Nitrox - 1.40 or 1.60 PO2?

[wve]

Isn't hyperbaric oxygen therapy more often for cases where the the blood O2 levels are fine, but that there is some sort of circulatory or tissue damage problem that is making it so that not enough blood is reaching the tissue in question ?

I was under the impression that one of the reasons for HBOT was to supplement or replace the blood/hemoglobin O2 transport mechanism with other mechanisms such as the oxygen dissolved in plasma, and the reason for pressure was to increase the amount of dissolved O2 up to useful levels.

(yeah, this is a bit of hijack from the original question, but that one has been beat to death)

I wasn't really thinking of hyperbaric oxygen therapy. I was thinking more of patients who require mechanical ventilation because their cardiorespiratory system is not working properly.

I'm not a hyperbaric oxygen expert, but I believe you are correct.

Hemoglobin is the primary way oxygen is delivered to the various body tissues. Normal arterial blood is more than 90% saturated. That works fine as long as you can get enough blood to the point where it is needed.

In cases where the plumbing is damaged, increasing the amount of oxygen in the blood can help. You still have inadequate blood supply, but there is more oxygen in the blood that does make it to the scene.

External oxygen under pressure can also help heal certain wounds by killing bacteria (which often don't like oxygen) and creating a healthier environment for the healing process to take place.

 

[hobodiver]

If you mean by "the same thing" to lower nitrogen exposure, Yes.

Again, I'm no tech diver, but as I understand it, the reason for adding a third gas, such as helium, is to lower the amount of nitrogen AND oxygen by adding yet another inert gas.

I understand the reasoning behind adding helium while diving deep, but I was wondering if adding helium (say 80% helium and 20 % oxygen) would assist in off gassing nitrogen to the same extent that that 100% oxygen does? i.e. the PPN2 is zero, hence the body will offgass more nitrogen.

 

[Rhone Man]

I understand the reasoning behind adding helium while diving deep, but I was wondering if adding helium (say 80% helium and 20 % oxygen) would assist in off gassing nitrogen to the same extent that that 100% oxygen does? i.e. the PPN2 is zero, hence the body will offgass more nitrogen.

Only problem is that your body absorbs He faster than it absorbs N2. You are better of with 80% N2 than 80% He.

Plus 80% He would be horrificly (sp) expensive!

 

[Charlie99]

I understand the reasoning behind adding helium while diving deep, but I was wondering if adding helium (say 80% helium and 20 % oxygen) would assist in off gassing nitrogen to the same extent that that 100% oxygen does? i.e. the PPN2 is zero, hence the body will offgass more nitrogen.

There isn't any free lunch. While putting helium in the mix means there is less nitrogen, the body also absorbs helium. You then have to offgas that helium on the way back up to the surface.

A crude, but reasonably accurate way to look at the effect of mixes that have multiple inert gases is to consider the ongassing and offgassing of each inert gas to be taking place independent of all other gases.

On the other hand, bubble formation (which can lead to DCS) is dependent upon the relationship between the SUM of all inert gases and the ambient pressure.

 

[Gene_Hobbs]

Nice post wve. If you are really wanting to know more about CO2 in diving, you might want to start here. This is from a longer post on another board.

Charlie99, you are right about the O2 content in HBO.

rstofer, there's not much on drugs and diving but we have almost all that exists in our database.

hobodiver, there is some evidence that treatments with heliox are not at all a bad option. (Hyldegaard's work is especially interesting.)

Back to the scary kids at the front door...

 

[nereas]

Why not?

Surely doing a Dill flush and not injecting any more O2 for a while is the same as an air break?

True, good point.

 

[HowardE]

True, good point.

Are you sure about that?

It would depend on several factors, the least of which is whether or not you are diving your CCR on manual or not.

 

[netmage]

True, good point.

I wish this would have let me quote some more.... so let me paint the typical air break scenario on open circuit..... you've just settled in at your 20' stop for a stretch, 18-20 mins into a 60 minute hang and you need to do an air break...

