Last Deco Stop Depth, Oxygen Window, and Deco Efficacy

[Michael Guerrero]

Recently I was party to a conversation regarding what depth to use for last stops and why. I want to outline my understanding of the relevant factors with respect to deco efficacy only, not environmental conditions (surge, temp, etc.) and solicit your comments on the accuracy of my statements.

1. Decompression occurs when the gas tissue tension exceeds the inspired partial pressure (pp) of the same gas.

2. Gases on- and off-gas independent of other inert or metabolic gases and rates are based on gas gradients, perfusion, diffusion, and solubility.

3. The principle way to create an inert gas gradient is to change ambient pressure (descending and ascending), and an alternative way is to change the fraction of inspired inert gas.

4. During ascent, ambient pressure decreases, lowering the pp of inspired gas relative to tissue tension of the same gas.

5. When the inspired pp of an inert gas is lower than the tissue tension of the same inert gas, the inert gas diffuses out of the tissue, into the venous bloodstream, to the lungs, and out of the body.

6. Off-gassing is dependent in pp, which is normally linked to ambient pressure.

7. When we use oxygen to accelerate decompression, we are decoupling pp from ambient pressure by creating a greater reduction in pp of a specific inert gas (increasing the gradient).

8. This is known to some people as the oxygen window (there are other definitions for the oxygen window), and allows accelerated inert gas off-gassing, potentially exceeding compartment m-values.

9. This does not increase our chances of creating bubbles because bubbles are affected by the relationship between tissue tension and ambient pressure.

10. The presence of oxygen in inspired gas does not in and of itself accelerate off-gassing.

11. It is only when oxygen (or another gas) displaces a portion of an inspired inert gas during decompression that accelerated off-gassing occurs.

12. Pp is affected by ambient pressure (depth), and reducing depth during decompression increases the inert gas gradient between the tissues and inspired gas (assuming no change in inspired gas).

13. A shallower depth leads to an increase in gradient, and greater off-gassing due to decreased inert gas pp.

14. Oxygen is considered dangerous at a pp greater than 1.6 during deco, and reducing depth reduces the pp of oxygen (i.e., the pp of oxygen is lower at 10ft than at 20ft if breathing pure oxygen).

15. Greater off-gassing does not occur between 20ft and 10ft on pure oxygen because the pp of inspired inert gas is 0, and thus the gradient cannot be increased once on pure oxygen.

16. Your risk of oxygen toxicity is less at 10ft than 20ft due to the reduced pp of oxygen (1.3 vs. 1.6).

17. A pp of 1.6 is used strictly for prevention of oxygen toxicity and not off-gassing efficacy (i.e., inspired inert gas displacement is what really accelerates decompression).

Given the above, and precluding any environmental factors increasing the risk of holding a 10ft stop vs. a 20ft stop, it is safer to do your last deco stop at 10ft rather than 20ft by virtue of reduced exposure to a high oxygen pp.

My sources for all this are Mark Powell's book Deco for Divers, Simon Mitchell's presentations at the 2008 DAN Tech Diving Conference and DAN's Rebreather Forum 3.0 conference, David Doolette's presentations at both conferences, as well as some others from the 2008 DAN conference.

 

[raftingtigger]

No expert here, but you have to balance the increased risk of oxygen toxicity at 20' vs. the decreased ambient pressure (and consequent push for more bubble formation) at 10'. My take from Deco for Divers was that given everything you explained then since there is no faster off gassing at 10' and less bubble formation pressure at 20' you are better off with the deeper stop. Doesn't address your OxTox issue however.

 

[Michael Guerrero]

Your deco algorithm (whichever one you choose to believe) should accomplish minimization of bubbling I think. I'm not advocating going to a stop earlier than the deco algorithm, only that given the choice of doing your full O2 deco at 20ft or 10ft, it's safer to go to 10ft when your algorithm allows based on oxygen toxicity (assuming same GF/conservatism upon surfacing).

 

[tomfcrist]

Assuming there is no physical exertion at the 20' stop on O2, that stop is the most efficient for offgassing(vice the 10' stop).

When considering oxtox, do your gas breaks preferably more often than the standard 20 on 5 off, and use a Jon line to make the physical exertion level as close to non existent as possible.

 

[Michael Guerrero]

Assuming there is no physical exertion at the 20' stop on O2, that stop is the most efficient for offgassing(vice the 10' stop).

Care to explain? Also, it's my understanding mild exertion is better than no exertion during all phases of deco because it increases blood flow and warms the body (almost always better for deco unless you're in danger of heat exhaustion).

Little exertion and cold on the bottom, mild exertion and warm during deco.

 

[tomfcrist]

Assuming there is no physical exertion at the 20' stop on O2, that stop is the most efficient for offgassing(vice the 10' stop).

Care to explain? Also, it's my understanding mild exertion is better than no exertion during all phases of deco because it increases blood flow and warms the body (almost always better for deco unless you're in danger of heat exhaustion).

Little exertion and cold on the bottom, mild exertion and warm during deco.

Little exertion on the bottom phase isn't very realistic...

Exertion on backgas during deco has it's merits, which may be where you got that concept from. Unfortunately I exertion also causes increased Carbon dioxide retention, in turn increasing the possibility of oxtox. Exercise while breathing 100% O2 is a really bad practice.

 

[Michael Guerrero]

I guess I should say the lower the level of exertion the better on the bottom (photographers and dpv drivers seem pretty lazy to me ).

I'd also say increased exertion increases CO2 production (probably not significantly for very mild exertion unless you're really out of shape and putting your shoes on makes you winded, in which case you should not be doing these dives anyway), but not necessarily retention. Increased CO2 production (for most people) results in increased respiration and elimination of additional CO2. Cerebral blood flow is controlled by arterial CO2, and if you can reduce it by ventilation you may mitigate the affect (per Dr. Richard Vann's presentation in 2008).

But all this is really a distraction from my main concern in this thread, which is the depth of the last stop on O2.

So how about your rationale for preferring the 20ft stop?

 

[tomfcrist]

The 20' stop...easy....o2 is most efficient in aiding offgassing at that depth(higher ppo2 with zero ppn2 or ppHe).

In relation to the previous point, light finning and moving wouldn't be enough exertion to worry about during o2 deco, but fighting current, or swimming around excessively could cause increased Carbon Dioxide levels....there are many, many, did I say many? Cases of oxtox during increased exercise while breathing high ppo2's. The only explanation I have seen for this is carbon dioxide.

 

[Michael Guerrero]

How does a higher ppO2 accelerate off-gassing if ppN2 (or ppHe) is already at 0? So which of points 10, 11, 15, and 17 above are you saying is incorrect?

 

[Dr. Lecter]

The 20' stop...easy....o2 is most efficient in aiding offgassing at that depth(higher ppo2 with zero ppn2 or ppHe).

I think he's looking for a defense of the concept that a 1.6ppO2 is somehow more efficient at offgassing N2/He than the lower ppO2 of the 10' stop, assuming 0.00pp of N2 and He at each ppO2.