O2 before a dive?

[Dr Deco]

Hello Readers:

Oxygen Prebreathe For Diving

As I wrote above, “oxygen breathing for a diver is less efficacious…” This I indicates that it must be somewhat efficacious – only less.

The interesting posting by JonKranhouse passed along indicates that Doppler bubbles are reduced in the dive series tried in the reported study. Their dive profiles were ones that extended some advantage.

It is always possible to calculate the compartment nitrogen gas loads during oxygen washout, and then calculate the compartment gas loads during nitrogen uptake during the dive. There are definitely some dives for which this confers some bottom-time advantage. For the recreational diver, this can be helpful to extend bottom time.

The problem is with the supply of oxygen – it is usually limited for recreational divers. For commercial or military divers, oxygen is generally not limited. They, however, go to nitrox for SCUBA since it is always advantageous under all circumstances or they use surface supplied gas since it is unlimited [diver fatigue limits generally].

“If oxygen supply were no object” is the diver’s equivalent of “if money were no object.” Something always limits.:depressed:

Research Today

The researchers indicate “but the effectiveness of normobaric oxygen before hyperbaric exposure has not been well explored.” Very true.

Regrettably, there are few barophysiologists left today. We had a wonderful program while I was at NASA, but it eventually ended. Since we eliminated DCS on orbit and reduce oxygen prebreathe to manageable time limits, NASA headquarters did not see the advantage of putting more money into studies. I wanted to perform research, but NASA HQ wanted only goal-oriented work.

Our sun set, and I retired.

Dr Deco :doctor:

 

[Bubbletrubble]

@Dr. Deco: You know, the data presented in the study Jon cited is consistent with some other mechanism(s) besides nitrogen washout being responsible for the lowering of VGE due to pre-breathing oxygen. Specifically, the KISS score for the second dive in an O2-air schedule was significantly lower than the KISS score for the second dive in an air-air setup (18.8 ± 5.2 vs. 35.6 ± 6.6, respectively). Perhaps pre-breathing oxygen somehow decreases the numbers of bubble micronuclei. I have no idea what the mechanism might be.

Interestingly, the KISS score for the second dive was very similar between the O2-air and air-O2 paradigms (18.8 ± 5.2 vs. 17.02 ± 5.2, respectively). This result suggests that pre-breathing oxygen has a sustained protective effect against VGE that lasted at least as long as 3 hours. One explanation of this could be the beneficial effect of prolonged slowing of blood flow (decreased cardiac output, decreased heart rate, and increased systemic vascular resistance) that accompanies breathing normobaric oxygen (shown in other studies). Such decreased blood flow might somewhat mitigate tissue on-gassing of nitrogen. If we extend the blood flow argument to include CNS tissue and cerebral blood flow, we might speculate that pre-breathing oxygen decreases the incidence of neurological DCS. Follow my train of thought here...increased arterial oxygen tension caused by pre-breathing oxygen could keep cerebral blood flow at a lower rate. AFAIK, CBF almost doubles when arterial oxygen tension drops below 6.7 kPa (50 mm Hg). This is an idiosyncrasy of the auto-regulation of cerebral blood flow. The end result is that it would minimize nitrogen on-gassing of CNS tissue.

My initial reaction to pre-breathing oxygen was that it wouldn't be beneficial for scuba divers. But I could be wrong about that.

Just a few thoughts...

 

[Dr Deco]

Hello Bubbletrouble:


There is no question that something other than nitrogen washout could be occurring. Washout is my first thought, and I guess I simply stopped there.

In recent years, oxygen prebreathe has generated interest, especially for astronauts. While I was at NASA, I proposed to a colleague at the University of Texas Health Sciences Center in Houston that we try a hyperbaric prebreathe to squeeze out the micronuclei faster than prebreathe at 1 ATA [ref below]. Rats were used as the test subjects. It worked [ie, less DCS], but to my surprise, the effect worked even when performed many hours before the ascent [depressurization] to altitude.

Rats were used and they are constantly in motion. There is really no way that compressed nuclei were not regenerated considering the physical activity of these rodents. Some other mechanism must be present, possibly involving the capillary endothelium.

Oxygen has been regarded as a passive agent that allows for nitrogen elimination and also as a treatment modality once decompression bubbles have grown. However, there is a very good possibility that something else occurs, and this "something" is of a biochemical nature.

