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The Case for Biochemical Decompression

How much do you fart during decompression? How about your teammates? It turns out that those may be critical questions if you’re decompressing from a hydrogen dive, or more specifically hydreliox, a mixture of oxygen, helium, and hydrogen suitable for ultra-deep dives (Wet Mules, are you listening?). Here the former chief physiologist for the US Navy’s experimental hydrogen diving program, Susan Kayar, gives us the low down on biochemical decompression and what it may someday mean for tech diving.

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by Susan R. Kayar, PhD
Header courtesy of A. Tocco Comex

Thirty years ago, the Naval Medical Research Institute (NMRI) in Bethesda, Maryland, hired me for what at the time I thought was the coolest job I could ever be asked to do.  I still think so.  I was hired to be the physiologist for their experimental hydrogen diving program.  Why dive with hydrogen?  A recent InDepth article by Reilly Fogarty, “Playing with Fire: Hydrogen as a Diving Gas”, does an excellent job of explaining this subject.  The short answer: because hydrogen is the smallest molecule. 

One  might think that in an era with excellent one-atmosphere hard suits, and multiple forms of submersibles and robotics, there is no need to send bare-naked divers to the sorts of depths involved in hydrogen diving, as will be described shortly.  If these alternatives to divers are so great, why do we still use commercial divers at all?  One needs to ask an operational person this question, rather than a scientist like me.  But I think the words “logistics”, “costs”, “safety,” and “the direct human touch” would figure in the answers.  

Just a snapshot of the enormous efforts needed to send works to the oceans floor. Photo courtesy of A. Tocco Comex.

Once a diver dives deep enough to exceed safe limits with regard to nitrogen narcosis, the usual gas switch for the diluent to oxygen is helium.  However, if a diver keeps on going into the range of 1000 to 2000 feet of seawater (roughly 300-600 msw), a helium-oxygen gas mixture becomes dense enough that the work of breathing becomes difficult.  Divers fight to move this dense gas into and out of their lungs, making the effort to breathe a serious source of fatigue and a distraction to their assigned jobs. (See “Maintaining Your Respiratory Reserve,” by John Clarke). Hydrogen is a diatomic molecule (i.e. H2) with two protons and no neutrons, and is therefore half the molecular weight of helium, a monatomic molecule with two protons and two neutrons.  Therefore  replacing helium with hydrogen, eases a diver’s respiratory distress i.e. work of breathing.  

There is also a phenomenon of ultra-deep diving known as High Pressure Neurologic Syndrome, or HPNS, (also known as High Pressure Nervous Syndrome) which is evidently a function of high pressure interfering with the transmission of signals in the nervous system.  Symptoms of HPNS can range from tremors to confusion to psychosis and are highly variable in depth at onset and from diver to diver.  For unknown reasons, hydrogen at high pressure is narcotic and can suppress HPNS.  Past a very high pressure that again varies with the diver, but generally on the order of 23 atmospheres partial pressure of hydrogen, its narcotic properties can become overwhelming and have their own psychotic effects.  

There are also serious issues involving the explosivity of hydrogen in combination with oxygen, but these issues are manageable with the care one always uses in handling oxygen and other combustible and hyperbaric gases.  Hydrogen and oxygen can be combined safely if the oxygen content is less than 4% of the gas mix, with dive operations usually opting for 2% oxygen as their safe upper limit.  A 2% oxygen mixture is breathable if the total pressure is 10 atmospheres (roughly 90m/295 f) or more.  This is normally accommodated by starting a pressurization with helium and then switching to hydrogen after 10 atm.  As a final consideration, the price of helium is rising, and may make hydrogen substitution increasingly attractive.  Consequently, for a variety of practical reasons, hydrogen has a potential place in ultra-deep diving beyond 10 atmospheres of pressure.

Investigating Biochemical Decompression

As the physiologist to the hydrogen diving program at NMRI, my assignments were two-fold: first, to determine if there are any dangerous biological effects that had been previously overlooked of breathing hyperbaric hydrogen,  and second, to look into something that NMRI was calling “biochemical decompression,” or “biodec,” a term they had coined themselves.  

