medical o2 on deck

[SDAnderson]

Mike said:

"There's a lot more involved in using O2 at 20' than most divers know about -- surprise, surprise. Extended periods on O2 at 20' will give you a congested feeling in the chest/lungs, but this is easily dealt with by taking back gas breaks (use the lowest PPO2 mix available). However, there is more to it than congestion -- extended periods of O2 will actually restrict offgassing. The best way is to go on O2 for 12mins (not 20mins) and break for 6mins on backgas. "

In my convoluted mind, what you're saying seems to indicate that EAN80 would actually be preferable to straight O2 as a deco gas at 20fsw. Wouldn't EAN80 eliminate the need to take an air break every 12 minutes? Am I on the right track?

Steven

 

[Lost Yooper]

Not really. The 1.6 PPO2 is what is really being talked about. If you stay on 80/20 at 32' (or whatever is 1.6) then you would need a break from it as well. 80/20 isn't a good idea at all for deco.

Mike

 

[SDAnderson]

Mike-

I’d like to start by stating what will rapidly become obvious: I am not a physician and what little understanding I have of gas physics and physiology is truly pitiful. I’ve been working on this and I think that I understand the problem as you stated it, however. To paraphrase: exposure to a ppO2 of 1.6 for 12 minutes results in some kind of saturation of blood gases, triggering an autonomic response that results in the restriction of alveolar blood flow, thus reducing the pressure gradient across the alveoli and thereby reducing pulmonary gas exchange. This is not the CNS oxygen toxicity problem that leads to nausea and convulsions, nor is it the edema problem that leads to the perception of a tight or full chest. (I wonder to what extent, if any, they may be related, however.)

If this isn’t what you said, I still need help wrapping my noodler around your thought, please be kind. That said, I understand, in layman’s terms, the idea of autonomic responses to changes in blood chemistry: one prime example being the role that elevated CO2 levels play in controlling respiration rates. Adding one more to the list of things that go bump in the night is not too difficult. This is a new concept to me and a quick search through the NOAA and Navy manuals and of the www didn’t turn up anything that appeared relevant – I’d love to read more if you can point me in the right direction. I’m especially interested in any empirical information about the time/exposure required to trigger this response and how far it’s necessary to back off to release or reverse the response.

Thanks - and a very happy holiday to you and yours!

 

[Dr Deco]

[sp][1.] Oxygen has been used in decompression for decades, and vasoconstriction has never been found to be necessary to factor into the calculations. I really doubt that it is a factor since physical activity of the diver will cause an opening of the arterioles. Perfusion to the tissues will be maintained.

[sp][2.] The problem of oxygen toxicity should not appear in technical divers under the conditions encountered to date. Commercial and military divers are exposed to higher partial pressures for longer durations (for many hours in a dry chamber) without physiological consequences. The oxygen does described by divers in this column are much lower.

Dr Deco
:doctor:

 

[aue-mike]

Commercial and military saturation diving is not really applicable here. Under those conditions where they have a dry home to hang out at for as long as they want (habitat), time is not an issue and they pad the time considerably. Furthermore, diving with hats allows the military to crank up the PO2.
In regard to technical diving applications, O2 toxicity can become an issue. As many times we are working to produce the fastest, safest, and the most efficient decompression possible, any scenario that can potentially hinder decompression needs to be factored in. Should your lungs not be working for you at 100%, which can happen after long exposures to even moderate PO2s, it can have a negative effect on your decompression.

Cheers,

 

[Dr Deco]

Dear aue-mike;

Regarding the question of hindered gas exchange in the lungs, I am not aware of any evidence for this unless lungs are quite edematous. I would certainly hope that divers were not using oxygen to the extent that they were experiencing respiratory problems.

In general, there is a large amount of reserve for gas exchange in the lungs. That is why gas uptake and elimination is very good even when one is exerting themselves considerably. The halftime for gas exchange is on the order of a few seconds between lung alveoli and lung capillaries. Compare this to tissue exchange halftimes of tens of minutes to several hours. I would be surprised that pulmonary gas exchanged would need to be factored into decompression calculations.

Dr Deco
:doctor:

 

[Lost Yooper]

Reelraff,

I've been pretty busy lately, but I'll look around for more "scientific" info. The long time standard for deco in tech diving was 20mins on O2 and 5mins off at 20'. This is still OK if all you had to do is 20mins or so (I'd do all 20mins on O2). If you have 30mins (or more), I would do 12mins on 6mins off, and 12mins on then surface on O2. I don't think this is an exact science, so I think there plenty of room for a fudge factor.
I don't know exactly where or how the 20mins on 5mins off came to be, but the base-info is out there somewhere. It is clear that extended exposures to O2 will cause a congested feeling in the lungs which is very counter-productive to deco. My information is based on some extreme technical divers which have found that 12 on 6 off is getting them out faster and cleaner (and with less lung distress) than 20 on 5 off. It wouldn't surprise me if there isn't official "scientific" evidence to back this up. It make sense to me and works for me, so that's what I do.

Take care.

Mike

 

[Dr Deco]

Dear Readers:

This is very interesting regarding the tech divers, oxygen usage, and lung congestion. The fact that oxygen problems with this small amount are not noted in professional and military divers as contrasted with tech divers may reflect the difference in dry chamber versus in-water decompression.

I am interested in why tech divers are noting lung congestion with what would be considered very small amounts of oxygen in terms of Unit Pulmonary Toxicity Doses (so-called UPTDs).

Dr Deco
:doctor:

 

[The Iceni]

This is for reefraf.

Take a look at this thread;

http://www.scubaboard.com/t4425/s.html