Hypothetical question

[doctormike]

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.

But ambient at the surface has nothing to do with this particular process that I am describing. The driving force is the drop in ambient pressure on ascent. Your tissues don't "know" what is happening on the surface when you ascend to a shallower depth, they just experience a drop in ambient pressure. And unless you are breathing 100% O2, there is always "something" to come out of solution, that something is inert gas (N2 for the OPs question).

Here are two thought experiments to better illustrate this.

1) You have a cylinder with a piston and some amount of dissolved N2 in solution, and a gas-liquid interface, in Henry's law equilibrium. Let's say the PPN2 is 0.79. The piston has the cylinder pressurized to 5 ATA. You suddenly drop the piston, resulting in a sudden drop in pressure to 2 ATA. Gas will come out of solution, even though the PPN2 is the (arbitrary) value found in surface air.

2) Lets say that oxygen toxicity doesn't exist, or that we are doing this with some animal model with no CNS to make it an issue. This is just about bubble formation. You are at 330 FSW (11 ATA), breathing EAN93, which gives you a PPN2 of 0.79, the same as on the surface. You suddenly ascend with no stops to just below the surface. Do you feel that there is no decompression risk, no bubble formation? Remember, your tissues don't "know" what is on the surface. They just have experienced essentially an explosive decompression, and they have N2 in them.

 

[EFX]

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.

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.

You bring up a good point here. Reducing ambient pressure relative to the tissue pressure will cause the gas to flow out of those tissues and create more silent bubbles. However, with our EAN60 dive, when breathing air on the surface the flow is reversed and some of those silent bubbles will get reabsorbed back into the tissues. The question is: how many of those bubbles remain should we fly immediately without a long SI. In the research I've read the bubbles take time to dissipate but the tissues are offgassing not ongassing.

 

[doctormike]

You bring up a good point here. Reducing ambient pressure relative to the tissue pressure will cause the gas to flow out of those tissues and create more silent bubbles. However, with our EAN60 dive, when breathing air on the surface the flow is reversed and some of those silent bubbles will get reabsorbed back into the tissues. The question is: how many of those bubbles remain should we fly immediately without a long SI. In the research I've read the bubbles take time to dissipate but the tissues are offgassing not ongassing.

Right. That's exactly the point of ANY dive-to-fly recommendation. The assumption is that silent bubbles - which ALL divers form unless we are diving 100% O2 - will be cleared by the time we experience ANOTHER drop in ambient pressure, which could cause silent bubbles to become clinically significant.

Saturated tissue compartments are saturated tissue compartments. We all have them right now, with PPN2 of 0.79. Putting us a 30 FSW on EAN60 might not cause FURTHER ongassing, but it doesn't eliminate tissue inert gas that is already there. THAT is what can bubble on ascent.

It's fine to say "I'm guessing that by diving a rich mix for a short time, that any bubbles that I generate will not cause symptoms if I fly an hour after surfacing". It may even be correct. But realize that it's just a guess based on very limited data. The fact that you are breathing a mix which at depth has the same PPN2 of surface air doesn't change the physics of ascent.

 

[Storker]

Hypothetical question

This?

Ascending from 30 FSW to the surface on EAN60 means a drop in ambient pressure, which means a risk for bubble formation. Unless you are breathing 100% O2, you have some N2 in your tissues, and it's the drop in ambient pressure that start bubble growth, which correlates with DCS risk.

That's a claim, not a description of the physical process.

Hypothetical question

This?

The best way I have understood it is that a drop in ambient pressure is what causes bubble formation

Sorry, I need a source for that.

Hypothetical question

This?

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)

Yes, but the situation changes dramatically as you surface, spit out your reg delivering EAN60 and start breathing air. See below.

Hypothetical question

This?

Unless you have no inert gas in your tissues (i.e. you have been breathing 100% O2 long enough to completely outgas all of your compartments), when you ascend, you can form bubbles.

How?

All the deco theory I've read, says that if the tissue tension is lower than the inspired partial pressure, the diver will ongas. If the tissue tension is higher than the inspired partial pressure, the diver will offgas. And as you have already alluded to, which also is supported by @RainPilot 's experience, we don't worry much about O2 bubbles, only inert gas bubbles. Otherwise, prebreathing O2 before going to hypobaric conditions would cause just as much bubbles, only that they'd be O2 bubbles instead of N2 bubbles.

