Oxygen window

[MattS]

I believe this is due to the non-linear O2-hemoglobin disassociation curve alluded to above (you did ask). Only a very small amount of O2 will be dissolved into plasma, the vast majority is attached to hemoglobin (Yes, its another decay curve).My guess is that o2 is handled more efficiently by the body in this manner.

The OHDC describes O2 Haem binding up to Haem saturation of 4 molecules O2 per Haem molecule.
http://www.ventworld.com/resources/oxydisso/dissoc.html

CO2 is dissolved into blood (plasma) and so this might explain why these two metabolic gasses are carried in different ways, one by hemoglobin, one by plasma to give a cleaner interchange.

You may need to rethink ;

"95% of the CO2 generated in the tissues is carried in the red blood cells:
...Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!) "
http://users.rcn.com/jkimball.ma.ult...s/B/Blood.html

I still can not work out what part plasma plays in O2 exchange in healthy tissue.

MarkC
It turns out that 50% is the lowest mix that will saturate Haem when breathed at 6m George seems to be saying that 100% is the gas of choice for a 'short' dive to 30m. Deeper than that he reckons you need two stages 100% + 50%...and of course Helium. So a single stage of 50% is not DIR (according to the quote)

I still think he is making a 35m dive too complicated.

.

[Mark]

If 32% could be used in the same way that we use our 50% (i.e. as a deco gas) then I would see it as a good thing. But, it is just a travel gas which allows you to get 10mins longer bottom times and use up more of your nice trimix back gas than we can. We travel up to 21m on our backgas so a larger reserve is required. But, even on backgas we are ongassing less N2 than you are when you switch to your 32% 'deco gas' at 40m.

What it boils down to in the end is that we are probably doing fairly similar overall deco times but you guys are carrying two stages and we are only carrying one (and our deco is worked into deeper and our shallow stops are much shorter than yours as a result).

Regards,

Mark

[Mark]

Originally Posted by MattS

So a single stage of 50% is not DIR...

Oh yes it is! It's very DIR actually.

regards,

Mark.

[MattS]

And attributed to George: If you're doing a nitrox or a triox dive that's under a 100ft and it's a short dive, then oxygen will be your first choice after that then you'd be adding 50/50.

MattS: So a single stage of 50% is not DIR...

Mark: Oh yes it is! It's very DIR actually.

Python:An argument isn't just contradiction....An argument is a connected series of statements intended to establish a proposition.

Anyone care to explain?

Why and in what circumstances would a DIR diver choose a single 50% rather than 100% or 50% + 100% ?

[And]

Hi

You are not taking into account bottom times. If you were doing a 30 mtr dive for say 90 mins (cave dive) you would use O2 for deco. That is what George is saying. In his reality normal bottom times are significantly longer than ours.

With a short bottom time like the ocean dives we (as in Mark and I) in the 35-50 mtr range there is no need to take two deco gasses, so we choose 50%. There are at least two reasons for this that I can see.

One. As I pointed out before run any GF program you like on 30/90 and look at where the stops start, at around 21, 18 or so metres. That is why we choose 50%. At that point the PN2 does not change much from the 21/35 or 18/45 we are breathing and we no longer require helium. The tissues are in a proper offgassing situation and we want to help them. We stay at that point for 5 mins or so to saturate the hemoglobin (O2 Window) and then our deco becomes very much like any other deco, a fibbonacci sequence if you like.

Two. Gas management. If we only took O2 we would be doing all those earlier stops on backgas at a very low Po2 level. We want a gas available to us earlier in the ascent.

If/when dives start to get longer/deeper then we would add O2.

Andy

[And]

Originally Posted by MattS

You may need to rethink ;

"95% of the CO2 generated in the tissues is carried in the red blood cells:
...Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!) "
http://users.rcn.com/jkimball.ma.ult...s/B/Blood.html

I still can not work out what part plasma plays in O2 exchange in healthy tissue.

Yes, sorry, I was obviously out of my tree last night. CO2 is meant to represent the Oxygen content of the blood and to say that the excess O2 is stored in plasma.

Plasma, well the plasma stores the oxygen before it is picked up by the hemoglobin. It is integral in the gas exchange itself.

On inspiration, the concentration of dissolved oxygen is greater in the alveoli than in the capillaries. Oxygen, therefore, diffuses across the alveolar walls into the blood plasma. In the reverse process, carbon dioxide concentration is greater in the blood than the alveoli, so it passes from the blood into the alveoli and is ultimately breathed out.

As oxygen diffuses into the plasma, hemoglobin in the red blood cell picks up the oxygen, permitting more to flow into the plasma. The oxygen-carrying capacity of hemoglobin allows the blood to carry over 70 times more oxygen than if the oxygen were simply dissolved in the plasma alone.