Your going from a pO2 of 1.6 assuming 100% O2 down to what... assuming air..., maybe a pO2 of 0.336 for a sustained period, maybe 5 minutes or so.... and then flipping back and forth as necessary to complete deco.

Relating this to a CCR dive is a little like apples and oranges, my intuitive response would be to plan to dive a different set point based on planned exposure and not introduce complexities like multiple dynamic set point selections..., but I'll humor a little, so you get to 20' and decide to do an air break..., you're going to need to drop the pO2 by an equivalent margin in order to make a difference in the inspired pO2 relative to current exposure. Say your a good CCR diver, and your not pushing 1.2 or 1.3, this means flushing down to something low.., say .21 or .4 and SUSTAINING that low pO2 on a manual unit, or your on an eCCR and you select a low set point and breathe it down or flush it down, and maintain it for a prolonged period to be effective, i.e. 5 minutes. (assuming you have a dil that can flush it to that point)

It just simply far easier to plan the dive to a .7 or 1.0 set point based on planned total exposure vs. flip flopping the loop around. (and corresponding tables/computer)

There's a few folks who take the approach of diving a lower setpoint on the bottom, or ingress into the cave (say 1.0), then once turning (1.2), and maybe at deco 1.4 to more evenly allocate and make most efficient use of the higher pO2 when it can be the most efficient while not burning up the clock.

My belief is most CCR folks simply aren't doing the massive exposures at the higher range of pO2's. Their selecting lower set points. Now that the GUE boys have taken a bite of the eCCR apple, its yet to be seen how they will apply the paradigm to their significantly longer exposures and dive profiles.

-Tim

 

[lamont]

If you mean by "the same thing" to lower nitrogen exposure, Yes.

Again, I'm no tech diver, but as I understand it, the reason for adding a third gas, such as helium, is to lower the amount of nitrogen AND oxygen by adding yet another inert gas.

The major reason to add helium is to reduce narcosis.

And while helium is faster at ongassing, it is also faster at offgassing, so on paper it shouldn't matter if you're using helium or nitrogen from a deco standpoint.

There's a bunch of different opinions on the use of helium, some believe that helium is a bad deco gas, some believe that helium is a good deco gas, some believe that you want to keep them balanced. I've seen some anecdotal evidence that divers on 21% have gotten bent alongside divers on 21/35, which inclines me towards the "balanced is better" or "helium is good" viewpoints. I've also heard that helium may be worse if you don't do your deepstops or have an explosive decompression accident, and don't discount that. Scientific evidence in this area seems to be sorely lacking.

 

[lamont]

The cause was unknown? The woman had a seizure underwater and drowned. That is both fact, and quite easy to understand. The coroner determined oxygen toxicity as the cause of the seizure. ALL the divers that participated said it was at 1.4 pp02.

So the only "maybe" here is the cause of the seizure. And that is pretty much conclusive itself.

On nearly the same day as that accident, Chief Justice John Roberts had a seizure that struck without any medical explanation or history of seizure disorder.

And you can't do an autopsy and show that oxtox occurred, there's no evidence left behind.

Also, the distribution of sensitivity to oxtox and distribution of seizure threshold is also going to have a tail on either side. Individuals aren't all throwing the same dice when they go diving with elevated ppO2 gas. Some individuals may have dice which are loaded against them, and some people may be fairly hard-headed when it comes to oxtox. There's some suggestion that the victim in this fatality had a history of funny reactions to high ppO2s and may have been unusually sensitive to O2.

So, given all the dives done on 1.4 ppO2s in the denominator and this one dive in the numerator, what is the probability on any given dive at 1.4 ppO2 of resulting in a fatality? And how does that compare with all the rest of the risks when you go diving, including the drive to the site? A lot more divers have probably died on the drive to the site than have died at 1.4 ppO2s...