Let's look at the situation at depth, just before surfacing and at the surface, and for simplicity I'm assuming that at least one tissue is fully saturated.

Depth: 9m (1.9 ATA)
Gas: EAN60
Inspired pPO2: 1.14 ATA
Tissue O2 tension at saturation: 1.14 ATA
Inspired pPN2: 0.76 ATA
Tissue N2 tension at saturation: 0.76 ATA

The general consensus seems to be that if the if the ratio between tissue tension and inspired partial pressure is higher than 1.58:1, there is an unacceptable risk of forming pathological bubbles. With a tissue tension of 0.76 ATA, this happens at 1.2 ATA, or 0.2m depth. But bubbles don't form immediately, it takes time. And a few seconds after the diver has reached a ratio of 1.58:1, they are at the surface, spitting out their reg and taking their first lungful of air at 1 ATA.

Depth: 0m (1.0 ATA)
Gas: EAN21
Inspired pPO2: 0.21 ATA
Tissue O2 tension at saturation: 0.21 ATA
Inspired pPN2: 0.79 ATA
Tissue N2 tension at saturation: 0.79 ATA

So, just after passing the 1.58:1 limit less than a minute after they left the bottom, the diver has a ratio between tissue tension and inspired partial pressure of 0.96:1 and is in fact ongassing. A principally similar, but of course more extreme, situation would be surface chamber deco (Sur-D), where the diver comes up owing a significant amount of deco, jumps in the chamber and is blown down to beyond their maximum depth. And Sur-D is - AFAIK - well known in commercial and military diving.

The driving force for bubbling isn't total pressure, it's the ratio between tissue tension and inspired partial pressure. IOW Henry's law. Cite: Decompression Theory - Part 2 - SDI | TDI | ERDI

 

[huwporter]

I think that the only part of this (hypothetical) dive that might, just plausibly, lead to a very brief shower of bubbles might be during the actual ascent itself, when you will briefly experience both a tissue oversaturation of N2 relative to inspired gas and a reduction in pressure. I think that these would be A) very minor, and B) would pass normally into the veinous system and to the lungs, where they would be exhaled. (From the study I quoted earlier, I'm completely comfortable in discounting any risk from O2 bubbles.)

On arrival at the surface, there would no longer be a relative oversaturation of N2 relative to inspired so I think the shower of bubbles would A) rapidly cease, and B) there would be no supersaturation to encourage further growth of any existing bubbles. So (assuming normal physiology, no shunts) I don't personally think there would be any further cause for concern. If the diver had a PFO or lung shunt, then I guess one of those bubbles might plausibly bypass the lungs and make it into the arterial system, but even then there would rapidly be no supersaturation gradient to cause further bubble growth until takeoff, and then the supersaturation would be very slight. We have micro bubbles in our bodies all the time, and people with PFOs (25-33% of the population) aren't constantly getting the bends on commercial flights.

However one observation to make off the back of this is that most people (myself included) are interpreting the dive description "90 minute dive to a maximum depth of 9 meters" as a square profile. If the dive was actually a severe sawtooth profile repeatedly alternating between 9m and a shallow depth, then the bubble forming conditions described above would be experienced for a longer aggregated time, and the pressure pumping just might be enough to aggravate the situation, particularly if combined with a shunt.

So I'm going to change my answer of what I'd do to:

Square profile, no shunts: Fly immediately, no concern at all.
Square profile, shunt/PFO: Fly immediately, negligible risk.
Sawtooth profile, no shunts: Still probably fine to fly immediately but I'm probably not confident enough about this to actually do it.
Sawtooth profile, shunt/PFO: Possibly a concern, would wait before flying.

Happy to be corrected by any experts on any of this...

 

[doctormike]

But bubbles don't form immediately, it takes time.

And you know that how? Where do the bubbles come from that are found by doppler in divers ascending from a recreational, no-stop dive?

The driving force for bubbling isn't total pressure, it's the ratio between tissue tension and inspired partial pressure. IOW Henry's law.

Right. It's the change in ambient pressure, not the total pressure. Which is why you don't bubble when switching to 100% O2 at 20 feet and staying there. Or do you think that decoing on O2 is a risk factor for DCS, given the sudden large increase in the ratio between tissue tension and inspired partial pressure?

I think that you are confusing offgassing with bubbling. I was doing exactly the same thing before that Shearwater GF thread that I mentioned.