Susan Kayar at her workplace. Photo courtesy of Susan Kayar.

The unknown dangerous biological effects portion of the research was addressed first.  The short answer to that was “none”.  We found no evidence that inhaled hydrogen could participate in any unwanted biochemical reactions in the body, discounting whatever reactions eventually make hydrogen narcotic.  We still do not know exactly why hydrogen becomes narcotic, but it is unlikely from the physical properties of hydrogen that its narcotic effects are permanently harmful post-dive.

Then we got to the really exciting part of the hydrogen research program at NMRI: biochemical decompression.  A few years before I was hired in 1990, a biochemist at NMRI, Dr. Lutz Kiesow, heard it was possible for divers to use hydrogen as a breathing gas.  He knew there were many microbes that possessed a hydrogenase enzyme allowing them to consume hydrogen gas as a metabolic source equivalent to the consumption of oxygen as a metabolic source for most land organisms.  End products for hydrogen metabolism can vary with the microbe, but is often methane (CH4).  Hence, as a class, such microbes are called “methanogens”.  

Dr. Kiesow proposed that NMRI establish a research project to isolate the hydrogenase from a methanogen, and insert it somewhere in the body of a diver to effectively create  a chemical scrubber unit for hydrogen.  If a diver could continuously scrub out some of the hydrogen going into solution in his body during the dive, the diver would have a reduced body burden of inert (to the diver) gas, and could subsequently decompress more rapidly with lower risk of decompression sickness (DCS).  

What a cool concept!  I loved it from the moment I heard it.  But the real challenge was to resolve Dr. Kiesow’s “somewhere in the body” requirement into a safe, readily reachable, functionally useful body location.  The director who hired me understandably warned me that divers would be opposed to receiving routine injections, or any sort of biological implant making them Bionic Men, permanently different from their former selves or from other divers. So what was left?  

Susan Kayar today, sharing her knowledge with the world. Photo courtesy of Susan Kayar

On my first musings with the scientific head of NMRI when I was hired, I wondered if we could perhaps encapsulate the hydrogenase enzyme, or better yet just whole methanogens, and swallow the capsules down for delivery to the large intestine as the working location for this scrubber unit. The scientific head instantly responded he had been thinking the same thing, but had not wanted to bias my thinking by saying it first.  The approach met all our criteria. Taking capsules by mouth is as easy and as non-invasive a way to get things into the body as there can be.  The large intestine has many microbial species living there safely and performing many jobs that we are slowly realizing are important to our health.  

Trust Your Gut?

Methanogens typically are anaerobic organisms that would die quickly if exposed to oxygen, and the large intestine is the only part of the body that provides an anaerobic environment.  Some species of methanogens are even a normal part of our intestinal flora, where they consume traces of hydrogen manufactured by other intestinal microbes.  We were therefore confident that adding more methanogens should do no digestive harm. The amplified population of methanogens in the intestine would be likely to stay high only for as long as the divers breathed hydrogen, and return to baseline shortly after the exposure to hydrogen ended. The methane end product of this hydrogen scrubbing has a safe means of escaping from the intestine.  

The methane-releasing issues were the only parts of this research that got a little weird at times. I was very carefully coached by Navy people to use lengthy euphemisms such as “the methane is released to the environment following the path of least resistance,” or “methane has an obvious means of egress from the intestine.”  I was warned never to use what I have come to refer to as “the four-letter f-word” for methane release.  But the euphemisms never helped.  All audiences instantly understood the euphemisms as such.  

The first dive to 701m. Photo courtesy of A. Tocco Comex.

Indeed, I came to consider it a sign that my audience was truly listening to me and following the science when they suddenly started squirming in their seats and trying with greater or lesser success to cover their laughter when I started explaining the fate of methane. Jokes followed. One Navy brass listener asked me if the implementation of hydrogen biochemical decompression meant a negation of the stealth intended for Navy SEALs when they used closed-system (i.e., non-bubbling) breathing rigs.  The only sensible thing for me to do was laugh along with the room.  

