Oxygen Window and deeper stops

[trheeltek]

I was trained to tack on an extra 5 minutes of runtime to my shallowest stop for safety (O2 at 20 ft, usually). After reading some about the oxygen window and the opinions of George Irvine, I'm wondering if I'm tacking the 5 minutes on the wrong stop.

Would it make more sense to do the extra time on a 50/50 mix at 70 ft, where presumably the O2 window is maxed out, and I'm disproportionately off-gassing fast compartments. I have the impression that fast tissues include lipids around nerves, etc.; do I lower my risk of type II DCS any by extending the deep stop instead of the shallow one? Or is is a wash (so to speak). Or is my grasp of theory still a little underwhelming.

 

[BIGJC]

I understand Irvine's theory to be that the proper decompression schedule is about hitting the right "curve" and that adding on safety time to one stop or another only serves to invalidate the schedule. I think he believes that traveling through the ranges of pressure gradients that occurr on ascent at the proper speed will allow for all of the tissues (fast or slow) to expel the proper amount of gas at each depth. Sort of like how air expands and vents in your halcyon rig (how did I know that??) when your on your way up the anchor line.

that being said....

It seems like having extra time at the deep 70 foot stop on 50/50 would allow gas to come out of your tissues faster (due to the high pPO2) and also allow for any bubbles to pass and be filtered without expanding to critical size. Both are valid concepts and ultimately impossible to prove... This idea is true (but much less so due to the limited gas compression) at 20 feet on oxygen... so in my humble opinion extra time on the deep stop is more valuable than at 20 feet on O2... but Hey why not do both? as long as you don't overexpose yourself to CNS toxicity??

 

[BigJetDriver]

I hope you'll pardon my intrusion, but here are some thoughts on the subject.

"Pearling fleets, operating in the deep tidal waters off northern Australia, employed Okinawan divers who regularly journeyed to depths of 300 fsw for as long as one hour, two times a day, six days per week, and ten months out of the year. Driven by economics, not science, these divers developed optimized decompression schedules empirically. As reported by Le Messurier and Hills, deeper decompression stops, but shorter decompression times than required by Haldane theory, were characteristic of their profiles. Such protocols are entirely consistent with MINIMIZING bubble growth and the excitation of nuclei through the application of increased pressure, as are shallow safety stops and slow ascent rates." (Technical Diving in Depth, B. R. Wienke, PhD., Los Alamos National Laboratory, as published by Best Publishing)(Emphasis mine.)

Both the deep stops, as popularized by Richard Pyle, of the Bishop Museum, and shallow stops, especially at high PPO2's, are useful.

Ascent rates are ALWAYS part of the decompression computation, no matter what computaion you are using. and slow ascent rates have proven safer.

There is nothing mystical about the shape of any decompression computation. If you plot it on a graph, it will have a "shape" of some kind. Usually, and I am sure you will have noted this, if plotted as depth against time, it will take the form of an arc that is almost vertical at one end, arcing and shallowing toward the other, albeit with some sawtooth or jagged edges.

------THE QUOTE ABOVE IS AN HISTORICAL REFERENCE AND IS IN NO WAY A RECOMMENDATION FOR DIVING PROCEDURES!----------

 

[Dr Deco]

Dear Readers:

Threading the Needle

There is the idea among divers that decompression is similar to threading a needle, that is, there is a very narrow passageway though which you must pass to reach the “promised land.” There are probably several reasons for this. In my years of experience with table designers, I might proffer the following:

[1.] For decades, divers have believed that all inert gas remains in solution in the tissues. The conclusion is that you can come within a “hair’s breadth” of the “metastable limit,” ascend to the surface, and live to see the next sunrise.

[2.] Table designers and decompression modelers are driven by the unflappable belief in the veracity of their model(s). They are idealists, and I have never personally known one driven solely by a profit motive. They know that, if you study what they have done, you will be drawn ineluctably to the same conclusion.

[3.] Modelers believe that their mathematics will capture the salient points. While other factors can be present, those included in the model are the major ones.

Conservatism

To allow for the considerable variance in dose/response (quantity of nitrogen/response to decompression), there is considerable conservatism built into tables (and deco meters) to allow for a safe and successful outcome for virtually all divers when the device is used under the conditions employed to collect the model parameters. Yes, you certainly get DCS even if you follow the table, as virtually all divers are aware.