Once again-

Totally reasonable: "Given the lack of ADDITIONAL N2 loading by this dive, I think that any bubbles formed on ascent are trivial, and will be cleared so rapidly on the surface that there is a clinically insignificant risk of a second decompression by ascent to altitude immediately after surfacing"

Not reasonable: "Given the fact that the diver isn't ongassing additional N2 during the dive, there is no physical process that can cause bubble formation when the diver is exposed to an ambient pressure drop"[/QUOTE]

 

[TrimixToo]

And you know that how? Where do the bubbles come from that are found by doppler in divers ascending from a recreational, no-stop dive?



Right. It's the change in ambient pressure, not the total pressure. Which is why you don't bubble when switching to 100% O2 at 20 feet and staying there. Or do you think that decoing on O2 is a risk factor for DCS, given the sudden large increase in the ratio between tissue tension and inspired partial pressure?

I think that you are confusing offgassing with bubbling. I was doing exactly the same thing before that Shearwater GF thread that I mentioned.

Once again-

Totally reasonable: "Given the lack of ADDITIONAL N2 loading by this dive, I think that any bubbles formed on ascent are trivial, and will be cleared so rapidly on the surface that there is a clinically insignificant risk of a second decompression by ascent to altitude immediately after surfacing"

Not reasonable: "Given the fact that the diver isn't ongassing additional N2 during the dive, there is no physical process that can cause bubble formation when the diver is exposed to an ambient pressure drop"

[/QUOTE]

And, here's where we need an expert to chime in. The risk falls somewhere between the risk of getting out of bed and base jumping. I think it's a lot closer to the latter; that is, your "Totally reasonable" statement.

I think the bubbles, if any, are small and filtered quickly given a reasonable ascent speed. I think the probability of stepping out of the water and climbing into an airplane for an immediate takeoff is very low outside military operations, too. There's some time involved in getting ashore, packed, to the airport, through security, entering the aircraft to be seated, taxiing, takeoff, and gaining enough altitude that the cabin pressure altitude reaches 8000'.

You have to be a bit careful with the absolutes. First, *everything* takes time (truly instantaneous processes tend to be bad for bystanders and participants both). This includes both bubble formation and bubble filtering in the lungs. So we can talk about the scale used to measure or project those times, but neither process is instant.

 

[Storker]

It's the change in ambient pressure, not the total pressure.

We might be talking across each others. What do you mean with "ambient" pressure vs "total" pressure?

For me, the ambient pressure is the total pressure the diver experiences where the diver is located. The partial pressure of one particular gas is the total (ambient) pressure multiplied by the fraction of that particular gas.

 

[doctormike]

The general consensus seems to be that if the if the ratio between tissue tension and inspired partial pressure is higher than 1.58:1, there is an unacceptable risk of forming pathological bubbles.

I'm not a deco expert, but that is not what that number represents. The 1.58:1 ratio that you are citing is from Dr. Robert Workman, a US Navy doctor who concluded in the 1960s that decompression sickness would happen if a diver was subject to a change in ambient (total) pressure of greater than 1:58:1 (a refinement from Haldane's 2:1).

The M-value line that is derived from these and other further overpressure risk refinements is what you see on that well known diagram. Gradient Factors are a percentage of the way from the ambient pressure line to the M-value line, NOT from the p(insp) line, which is below the ambient pressure line (the richer the mix, the further below the ambient line).

When you are considering DCS risk and bubble formation, the relevant thing is the change in total (ambient) pressure, not partial pressure. Partial pressure varies with mix as well as depth. Total pressure only varies with depth.

Note from the graph that bubble formation is relative to the ambient (total) pressure line (orange) not the inspired inert pressure line (green). You can drop that green line all the way down to the X axis if you use 100% O2 and increase that gap, but you don't increase DCS risk until you ascend, and drop ambient pressure.

 

[doctormike]

We might be talking across each others. What do you mean with "ambient" pressure vs "total" pressure?

For me, the ambient pressure is the total pressure the diver experiences where the diver is located. The partial pressure of one particular gas is the total (ambient) pressure multiplied by the fraction of that particular gas.

Total pressure (sum of all partial pressures, Dalton's law) and ambient pressure are the same thing. I said it was the change in ambient (or total) pressure that caused a DCS risk, not the absolute amount of ambient (or total) pressure, which is what I though you were claiming.