An interesting phenomenon happened as soon as people got over their initial laughter at this childishly scatological word that I did not say but that they obviously thought of themselves. They started thinking about the physiology and the gas transfer physics I was describing, and they liked it.  No more laughter after that moment of enlightenment arrived. So go ahead and laugh now. “Better out than in” applies to laughter also.  I got a million of ’em. I am known in some circles as the “Queen of Farts” with good reason. 

Measuring Flatulence err Farts

I retired from Navy civilian service years ago, so I can say whatever I wish.  I measured farts. Measuring farts is funny. And measuring farts in rats and pigs is exactly how my NMRI team and I succeeded in demonstrating the feasibility of hydrogen biochemical decompression to reduce the incidence of DCS following hydrogen dives by roughly half. As far as we know, methane release rate is the only variable that can be biologically manipulated with a measurable effect on DCS incidence following any kind of dive. There is nothing humorous about reducing DCS incidence.  

Photo courtesy of Aqua Magazine, Susan Kayar.

The methanogenic species we chose has a rather grand first name but oddly mundane last name: Methanobrevibacter smithii.  It is native to the intestinal flora of many mammals, including humans and pigs, and thus does not cause digestive issues when added to the intestines.  The metabolic equation for M. smithii is the following: 

4H2 + CO2 = CH4 + 2H2O

To speed things along in the lab, we surgically injected M. smithii cultures into the upper end of the large intestines of our lab animal models of divers, which were initially rats and later pigs.  The animal-divers were then placed in a hyperbaric chamber which we pressurized with hydrogen and oxygen.  Some hydrogen and oxygen breathed by an animal-diver dissolves in the blood for transport throughout the body.  When the blood circulates through the vasculature of the intestinal wall, some hydrogen diffuses down its partial pressure gradient into the intestinal cavity, where the M. smithii are housed.  

Figure 1. Sample hydrogen dive with a rat using biochemical decompression.  A rat with a culture of M. smithii in its intestines was placed in a hyperbaric chamber.  As the pressure of hydrogen (green squares) increased in the chamber, increasing quantities of methane (red dots)  were released from the rat.  When the chamber was decompressed, methane release initially spiked as hydrogen became super-saturated in the rat, and then fell as hydrogen was removed from the chamber. Diagram courtesy of Susan Kayar.

Oxygen is taken up by the cells of the intestinal wall and aerobically metabolized to carbon dioxide (CO2), some of which also diffuses into the intestinal cavity. M. smithii metabolizes the hydrogen and carbon dioxide to methane and water. The animal-diver safely absorbs the water. It is a real scientific benefit that the methane exits the body as easily as it does. Since no mammalian cell manufactures methane, we could track the metabolism of our methanogens inside our animal-divers simply by measuring the rate of release of methane from them to the surrounding environment by gas chromatography. As the hydrogen pressure in the chamber increased, we measured increasing quantities of methane in the chamber gases 

Figure 2. Risk of DCS was significantly reduced in rats with methanogens following dives in hydrogen. Rats with M. smithii in intestines had significantly fewer cases of DCS (5/20) compared to untreated control rats (28/50) and rats undergoing the same surgical procedure as the treated rats but without M. smithii injections (13/20). Diagram courtesy of Susan Kayar.

When we then decompressed our animal-divers, on average, the animals with supplemental methanogens had approximately half the incidence of DCS as those without supplements. As the volume of methane they released during the dive increased, their incidence of DCS decreased.

Figure 3.  Injected activity of methanogens correlates with methane release rate and lower incidence of DCS in pigs following hydrogen dives (Kayar et al., 2001).  

