The other guides deal with ascending faster than the tissues can release inert gas. Isobaric counterdiffusion can occur with no change in depth. The diver stays at one depth and switches from one gas to another, and the switch itself can cause decompression sickness.
Isobaric means at the same pressure. Counterdiffusion means 2 gases diffusing in opposite directions: helium leaves the tissues while nitrogen enters them. The depth gauge shows no change.
Superficial and deep tissue counterdiffusion
In the 1970s Christian Lambertsen and his colleagues at the University of Pennsylvania studied subjects in chambers who breathed one gas while surrounded by another. Surrounded by helium and breathing nitrogen or neon, they developed intense itching and skin lesions with no change in pressure. Lambertsen called this superficial counterdiffusion.
He also described a deep tissue form, which is the one relevant to divers today. The surrounding gas plays no part. The diver changes breathing gas, and the 2 gases diffuse in opposite directions inside the body.
- Nitrogen
- Helium
A deep trimix dive
The dive is 20 minutes at 60 m on 15/55: 15 % oxygen, 55 % helium, the rest nitrogen. The tissues hold a large load of helium and less nitrogen than an air dive would leave.
The figure shows one fast compartment from the DiveLogic engine at the start of the ascent. Helium begins to leave as the pressure falls. The nitrogen level changes very little.
The switch
At 36 m the diver switches to EAN32, which has no helium and 68 % nitrogen. The depth is unchanged, but the direction of the nitrogen gradient reverses.
With no helium in the lungs, helium leaves the tissue quickly. With much more nitrogen in the lungs, nitrogen enters the tissue. The 2 gases move in opposite directions in the same tissue.
Total inert gas
Bubble formation depends on the total inert gas pressure in a tissue. In this compartment helium leaves faster than nitrogen enters, so the total continues to fall. The model does not flag a problem.
This is a limitation of the model. A compartment model treats each tissue separately, supplied only by blood. In the body, counterdiffusion problems occur where tissues with different properties lie next to each other. The model does not represent this.
All 16 compartments
Every compartment shows the same pattern: helium falls while nitrogen rises. The fast compartments change the most.
Whether the change causes harm depends on the tissue. The evidence points to the inner ear.
The inner ear
Divers who made deep switches from trimix have had inner ear decompression sickness: sudden vertigo, nausea and vomiting, sometimes with hearing loss, and sometimes with no other symptoms. Doolette and Mitchell modelled the inner ear as a small tissue supplied partly by blood and partly by the surrounding fluid. Their model showed that a switch from a helium mix to a nitrogen-rich mix can briefly make the inner ear supersaturated while the rest of the body is not.
For this reason counterdiffusion is managed with rules on the gas switch itself, not with the decompression model.
Try it
Set a bottom mix, a deco gas and a switch depth. The DiveLogic engine plans the dive, runs its 6 counterdiffusion checks on the switch, and finds the gas with the smallest nitrogen rise that passes all 6. The rule of fifths is shown with them. Compare 18/45 to EAN50 at 21 m, then EAN32 at 36 m, then a 50 % oxygen mix with some helium.
- Nitrogen gained
- Helium lost
- Net change
- N₂ fraction risepasses
- He fraction fallpasses
- ppN₂ risepasses
- ppHe fallpasses
- Switch at the ceilingpasses
- EAD mismatchpasses
- Rule of fifthsN₂ +13, allows +9
What to remember
- Isobaric counterdiffusion is 2 gases diffusing in opposite directions at a constant depth. The depth gauge gives no warning.
- Lambertsen described 2 kinds: superficial, a different gas around the skin, and deep tissue, a switch of breathing gas.
- After a switch from helium to nitrogen, every compartment loses helium while it gains nitrogen. A compartment model shows these changes but cannot show the resulting harm.
- The tissue most at risk is the inner ear, which is why vertigo after a deep switch is taken seriously.
- Divers manage it with rules on the switch itself: DiveLogic’s 6 checks, and rules of thumb such as the rule of fifths.
Gas switches in DiveLogic
DiveLogic runs the same 6 checks on every gas switch in a plan, including bailout switches on a rebreather, and flags any that fail. The checks do not change the schedule. They identify switches of the type associated with inner ear decompression sickness.
Sources
- Lambertsen C. J., Idicula J. (1975). A new gas lesion syndrome in man, induced by “isobaric gas counterdiffusion”. Journal of Applied Physiology 39(3): 434-443.
- Doolette D. J., Mitchell S. J. (2003). Biophysical basis for inner ear decompression sickness. Journal of Applied Physiology 94(6): 2145-2150.
- Bühlmann A. A., Völlm E. B., Nussberger P. (2002). Tauchmedizin, 5th edition. Springer, Berlin. (ZHL-16C compartments used by the figures.)