By B.R. Wienke & T.R. O’Leary
NAUI Technical Diving Operations
Fact — Isobaric counterdiffusion is a real gas trans-
port mechanism. Please pay attention to it in mixed gas deco diving.

Fiction — Isobaric counterdiffusion is a theoretical
concoction and doesn’t affect divers at all.

Why— Observed in the laboratory by Strauss and Kunkle in bubble experiments and is a simple physical law porting to diving. Studied first by Lambertsen and Idicula in divers in 1975, published and reported in many diving medical and physiology journals, and now accepted by the deco science community worldwide.

WHAT is isobaric counterdiffusion (ICD)?

Isobaric means equal ambient pressure.

Counterdiffusion means two (or more) gases diffusing in opposite directions.

For divers, the two gases are nitrogen and helium. And that is where concern is focused — inert gases, and metabolic gases like oxygen, carbon dioxide, and water vapor, nor trace gases in the atmosphere. Thus, ICD in diving underscores two inert gases moving in opposite directions under equal ambient pressure in tissues and blood. But what’s important are relative speeds for counterdiffusion. Lighter gases diffuse faster than heavier gases. In the case of helium and nitrogen, surrounding nitrogen loaded tissue and blood with helium will result in greater total gas loading because helium will diffuse into tissue and blood faster than nitrogen diffuses out, resulting in higher total inert gas tensions. Surrounding helium loaded tissues and blood with nitrogen will produce the opposite effect, helium will outgas faster than nitrogen ingases and total inert gas tensions will be lower.

Perhaps a better descriptor is isobaric countertransport, because diffusion is only one of a number of different movement mechanisms. Historically, both terms have been used, with the former mostly employed in the decompression arena. Countertransport processes are a concern in mixed gas diving when differing gas solubilities and diffusion coefficients provide a means for multiple inert gases to move in opposite directions under driving gradients. While ambient pressure remains constant, such counterdiffusion currents can temporarily induce high tissue gas supersaturation levels and greater susceptibility to bubble formation and DCS.

In general, problems can be avoided when diving by employing light-to-heavy (breathing) gas mixture switches and by using more slowly diffusing gases than the breathing mixture inside exposure suits (drysuits). Such procedure promotes “isobaric desaturation,” as termed in the lore. The opposite, switching from heavy-to-light gas mixtures and using more rapidly diffusing gases than the breathing mixture inside exposure suits, promotes “isobaric saturation” and enhanced susceptibility to bubble formation. More simply, the former procedure reduces gas loading, while the latter increases gas loading. The effects of gas switching can be dramatic, as is well known. For instance, a dive to 130 fsw for 120 min on 80/20 heliox with a switch to 80/20 nitrox at 60 fsw requires 45 min of decompression time, while 210 min is required without the switch (Keller and Buhlmann in famous mixed gas tests in 1965). Yet, skin leisions and vestibular dysfunctionality have developed in divers breathing nitrogen while immersed in helium (test chambers and exposure suits). And nitrogen-to-helium breathing mixture switches are seldom recommended for diving, particularly extended periods of time.
In the case of exposure suits filled with light gases while breathing heavier gases, the skin leisions resulting are a surface effect, and the symptomology is termed “subcutaneous ICD” Bubbles resulting from heavy-to-light breathing gas switches are called “deep tissue ICD”, obviously not a surface skin phenomenon. Bottom line, if you don’t want to read further is simple. Don’t fill your exposure suits with a lighter gas than you are breathing, and avoid heavy-to-light gas switches on a deco line or lift bag. In both cases, bubble risk tracks with exposure time.

But what, you say, about detox switches from deco nitrox to trimix or heliox back gas? We all know it’s been done since time immemorial, and is still done. For most of tech diving in the 200 fsw to 300 fsw range, for periods of time not exceeding 60 min or so, short detox switches off nitrox to heliox or trimix are not high risk, so long as cumulative detox times stay below 40 min roughly. But the statements above are still true – switching from nitrox back to trimix incurs risk versus other alternatives that can be used. And for very deep dives in 500 fsw range and beyond (like the dives Mark Ellyat clocked), isobaric switches off nitrox back to trimix are not a good idea. In fact, because of the depths and pressures, increased ingassing gradients for one or other gases lead to “isobaric slam” as we have coined the word. Slam is mitigated by making isobaric gradients as small as possible within the deco plan. Slam also shows up on deep dives as inner ear vertigo with fluid shifts (bad). Or simply, ICD problems increase with depth as ingassing gradients for one or the other gases increase. Avoid this by careful selection of switch mixes, minimization of nitrogen, and washout with oxygen in the shallow zone.

