O2 administration: time vs toxicity?

[dlegros]

There was a related thread over on Dr Deco http://www.scubaboard.com/showthread.php?s=&threadid=37314

I dug through the little info we had at work on pulmonary oxygen toxicity - a salient point being that it occurs when pp02 is above 0.48bar (so roughly 50% O2).

Even acconting for leaks around the face and the safety vents in a non-rebreathing mask with reservoirm these can easily deliver 80+% O2.

The time factor is suggested as 6h rather than 24.

Unfortunately, the copy I was working from has had the citation chopped and no-one can remember where it came from (our tox library has several thousand articles - so the odd reference gets missed!)

Dr D also posted some interesting results on POT in rats.

Dom

 

[The Iceni]

divemed06 once bubbled...
Let's say that I'm on a charter, very far away from shore and EMS, what is the "safe" duration of O2 administration for an injured (ie. DCS) diver? .

Hi guys,

Can I put this a little more into perspective?

Oxygen is used in th treatmant of DCI because it reduces the nitrogen offgassing counterpressure to zero. If the casualty is breathing air there exists a conterpressure of 0.79 bar which slows offgassing and worsens DCI.

Scubadoc, of course is quite right. However, one thing that irritates me is the quite unfounded fear of using surface oxygen because of the risk of making matters worse.

I do not see how there can ever be any such risk during a rescue.

There are no risks of CNS oxtox with surface O2 but there is indeed a real risk of pulmonary oxygen toxicity which is measurable and quite predictable. The Royal Navy (and NOAA) recommends a safe oxygen exposure in any 24 hour period to be about 1440 (oxygen toxicity units) OTUs i.e. 24 hours at 1 bar ppO2 (24 hrs x 60 mins = 1440).

After 24 hours of 100% surface oxygen a reversible reduction in lung function of about 10% can be measured.

The standard RN treatment table 62 uses 637 OTUs and it is very unlikely indeed that the risks of pulmonary oxygen toxicity will prevent the chamber doctor from providing the required treatment of a life-threatening neurological DCI, even if this limit is exceeded.

Even a fully extended RN treatment table 62 uses only 1062 OTUs.

Thus two full hours worth of surface oxygen adds only 120 OTUs to the pulmonary oxygen clock but could prevent a severely incapacitating (or even fatal) neurological DCI.

It is wise that technical divers monitor their exposure limits to prevent pulmonary symptoms, particularly on multiple dives over a number of days, but there is only one immediate treatment of an incidence of DCI and that is continuous 100% oxygen (followed by recompression).

So I would recommend an oxygen DV since it delivers 100% oxygen and the limited supplies carried on the boat will last longer. See this thread.

As for CNS hits in a chamber, Gary, these are quite benign, easily teated (by an air break) and almost routine.


:doctor:

 

[Otter]

mwilding once bubbled...
unless the boat is on the bottom...

And in water deeper than 20 fsw.

 

[Griff]

Dr Paul Thomas once bubbled...
Hi guys,

Scubadoc, of course is quite right. However, one thing that irritates me is the quite unfounded fear of using surface oxygen because of the risk of making matters worse.

I do not see how there can ever be any such risk during a rescue.

Paul seems to have the simplest and most practical approach to this scenario. It is impotant to note that the scenario revolves around a simple charter boat for what seems to be recreational diving purposes, without immediate access to any DDC (deck decompression chamber).

Of course there are risks in the use of oxygen but they are easily countered when the patient finally arrives at the hyperbaric centre. A rapid assessment by the diving physician on amount of oxygen the patient has been exposed to on the surface (FIO2) combined with the recent dive profiles and clinical assessment of the patient's lungs as well as the blood gas levels will give the dr all the information he/she needs to give the necessary treatment and reduce the chances of Ox Tox.

Put simply the Dr will wiegh up the risks and the benefits of the hyperbaric treatment neccessary. During any rescue or first aid after a suspected diving accident the simple rule of thumb should be: ABC's then to give the highest possible concentration of surface O2 for as long as possible. If transfer to medical aid is an extreme time; then better, more advanced training in dive accidents in general may need to be considered as well as special procedures prior to departure