Hypothetical question

[doctormike]

Just from physics point of view, the nitrogen Henry’s law constant in water (don’t know the value in the blood) is 1600 atm / (mol/litre) at 25C. For discussion point of view, let’s assume the nitrogen Henry’s Law constant in the water is the same as in the blood. Then nitrogen solubility in the blood at the surface would be about 13.8 ppm (0.79 x 28 / 1600 / 1000). At 9 m depth with EAN60 the amount of soluble nitrogen in the blood would be 13.3 ppm, still below saturation. As you ascend to the surface, you will load less nitrogen (5.52 ppm). At any point during the dive you have not pushed your body beyond the maximum nitrogen loading that your body can take. So, how would the bubbles form if all of the dissolved nitrogen is still below saturation? I’m confused. Am I missing something?

With the OPs hypothetical dive, the diver is breathing gas with a similar (slightly lower) PPN2 as in surface air, so no ongassing (slight offgassing at 30 FSW, actually, 0.76<0.79, until he reaches equilibrium again). In the steady state, the compartments are saturated, first the fast ones and then the slow ones.

But as you ascend, ambient pressure decreases, so your PPN2 in your inspired gas is now lower than that, creating an offgassing gradient between p(tis) and p(insp). Compartment saturation isn't an absolute number, it's relative to ambient pressure. If you take a saturated tissue compartment and drop the ambient pressure, you create a PPN2 gradient and offgass. Ascending makes the tissue compartments supersaturated, until they reach a new equilibrium.

 

[Dan]

...If it was just the gradient that was the risk factor, switching to your 100% O2 deco bottle at 20 feet would cause DCS which it doesn't - even though it causes a sudden large PPN2 gradient..

I think this is because you will load 0 ppm of dissolved nitrogen into your body. Therefore you no longer adding more dissolved nitrogen that you already have at that point.

 

[Storker]

Hypothetically as an instructor I’d stick to the industry standard.

Which is? (Just FTR). 24 hours before flying after any dive? At least, that's what my computer usually tells me.

Not that I necessarily disagree; what I'd allow for myself isn't necessarily what I'd recommend for others.

 

[shurite7]

The Question: Someone tells you that he or she needs to do a 90 minute dive to a maximum depth of 9 meters/30 feet using 60% nitrox. This person wants to know how long he or she should wait before flying on a commercial airline. What is your answer, and why?

I did not place a vote in the options above because there wasn't an option for a couple of hours. When I lived in GCM some of the staff would fly to Cuba in the evening for a weekend of fun. Quite often they dived in the morning or early afternoon. Before the dive they blended 50% O2. None got the bends. The dives were usually less than 50 feet. This wasn't normal practice, just once in a great while.

 

[Dan]

With the OPs hypothetical dive, the diver is breathing gas with a similar (slightly lower) PPN2 as in surface air, so no ongassing (slight offgassing at 30 FSW, actually, 0.76<0.79, until he reaches equilibrium again). In the steady state, the compartments are saturated, first the fast ones and then the slow ones.

But as you ascend, ambient pressure decreases, so your PPN2 in your inspired gas is now lower than that, creating an offgassing gradient between p(tis) and p(insp). Compartment saturation isn't an absolute number, it's relative to ambient pressure. If you take a saturated tissue compartment and drop the ambient pressure, you create a PPN2 gradient and offgass. Ascending makes the tissue compartments supersaturated, until they reach a new equilibrium.

OK. So there is a delay due to nitrogen diffusion into the tissue, not an instantaneous event.

 

[DavidFL]

Tempted to hijack the thread by asking if the commercial airline flight is on a 737 MAX, in which none of the poll answers seem long enough for that current quandary in my opinion....

 

[doctormike]

I think this is because you will load 0 ppm of dissolved nitrogen into your body. Therefore you no longer adding more dissolved nitrogen that you already have at that point.

Yeah, but bubble formation doesn't happen when ongassing, only when offgassing, so N2 loading isn't the issue in that case. The point is that there are two ways to cause a PPN2 gradient and offgas:

1) Ascending breathing the same mix

2) Switch to a richer mix without changing depth

In both of these situations, p(insp) is going to drop, and p(tis) is going to follow it. But only in #1 is there a change in ambient pressure, which is why there isn't a DCS risk with 2 but there may be with 1.

 

[doctormike]

OK. So there is a delay due to nitrogen diffusion into the tissue, not an instantaneous event.

Right, those are the different tissue compartments, fast to slow. With a "normal" dive (not the one in the OP), you may finish the dive before your slow compartments have fully saturated.

 

[tbone1004]

I'm sure it's been said, but I'm not reading 4 pages of answers.
Max ppN2 of EAN60 at 2ata is .8
Normal ppN2 of air is .8
You will have less total nitrogen after that dive than you did before you got in

 

[dmaziuk]

Theoretically, one might be concerned about O2 bubbles forming, though. At 9m on EAN60, the pPO2 is 1.14 bar, compared to 0.21 at the surface on air.

I can't find the cite anymore, somewhere on PubMed there's an old study that concluded that oxygen doesn't bubble nearly as much as nitrogen. I could never find much about how or why.

Isn’t the body metabolism consumes the oxygen and keeps the gas from coming out of solution (blood)?

Yes but you're not going to metabolize 3 times as much O2 if you triple its content in your breathing mix. The "oxygen window" PP drop in the formula is only around 0.05. "Not bubbling" appears to be the key from what I read so far.