Knowing from the metabolic equation above that four hydrogen molecules are consumed for each methane molecule manufactured, we could easily estimate the rate of hydrogen-scrubbing inside our animals.  Based on the solubility of hydrogen in body tissues (which we guesstimated as being similar to water), and the time at pressure of the dive, we could estimate how much hydrogen would dissolve in an animal of a given body mass by the end of the bottom time, and what fraction of that body burden of hydrogen had been eliminated by our process.  We computed that when M. smithii eliminated approximately 5% of the hydrogen dissolved in our animal-divers’ bodies, DCS incidence was reduced by 50% (Fahlman et al, 2001).   

Human Biodec

Having succeeded in demonstrating hydrogen biochemical decompression in a small animal model, the rat, and a larger animal model, the pig, we are at least scientifically prepared to extend this work to human divers.  A diver would make a saturation dive (commonly abbreviated to “sat”, meaning a dive sufficiently long i.e. 24 hours or more, to saturate the diver’s tissues with the breathing mixture) using a hydrogen-oxygen blend we usually call “hydrox”, or a hydrogen-helium-oxygen trimix which goes by the awkward name of “hydreliox”, depending on practicalities.  

Dive operations may even opt for a quad-mix including nitrogen.  The ultra-deep diving trials at Duke University found the narcotic properties of nitrogen helped to suppress HPNS, which was so problematic for their divers breathing heliox. However, the interaction is complex. Since we are still working out the exact mechanisms that make nitrogen and hydrogen narcotic under pressure, it remains to be determined if combining nitrogen and hydrogen for deep sat dives makes narcotic issues better or worse.  The issue deserves testing.

What oral supplements might look like. Photo by JESHOOTS.com from Pexels.jpg.

Regardless of the other gases in the sat diver’s mix, if there is hydrogen, then hydrogen biochemical decompression could be considered.  A couple of days before the end of the bottom time, the diver would prepare to biochemically decompress as a supplement to the physical decompression.  The basic process would be identical to that for our animal models, except for a gentler way of delivering the methanogens to the diver.  We would freeze-dry cultures of M. smithii and pack them into oral-delivery capsules designed to dissolve only under the conditions inside the large intestine.  It would take around 24-36 hours to have a capsule arrive in the intestine, dissolve, and re-activate the methanogens.  We would know that the M. smithii were on site and sufficiently active by chemically analyzing the sat chamber gases for methane output.  Then we would get to watch the diver not bend as he decompressed faster than divers in other hydrogen diving operations without biochemical decompression.  As I said, coolest job ever, or what?

But wait!  

There is one more really exciting finding to report.  We have evidence that even the quantity of methanogens native to the intestinal flora of a pig can provide sufficient hydrogen-scrubbing activity to reduce DCS incidence from a hydrogen dive (See Fig. 4 below).  Humans and pigs are similar in many respects, including basic intestinal flora.  It may well be that any human divers on a hydrogen dive, such as those at COMEX , have already benefited from hydrogen biochemical decompression without realizing it.  They have only to test for methane in their chamber gases to know.  

Figure 4.  Native methanogens in untreated pigs significantly reduced DCS incidence.  As untreated pigs were exposed to various dive profiles in hydrogen, increasing pressures of hydrogen elicited increasing quantities of methane released by methanogens native to the pigs’ intestinal flora.  Open circles represent pigs with subsequent DCS, closed circles represent pigs without DCS.  DCS incidence was significantly lower as the pigs released more methane.  

Skeptics have argued that the relatively small percentage of hydrogen scrubbing we have computed may be far too little to have any impact on DCS risk in human divers or to make a worthwhile reduction in decompression times. In addition to pointing to our DCS incidence data, we note that all divers are familiar with how important small differences in gas loads can be in DCS risk. If we dive within the time at depth limits of our chosen algorithms, we are confident to a very high level of probability that our dive will end safely. But exceeding our planned no-decompression limits by even a few minutes, and thus adding only a relatively small percentage increase in our inert gas load beyond what we think of as safe, makes our dive profile much riskier.  [Ed. Note: These are computational risks not necessarily operational ones i.e. small changes in times/depths are unlikely to result in DCI] Likewise, we are all in the habit of making what we term a “safety stop” in 3-5m/10-15 ft even from a low-risk, no decompression time-requiring dive. 