What Else?

As one who does not design tables, I am free to talk, and I believe that the system is different both for supersaturation (Haldane models) and even bubble size/number (two-phase models). We have question of the surfactants and the nature of these, the activity levels tissue perfusion, the ease of generation of nuclei, and the apparent interplay of nitric oxide and exercise (possibly the same as the preceding condition).

I do not believe that there is a “right curve.” Certainly there are generalities such as the “oxygen window” is larger at deeper depths and quick ascents will cause Boyle’s law expansion of microbubbles and reduce the Laplace pressure. This is not quite the same thing.

Whenever decompression is performed entirely in the water with adynamic divers, the whole nuclei generation rate is changed over the same decompression in a chamber (I believe). Much success is thus attributed to “the special method.”

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[BigJetDriver]

Doc Deco, et al,

I have to say, after all of the study I have put into the subject of decompression theory (and continue to do so), that is one of the finest, IF NOT THE FINEST, capsule comments on the whole subject.

RIGHT ON, DOC!!!

 

[Kendall Raine]

Lots of interesting thoughts here. I'd like to add to two of them: Does adding extra time to the 70 foot stop on 50% (PO2=1.6 ATA) yield added decompression benefit over adding extra time to a 20 foot stop on 100% (PO2=1.6 ATA)? And, is there a "right" curve?

First, the oxygen window is NOT wide open at 1.6 ATA. It maximises at about 3.0 ATA. Hence, Table 6A puts divers on 100% at 60 fsw (2.8 ATA). 1.6 ATA is the "widest" it gets within generally accepted CNS toxicity limits.

As Dr. Powell discussed, there is much support for the position that deco is a balance between harmlessly removing inert gas from existing gas and dissolved phases. Deep stops are designed for the first, shallow stops the second. It's important to resolve inert gas in bubble form before the bubble radii exceed Laplace cut off. Hence, deep stops need to be deep enough and long enough to accomplish this without creating new propblems from additional inert gas loading. Extending the 70 at 1.6 ATA stop is designed to "clean up" some of the gas in bubble form before they exceed their Laplace limits and start to grow from further decompression causing other problems. The 20 stop on 100% is designed to maximize the pressure gradient in two ways: 1) reducing external pressure, and 2) removing inspired inert gas. This then hastens dissolved gas elimination.

What does this mean? From personal experience with using a Doppler after dives on trimix in the 250-300 range, I've found that by including deep stops starting at about 4/5 of bottom depth and lengthening intermediate stops over what would be prescribed by Buhlmann (neo-Haldanian), I have been able to significantly shorten my 20 stop while simultaneously reducing my post dive bubble grades. I have gradually changed the shape of my deco over time and recorded the Doppler results as a way to test relative efficacy of different curves. In particular, I have lengthened the 70 from that suggested by RGBM and shortened, slightly, the 20. I also ascend from 20 at 2 fpm to surface. I breathe O2 at surface for a few minutes before exertion. I have also done this for various diving partners and the results have been similar. What do I conclude from this?

1. Deep stops seem to work
2. The shape seems to matter
3. There are not strong correlations since many other factors play into post dive scores. Work load, sleep and hydration, not surprisingly, appear to have an important role to play.

A note of caution! I am an amateur. I do this for myself and my friends and not part of a disciplined scientific process. Other shapes may work better still. I'm not saying one shape is right and others wrong. I don't know.

 

[daylight]

Thanks to all for the information. Very helpful.
Dive safe,
Larry

 

[Saturation]

Kendall Raine once bubbled...
Lots of interesting thoughts here. I'd like to add to two of them: Does adding extra time to the 70 foot stop on 50% (PO2=1.6 ATA) yield added decompression benefit over adding extra time to a 20 foot stop on 100% (PO2=1.6 ATA)? And, is there a "right" curve?

On your long dissertion from which the above was quoted, KC, kudos on your logic. You should collate your data and publish it, especially if you're fine tuning your profile from a bubble model table.

As for the above question, I would eagerly await Dr. Deco's comment, but current bubble theory suggests that the deep decompression has added benefits over the shallow. So if you prolong your deep stops on higher pp02 than your backgas, there should be added reductions to your 20' stop.

