Pre-breathing O2

[Saturation]

Dr Deco once bubbled...
Dear Readers:

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

[On vacation this week]

Thanks Dr.Deco. Have a happy vacation. These questions can wait for your return.

Taking the concepts I quoted above then, assuming NSL dives with an ascent to 8000':

There is data to show reduction of DCI episodes in pre-breathing 1 ATA of 02 immediately post dive[say 60' 60min] over a delay in administering the pre-breathe after a NSL dive?

I agree fully with the kinetics involved and would recommend 02 immediately post-NSL dive more for DCI prophylaxis if not for FAD risk reduction. Its logical to assume that with any reduced inert gas load, the risk of bubbling is substantially reduced at 0 altitude, and logically lessened too at 8000 ft. However, Im ignorant of any human dive studies that specify when and how long to breathe the 02 in FAD. Can you point me to an article with this data?

I've read a lot of extrapolations taken from aerospace medicine protocols but a lot of data is based on patients exposed to 1 ATA prior to the pre-breathe or shallow, i.e., <= 60' dives. I also read Dr. Vann's November DEMA 2002 presentation on FAD and mentions the Neutral Buoyancy Lab protocol with 02 prebreathes followed ~ immediate flight. Are you involved here?

What duration do you think best for an 02 prebreathe after a NSL dive and how do you calculate the duration?


Many thanks,

marv

 

[richhagelin]

Can anyone refresh my memory. I can't remember the time period for ore-breathing O2 on the Air Force altitude chamber profile (I've been retired now for 10 years). It seems like it wasn't too long (15 or 20 min max). Anyone current or work in that environment who can refresh my memory?

Rich

 

[Saturation]

Typically at least 30 min 100% 02 [1 ATA] pre-flight. Longer is some instances.

richhagelin once bubbled...
Can anyone refresh my memory
Rich

 

[Dr Deco]

Dear Readers:

OXYGEN AT ALTITUDE

Prebreathing would be more efficient when performed earlier in the post dive interval. I do not have the reference, but the Brooks Air Force Base Group presented some at a meeting of the Aerospace Medical Society. They found experimentally that prebreathing efficiency was reduced if performed at increasing altitude. [ I do not have the reference at hand.]

The explanation most likely rests with microbubble growth. Dissolved inert gas will l be sequestered in the microbubbles and this will be at ambient pressure (since the bubble is flexible and can expand). When the gaseous nitrogen is at ambient pressure (plus a little from bubble surface tension), the driving force for elimination is not nearly as large as if the nitrogen were dissolved. Thus a rise to altitude, and microbubble expansion, is a bad method.

The same obtains with breathing oxygen as you exist the water versus breathing oxygen later, or as you ascent to altitude. That is the only test of this procedure of which I am aware. It is also know that oxygen breathing in association with VIGOROUS exercise is a very inefficient process (see Fulton’s “Decompression Sickness”).

NASA PREBREATHE :mean:

As far as the prebreathe process for astronauts at NASA is concerned, yes, I was the developer of the experimental basis of that procedure. It is now under development as an operational method with a different scientist as head.

Dr Deco :doctor:

On vacation this week.

 

[Saturation]

Thanks Dr. Deco for your replies from vacation

The Air Force study sounds consistent with what is known: lower pp02 lowers the efficiency of decompression.

The stability of microbubbles post dive is an interesting hypothesis for FAD induced DCI.

Did this factor in determining your 02 prebreathe protocol for the astronauts? Vann's notes say for an exposure of 40' for ~ 60min on 46% nitrox, the o2 pre-breathe was about 30min before flight.

Care to comment on how this protocol was estimated?

Hope you are having a restful warm vacation!

Best,


Marv

Dr Deco once bubbled...
Dear Readers:

OXYGEN AT ALTITUDE

Prebreathing would be more efficient when performed earlier in the post dive interval. I do not have the reference, but the Brooks Air Force Base Group presented some at a meeting of the Aerospace Medical Society. They found experimentally that prebreathing efficiency was reduced if performed at increasing altitude. [ I do not have the reference at hand.]

The explanation most likely rests with microbubble growth. Dissolved inert gas will l be sequestered in the microbubbles and this will be at ambient pressure (since the bubble is flexible and can expand). When the gaseous nitrogen is at ambient pressure (plus a little from bubble surface tension), the driving force for elimination is not nearly as large as if the nitrogen were dissolved. Thus a rise to altitude, and microbubble expansion, is a bad method.

