Increased nitrogen off-gassing 10ft/3m VS 20ft/6m on 100% oxygen

[Brett Hatch]

@boulderjohn is it fair to say that if the only thing we cared about was a gas exchange between the blood and the lungs, then it would be perfectly safe to:
- ascend to 20ft (following safe ascent rates and any deco obligations and gas switches along the way)
- switch to 100% O2
- ascend directly to the surface

And that the problem with this simplified model is that in reality, we also care about the gas exchange between the tissues and the blood, which is depth-dependent?

Because without that (or perhaps some other thing I didn't think of) I cannot see the mechanism that forces a long hang time after switching to O2. After all, the shallowest stops are the longest, so if some reasoning leads us to conclude that we can skip it, we must have gone really wrong somewhere along the way.

 

[L13]

tes

@boulderjohn is it fair to say that if the only thing we cared about was a gas exchange between the blood and the lungs, then it would be perfectly safe to:
- ascend to 20ft (following safe ascent rates and any deco obligations and gas switches along the way)
- switch to 100% O2
- ascend directly to the surface

And that the problem with this simplified model is that in reality, we also care about the gas exchange between the tissues and the blood, which is depth-dependent?

Because without that (or perhaps some other thing I didn't think of) I cannot see the mechanism that forces a long hang time after switching to O2. After all, the shallowest stops are the longest, so if some reasoning leads us to conclude that we can skip it, we must have gone really wrong somewhere along the way.

Once you switch to 100% O2, off gassing is maximized. This DOES NOT MEAN THAT DCS RISK IS ELIMINATED!

Even at the maximum rate, it takes time to off gas to a safe surfacing level.

You spend time at 20' to allow that maximized off gassing to reduce dissolved inert gas in your tissues. This takes time even at the maximized rate. Once the dissolved inert gas is reduced to a safe level you can progress to 10' to reduce CNS risk, or wait 20'. In either case you wait more time till the maximized off gassing reduces the dissolved inert gas to a level safe for surfacing.

 

[boulderjohn]

@boulderjohn is it fair to say that if the only thing we cared about was a gas exchange between the blood and the lungs, then it would be perfectly safe to:
- ascend to 20ft (following safe ascent rates and any deco obligations and gas switches along the way)
- switch to 100% O2
- ascend directly to the surface

And that the problem with this simplified model is that in reality, we also care about the gas exchange between the tissues and the blood, which is depth-dependent?

Because without that (or perhaps some other thing I didn't think of) I cannot see the mechanism that forces a long hang time after switching to O2. After all, the shallowest stops are the longest, so if some reasoning leads us to conclude that we can skip it, we must have gone really wrong somewhere along the way.

We need to stay at a safe depth as the nitrogen leaves our tissues. If enough nitrogen as left our tissues at the end of our 30 foot stop, then we can ascend to 20 feet. Once there, we can switch to oxygen, but we cannot go directly to the surface because we still have lots of excess nitrogen in our tissues. the fact that we have switched to oxygen allows our nitrogen levels to drop more quickly. Note that I said "nitrogen levels to drop more quickly." I did not say "nitrogen leaves our tissues more quickly." It does not. What is different is that we are not getting any more nitrogen once we have switched to oxygen.

The rate of gas exchange is depth dependent because we inhale differing amounts of gas at different depths. When we go to 100 feet, 4 times as much nitrogen is entering our lungs as at the surface.

You cannot ascend immediately after the oxygen switch because it is the pressure differential between your tissues and the environmental pressure (water plus atmosphere) that causes DCS. That has nothing to do with the differential between the tissues and the gas you are breathing.

 

[boulderjohn]

Once the dissolved inert gas is reduced to a safe level you can progress to 10' to reduce CNS risk, or wait 20'.

Yes, but there are also other considerations in deciding depth for the oxygen stop(s). There are depths in between those two, and there is no harm in going there.

I hate the 20 foot hang because I have to make sure I don't drop a foot or two or ascend a foot or two. My computer just gets pissed either way. I have my computer set for a 10 foot final stop, and once the 20 foot stop is over, I'm heading up--but not to 10 feet.

Mild exercise during the final stop is recommended to increase perfusion (blood flow), which improves off gassing. I have seen videos of the divers with the Richard Pyle team doing what looks like calisthenics or dance moves during those stops. There is no benefit and some harm doing a motionless hang. Where I do most of my diving, we do a long, slow swim once we reach the 20 foot stop. For a while we have to be careful with our depth, but once we are clear of 20 feet, we go to about 15 feet so we can swim along without having to pay any special attention to our depth.

 

[inquis]

we also care about the gas exchange between the tissues and the blood, which is depth-dependent?

As to whether that gas exchange is depth-dependent... well, it depends.