Another important consideration is nitrogen level. Nitrogen is not your friend for diving, with risk of DCS increasing with nitrogen fraction across multifaceted diving. We come back to all of this in discussing Ellyat’s record OC dive. Turns out most of this is an efficient deco strategy, too, within dual phase bubble models.

Details of ICD cannot obviously be recounted here in gory detail, but rudiments, time scales, and mechanistics are found Technical Diving In Depth, Reduced Gradient Bubble Model In Depth, and Basic Decompression Theory and Application. Also, released NAUI RGBM Tec Tables embody all discussed herein, but with a more simplistic approach to training. Minimization of intermediate and deco stage bottles for OC training is a NAUI RGBM Tec Table mainline, with just a single switch to pure oxygen at 20 fsw.

A closer look at the isobaric countertransport phenomenon is interesting. Particularly interesting is the “isobaric saturation” scenario depicted. Figure 1 tracks gas supersaturation following nitrogen-to-helium switching due to the isobaric counterdiffusion of both gases. For helium-to-nitrogen switching (hopeful case for technical and commercial divers), a state of “isobaric desaturation” would ensue due to isobaric counterdiffusion.

Depicted in Fig 1 is a comparative representation of the time courses of changes in helium, nitrogen, and sum of the two, tissue tensions for 480 min nitrogen tissue compartments and 240 min helium tissue compartments. The depth is 200 fsw with abrupt change from normoxic nitrox to normoxic heliox.  Note the buildup in time of total inert gas tension, with a maxima after some 400 min.  With faster tissue compartments, this maxima builds more quickly, on time scales of the slowest tissues involved.  Actually the curves remain the same as shown, but axis time scales are shortened by the ratio of the fast tissue halftime, t, divided by 240 for the helium compartment, t/240, and similarly for the nitrox compartment, tissue halftime, t, divided by 480, that is, t/480.  This is quite obviously not a good scenario for the mixed gas diver. If the gases were flip flopped, a minima would develop, identical in shape to inverted
Fig 1.  For faster tissue compartments, usually the case in mild deco (non sat) diving, effects seen in Fig 1 occur much more rapidly, like in the 10 min to 50 min time frame.
Mark Ellyat (UK) has made a number of dives beyond the 500 fsw mark.  A recent dive to 1040 fsw on OC trimix is a record and a spectacular accomplishment.
With a total run time in the 420 min range.  Stops are made every 10 fsw or so beginning near 750 fsw.  With rapid descent, a strawman RGBM schedule follows in Fig 2. As far as ICD (and HPNS because of the extreme depth), Ellyat’s strawman schedule exhibits a number of interesting features.
Dives exceeding 400+ fsw require high helium and low nitrogen.  On the way up, oxygen is increased in roughly the same proportion as helium is decreased, while keeping nitrogen fairly constant and in the 15% to 25% range (the lower the better for deco, but higher than 10% to address HPNS concerns).  Oxtox management falls in a ppO2 = 1.2 atm region.  Pure oxygen is employed at 20 fsw.  Note, there are NO isobaric switches to nitrox anywhere. Not EAN50 at 70 fsw.  At 30 fsw, 80/20 heliox (no nitrogen) is the switch mix. Rather than EAN50 at 70 fsw, heliox 50/50 would be a better choice.

Also HPNS and rapid descents can be a problem on trimix below 600 fsw, just like heliox below 400 fsw.  Though 10% nitrogen is popular for mitigating HPNS, it’s not foolproof, and nitrogen thus is above 10%. Bottom line here is ingassing gradients for nitrogen have been minimized by avoiding isobaric switches, and the transitions from richer-to-leaner helium mixes are also smoother.  No slams.
Remind you of a rebreather in some ways?  Yea, guess it does.
Happy and safe tech diving.