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I recall a number of years ago we had a visiting diver during dinner state that his expert friends in the Pacific northwest said that if he switched from air at 200ft to trimix, it would kill him because of isobaric inert gas counter-diffusion. Not wanting to pass up this opportunity we mixed some gas and did a dive to see if that was true. So about midnight that night we dropped to 200ft, switched to trimix and the rest is left to posterity. Needless to say nothing happened; although he was bit by a turtle the next day during deco at DiePolder. I almost died of laughing too hard. End of story. We must recognize that until recently, the prevalent theories used for constructing decompression models were limited to the classical decompression theories of Haldane, Buhlmann, and Workman. These theories assumed either the body was bubble free, or it wasnt. If bubbles were present, then the subject was bent. Even though Doppler studies are not without their own set of problems (e.g. false readings, inability to distinguish between dislodged bone or fat particles, or overall correlation to DCS), it became apparent through the use of this technology that simply the presence of bubbles did not necessarily mean that DCS was imminent. A second contributor to DCS was identified to include free-phase bubbles. In fact the notion of bubble seeds has been known for quite some time, but until recently this phenomenon has not received much attention in diving applications. More modern theories suggest we must not only account for dissolved gases (classical theories) but must also consider free-phase bubble growth (Hill, Gernhardt, and Weinke). Therefore the excitation of free-phase bubbles into growth and collapse must also be accounted for in decompression modeling. Digressing a bit, I recently read the following passage by a hyperbaric specialist on diver physiology who writes for Underwater Magazine, the journal for The Association of Diving Contractors International that further illustrates the complexity of decompression theory: . We also find that the correlation to circulating bubble emboli is imperfect. Large numbers of detected bubbles are not necessarily accompanied by decompression illness, and the absence of detected bubbles does not necessarily accompany a symptom free experience. In surface decompression using oxygen (a pervasive commercial diving practice), decompression illness cases occur without any observable bubbles more frequently than in decompression procedures that do not use oxygen. Saturation dives also exhibit a weaker correlation when compared with typical non-saturation diving. The correlation between Doppler detection and decompression illness in saturation diving is so bad that some investigators have concluded that Doppler monitoring is useless in saturation diving .. Hummm! Bubble growth/collapse mechanics appear very complex. If we are to consider this phenomenon as part of decompression procedure, and the indications are we should, we must evaluate this process, even when it tends to provide a contradiction to conventional theories [dissolved gas mechanics]. Bubbles will grow (or collapse) though various mechanisms that include, but not limited to, ambient pressure, osmotic tension, and size/surface tension. There is a critical size at which the bubble may grow or collapse depending on the parameters. Risking the possibility of placing my foot in my mouth, I recall discussions on the effects of osmotic pressure on bubble growth. Specifically a bubble may be stimulated into growth if subjected to an certain osmotic pressure differences. Therefore a bubble excited at depth by one particular gas mix, may tend to grow if subjected to a deco mix containing a different gas (in addition to the lowering ambient pressure effects and surface tension reduction as bubble radius increases). In essence, N2 or even O2 may diffuse into the bubble thereby increasing its size, or total distributed volume?? This effect may reinforce an argument for using more helium during decompression for long and deep dive profiles that tend to excite small bubble seeds not considered a factor in shallower or shorter duration dives??? While classical Buhlmann theory may suggest that decompression using increasing N2 fractions, and O2 is most beneficial, free-phase bubble mechanics may indicate otherwise for some instances. We have already alluded to this possibility when it became understood that gas bubbles in the circulation system have an effect on the bodys efficiency on off-gasing thereby further indicating the classical exponential ongas-exponential offgas computations to be over-simplified. Suffice to say there is more to the process than satisfying Buhlmanns equations as provided. Getting back to the subject of isobaric inert gas counter-diffusion, and speaking strictly in regard to the dissolved gas phase dynamics (no consideration for free-phase bubble growth) on which Haldane, Buhlmann, and Workman based their work, it may be concluded that a technical diver is not at risk from isobaric inert gas counter-diffusion. I include this discussion based only on their work and not to include more modern approaches to demonstrate that even with their findings that isobaric inert gas counter-diffusion as they define it does not appear to pose a risk to technical divers. The concept of counter-diffusion is applied and demonstrable in the technical diving community. Hans Keller and Albert Buhlmann in their work demonstrated that counter-diffusion can be used constructively to significantly reduce decompression times [1]. Their application illustrated the principle that inert gases of differing molecular weights will diffuse into and out of tissues at differing rates (comparable half-times based on the reciprocal of the square root of their molecular weights). Furthermore Buhlmanns [and others] theory assumes the decompression obligation, or degree of super-saturation above ambient pressure, is based on the total sum of all the inert gas partial pressures of the constituent inert gases. Appling this theory which inherently embodies the counter-diffusion principle he was able to demonstrate that decompression times can be shortened by switching to inert gases of greater molecular weight (N2,Ar) during deco. In effect, the helium would diffuse out of the tissues at a faster rate than the N2 or Ar would diffuse into the tissues. This in effect would create a degree of sub-saturation that proves beneficial to the decompression event. For example, Hans Keller in an open ocean dive to 1000ft, with 3 minutes of activity at depth, was able to decompress in 61 minutes. Buhlmann further noted that on relatively short duration dives (<2 hours), a high helium content breathing medium would typically result in longer decompression time than one done on a N2-O2 mix [1]. However after about 2 to 3 hrs duration a He-O2 dive would result in shorter decompression time. This may be explained by counter-diffusion as during the longer duration dives the N2 inert gas tensions in the tissues would be somewhat diminished and that during deco, the He inert gas would flush out of the tissues about 2.6 times faster than if the tissues were otherwise loaded with N2. Buhlmann also commented on the potential problem of super-saturation at an isobaric state if one switches to a lighter inert gas at depth. Very good discussions of isobaric inert gas counter-diffusion can be found in C. Lambertsen, M.D. [2], and C.A. Harvey and Lambertsen [3]. However one must keep in mind that the applications of these papers, as well as many of Buhlmanns [et.al.] on this subject are directed to commercial saturation diving applications (e.g. long duration exposures, great depths, high helium/hydrogen content bottom mixes, and chamber/bell environments). This I believe is where a bulk of misinformation occurs as sometimes information garnished from this sort of research is incorrectly applied to technical applications. Like comparing apples and oranges. As George I. mentioned, some of the research on the isobaric counter-diffusion principles involved farm animals. For example one experiment showed that pigs will develop venous gas embolisms after about 30 minutes of breathing a N2-O2 mixture at 1 ATA while surrounded by helium. In this experiment, the blood vessels contained more gas than blood [2]. Similar experiments with rabbits while breathing air with a helium atmosphere (@200 ft ambient pressure) indicated death will occur in about 1.5 to 2 hours. What does this mean? Dont take your pet pigs and rabbits diving? Perhaps, but a key factor outlined in these sorts of tests almost always includes the test subject being enclosed in a light inert gas environment while breathing a heavier inert gas mix. Human experiments have indicated that itching of the skin and vestibular effects (middle-inner ear diffusion) may occur as a result of isobaric inert gas counter-diffusion under similar parameters as the animal testing. Again this form of superficial isobaric inert gas counter diffusion is primarily a product of commercial diving procedure involving chambers or bells where it is possible for the occupant to mask breathe a heavier gas mix while being surrounded by a much lighter heliox mixture for whatever reasons we may or may not understand. However one should note this phenomenon will not occur in a submerged diver wearing a wetsuit, but may occur if the diver is wearing an inflated suit (drysuit) that is filled with a lighter inert gas than what is breathed. This latter consideration provides additional incentive to not use helium or high helium (backgas) as an inflation medium for your drysuit, beyond the thermal considerations that hopefully common sense would indicate. According to conventional theory, there is the