The same obtains with breathing oxygen as you exist the water versus breathing oxygen later, or as you ascent to altitude. That is the only test of this procedure of which I am aware. It is also know that oxygen breathing in association with VIGOROUS exercise is a very inefficient process (see Fulton’s “Decompression Sickness”).

NASA PREBREATHE :mean:

As far as the prebreathe process for astronauts at NASA is concerned, yes, I was the developer of the experimental basis of that procedure. It is now under development as an operational method with a different scientist as head.

Dr Deco :doctor:

On vacation this week.

 

[Dr Deco]

Dear Marv:

Efficiency

The reduced efficiency of the prebreathe when performed during the ascent (Brooks study) was hypothesized to be the result of the grown of tissue bubbles and not because of the reduced ppO2. The only factor in that case is that the partial pressure of nitrogen in the breathing gas be zero. The partial pressure of oxygen is not critical, only that the individual not be hypoxic. The tissue nuclei contain the nitrogen at ambient pressure, and there is thus very little driving force for excretion into the tissue and capillaries.

Nuclei Lifetimes

The lifetimes indeed play a role. They are on the order of hours, at least those generated by physical activity. The study is described in the following:
Dervay JP, Powell MR, Butler B, Fife CE. The effect of exercise and rest duration on the generation of venous gas bubbles at altitude. Aviat Space Environ Med 2002 Jan;73(1):22-7

Currently I do not have enough information to comment on the times derived by Dr Vann.

Dr Deco :doctor:

On vacation this week :mean:

 

[Saturation]

Thanks a ton Dr. Deco.

Would it be right to say then from a biophysical and practical view that breathing 02 should be underwater, limited by safety of course, to maximize the 02 window? And that this is more efficient that the pre-breathe on the surface?

It would be the earliest time to breathe 02 even before surfacing.

How would one estimate how long to breathe this 02 after a NSL dive?

Enjoy your vacation!

Best,

Marv

Dr Deco once bubbled...
Dear Marv:

Efficiency

The reduced efficiency of the prebreathe when performed during the ascent (Brooks study) was hypothesized to be the result of the grown of tissue bubbles and not because of the reduced ppO2. The only factor in that case is that the partial pressure of nitrogen in the breathing gas be zero. The partial pressure of oxygen is not critical, only that the individual not be hypoxic. The tissue nuclei contain the nitrogen at ambient pressure, and there is thus very little driving force for excretion into the tissue and capillaries.

Nuclei Lifetimes

The lifetimes indeed play a role. They are on the order of hours, at least those generated by physical activity. The study is described in the following:
Dervay JP, Powell MR, Butler B, Fife CE. The effect of exercise and rest duration on the generation of venous gas bubbles at altitude. Aviat Space Environ Med 2002 Jan;73(1):22-7

Currently I do not have enough information to comment on the times derived by Dr Vann.

Dr Deco :doctor:

On vacation this week :mean:

 

[Dr Deco]

Dear Dr Marv:

How Long?

You are correct in putting the oxygen breathing in the water as this does maximize the oxygen window and put the greatest internal pressure on any separated nitrogen (in bubbles). Unfortunately, I do not know how long a breathing period in the water would be since gas-loading analysis already tells one that surfacing is safe. We are therefore left with the case of supersaturation plus strenuous physical activity (should that occur). This would require laboratory trials and these have not yet been performed.

Table Testing

As a matter of fact, strenuous physical activity post dive is not built into any table-testing regimen. It is for this reason that a diver must be careful and avoid excessive physical activity unless they know that they are not overly saturated. This means not too much “work” in the first 30 to 45 minutes, roughly.

Dr Deco :doctor:

[On vacation this week] :mean:

 

[Saturation]

Thanks for the insight, Dr. D. This is a very enjoyable and informative discussion, so I appreciate taking the time to share your expertise here. I've more time on my hands as we are on emergency staff levels [and no patients!] considering the blizzard in the northeast.

Also, to avoid duplication, I've perused your history of posts around scubaboard.com, there's about 800+ posts =-) so I'm reasonably certain the core of this thread is a new discussion

By 'gas loading analysis' I understand it to mean mechanisms via models from Haldanian flavors to bubble models, is this correct? If so, I understand your statement to mean these models say its safe for the diver to surface after a no-stop dive.

