Last Deco Stop Depth, Oxygen Window, and Deco Efficacy

[Dr. Lecter]

Kev makes the case fairly clearly for the only difference in final stop depth being CNS loading (how much stock, if any, one puts in that metric is obviously quite varied...see, e.g., rjack's asinine echoing of AJ from an earlier thread), but one aspect remains at least theoretically unaddressed and is worth chewing on for the sake of the OP's point. Though I'm not aware of any model considering it important enough to alter run times when toggling between a 20' and 10' final stop depth.

While tissue gradients may not fluctuate between 20' and 10' on 100% O2, tissue tensions certainly still do. Theoretically, of course, as long as you're at 20' long enough to satisfy your model, it does no harm to do the remainder of your time at 15' or 10' or wherever's below the ceiling. Realistically, you're going to have bigger bubbles and more bubbles doing 20' and 10' stops than if you'd simply stayed at 20' the whole time. And we know it's harder to eliminate any undissolved gas, much less larger bubbles of it, than it is to off gas dissolved gas - thus, there's a theoretical efficiency case for doing the whole thing at 20'. But absent truly insane seas, I'll take the lower CNS loading for decos much past 2hrs.

 

[Kevrumbo]

Kev makes the case fairly clearly for the only difference in final stop depth being CNS loading (how much stock, if any, one puts in that metric is obviously quite varied...see, e.g., rjack's asinine echoing of AJ from an earlier thread), but one aspect remains at least theoretically unaddressed and is worth chewing on for the sake of the OP's point. Though I'm not aware of any model considering it important enough to alter run times when toggling between a 20' and 10' final stop depth.

While tissue gradients may not fluctuate between 20' and 10' on 100% O2, tissue tensions certainly still do. Theoretically, of course, as long as you're at 20' long enough to satisfy your model, it does no harm to do the remainder of your time at 15' or 10' or wherever's below the ceiling. Realistically, you're going to have bigger bubbles and more bubbles doing 20' and 10' stops than if you'd simply stayed at 20' the whole time. And we know it's harder to eliminate any undissolved gas, much less larger bubbles of it, than it is to off gas dissolved gas - thus, there's a theoretical efficiency case for doing the whole thing at 20'. But absent truly insane seas, I'll take the lower CNS loading for decos much past 2hrs.

. . .and absent insane Great White Shark encounters :

(LAUE/GUE Divers encounter at depth & deco over a week ago):
http://www.scubaboard.com/forums/socal/490629-great-white-shark-encounter-off-catalina-island.html
___
There's also the conservatively slow 1meter-per-minute final ascent from 6m/20' on Oxygen to the surface that's currently "in vogue", to keep those residual bubbles in check. . .

 

[Michael Guerrero]

Is the assumption that we want to minimize deco time also? From an oxtox standpoint 10' seems safer, but what about pushing the boundaries on the M-Values?

Like Tom mentioned above, I don't think you can factor in minimizing deco in this scenario since whether you do your last stop at 20ft or 10ft, using the same gas you pretty much have the same deco.

As far as pushing the m-values, I kind of feel like if you're getting out of the water at 85% GF, you're getting out of the water at 85%, however you got there, but that may be untrue.

The exertion thing was not related to my original question.

---------- Post added September 3rd, 2014 at 03:43 AM ----------

I have found that if i take 3 or 4 deep breaths from my air tank (exhaling fully on each cycle) - which is essentially a very mild hyperventilation- I can then switch to my oxygen tank and have a VERY low gas consumption rate for a couple of minutes. I mean I only feel the need to breath maybe 3 or 4 times per minute. After maybe 5 minutes, my breathing rate seems to go up, presumably as Co2 rises and I "feel" like i can blow it back down with another cycle of deep breaths from my air tank.

Is this common knowledge or does this have some significant hazard I am not aware of? I am familiar with hyperventilation and the dangers it presents with breathhold diving.