Sat dive operations currently using heliox and contemplating a shift to adding hydrogen will be dismayed to realize that hydrogen is considerably more potent at inducing DCS than is helium (Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901). This would mean that costs saved by substituting relatively inexpensively manufactured hydrogen (by electrolysis of water) for increasingly expensive imported helium could be overwhelmed by the costs added in significantly longer decompression time. This is where hydrogen biodec may provide its greatest advantage: in shaving down the extra time needed for safe decompression from a hydrogen dive to something closer to that of a heliox dive.  Until someone takes the step of testing hydrogen biodec in human subjects, we will not know to what extent operational decompression times could be reduced.  

Nitrogen Biodec?

What comes next?  In an ideal scientific world, our research in animal models would be followed by equivalent studies in human divers.  However, for the time being in the post-Russian Cold War Era, the US Navy has expressed no further interest in hydrogen diving and has not offered to support human studies in hydrogen biochemical decompression.  To assuage my disappointment, I wrote a novel in which hydrogen biochemical decompression is used to help save the day in a submarine rescue scenario.  The novel is entitled “Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship,” available as a paperback and Kindle e-book on Amazon.  

But I am still dreaming bigger than that.  Since hydrogen biochemical decompression works, why not shoot for something everyone in the diving world could use?  Nitrogen biochemical decompression!  There are nitrogen-metabolizing microbes native to our intestinal flora.  But the problems of experimentally making nitrogen biochemical decompression work are staggeringly complicated.  One of many is that in nitrogen metabolism, usually referred to as nitrogen fixation, the end-products are molecules such as nitrites, nitrates, and ammonia, which are not gases that would just come bubbling out for us to measure.  

Susan submerged. Photo courtesy of Susan Kayar.

These fixed nitrogen compounds would stay dissolved in the fecal material and join many more such molecules already there from protein digestion.  (If you think the fart jokes are bad, consider the fecal jokes. “No shit!”-Ed.) The presence of fixed nitrogen products in feces (also known as “fertilizer” under other circumstances) suppresses the nitrogen-fixing microbes from fixing even more, since the process is energetically expensive to the microbes and done only by necessity.  It would take some genetic manipulation of the microbes to get them to work for us, and some form of special molecular labeling to measure how much end products they are making.  I leave those problems to future scientists to solve, while I enjoy my retirement in New Mexico, the Land of Enchantment, and go on dive vacations to Hawaii, Papua New Guinea, Tahiti, Fiji, and Raiatea to keep my vestigial gills damp. I may even write another novel. 

Additional Resources

Operation SECOND STARFISH, A Tale of Submarine Rescue, Science, and Friendship

References

Bennett, P.B., R. Coggin, M. McLeod, 1982.  Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft).  Undersea Biomed. Res. 9(4)335-51.

Fahlman, A., P. Tikuisis, J.F. Himm, P.K. Weathersby, and S.R. Kayar, 2001.  On the likelihood of decompression sickness during H2 biochemical decompression in pigs.  J. Appl. Physiol. 91:2720-2729.  

Imbert, J.P., C. Gortan, X. Fructus, T. Ciesielski, and B. Gardette, 1988.  Ch. 13.  Hydra 8: Pre-commercial Hydrogen Diving Project.  Advances in Underwater Technology, Ocean Science and Offshore Engineering, Vol. 14, pp 107-116.  

Kayar, S.R., M.J. Axley, L.D. Homer, and A.L. Harabin, 1994.  Hydrogen gas is not oxidized by mammalian tissues under hyperbaric conditions.  Undersea Hyperbaric Med. 21(3):265-275. 

Kayar, S.R. and M.J Axley, 1997.  Accelerated gas removal from divers’ tissues utilizing gas metabolizing bacteria.  U.S. Patent No. 5,630,410.  

