Hypothetical question

[markmud]

And if your N2 pressure after the dive is the same as if you'd canceled the dive and had a beer on the boat instead, how likely is it that you'll bubble during a flight?

About nil.

Just a small FYI if you're going to shorten the name & Anglicise it, better make it "Alex"

Thanks for that. I could not figure out how to type those letters. I should have cut'n'pasted.

If you breathe 100% O2 for long enough, eventually all of your tissue compartments will be free of inert gas. Once that happens, any reduction in ambient pressure (scuba ascent, air travel, explosive decompression due to loss of cabin pressure or spacewalk) will not put you at risk for bubble formation, since there is no inert gas to bubble.

Thanks for responding,
I am now changing my vote. The person could fly right away.

cheers,
mm

 

[Storker]

About nil.

My point exactly

 

[doctormike]

I think inner ear isobaric counterdiffusion DCS https://www.physiology.org/doi/pdf/10.1152/japplphysiol.01090.2002 might be the only partial exception to the 'drop in ambient pressure' requirement, and this requires a switch from high-He gas mix to high-N2 gas mix part-way through a significant deco exposure. I don't think that switching to oxygen would have any such risk associated - happy to be corrected.

True, but beyond the scope of the OPs question. Good point!

 

[doctormike]

Yep. And if the N2 pressure in your tissues is the same as it is on the surface, how likely is it that you'll bubble if you suddenly find yourself at the surface? There just isn't any driving force pushing the N2 out of solution. And if your N2 pressure after the dive is the same as if you'd canceled the dive and had a beer on the boat instead, how likely is it that you'll bubble during a flight?

I don't know how likely. But you don't either. There may well be no clinical risk, but you should understand why, and it's not just because of what you describe.

The point of thought experiments like this are to get us to work through the basic science of diving and come to a better understanding underlying mechanisms. It’s not really about whether an actual diver can fly or not.

There are three levels of understanding in answering this question, resulting in three answers.

1) No, the diver shouldn’t fly, because all of the agencies say that you should wait 17-24 hours to fly after diving.

2) Aha! The OP has described a dive where the PPN2 of the breathing gas is about the same (or even a bit lower) than the PPN2 in the atmosphere at surface level. PPN2 gradients are what cause DCS, and since there is no gradient, there CAN be no DCS. There is no physical process to cause bubbling.

3) Wait a minute. Since during ascent there is a drop in ambient pressure, and since the diver is not breathing 100% O2, that means that there is some inert gas in the tissue, and some bubbles will form (as evidenced by experimental doppler data). Those bubbles are probably silent, meaning that the diver is at no risk of DCS when they surface, but there is no consensus on the limited (military) data about what happens when you take a subject with subclinical bubbles and then immediately expose them to a second drop in ambient pressure (air travel). My gut feeling tells me that there probably isn’t much of a risk, but it’s just that, a gut feeling based on no actual data. But I do understand that there is a physical process that can cause bubbling even though there is no difference in N2 loading between this diver and someone who played tennis that afternoon. The difference is that the diver was exposed to an ambient pressure drop.

 

[Storker]

I do understand that there is a physical process that can cause bubbling even though there is no difference in N2 loading between this diver and someone who played tennis that afternoon.

I don't, so if you could describe that physical process I'd appreciate that.

 

[RainPilot]

there is some inert gas in the tissue, and some bubbles will form (as evidenced by experimental doppler data).

But what would cause the bubbles? If the inert gas pressure in the tissues is the same as the ambient at the surface, what would drive those bubbles to form? There is nothing coming out of solution.

The high altitude prebreathe is because there is still an inert pressure differential. We had to do them for unpressurised flight above 18000’ since that was the 1/2 ATA point, PPNO2 would be 0.4 there and 0.8 in your surface saturated tissues so there was potential for DCS especially in high speed ascents. We also did them for HALO for the same reason.

 

[Dominik_E]

As also in the thread in the other forum, I am in the "I would fly immediately" camp as far as my personal safety is concerned.

But, very importantly, I share doctormike's opinion that we simply do not have sufficient knowledge about the location and exact physical mechanism of microbubble formation to definitely rule out bubble formation during ambient pressure drop even when far away from (super-)saturation. All models we have regularly fail with regard to preventing bubbles (while still succeeding in preventing DCS symptoms), so their inherent logic surely is not a strong foot to stand on.

This being said, even if there are bubble formed, I think is is safe to assume that bubble formation will terminate once breathing normal air on the surface (no supersaturation). Circulating bubbles will be eliminated in relatively short timeframes. As we do not have transporter technology to beam the diver in our thought experiment onto the airplane, I thus think that the question of bubble formation on ascent is a highly interesting one, but does not imply a real risk.

 

[Dominik_E]

RainPilot, I think a very important unanswered question is "Will all parts of the human body be able to hold 0.79bars pp of inert gas without bubble formation at 1bar ambient pressure, even if the body is arriving in this conditions from a higher ambient pressure environment?". Without the ambient pressure drop, it is trivial to see that our bodies are able to do this, else we would be bubbling sitting here and posting. However, if we were to be teleported from here to 14km altitude, we would bubble, so there is for sure not an ambient pressure independent compartment for 0.79bars inert gas in our bodies. And this pressure dependence is the unknown factor that in my view makes the direct conclusion (that there will be no bubbles as long as ppN2 is not larger than 0.79) impossible.

 

[TrimixToo]

I think the differential N2 saturation is a rounding error. I'd fly right away without pause. But that's me. Mike has injected some doubt, but I cannot see how there is anywhere near enough difference to cause DCS at .75 ATA, an inert gas saturation of about 1.3 ATA, given that the PADI saturations allowed by NDLs are, IIRC, somewhere around 1.6. But I'd love to hear from Simon if he's reading this.

 

[doctormike]

I don't, so if you could describe that physical process I'd appreciate that.

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