So it is indeed more efficient. In a normal surface breathing situation, saturating the hemoglobin won't really happen. You'd have to be in a very oxygen rich environment. As the plasma can store o2 then it will once the hemoglobin is saturated. Whether that actually plays any major part in the tissue diffusion of gasses I don't know.

Andy

[Andy Phillips]

Originally Posted by Mark

If 32% could be used in the same way that we use our 50% (i.e. as a deco gas) then I would see it as a good thing. But, it is just a travel gas which allows you to get 10mins longer bottom times and use up more of your nice trimix back gas than we can. We travel up to 21m on our backgas so a larger reserve is required. But, even on backgas we are ongassing less N2 than you are when you switch to your 32% 'deco gas' at 40m.

What it boils down to in the end is that we are probably doing fairly similar overall deco times but your are carrying two stages and we are only carrying one (and our deco is worked into deeper and our shallow stops are much shorter than yours as a result).

Regards,

Mark

Mark,

Think this through a little mate, (and trust me I am willing to be corrected), there are two reasons we use 32%.

1) It gives us a quicker and safer bail out option (don't forget Mark and I dive a buddy system based on self sufficiency) if we loose our back gas we can get onto the 32% at 2.0 deeper than we could 50%

2) Our high PPO2 deco is started deeper than yours. I just can't for the life of me see what the difference is, yes, it is true that during the deco we will be on gassing a certain amount of N2, but when you work out the deco the difference is so minimal as to be out weighed by the consideration in point 1 above.

Mark and I started doing these deep dives useing 50%, over a period of time we tried various gasses and arrived at 32 and 80. We consider this to be the safest options based upon practical experience.

As for the rest of this thread, woof, woof. I have just e-mailed this all to my mate at UCL, hopefully he will find the time to reply to me, cos I am lost.

Take care,

Andrew

[Mark]

Originally Posted by imported_andyp

Mark,

Think this through a little mate, (and trust me I am willing to be corrected), there are two reasons we use 32%.

1) It gives us a quicker and safer bail out option (don't forget Mark and I dive a buddy system based on self sufficiency) if we loose our back gas we can get onto the 32% at 2.0 deeper than we could 50%

2) Our high PPO2 deco is started deeper than yours. I just can't for the life of me see what the difference is, yes, it is true that during the deco we will be on gassing a certain amount of N2, but when you work out the deco the difference is so minimal as to be out weighed by the consideration in point 1 above.

Hi Andy,

I don't go for this deep bailout option I'm afraid. I wouldn't fancy switching to a PO2 of 2.0 in an buddy-less emergency scenario (or any other in water scenario). You could tox VERY easily! If the shit has really hit the fan you would have an increased heart rate and would be breathing heavily, a PO2 of 2.0 is just about the last thing you need to add to the situatition. It would be much better to rely on ones buddy and their safe to breath backgas. The worst buddy in the world could save you simply by donating a backgas reg (preferably long hosed!). Give me an realistic example of when you would 'need' to switch to 32% in an emergency.

I have never even come close to getting separated from Andy BTW. Short of being beamed up by aliens I find it hard to imagine a situation where we would split up. All I have to do is give him a quick 'help' flash with my torch and a long hosed reg is instantly thrust towards me.

When you switch to 32% at 40m you do have a PO2 of 1.6 for a short time but you do not use this high PO2 to accelarate your deco. It takes a good couple of minutes for the gas to circulate the body so we extend our gas switch stops to make the most of the high PO2. In that way we are maximising the benefit from the gas (and the O2 window). Our deco profiles are designed to get the maximum deco benefit from the deco gas. So are yours but only when you get onto the 80%. I'd be interested in what effect the sudden spiking of N2 at 40m has on the offgassing process. If offgassing normally starts due to a decrease in the ambient pressure on ascent then spiking the PN2 must be like going deeper? So 32% can't be an effective deco gas, can it?? I think we must offgass more effeciently upto the gas switch due to the lower N2 content of our backgas and the reducing ambient pressure.

Regards,

Mark

[Andy Phillips]

Originally Posted by Mark

Hi Andy,

I don't go for this deep bailout option I'm afraid. I wouldn't fancy switching to a PO2 of 2.0 in an buddy-less emergency scenario (or any other in water scenario). You could tox VERY easily! If the shit has really hit the fan you would have an increased heart rate and would be breathing heavily, a PO2 of 2.0 is just about the last thing you need to add to the situatition. It would be much better to rely on ones buddy and their safe to breath backgas. The worst buddy in the world could save you simply by donating a backgas reg (preferably long hosed!). Give me an realistic example of when you would 'need' to switch to 32% in an emergency.



OK, for and example, we did a dive earlier this year, we could not see each other at the bottom of the shot, we had in fact lost each other at the very begining of the dive (more to the point I lost the shot the moment I let go, the vis was literally zero). There have been one or two other occasions where we have lost each other, for whatever reason, admitidly, we found each other again because we have an agreed proceedure to do this. Poo happens as we all know, we plan our dives on this bases. Hence the bailout option.



I have never even come close to getting separated from Andy BTW. Short of being beamed up by aliens I find it hard to imagine a situation where we would split up. All I have to do is give him a quick 'help' flash with my torch and a long hosed reg is instantly thrust towards me.



Couldn't see my own torch let alone Marks.



When you switch to 32% at 40m you do have a PO2 of 1.6 for a short time but you do not use this high PO2 to accelarate your deco. It takes a good couple of minutes for the gas to circulate the body so we extend our gas switch stops to make the most of the high PO2. In that way we are maximising the benefit from the gas (and the O2 window). Our deco profiles are designed to get the maximum deco benefit from the deco gas. So are yours but only when you get onto the 80%. I'd be interested in what effect the sudden spiking of N2 at 40m has on the offgassing process. If offgassing normally starts due to a decrease in the ambient pressure on ascent then spiking the PN2 must be like going deeper? So 32% can't be an effective deco gas, can it?? I think we must offgass more effeciently upto the gas switch due to the lower N2 content of our backgas and the reducing ambient pressure.

Regards,

Mark

OK, I understand the GUE theory on this, I just don't get it. We hold the first stop for 2 mins to get one complete cycle then off we go. As I said, you could argue that we are on gassing N2 but the penalty for this is minimal and is out weighed my the bail out arguament as far as I am concerned. In addition to this, we are keeping our PPO2 high for quite a long period of the deco, and switch earlier than you onto 80%. As for the 2.0 arguament, this is the earliest I can realistically get onto a gas, it is the max recommended PPO2 by the US Navy. If push came to shove and I had a choice of definitly drowning, or possibly having an O2 hit. I would take the option where there was even the hint of a chance of survival. It is true though, If I was running a single deco gas, it would be 50% on a 50 ~ 60M dive.


As for N2 spike's, OK this is a different matter altogether, and is an arguament that is only just begining. In truth if we followed the latest theory we would replace N2 with He and run 100% O2 on our final stop to wipe the He out of our bodies. From the discussions I have had with various people who have done deep (100M ++) dives and with my mate at UCL it really doesn't matter on dives to 70M, past that at least the intermediate (travel) gas should have He in to reduce this "spike" from the N2 as we assend.

We did this on the 100 M dive we did last year and intend to repeat the process later this year when we do a couple of others down to this again

I don't expect you to agree with me, but I have formed these opinions over a period of time. The one area where I am completely mailble at the moment is the issue of adding He to the deco mix.

Andrew

ps, there is a bit of reply in the quote bit above, I just couldn't work out how to copy little bits.

[Dave Crampton]

I took Andy's initial post of:

1) It gives us a quicker and safer bail out option (don't forget Mark and I dive a buddy system based on self sufficiency) if we loose our back gas we can get onto the 32% at 2.0 deeper than we could 50%

to mean we can get onto EAN32 2 times deeper than we could on EAN50, as EAN32 and EAN50 gives 1.6 PPo2 at 40m and 22m respectively, roughly 2 times, until I read his reply to your post Mark.

I read Andy's post to mean that he can get onto deco earlier/deeper, and has the option of bailout onto EAN32 if he needs it. This 'only if he needs it is re-enforced by the last statement in point 2:

but when you work out the deco the difference is so minimal as to be out weighed by the consideration in point 1 above.

I was a little confussed then Andy, in your reply to Mark you state that:

As for the 2.0 arguament, this is the earliest I can realistically get onto a gas, it is the max recommended PPO2 by the US Navy.

I think you may have mis-interpreted Mark's reply about the number 2.0 which I took you to mean 2 times, as Mark appears to have read 2.0 as a PPo2 level. Now I claim to be a complete novice here but I do recall Andy Hayhurst saying on my Nitrox course that although US Navy divers do dive PPO2 levels higher than our recreational max of 1.6, they are young, very fit men in their late teens/early twenty's, and that they are quite expendable, i.e. this one's expired, send the next guy to put his equipment on and go back down. Now I'm not trying to be be rude or disrespectfull here Andy but I don't beleive your in your early teens, especially as I think your a director in your own company right?

Do I have the right impression of what you ment when you stated 2.0, 2x the depth of gas switch instead of a switch on to deco at 2.0 PPO2, especially as you said you could use it as a bailout/earlier deco start depth?

By the way I'm reading all this with interest guys, partly, as I'm diving with a club buddy on Tuesday who is a Consultant Anaesthetist. I expect he'll be expecting the usual, dive chit chat and this will throw him when I start asking about hemegloben saturation, plasma O2 carrier concentration and oxygen windows!

I'm not advocating anyones choice of Deco mix here. I'm just trying to soak up as much off you guys as I can.

Dave C