Don't know if it is a hazard, but the drive to ventilate is controlled by arterial CO2, and hyperventilation lowers arterial CO2 (which is why it helps breathhold divers) so it sounds plausible to me. I'm not a physiology expert though. You should watch the Dr. Vann video from 2008, he talks about an early rebreather researcher who would feel symptoms of oxtox coming on (tremors mostly if I recall), and would hyperventilate and the symptoms would go away. Very interesting. I can send you the link if you want it.

--------------------------

And thanks Kevrumbo for posting that. Do you know who provided the response to Chickdiver, I didn't see it in the repost? Will read thoroughly when I get home from work.

I also need to look at tissue tension more. I'm unclear as to whether tissue tension is a function of abmient pressure and total gas tension or if ambient pressure is just kind of holding it all into the tissue while the volume of absorbed gas is being depleted through off-gassing. My thinking is definitely slanted in the supersaturation model vs. the bubble model theory, but I need to look at this further.

At the very least it would seem I don't have a ton of problems in my understanding of the current theories since the discussion is revolving around bubbles vs. oxygen exposure, so that at least makes me feel better.

 

[dreamdive]

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.

Here are some studies to the contrary:

Dujic Z, et al - Exercise during a 3-min decompression stop reduces postdive venous gas bubbles.Medicine and Science in Sports and Exercise 2005; 37: 1319-1323


Jankowski LW, [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Nishi RY[/COLOR], [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Eaton DJ[/COLOR], [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Griffin AP [/COLOR]- Exercise during decompression reduces the amountof venous gas emboli.
Journal of Undersea and Hyperbaric Medicine 1997; 24: 59-65




To "make the physical exertion level as close to non existent as possible." is not a good idea as Michael already pointed out.

BTW: it appears that fit divers are off-gassing more efficiently than non-fit ones.

 

[tomfcrist]

The exertion thing was not related to my original question.

It IS related....specifically as it relates to OXTOX.

Here are some studies to the contrary:

Dujic Z, et al - Exercise during a 3-min decompression stop reduces postdive venous gas bubbles.Medicine and Science in Sports and Exercise 2005; 37: 1319-1323


Jankowski LW, [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Nishi RY[/COLOR], [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Eaton DJ[/COLOR], [COLOR=rgb(14.901960%, 14.901960%, 14.901960%)]Griffin AP [/COLOR]- Exercise during decompression reduces the amountof venous gas emboli.
Journal of Undersea and Hyperbaric Medicine 1997; 24: 59-65




To "make the physical exertion level as close to non existent as possible." is not a good idea as Michael already pointed out.

BTW: it appears that fit divers are off-gassing more efficiently than non-fit ones.

REF A Method.....[h=4]METHODS:[/h]Ten healthy, military male divers performed an open-sea field dive to 30 m of sea water breathing air, remaining at pressure for 30 min. During the bottom and decompression the subjects performed fin underwater swimming at about 30% of maximal oxygen uptake. Each diver underwent two randomly assigned dives, one with and one without exercise during the 3-min decompression period. Monitoring of venous gas emboli was performed in the right heart with ultrasonic scanner every 20 min for 60 min after reaching surface pressure in supine rest and during forced two-cough procedure.


This deco was done on AIR.....
REF B abstract........To determine the effects of moderate, intermittent exercise during decompression on the Doppler detectable amount of venous gas emboli (VGE), 29 healthy male volunteers performed 44 wet (8 degrees +/- 2 degrees C) dives to 45 msw (450 kPa) for 30 min with standard air decompression. During compression and the bottom period, all subjects were inactive; during decompression, 28 remained inactive, 11 performed leg exercise, and 5 did arm exercise. Intermittent exercise was controlled at approximately 50% of each subject's arm or leg aerobic capacity. At 30-min intervals after surfacing, subjects were monitored with a Doppler ultrasonic bubble detector. The Doppler scores were used to calculate the Kisman Integrated Severity Score (KISS). The KISS were log transformed (with zeroes being equivalent to log 0.01) and analyzed with a one-way analysis of variance. No significant differences (P < or = .05) between mean KISS scores after arm or leg exercise were observed, thus these data were pooled and compared to those of the inactive controls. The mean pooled KISS after exercising during decompression were significantly lower than those of the inactive controls. Moderate, intermittent exercise during decompression apparently reduces the amount of Doppler-detectable VGE after diving. The incidence rate of decompression sickness in both groups was not significantly different (P < 0.05).


Once Again....DECO on AIR.....


If you want to really look into the exercise option to reduce bubble micronuclei, thus reducing DCS risk, try some good aerobic exercise 24hrs prior to the dive....it has the same effect, without increasing your OXtox risk.

 

[shoredivr]

And thanks Kevrumbo for posting that. Do you know who provided the response to Chickdiver, I didn't see it in the repost? Will read thoroughly when I get home from work.

It says clearly Deepstops2003.

 

[dreamdive]

Tom,

I was taught the same thing: have to be perfectly still on deco and perfectly horizontal or else. Not so!

The studies are reporting mild to moderate exercise during your deco and, unless one is totally out of shape, should not be contributing to any CO2 retention with subsequent risk to seizure from high PO2 (1.6 ATA)

As Michael already mentioned, the idea is to increase perfusion during your deco to facilitate off-gassing. One Navy study with impressive numbers has demonstrated that the best on/off-gassing scenario is being cold during descent and bottom phase and warm on your ascent/deco. Obviously that is a difficult to achieve as one works harder during phase 1 and 2 and typically the least on ascent.

Considering that we have data to support the moving around during deco has helped me stay warmer and less bored.

Cheers,

Claudia

PS: I realize this is totally anecdotal, by I run my PO2 at 1.2 to 1.3 during the dive and increase it to 1.6 on deco while "exercising" with no seizures.

I posted the studies not to disprove your claim that exercise during deco decreases your seizure threshold, but to show that exercise during deco improves off-gassing.

 

[TSandM]

The question of 10 foot and 20 foot stops isn't a new one. In terms of decompression efficiency, it doesn't matter. CNS loading is higher at 20 feet. If you are diving in a mirror-calm pond, pretty clearly you should plan to split your O2 time between the depths, to control bubble growth but at the same time reduce CNS exposure.

In other diving circumstances, however, it may be preferable to do the stops deeper. If surface water conditions are rough, or if there is a lot of boat traffic, you might be better off staying at 20. I've done dives in Puget Sound where there was so much surface current we could barely get to the downline, but that current absolutely died at 20 feet.

 

[boulderjohn]

The idea of the oxygen window as defined by GI3 as an added benefit was based on a single piece of research that has been oft discussed. I read it carefully during those discussions, and even with my lack of true science background, I could see a problem. The only paragraph in which it is mentioned leaps out of nowhere. The conclusion is not actually based on any of the data that came before (although it seems to be if you don't look carefully), and it violates Dalton's Law. I was at that time training with an agency (UTD) that was thoroughly wedded to that idea in its polices, and I argued strenuously with its leadership about it. When I finally took the formal Ratio Deco course from its owner (Andrew Georgitsis), he (knowing I was in the class and bringing my concerns with me) said that they had decided that the study was indeed flawed and the old idea of the oxygen window may not be true. He did come up with two other reasons (and I can't remember them) for holding to their idea of the S-curve profile, though.

At about the same time, I had an email exchange with Jarrod Jablonski of GUE about this. He, too, said that they realized that the oxygen window theory was flawed, but they, too, were holding to the S-curve model that was based on it because it had worked effectively for a number of years. I have since been told they have abandoned it altogether.

In short, the idea that a high PPO2 during decompression held an additional advantage beyond the loss of nitrogen, however brief, was based on a single discredited piece of research, and it has been abandoned by most of its former supporters.

 

[PfcAJ]

The s curve has been more or less replaced with a linear deco. So there is more time at the gas switch stops than what buhlmann or vpm prescribe, but it's not as pronounced as the older a curve ascent strategy.