Speaking of Oxygen...

[Rick Murchison]

In the Navy, flying off the carrier, (especially on test hops where part of the routine was to dump cabin pressure at 40,000 ft - a rapid ascent indeed) we used to "pre-oxygenate" before flight, the idea being to blow off as much nitrogen as possible before going to altitude, to reduce the possibility of high altitude DCS.
Does anyone know if any work has been done with a pre-oxygenation routine for divers? It seems that by starting with a lower nitrogen load in the first place there may be some anti-DCS benefit to such a routine.
Rick

 

[omar]

What would be the point?
You would have a lower nitrogen load at the start but the ongassing of nitrogen would occur at a faster rate once you were exposed to higher partial pressures of nitrogen. This is because the gradient of the partial pressures would be greater. It would be similar to the reverse of the oxygen window for deco.

Eliminating nitrogen in your slower tissues would also take a considerable amount of time to occur. By the time that this happened your lungs would be irritated and it would not be wise to go diving. The easiest way to do this would be to hang out at altitude in the mountains for a couple of days and then zoom down to sea level and go diving. In the end you will be limited to the pressure differential between ambient pressure and tissue pressure when you surface.

Dr Deco discussed this some time ago with respect to astronauts and the low pressures involved in spacesuits during spacewalks (4 psi!). The same question came up and I don't remember the full discussion

omar

 

[Rick Murchison]

Originally posted by omar
What would be the point?
You would have a lower nitrogen load at the start but the ongassing of nitrogen would occur at a faster rate once you were exposed to higher partial pressures of nitrogen.

True, the rate of ontake would be higher, but the sat levels would be lower (lower absolute levels of nitrogen in the tissues with attendant lower DCS risk is the point). My question is "Does anyone know if any work has been done" in this area re: diving. I'm well aware of the low pressure work as I was one of the workers. (4 psi is still .27 ATM, precious plenty for the oxygen side of the problem when breathing 100% oxygen - the carbon dioxide side is where we run into trouble and need positive pressure breathing of oxygen at very low ambient pressures - but *that* really doesn't have diving implications)
I interpret your answer to mean you aren't aware of any work on preoxygenation routines relative to diving.
Rick

 

[omar]

Originally posted by omar
What would be the point?

In the end you will be limited to the pressure differential between ambient pressure and tissue pressure when you surface.

Yep,
Rick you would only have a lower total saturation if the oxygen breathing period is less than the product of the period at elevated pressure and time.

The driving force is the pressure gradient.

For air the gradient at a depth of 66 feet for tissue with no N2 would be 1.5 times greater than the one for tissue at surface P. This would be the same as doing a dive to 66 with a tissue PN2 at 0.79 and a dive to 33 with a tissue PN2 at 0.0. A dive time of 30 min at these initial tissue partial pressure will result in very similar total loading. The result is you are giving yourself a depth or time penalty by reducing your initial loading.

Like I said very similar to altitude diving.

omar

 

[Rick Murchison]

Ok, Omar, let's take your example and put a few numbers to it:

For air the gradient at a depth of 66 feet for tissue with no N2 would be 1.5 times greater than the one for tissue at surface P. This would be the same as doing a dive to 66 with a tissue PN2 at 0.79 and a dive to 33 with a tissue PN2 at 0.0. A dive time of 30 min at these initial tissue partial pressure will result in very similar total loading. The result is you are giving yourself a depth or time penalty by reducing your initial loading.

Let's assume, for simplicity
Our breathing air is 80% nitrogen, 20% oxygen
Bubble threshold is 1:1 (AmbientP)
Dive to 3ATM for 30 minutes
Assume diver 1 does a 30 minute preoxygenation; diver 2 none
Since it's the easiest to work with, look at the 30 minute tissue.
-------------------
Diver 1 starts at .4 ATM nitrogen; Diver 2 at .8
Diver 1 completes at 1.4 ATM, Diver 2 at 1.6 (diver 1 has taken on .2 ATM more than diver 2 due to the increased differential)
Diver 1 has 33% less nitrogen to offgas (.4 ATM) than Diver 2 (.6)
I think that warrants more than a passing glance.
As you can see, a fairly short pre-ox (with no significant penalty on the CNS clock) can have a pretty significant effect, and there is no depth or time penalty at all.
Rick

 

[blacknet]

Hello,

Yes there is some info on the subject in the usn diving manual. Check in the rb section as that's where it's typicaly used.

Ed

 

[Rick Murchison]

Originally posted by blacknet
Check in the rb section

Ye got me there Ed... "rb" section?
Standing by for education...
Rick

 

[blacknet]

Hello,

Sorry, rb = rebreather.

Ed

 

[Rick Murchison]

Thanks.
I'll check it out.
Rick

 

[WYDT]

I would think the much larger concern would be of CNS exposure. I wouldn't want to risk an O2 tox just to eliminate the little Nitrogen I had to start with. I can't imagine it would make much of a difference anyway.

JMHO