Question #1: If a model says its safe to surface at 1 ATA, how much credence to use its assumptions to surface at 0.5ATA instead?

This would appear like a diver would go straight to altitude after completing decompression. The model would thus have to increase decompression time to compensate. It could include even a 'stop' at 1 ATA, which effectively calculates the surface 02 breathing time. The idea is is if successful decompression has been completed for many years using say Buhlmann's algorithm to 1 ATA, what does the same model say if we reduce ambient pressure to 0.5 ATA.

I presume there is no data to support such extrapolation, but looking at your opinion on the subject.

By your statement "We are ...left with .... ..supersaturation plus strenuous physical activity", divers are told not to exert themselves post dive for several hours to most a day.

Question #2: Did your work attempt to quantify the exertion that is safe in some known 02 consumption scale? Like METS or V02 to represent the rate of exertion? Walking in mph? From a level of activity chart [from Bove or Bennett's book] the level of exertion you recommend appears not to exceed 2mph at maximum.

This has practical consequences for divers. I don't realistically see divers flexing arms and legs for 45 minutes. However, a leisurely walk through a beach, woodlands, resort etc., is a high possibility, and we can estimate walking speed without meters and tools.

Dr Deco once bubbled...
Dear Dr Marv:

How Long?

You are correct in putting the oxygen breathing in the water as this does maximize the oxygen window and put the greatest internal pressure on any separated nitrogen (in bubbles). Unfortunately, I do not know how long a breathing period in the water would be since gas-loading analysis already tells one that surfacing is safe. We are therefore left with the case of supersaturation plus strenuous physical activity (should that occur). This would require laboratory trials and these have not yet been performed.

Table Testing

As a matter of fact, strenuous physical activity post dive is not built into any table-testing regimen. It is for this reason that a diver must be careful and avoid excessive physical activity unless they know that they are not overly saturated. This means not too much “work” in the first 30 to 45 minutes, roughly.

Dr Deco :doctor:

[On vacation this week] :mean:

 

[Dr Deco]

Dear gozumm:

Gas Loading Analysis

By this, I mean only the Haldanian considerations of dissolved inert gas. Two-phase models are better, but, generally , they will produce the same results for no-decompression (i.e., SCUBA) dives. This is because the Haldane method works well for these situations, even if its underlying hypothesis (or mechanism) is incorrect. The two-phase models begin to show a difference with deep dives and this difference manifests itself in the appearance of deep stops, contrary to the Haldane hypothesis.

The difference in (i) the radii of decompression bubbles and (ii) their number can result in a difference in dive outcome in many situations. The first will govern eventual bubble off gassing because of a reduction in the Laplace pressure and the second will influence the number of nitrogen molecules that will be sequestered in a bubble instead of entering into the capillaries.

Surfacing At Altitude

If one makes an extrapolation of the m-value lines, one finds that they should be able to go to higher altitudes (as an aviator) than is found to be true practice. The reason for this might well be in the number of micronuclei. In any case, the design of flying-after-diving (FAD) rules has been difficult. Surfacing “at altitude” is simply a FAD situation with a very short, or no, surface interval.

Buhlmann’s Altitude Tables

Because of difficulties, Buhlmann found it necessary to design and test the tables he derived for diving at altitude in the lakes of Switzerland. Simple extrapolations were not found to suffice. One can make estimates, but apparently, they are not foolproof.

Post-dive Exercise

The level of exercise is important, and for some situations, it has been quantified. The earliest work on the subject was by Franklin Henry, an exercise physiologist working during WW II. His contribution to the war effort was to study flyer’s bends. He found that work as measured in oxygen consumption correlated with DCS. This material is in Fulton’s book Decompression Sickness (1951). The type of activity was less important that the work load. Micronuclei are mentioned as a possible mechanism by E N Harvey (in another chapter).

This exercise must be balanced against the desire to increase regional blood flow, primarily through the mechanism of the muscle pump. When I speak of activity at the surface, I refer to the avoidance of heavy activity, such as lugging SCUBA tanks around or playing beach volleyball. On the other hand, I also believe that it is a bad idea to sleep during the surface interval since this is the may to a very large drop in perfusion of the tissues. Divers should move around in a normal fashion, avoiding both strenuous activities and sleep, It is true that it is not practical for a diver to continue to move arms and legs during the surface period.

More

More appears on this topic in the supplements to SUBASOURCE. These can be found at
http://www.doctordeco.com/

Dr Deco :doctor: