Pre-breathing O2

[richhagelin]

In the altitude chamber training of AF pilots, we pre-breath O2 before beginning the chamber ride (which typically, includes a rapid decompression from 8,000ft to 20,000ft+).

Has any work been done on pre-breathing O2 before diving to reduce susceptibility to DCS; to increase bottom times; to reduce decompression stops?

Rich

 

[Dr Deco]

Dear Rich:

Prebreathe for Divers

It is my understanding that some oxygen prebreathe was tested decades ago for divers. The problem with this method is that it does not take very long before the tissues refill with inert gas after they have been “washed” clean.

It is possible that, in some special circumstances, and with certain exercise procedures during the prebreathe, one would be able to accelerate the decompression. This does not mean that the entire process would require less time. That is, for the real cost of the deco procedure, we must count washout time plus the (hopefully) reduced post dive decompression.

Dr Deco :doctor:

 

[richhagelin]

Thanks, just curious. Another question: which tissues typically are the longest half-time tissues for figuring decompression time?

Rich

 

[Dr Deco]

Dear Rich:

Longest?

It depends whom you ask. Four hundred minutes is typically a long half time. Professor Buhlmann had 600 minutes tissues, I believe. If you consider only SCUBA, 200 minutes might do it. Another aspect of this question is, what are the tissues ?

Generally speaking, most in the know do not use the term ”tissue” any longer as it speaks of an actual anatomical entity. Rather one considers the gas uptake and elimination scheme as a mathematical construct that is useful in creating a “map” of the gas partial pressures in your body. In a sense, it is similar to those pie graphs that one sees describing, for example, the type of trees in a forest. We all are aware that this is simply a representation, and no one thinks that all maples grow in one section, only oaks grow in another, etc. Compartments are similar useful "pictorial" constructs.

Long halftimes

The blood flow characteristics of tissues would lead one to believe that the gas should enter and exit fast than it does. While the inert gas certainly enters and leaves with the blood flow, the flow to tissues appears to be very non-uniform. This is referred to as :heterogeneous perfusion.” In addition, there is the very real possibility that long halftimes are in reality the result of gas phase formation. They probably do not exist in many divers and are virtual compartments.

Dr Deco :doctor:

 

[Saturation]

You could reduce your flying after diving risk of DCI with an 02 prebreathe.

 

[Dr Deco]

Dear Readers:

Oxygen prebreathing is definitely a possibility prior to flying and for flying after diving. It has had some use in the military and at NASA.

Dr Deco :doctor:

 

[Rick Murchison]

We discussed this about two years ago - and plugged some numbers using Haldanean theory just to see how it might work. Bottom line is that while you should be able to theoretically reduce your Nitrogen uptake by prebreathing oxygen as we do prior to high altitute flight, the benefit is accelerated at the end of the dive, so with a finite oxygen supply it makes more sense to use it for decompression rather than for prebreathing, or even on the surface post dive rather than predive (short answer is greater delta P means quicker nitrogen elimination... and ongassing).
Rick

 

[richhagelin]

Doc,

Then......... if I were to use the O2 breathing to reduce the time before flying after diving (rather than to reduce decompression obligation at the end of the dive), it seems that the most benefit would occur immediately after diving (as opposed to immediately before flying xxx hours later), since the N2 saturation would be at its maximum at that point. Is my logic correct?

Is there any way to quantify the effect in reducing the safe time before flying? Any sources that might help me figure it out?

Thanks,

Rich

 

[Saturation]

Hi Rich:

It would help to breath the 02 post dive ASAP not just for FAD but to reduce you DCI risk after a dive, but exactly when and for how long, using biomedical studies data, is a work in progress.

Dr. Vann has summaries of current recreational FAD data and their work in progress: http://www.scuba-doc.com
see Flying After Diving link and his powerpoint slides.

There is existing data to show decompression inefficiency using backgas at different stop depths, so its follows that at 1 ATA breathing air, offgassing to allow an ascent to 8000ft would take many hours. However, once 02 is initiated, even hours after a dive, you accelerate tissue desaturation.

Interested technical divers can view their tissue inert gas [or for air, N2 ] loads by using software to show a models' estimate for various tissue compartment saturations. The effects of decompression protocols on these compartments are also shown graphically. If one runs such software for a NSL dive, and then force the model to say one is surfacing at 8000' above sea level, it calculates a decompression profile and show which tissues are affected.

Technical divers breath pp02 1.6 at the 20' stop, and accelerate their desaturation even faster. Many then follow the recreational guidelines of 12-24 hours PFSI, on the assumption their surfacing tissue saturations are no worse than a recreational diver after multiple dives without formal decompression. This information is empiric, but you can show yourself the effects of their protocols through the software, some of which is free: www.gap-software.com

richhagelin once bubbled...
Doc,

Then......... if I were to use the O2 breathing to reduce the time before flying after diving (rather than to reduce decompression obligation at the end of the dive), it seems that the most benefit would occur immediately after diving (as opposed to immediately before flying xxx hours later), since the N2 saturation would be at its maximum at that point. Is my logic correct?

Is there any way to quantify the effect in reducing the safe time before flying? Any sources that might help me figure it out?

Thanks,

Rich

 

[Dr Deco]

Dear Readers:

The flying-after-diving question appears in many incarnations. This is a good version and treats the question of oxygen breathing.


OXYGEN BREATHING

Oxygen will always be beneficial with respect to hastening elimination of inert gas. One can find from gas loading analysis that more inert gas (that is, greater partial pressure) is removed with the partial pressure is greatest. However, it can also be shown that the fraction is equal. It is more more benefit to drop from 80% to 40% than from 8% to 4%, however.

One really desires to reduce the partial pressure of inert gas early in the off gassing period because the problem of microbubble formation is reduced. The early workers on the Eads Bridge and the Brooklyn Bridge found that there was a difference in whether you climbed the ladder from the caisson and then locked out (the better method) or locked out and then climbed the ladder (the most deleterious method). We see that the first system would produce micronuclei but they were not in a supersaturated tissue (since this was before decompression). Locking out first resulted in the workers climbing ladders in a supersaturated condition and the nuclei thus produced grew very fast.

If you breathe oxygen immediately after reaching the surface , the partial pressure of tissue inert gas is reduced, and, MORE IMPORTANT, the tendency to form stable micronuclei is reduced.


INCREASING WASHOUT EFFICIENCY

If one desires to increase the efficiency of the elimination process, you can be seated and move your arms and legs during the oxygen breathing period. We are not talking about vigorous activity but rather simply moving arms in and out and straightening and withdrawing your legs. Do not do what was tried during WW II, namely, running in place and “jumping jacks”.

This procedure will increase the blood flow in the arms and legs though the mechanism of the muscle pump and an increase in metabolic products that exert local control of the capillaries.

Dr Deco :doctor:

[On vacation this week]