As I think everyone would agree, it's the gradient between inert gas pressure in the source & destination that governs the rate of transfer. Again, by virtue of speaking of "inert gas", I maintain that means partial pressure. In the lungs, the (inspired) ppN2 is mix*absolute pressure. When any nitrogen is in the mix, that leads to a depth-dependent inspired ppN2 (as you said). However, when there is no nitrogen in the mix, that "mix" term is 0 and the ppN2 is 0, no matter what the absolute pressure is. Thus, inspired ppN2 is NOT depth-dependent when breathing O2.

What about the "source" side of things (tissues)? Tissue tension does not change very quickly, and is therefore NOT terribly depth-dependent as you ascend or descend. Think about it, when you drop down to 30 m breathing air, the tissue tension (i.e., ppN2 in the tissues) is most definitely NOT 0.21*4 atm (0.84 atm) when you level off. Yes, wait long enough and it will be that, but then we go and ascend which changes everything again. After reducing depth, the tissue ppN2 doesn't change just because the ambient pressure drops. It changes because the gradient drives the gas to move. The nitrogen moves within tissues, between tissue & blood, and between blood & alveoli based on the (difference in) ppN2 in each.

 

[inquis]

You cannot ascend immediately after the oxygen switch because it is the pressure differential between your tissues and the environmental pressure (water plus atmosphere) that causes DCS. That has nothing to do with the differential between the tissues and the gas you are breathing.

Quoting this for emphasis. How big a bubble gets (or even if one forms) is governed by the former. How fast gas moves from place to place is governed by the latter.

 

[dmaziuk]

As to whether that gas exchange is depth-dependent... well, it depends.

Well, , the case of decompressing on pure O2 is the clear proof by counter-example that it doesn't: it's driven solely by PPN2 in the tissues, at any depth.

 

[inquis]

I cannot see the mechanism that forces a long hang time after switching to O2.

The long half-times of some of the tissue compartments drive the long hang times. Gas moves out of such tissues very slowly, in spite of the maximum gradient achieved by breathing O2. We switch to O2 because anything else would be even longer!

 

[Brett Hatch]

Alright, it sounds like I was thinking about it wrong in two ways. One, ignoring the gradient between the tissues and the blood. And two, ignoring that DCS does not occur because of a large blood:lung gradient, or a large tissue:blood gradient for that matter, it occurs because of a large volume of expanding gas in either the tissues or the blood.

Tell me if it makes sense to think of this way: for each tissue compartment, there are 2 relevant gradients. One gradient from lungs to the blood, and a second gradient from the blood to the tissue. By inhaling O2, the lung:blood N2 gradient is maximized, which facilitates N2 exiting the blood as quickly as possible, which drops the ppN2 in the blood as fast as possible. As the ppN2 in the blood drops, it will create the second blood:tissue gradient; the faster the blood ppN2 drops, the larger this second gradient, which facilitates N2 exiting the tissues and entering the blood as fast as possible.

But nothing in the above paragraph has anything to do with DCS at all (at least, not directly). DCS occurs when the volume of inert gas reaches a critical point. The mechanism for the large volume of gas is to first descend, and the N2 dissolved in the tissues will asymptotically approach the partial pressure of whatever a diver is breathing. Then as the diver ascends, the gradient changes such that the diver begins to offgas and whatever N2 is still dissolved in their tissues will expand. That expansion is the cause of DCS, not the gradient and offgassing.

Do I have that right? If so, I think I can see where I got confused: if you only ever breathe 1 gas in a dive, then pressure gradients always increase at the same time as gas expansion, and vice versa. But with gas switching, these no longer go hand in hand. Case in point, switching to O2 greatly increases the gradient and offgassing, but it does nothing at all to the volume of the dissolved gas in the tissues.

 

[Nick_Radov]

I think you're asking about the oxygen window? This article might answer your questions, at least from a theoretical perspective.

The effect of hyperbaric oxygen on the oxygen window | GUE

www.gue.com

But I don't think anyone has done a large-scale study to empirically determine whether it is better to do part of your oxygen deco at 10 ft, so we don't really know for certain. Obviously on an open-circuit deep dive you have to stop at 20 ft to do your gas switch and then stay for some time to prevent too many bubbles from forming. But after that point is it better (from the perspective of minimizing DCS risk or minimizing total deco time) to stay at 20 ft for the remainder of your shallow deco time, or move up to 10 ft and spend a significant amount of time there? Who knows.

From a practical standpoint for ocean diving in rough conditions, buoyancy control at 10 ft is kind of a hassle. I mean I can do it but I'm constantly watching my depth gauge and making minor adjustments. Whereas sitting at 20 ft is more relaxing.