not needed. I would also expect that the diver would be relatively quite in the water and not generating a lot of carbon dioxide, a cerebral vasodilator.

Dr Deco

 

[The Iceni]

Ta!

 

[Lost Yooper]

Dr. T, I have a ton of stuff that I've collected on the subject of oxygen toxicity from leading technical divers. There seems to be a big discrepency between the theory and the reality of oxygen toxicity.

The gist of it is that this is a fairly imprecise subject which has more questions than answers and that breaks are the way to go -- especially on helium based mixes.

I very much want to show you this stuff and get your imput. E-mail me if you're interested.

thelostyooper@cs.com

Mike