Lillo R.S., E.C. Parker, W.C. Porter, 1997 Decompression comparison of helium and hydrogen in rats. J. Appl. Physiol. 82(3) 892-901

Kayar, S.R., T.L. Miller, M.J. Wolin, E.O. Aukhert, M.J. Axley, and L.A. Kiesow, 1998.  Decompression sickness risk in rats by microbial removal of dissolved gas.  Am. J. Physiol. 275 (Regulatory Integrative Comp. Physiol. 44):R677-682.  

Kayar, S.R., A. Fahlman, W.C. Lin, and W.B. Whitman, 2001.  Increasing activity of H2-metabolizing microbes lowers decompression sickness risk in pigs during H2 divesJ. Appl. Physiol. 91:2713-2719.  

Kayar, S.R. and A. Fahlman, 2001.  Decompression sickness risk reduced by native intestinal flora in pigs after H2 dives.  Undersea Hyper. Med. 28(2)89-97.  

Valée, N., Weiss M., Rostain JC, Risso JJ, A review of recent neurochemical data on inert gas narcosis. Undersea Hyper. Med. 38(1)49-59


Susan grew up in the St. Louis, Missouri, area.  An early fascination with the films of Jacques Cousteau inspired her to become certified as a scuba diver while still in high school.  Her diving in Missouri was confined to artificial lakes with sunken rowboats, lost Coke bottles, and a few carp as the thrills.  She persevered in her interests in marine sciences and attended the University of Miami as a biology major, remaining at that institution all the way through to a doctorate.  After graduation, it did not take long to realize she would starve if she insisted on a job in marine biology, so she moved into studying physiology in extreme environments and exercise stress.  Postdoctoral research appointments sent her from Colorado to Switzerland to New Jersey.  Her dream job finally materialized in an appointment with the US Navy in the Washington, DC area, where she studied decompression sickness risk in animal models of ultra-deep diving.
Susan was inducted into The Women Divers Hall of Fame in 2001 in recognition of her Navy diving research.  When funding for her Navy program ended, she managed research funding efforts for the National Institutes of Health (NIH), Defense Advanced Research Programs Agency (DARPA), and the Office of Naval Research (ONR).  Now in retirement, she has written a diving-themed novel, “Operation SECOND STARFISH.”

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Isobaric Counterdiffusion in the Real World

Isobaric counterdiffusion is one of those geeky, esoteric subjects that some tech programs deem of minor relevance, while others regard it as a distinct operational concern. Divers Alert Network’s Reilly Fogarty examines the physiological underpinnings of ICD, some of the key research behind it, and discusses its application to tech diving.

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By Reilly Fogarty

Header image by Derk Remmers

Most divers don’t spend much time thinking about isobaric counter diffusion (ICD), and it’s not just because it has an inconveniently long name. The phenomenon is complicated to understand, depends on mechanisms that are partially or wholly theoretical, and falls squarely in the unknown gray area of decompression science. As a result, students in technical courses receive varying information about the subject, depending on their instructor’s understanding of it. 

Some consider the topic irrelevant to divers while others recognize that a functional understanding is mandatory for modern mixed-gas diving. A combination of infrequent exposure, lack of research and widely spread misinformation have made ICD an unapproachable subject for many divers. Here’s what we know and what we’re still researching. 

The Physiology

Isobaric counterdiffusion is not a concept that’s limited to diving. The phenomenon describes the diffusion of different gases into and out of tissues after a change in gas composition and the physiological effects of those gas switches. This is relevant in hyperbarics, anesthesia and diving and aerospace. As divers we’re most concerned with what happens with a gas switch during a mixed-gas dive, and research in related fields can provide useful data. 