Dr Deco :doctor:


Butler BD, Little T, Cogan V, Powell M. Hyperbaric oxygen pre-breathe modifies the outcome of decompression sickness. Undersea Hyperb Med. 2006 Nov-Dec;33(6):407-17.

 

[DocVikingo]

An article I wrote for the issue Aug '09 of Undercurrent
http://www.undercurrent.org/

"Should You Breathe Oxygen Before a Dive?

Physically fit sport divers doing conservative profiles (e.g.,
slow ascents, deep stops and safety stops) have only a tiny
risk of getting decompression sickness. Still, DCS incidents
range from inconvenient to disabling. Any additional steps
for protection are worthy of consideration - - some hyperbaric
medical researchers from France think breathing oxygen
before a dive may be a good one.

Focusing on a technique to reduce altitude decompression
sickness risk in aviators and astronauts, the researchers
recently published an interesting study on the effects of
breathing oxygen (O2) prior to a dive, and what it did to
subsequent bubble formation in the bloodstream. Divers performed
two openwater dives to 98 feet for 30 minutes, with
a six-minute safety stop at 10 feet and surface interval of 100
minutes. Beforehand, researchers broke the divers into four
groups with these pre-dive conditions: (1) Air administered
prior to both dives; (2) O2 administered prior to both dives;
(3) O2 administered prior to the first dive and air prior to the
second; and (4) air administered prior the first dive and O2
prior to the second.

Post-dive Doppler venous gas bubble scores were reduced
in all pre-oxygenation dives. This effect was maintained after
a second dive, without needing to pre-breathe additional
oxygen. Not surprisingly, O2 pre-breathing for every dive
produced the greatest reduction in bubbles. The authors
concluded that O2 pre-breathing has a prolonged protective
effect because it decreases venous gas emboli formation, and
could therefore be beneficial for multi-day repetitive diving.
So are these results meaningful and practical to the recreational
diver? Possibly, but not yet practically. The amount
of O2 pre-breathed in the study - - 10.5 cubic feet per dive
- - would be too much for most operators to supply, except
if they charged hefty fees. If you brought your own oxygen
bottles along, we can see other divers getting jealous and
wanting a toke.

The protective effects of pre-breathing O2 against suffering
actual DCS remain to be proven. So while you may feel it
worthwhile to bring your own oxygen tank, we don’t see this
catching on with dive boats unless future studies show blockbuster
results or the amount of oxygen can be reduced.- - Doc Vikingo

“Pre-dive normobaric oxygen reduces bubble formation in scuba
divers,” by O. Castagna, E. Gempp and JE Blatteau, European
Journal of Applied Physiology, February 14, 2009.

Regards,

DocVikingo

 

[fppf]

So, that would be a 3-4 minute pre breath.
I would like to see a comparison study between pre breath and use O2 on a safety stop. I'm guessing you will see much more benefit using it post dive then pre dive.

 

[Bubbletrubble]

This entire discussion got me thinking. I realize that the study by Castagna et al. looked at the effects of pre-breathing 100% O2 at 1 ata. I wonder: (a) whether the same benefits would be seen if the oxygen breathing session occurred at 1.5 ata and (b) how long the interval between "pre-breathing" and the actual dive might extend. The answer to these questions might have some relevance to divers who breathe O2 as a part of their "deco gas" and then proceed to do repetitive dives. Perhaps we are looking at a powerful 2-for-1 effect when breathing O2 during a shallow deco stop.

 

[Bubbletrubble]

So, that would be a 3-4 minute pre breath.
I would like to see a comparison study between pre breath and use O2 on a safety stop. I'm guessing you will see much more benefit using it post dive then pre dive.

Bear in mind that "post dive" might be considered "pre dive" for subsequent dives.

 

[fppf]

Bear in mind that "post dive" might be considered "pre dive" for subsequent dives.

Not if you only do one dive.

 

[Bubbletrubble]

Bear in mind that "post dive" might be considered "pre dive" for subsequent dives.

Not if you only do one dive.

We really don't know how long these benefits from oxygen breathing might last. It clearly lasts at least 3-4 hours. Perhaps it lasts for days. If so, then maybe a diver wouldn't need to pre-breathe before every dive...maybe just pre-breathe O2 at the beginning of the dive week or every few days. That would definitely cut down on how much O2 a liveaboard dive op would have to carry in order to service all of its dive guests. Alternatively, liveaboards could think about installing one or more oxygen concentrators on the boat. Good for treating and preventing DCI, eh? Just thinking out loud here...