I am unsure of the exact relationship, extend 1 min at 70' reduces your 20' by 5 min for example, nor when the ongassing effect begins to trade off on the higher pp02.

A word of caution, the relationship between doppler grades and DCI are not 1:1 but highly correlated. Since missed deco commonly manifests with joint pain, should you reach that point despite what the doppler readings say, that again would be publishable material.

If you need help in writing such a paper contact me in private.

 

[Dr Deco]

Hello Kendall:

Deep Stops

Please do not confuse the “deep stop” issue with the idea of removing free gas from the microbubbles. During the deep stops, you are favoring the diffusion of dissolved nitrogen (or helium) from tissue fluids to the capillaries. The gas microbubbles remain small, held that way by the Laplace pressure. If you were to ascent too rapidly, the microbubbles would enlarge to the point where they would grow explosively and the gas inside would come to essentially ambient pressure for that depth and all depths up to the surface. That means sequestered free gas with virtually no driving potential.

Thus all stops, deep, medium, and shallow are attempting to eliminate dissolved nitrogen or helium and prevent the enlargement of the microbubbles. We want small radii and the Laplace pressure to “pump them up.”

Oxygen window

Yes, it is bigger the more oxygen is present in the breathing mix as long as metabolism is occurring (which we will assume is occurring in a living diver). It is greater at deeper depths.

Doppler and Decompression Procedures

I find that fascinating that you are investigating decompression bubble formation and diving. I would not have thought that such was occurring. I have never suggested that divers do this because it would eventually lead to the removal of “safety margins” which are built into all decompression tables and meters.

I must caution however, that these procedures must be quite regular since small variations can produce large changes. I offer these examples from my research.

[i.] When test subjects were exercising on a bicycle to promote inert gas washout (oxygen prebreathe), it was possible to tell by Doppler bubbles who rode the Exercycle first even when one preceded the other by only ten minutes.
[ii.] When the same Doppler monitoring technician was in the chamber with the test subjects, it was possible to tell whether he was with two or three subjects on the basis of Doppler bubbles (subject number influenced technician movement).
[iii.] In development of the oxygen prebreathe, the incidence of DCS was modified by up to eight fold (800x) by whether the subjects were standing or sitting throughout the decompression.

Experimentally we could find bubble differences (sometimes big differences) with small movement changes. If one were to “fine tune” a decompression and make a modification (that you thought was trivial), it might have a disastrous outcome in the absence of the padding of the “safety margin.”

A different Case

I recognize that you are not trying to develop the fastest ascent time in the world but rather are simply looking at the variations in bubbles with changes in stop time. That is truly fascinating. My cautions are going to others who might say to themselves, “I can set dive records with Doppler monitoring such as this.” OhOh :nono:

I await the day when recreational divers will make contributions to the field of barophysiology in the same way that amateur astronomers contribute to contemporary astronomy. That would really increase the amount of information that we have. In many cases, they have already.

Professionals are often very rigid in their ways of thinking. Amateurs generally do not provide the necessary controls. I will be very interested in seeing how this plays out. Possibly the day of the Doppler monitor and the oxygen uptake analyzer incorporated into my mythic Bends Buster decompression meter is not so far off after all.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Kendall Raine]

Thank you, Mike. I see where my remarks about deep stops addressing free gas phase may have been expressed simplistically and inartfully.

Just so you know, I'm not after creating a new set of tables or getting out of the water in the shortest amount of time. I started using the Doppler nine years ago out of curiosity. When I switched from deco schedules based on Buhlmann to RGBM, I did so gradually with the Doppler serving as a safety blanket. That is, I only shortened the 20 stops incrementally using the Doppler to affirm that I wasn't raising deco stress. I then started extending the 70 with only slight shortening of the 20 after that. The schedules I'm using now are still close to those prescribed by RGBM. In that sense, the approach is similar to other table design-theory then empirical validation and refinement. The total run times are typically not shorter than RGBM.

I was still seeing large variations in post dive bubbles based on work load, so started using a scooter for any significant distances at depth. As you would certainly expect, this had the result of reducing post dive bubbles.

As with deep stops, nothing new here. I'm simply finding out for myself what others, e.g. yourself, George Irvine, Bruce Wienke and others have been saying for a long time. I'll also try to never forget Dr. Hamilton's First Law of Decompression: S**t Happens!