The effects of ICD as they relate to divers primarily involve the movement of two inert gases in opposite directions at equal ambient pressures, hence the term “isobaric,” in tissues and blood. The relative speeds of counterdiffusion are affected by many factors including density, surface tension and viscosity in fluids, as well as a variety of physiological factors, membrane properties and specific gas interactions (Oswaldo, 2017). Gas diffusion itself is a fascinating and broad topic but one for another day, so for the sake of understanding ICD it can be simplified somewhat. Fundamentally the issues related to ICD center around a disparity in the speed at which one inert gas diffuses into the body while another diffuses out. This can occur with a slow saturated gas exiting a tissue and a fast saturating gas entering or vice versa. 

Superficial ICD occurs when the inert gas breathed by a diver diffuses more slowly into the body than the gas surrounding the body. Because this requires being surrounded by a gas with a high diffusivity it is most often seen in saturation divers breathing air or a low-helium content gas in a heliox environment. This can theoretically occur in diving and is the reason that new mixed-gas divers are told to avoid using a suit-inflation gas containing helium (besides that fact that it has low thermal conductivity i.e. its cold!). 

Helium has a diffusivity that’s approximately 2.65 times that of nitrogen (Lambertson, 1989), and because of that disparity it can diffuse into the skin quickly while nitrogen diffuses more slowly. The slow diffusion of nitrogen from the fluids and tissues of the body while the helium saturates the skin can cause supersaturation in some superficial tissues that can result in gas bubble formation. These gas bubbles can cause painful, red lesions on the skin, but the phenomenon does not occur when the gases are reversed and the breathing gas has a greater diffusivity. 

Figure by InDepth.

If ICD is a concept you’ve encountered before it’s likely deep tissue ICD that you’ve been exposed to. This second type of ICD occurs when one breathing gas is exchanged for another of different diffusivity, as in a gas switch from say a nitrox travel gas to a trimix bottom mix or from a trimix bottom mix to a nitrox decompression gas. As with superficial ICD, this occurs when a gas with high diffusivity is transported into a tissue more rapidly than a slower-diffusing gas is transported out. The result is the same: supersaturation of some tissues and bubble formation. These bubbles can cause itching followed by joint pain and have been more recently associated with inner-ear decompression sickness, although the bubble formation could contribute to other types of decompression sickness as well. 

The Research

Quantifying the risk of ICD and identifying cases of decompression sickness (DCS) that resulted from ICD rather than other risk factors can be difficult, but there is significant research correlating several proposed mechanisms to increased bubble counts and DCS in human and animal models. Like DCS, ICD is fairly well accepted academically on a correlational basis, but the specific mechanisms require additional research to confirm. 

Data from as early as 1977 indicates a risk of ICD in divers even within recreational depths, with increased bubble counts observed in goats saturated at 5 atmospheres and switched from a breathing gas containing 4.7 atmospheres of nitrogen to 4.7 atmospheres of helium (D’Aoust, 2017). Similarly, saturation divers on the Hydra V mission experienced DCS following a gas switch from hydreliox to a faster-diffusing heliox mixture, with the gas switch thought to have caused the DCS (Rostain, 1987). 

More recent work on decompression models of the inner ear have indicated that even a transient increase in gas tension (the relationship between breathing gas and gas saturated in the body) related to a switch from a high-helium-content gas to a nitrogen mix may increase the risk of inner-ear decompression sickness (IEDCS). This model is particularly interesting because the diffusion of gases across the round window is extremely low (bordering on negligible), which complicates the transport of inert gases in the ear. Data from Doolette and Mitchell suggests that these gas switches could result in a temporary increase in gas tension as the nitrogen input exceeds the removal of helium via perfusion in the vascular compartment and diffusion in the peri- and endolymph causing bubble formation and growth (Doolette, 2003). 

Photo courtesy of the GUE archives.

There are several variables to consider with this model, but the data appears sound and the mechanism provides a likely explanation for the well-documented cases of IEDCS related to gas switching in technical diving. Other models have been proposed to explain these incidents, and these vary by physiological models and diffusion constants used, but most focus on tissue supersaturation as a result of varying gas tension following a gas switch (Burton, 2004).

