Underwater off-gassing equivalent to a surface interval on air

[Alaskan Scuba Dude]

@Duke Dive Medicine: could two staged decompression dives per day at a depth of 165 ft per dive over ten to fourteen subsequent days resulting in high CNS toxicity as a consequence. Would those types of dives prevent you from using a hyperbaric chamber if you were to get bent?

 

[Duke Dive Medicine]

Thank you. What is isobaric counterdiffusion?

It's where a gas diffuses in as another is diffusing out. In practice, it can happen if a diver switches from a breathing mix that does not contain helium to one that does. The helium diffuses in quickly and can result in bubble formation.

Best regards,
DDM

 

[Duke Dive Medicine]

I don't understand how the same nitrogen gradient in both cases can result in a different rate of off-gassing. Can you please elaborate on the mechanism behind this (how higher O2 pressure influences the N2 rate)?

Sorry - my last response lacked detail. Widening the O2 window inside the body by breathing higher partial pressures of O2 results in less likelihood of bubble formation but not overall faster off-gassing. Mark Powell does a much better job of explaining it than I can, along with diagrams, in Deco for Divers, p. 131-135. Thanks for the callout.

Best regards,
DDM

 

[lowwall]

Sorry - my last response lacked detail. Widening the O2 window inside the body by breathing higher partial pressures of O2 results in less likelihood of bubble formation but not overall faster off-gassing. Mark Powell does a much better job of explaining it than I can, along with diagrams, in Deco for Divers, p. 131-135. Thanks for the callout.

Best regards,
DDM

Whew, no need to rework ZHL-16

The trade-off for decreased likelihood of bubble formation are the usual drawbacks of time spent at higher PPO2 exposure. In this case 1.6 vs 1.3. And it's not a trivial amount of time. If you specify 3m/10’ as your last stop, roughly 2/3 of the 100% portion will be spent there.

 

[inquis]

@Duke Dive Medicine (or anyone else that knows), we're taught that one of major reasons to take air breaks during lengthy O2 stops is to reduce mucus buildup (improving subsequent gas exchange). Does partial pressure drive that buildup or is it more tied to the fraction of O2 breathed?

 

[EFX]

It's where a gas diffuses in as another is diffusing out. In practice, it can happen if a diver switches from a breathing mix that does not contain helium to one that does. The helium diffuses in quickly and can result in bubble formation.

Best regards,
DDM

I've read something different. ICD typically happens on ascents when divers are switching from a gas with a low N2 mix to a gas of a richer N2 mix. Here is what I've read:

ICD (Isobaric Counter Diffusion) can be caused by the absorbtion of one inert gas of high solubility while another inert gas of lower solubility leaves the tissues. This can happen when switching from a helium mix (trimix or heliox) to nitrox during deco stops. Isobaric means at constant pressure. It is important to realize that one inert gas can enter a tissue at the same time that another inert gas is leaving the tissues. The transfer of inert gas into and out of tissues does not depend on the total pressure of the gases but just on the partial pressure of the particular inert gas. The reason that a switch from trimix or heliox to nitrox can be so dangerous is that nitrogen being of higher solubility will enter the tissues faster than helium can leave increasing the total inert gas pressure beyond a safe value. The higher the positive value of ICD the greater the risk for a DCS hit. One rule that is used with great success when switching from a helium mix to nitrox is that any increase in the fraction of nitrogen should not exceed 1/5 of the decrease in the fraction of helium.

 

[inquis]

The reason that a switch from trimix or heliox to nitrox can be so dangerous is that nitrogen being of higher solubility will enter the tissues faster than helium can leave increasing the total inert gas pressure beyond a safe value.

This is contrary to what I've read in that helium moves faster than nitrogen (smaller molecule, higher diffusivity). Thus, switching to helium rich gas is not recommended when tissues are already heavily loaded with nitrogen because helium enters faster than nitrogen can leave. This isn't usually a problem, though, because the normal case is switching from back gas to deco gas (less helium). For MOST tissues in such cases, the helium leaves faster than the nitrogen enters (no problem).

You're correct, though, that switching to a higher N2 fraction can be a danger in some cases, because of vestibular (inner-ear) DCS. A reservoir of helium-rich gas is trapped on one side (somewhat replenishing the helium that leaves) and the increased N2 fraction is also entering from the other side. From what I've read, switching from helium rich gas to deep-air could have the necessary N2 gradient that would increase the total pressure dangerously above ambient. However, it's not really a concern for "normal" deep-deco gas/depth choices (e.g., 21/35 at 190 ft). Compare that (ppN2 of 3.0 atm) to the outdated practice of air at 215 ft (ppN2 of almost 6 atm). (Don't hold me to those specific numbers, as I wasn't doing deco dives then, but the general direction holds.) The reduction of the standard for max ppO2 on deco (to 1.6 atm from whatever it used to be) also helps reduce the likelihood.

 

[dmaziuk]

And then there is the take on it by a physicist who writes dive planning software:

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

Isobaric Counter Diffusion Criteria – The Theoretical Diver

thetheoreticaldiver.org

 

[rx7diver]

And then there is the take on it by a physicist who writes dive planning software:

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

Isobaric Counter Diffusion Criteria – The Theoretical Diver

thetheoreticaldiver.org

Hmmm. Do you have links to pertinent peer-reviewed articles written by a physiologist (rather than to blogs written by a theoretical physicist)?

rx7diver

 

[EFX]

The Wikpedia article on ICD includes cases where gas switches were made going from helium mixtures to nitrox and vice-versa. It also talks about the specific causes of inner ear DCS and skin bends due to breathing nitrox while the skin is surrounded by a helium mix. Here is the link for your convenience:

Isobaric counterdiffusion - Wikipedia

en.wikipedia.org