Off gassing at 4 vs 6m (from the complacency thread)

dteubner

I would like someone to explain (in simple words) why they think there is a difference in off gassing rates at different depths with a FIO2 of 1.0.

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Dave T

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Sutty

At 6m inert gas tissue pressure is x, inspired gas inert gas pressure is zero.

At 4m inert gas tissue pressure is x, inspired gas inert gas pressure is zero.

Only real difference is O2 exposure, ie PO2 1.6 Vs PO2 1.4.

Presume this was the difference you were meaning?

Neil

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Dutchy

Same thing could be accomplished by using EAN80@ 6m (EAN83 to be precise) is there more to it?

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Janos

From a deco perspective, you will off-gas inerts in bubble form as you ascend, so ascending straight to 4m (not 6m) will lead to more bubbles which is good if you get away with it but not if you don't.

There are obviously CNS issues.

Some people believe in the "Oxygen Window" meaning you off gas quicker at high ppO2s but personally I think this is cobblers and have seen no evidence backing it up in diving physiology, other than the very simplistic paper doing the rounds which only talks about the lack of inerts.

Me. I go to 6m, "switch" to 100% O2, wait for a bit, then go as shallow as is practical.

Janos

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dteubner

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Dave T

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Sutty

NO, in that case the pressure gradient would be reduced by the inert gas in the inspired gas at 6m. Therefore offgassing would be slower (assuming same inert gas).

Off gassing depends on the pressure gradient between pulmonary capillary blood and alveolar gas. Bubble formation will be governed by tissue over-saturation at ambient pressure. So as long as you are breathing 100% O2, then the deeper the better as far as bubble formation is concerned, off-gassing will be the same.
How deep you stop on 100% O2 will obviously be limited by how high/long you are prepared for your PO2 to be from a toxicity point of view.

I.e. Once you are on 100% O2 you are off-gassing maximally, if you can stay deeper (and are happy with O2 toxicity), then bubble formation will be less, Go shallower to reduce PO2, but you may increase bubble formation. However there may also be an issue with Varying ambient pressures as you go shallower in a swell.

Personally I think Janos has the right idea (as long as you can hold the depth).

Neil

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Mike

I guess I should have a go, given I made the origional statement.
Decompression rate is driven by the degree of overpressurisation - increase the overpressurisation, and you increase the rate of inert gas removal (both as dissolved gas, and as bubbles if you are in to that sort of thing) - up to a point anyway. (where bubbles start forming in tissue and disrupting the process - apparently that hurts).

As a huge oversimplification, consider just the case of a dissolved inert gas moving across a semi permiable membrane from an imaginary tissue, into blood. Now we are breathing O2 at 10m, so there is no inert gas coming in from the lungs, however there is still inert gas dissolved in the blood due to the ongoing decompression process. In this simplified model we will consider the pressure of dissolved gas in the blood to be relative to absolute pressure, ie the lungs are perfectly efficient inert gas exchangers, the only reason there is dissolved gas in the blood is due to the time taken for it to circulate to the lungs to offgas.
Lets say that the (dissolved gas)pressure in the tissue is X, and the pressure in blood is Y. Overpressure gradient driving rate of decompresion is X:Y.

Now lets go instantly from 10m depth to the surface (this is just for easy maths), assuming we were breathing O2 at 10m and will continue breathing O2 at the surface.

In my simple model, the inert gas pressure in the tissues has not changed, it is still X. The pressure of dissolved gas in the blood (as it tracks absolute pressure) is now HALF Y.

Therefore the overpressure gradient is now X : Y/2 (ie it has doubled).
As rate of gas exchange is driven by this pressure ratio, it will increase.
Hence we will offgass faster at the surface than at 10m, breathing O2 at both points. Same theory for 6m v 4m, just not as big an effect.

Obviously a gross oversimplification, but I think the idea is sound. Fockie is supposed to be dropping in for a beer this afternoon, I'll see if I can convince him

Mike

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Sutty

Why would the gas pressure in the blood have halved? where would the gas have gone? Blood is just another tissue - it just happens to be the one that equilibrates with alveolar gas. It has to off-gas to reduce inert gas pressure!

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Mike

Because the blood is a really fast tissue, and will very rapidly equalise to ambient pressure.

I wasn't really meaning that the pressure in the blood actually halves instantaneously, rather I was trying to describe the concept that there will be differences in pressure changes as you ascend across different tissues types, and this results in there being a greater gradient driving offgassing at 0m than at 10m (or 4m vs 6m).

BTW- I also think that hyperbaric O2 has a positive effect that is greater than just being a lack of inert gas - which would tend to confuse things further.

Mike

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dteubner

First, I must apologise for being a smart arse.

Unless you are making some theoretical sophisticated argument about microscopic bubble formation in tissues then depth has absolutely no effect on off gassing if you are breathing 100% O2.

Breathing 100% O2 means that there is essentially no inert gas in the arterial blood and therefore there is a large gradient from the tissues to the blood for the inert gas to move down. This gradient does not change with depth as there is no arterial inert gas at an depth (6m, 3m, surface, 20m) as long as the FiO2 is 1.0.

Of course the deeper you start breathing 100% oxygen the less the chance that bubbles will form (and the smaller any bubbles will be) which is why deeper is better from a decompression perspective but it is not about the off gassing rate.

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