Academic Versus Application

The challenge with the varied research into the mechanisms of ICD is that it can be difficult to determine what’s prudent to include in your dive planning and what data might not reflect reality. The good news is that the general aspects of ICD are fairly well understood, even if the specific mechanisms are theoretical. Reducing variances in gas diffusivity and transient tissue tension via conservative dive planning is relatively easy to do and poses no significant additional risk. Decompression obligations may be increased in some instances, but some mixed-gas courses are already including some ICD considerations, primarily related to IEDCS. 

Extending this to minimize the risks associated with ICD is not complicated, but specific recommendations are unfortunately not readily apparent. Lambertson proposed that switching from a helium mixture to a nitrogen mixture would be acceptable but the reverse should include recompression — something unlikely to be an option during a dive charter. Doolette and Mitchell propose a more practical approach: minimizing the switch from trimix to nitrox on ascent or planning to perform these switches at depth or in shallow water to minimize supersaturation. 

Doolette and Mitchell propose a more practical approach: minimizing the switch from trimix to nitrox on ascent or planning to perform these switches at depth or in shallow water to minimize supersaturation. 

There are some specific recommendations for preventing ICD (using the rule of fifths, calculating theoretical helium and nitrogen compartments, etc.), but these lack evidence and may or may not prevent incidents. What we can say is that planning your gas switching to minimize supersaturation due to transient tissue tensions, minimizing your switches from high-helium-content gases to low (as appropriate for your acceptable risk levels), and increasing conservatism as depth and dive time increase (due to increased tissue saturation levels) are good ways to keep yourself safe. 

The mechanisms may not yet be definite, but the data can back up these recommendations. And when there is no increase in risk with the conservative approach, it makes sense to go that route. Keep an eye on upcoming research in this area; while ICD can cause problems, some researchers are proposing that isobaric underdiffusion could decrease risk on technical dives, so it’s possible that your gas planning might be in for a shakeup in the near future. 

References

  1. Oswaldo, C. Gas diffusion among bubbles and the DCS risk. (November 24, 2017) 
  2. Lambertson, Christian J (1989). Relations of isobaric gas counterdiffusion and decompression gas lesion diseases. In Vann, RD. “The Physiological Basis of Decompression”. 38th Undersea and Hyperbaric Medical Society Workshop UHMS Publication Number 75(Phys)6-1-89. http://archive.rubicon-foundation.org/6853. 
  3. D’Aoust, B. G., Smith, K. H., Swanson, H. T., White, R., Harvey, C. A., Hunter, W. L., … Goad, R. F. (1977, August 26). Venous gas bubbles: production by transient, deep isobaric counterdiffusion of helium against nitrogen
  4. Rostain, JC; Lemaire, C; Gardette-Chauffour, MC; Naquet, R (1987). Bove; Bachrach; Greenbaum (eds.). “Effect of the shift from hydrogen-helium-oxygen mixture to helium oxygen mixture during a 450 msw dive”. Underwater and Hyperbaric Physiology IX. Bethesda, MD, USA: Undersea and Hyperbaric Medical Society.
  5. Doolette, David J; Mitchell, Simon J (June 2003). “Biophysical basis for inner ear decompression sickness”. Journal of Applied Physiology. 94 (6): 2145–50. doi:10.1152/japplphysiol.01090.2002. PMID 12562679.
  6. Burton, Steve (December 2004). “Isobaric Counter Diffusion”. ScubaEngineer. 

Additional Resources:

Tale of Two Agencies: How two tech agencies address isobaric counterdiffusion

Note: The British Sub Aqua Club (BSAC) recommends that divers allow for a maximum of 0.5 bar difference in PN2 at the point of the gas switch. According to former BSAC Tech lead Mike Rowley, “The recommendation isn’t an absolute but a flexible advisory value so a 0.7 bar differential isn’t going to bring the Sword of Damocles down on you.” 


When he’s not working with DAN on safety programs, Reilly Fogarty can be found running technical charters and teaching rebreather diving in Gloucester, Mass. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.

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