possibility of a deep tissue problem resulting by breathing a high-helium content gas following a prolonged exposure to a N2-O2 mixture. For example at 200ftsw constant ambient pressure with tissues saturated, or near-saturated, with N2, a 50ft supersaturation gradient will be achieved after about 480 minutes of switching to a high helium content mix (for the 480-N2/240-He minute tissue half-times). It takes several hours after reaching this supersaturation level for it to diminish to ambient (for the selected tissue halftimes). Depending on the tissue compartment in question, this may pose a scenario for a supersaturation above the allowable to prevent DCS. Again, however, we must be reminded that this example requires a near-saturation of the tissues with N2 (near-saturation occurs roughly after a period of 4 to 5 halftimes for this example about 40 hours). This is not a scenario that is remotely likely to occur in a technical diving application. It however may be an issue for some commercial applications. The only potential problem area I have noted for technical divers after long duration N2 mixes is if a diver is being treated for DCS at a recompression depth considered unsafe for air or N2 mixes (narcosis, O2 toxicity) and is switched to He mixes. However an increase in the ambient pressure (compression) at the switch will reduce the likelihood of bubble growth or development according to conventional theory [2]. This should not be an issue with experienced chamber operators or people willing to call DAN (Duke University hyperbaric research) for assistance. Given the duration, depths, light inert gas fractions, and environment, it just doesnt appear likely that a technical diver is at risk due to isobaric inert gas counter-diffusion [IMHO] even considering conventional theory. With the remote exception being a situation where the diver is breathing air or N2-O2 mix during deco while being surrounded in a high helium medium in his or her drysuit. However, using a high helium inflation gas in a drysuit would be a very stupid and ignorant thing to do considering the purpose of the exposure suit is to keep the diver warm. So this should be a non-issue unless the diver is an idiot and inflates the drysuit from a backgas containing a high helium fraction again not wise. Since a more acceptable practice is to use a heavy inert gas for drysuit inflation, such as Argon; if any isobaric counter-diffusion process is occurring, it would tend to create a sub-saturation condition thru the skin and therefore produce a desirable effect. Again posing no problems for the technical diver. As for switching to a trimix or heliox mix at depth from air or N2-O2 mixes, it is unlikely to result in a problem (re:deep tissue problems) because the degree of N2 saturation at this point (v.s. ambient) for technical divers is very low and any increase of the total inert gas loadings due to the switch at a constant depth (pressure) should not prove to be significant. Furthermore most technical diving applications involving a gas switch from air/N2-O2 to trimix or heliox also involve a continuation of pressurization that further renders the phenomenon inconsequential. According to conventional theories, a switch to a heavier (slower) gas from a lighter (faster) gas on ascent is desirable as it tends to create a sub-saturation condition during deco and therefore shorter deco time. However this switch must be done in such a way that it does not contribute significantly to the decompression obligation (switching too deep). However this dissolved gas theory does not account for free-phase bubble mechanics and its effect on decompression directly, and indirectly on its modifying effect on the conventional theories. It also does not account for other factors one may wish to consider during decompression such as the likelihood of less long-term tissue damage if DCS occurs with helium mixes compared to N2 mixes. In any case it appears that isobaric inert gas counter-diffusion is just not a consideration worth worrying about in technical diving, IMHO. I would like to read more references on free-phase bubble mechanics as applied to decompression diving if anyone can suggest them. Take care, Doug References: [1] Deep Diving and Short Decompression by Breathing Mixed Gases, H. Keller and A.A. Buhlmann, J. Appl. Physiology,20: 1267-1270 (1965). [2] Advantages and Hazards of Gas Switching: Relation of Decompression Sickness Therapy to Deep and Superficial Isobaric Counterdiffusion, C.J. Lambertsen, M.D., Institute for Environmental Medicine, University of PA. [3] Deep-Tissue Isobaric Inert Gas Exchange: Predictions During Normoxic Helium, Neon and Nitrogen Breathing at 1200 FSW, C.A. Harvey and C.J. Lambertsen, Proceedings of the 6th Symposium on Underwater Physiology, FASEB, Bethesda, MD (1978). -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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