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What is A Ventilator And Why Is It So Important?

Forget rebreathers. With the pandemic still spreading, ventilators have arguably become one of the most essential pieces of life-support kit on the planet. We asked anesthesiologist and director of Karst Underwater Research (KUR) Andy Pitkin to brief us on these life-saving devices. Take a deep breath.

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by Andrew Pitkin MBBS MRCP FRCA
Header photo by Sungmin Cho from Pixabay

The COVID-19 pandemic has highlighted one piece of medical equipment above all: the ventilator. What are these life-support devices, and how do they work? 

Until the 20th century, with a few exceptions, any patient unable to breathe for themselves was doomed to die. The large polio outbreaks that swept through the world in the 1930s and 40s prompted the design and manufacture of the first reliable means of respiratory support, the iron lung. Properly termed a negative pressure ventilator, and commonly referred to as a ‘tank’ ventilator, the iron lung and some smaller derivatives enclosing just the chest (‘cuirasse’ ventilators) were developed by Drinker1 and Emerson into the only equipment able to keep patients alive when their respiratory muscles had been paralyzed by the poliovirus. Well-funded hospitals in the USA were able to set up large wards full of iron lungs (Fig 1), but it was across the Atlantic, in the northern European country of Denmark, that the modern intensive care unit with its ubiquitous positive-pressure ventilators was born. 

Fig 1. At the height of the 1948 poliomyelitis epidemic 82 patients required ventilation in iron lungs at the same time in Los Angeles. Photo courtesy from “Patenting the Sun” by Jane S. Smith.

In the mid-1950s, Copenhagen, Denmark’s capital city, was ravaged by a severe polio epidemic that completely overwhelmed its handful of iron lungs and forced the physicians there to adopt methods borrowed from the operating room just to keep people alive. In desperation, the director of the infectious diseases hospital, the Blegsdamhospitalet, asked his anesthesiologist colleague Dr Bjorn Ibsen for help. Under Ibsen’s direction,  patients received tracheostomies and intermittent positive-pressure ventilation using a rebreathing system called a Waters circuit (Fig 2). 

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Fig 2. A Waters circuit used for hand ventilation of poliomyelitis victims in the 1952 outbreak in Copenhagen, Denmark. A simpler system without the soda lime canister was used later, with the disadvantage that a higher flow rate of oxygen/nitrogen mixture was required to prevent excessive rebreathing of carbon dioxide. Illustration by InDepth.
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Bjorn Ibsen. Photo courtesy of P.G. Berthelson.

Rebreather divers will easily recognize this as a pendulum rebreather system, but in this case the gas moved in and out of the patient’s lungs not through their own efforts, but by a medical student squeezing and releasing the bag (counterlung) 20 to 25 times per minute, 24 hours a day, for periods of up to several months. In the absence of enough mechanical ventilators, hundreds of medical students risked exposure to the much-feared polio virus to keep these patients, many of them children, alive (Fig 3). The success of positive-pressure ventilation in dramatically reducing mortality in Denmark rapidly spread throughout the world and the modern intensive care unit was born.

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Fig 3. A patient with poliomyelitis being hand ventilated with a Waters circuit through a tracheostomy. Medical students were paid 30 shillings (about $100 today) for an 8-hour shift (with a 10-minute break for a cigarette in the middle!). Photo by Lassen HCA.

Human muscle power was soon replaced with mechanical power using either motorized bellows or a pressure-regulated gas source controlled by an automatic valve (like pressing the purge valve on a regulator’s second stage). The first classification by Mapleson2 divided ventilators into flow generators and pressure generators; both could be further subclassified into time-cycled, pressure-cycled and volume-cycled units, depending on what parameter the machine used to switch from inspiration (breathing in) to expiration (breathing out) and vice versa. An example of a flow generator is a typical bellows ventilator such as the Oxford Penlon unit (Fig 4), still used in some hyperbaric chambers because of its mechanical simplicity, and many anesthesia machines in operating rooms have similar ventilators built in. 

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Fig 4. The venerable Penlon Oxford ventilator is a simple bellows system driven by a 60 psi compressed gas supply. A smaller set of bellows can be substituted for use in children. Photo courtesy of A. Pitkin.

Modern Ventilators

With advances in microelectronics in the 1970s it became feasible to integrate gas control valves with flow and pressure sensors to allow a single ventilator to work in a flow-generating mode as well as a pressure-generating mode, and many other modes became possible that could not be realized with a simple mechanical ventilator. Units such as the Siemens Servo 900 (Fig 5) and the Puritan Bennett 7200 were in the vanguard of this approach.

Fig 5. The Siemens Servo 900C, a ventilator I became intimately familiar with while training in intensive care in the early 1990s. Introduced in 1971, its use of electronically-controlled scissor valves to regulate gas flow based on input from pressure and flow sensors represented a major advance over earlier designs. Photo courtesy of Siemens Healthineers. Special thanks to Healthineers’ media team divers!

Modern intensive care ventilators have evolved into complex computer-controlled systems that operate with a degree of precision, flexibility and sophistication that would astonish Ibsen and his contemporaries (Fig 6). In the rare cases that adequate oxygenation and/or carbon dioxide clearance cannot be achieved with conventional ventilators, highly specialized options such as high frequency oscillatory ventilation (HFOV) and extracorporeal membrane oxygenation (ECMO) also exist.

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Fig 6. Simplified schematic of the pneumatic system of a modern intensive care ventilator. For simplicity, this diagram omits many important components, including non-return and safety valves, calibration and alarm systems. Diagram courtesy of A. Pitkin.

A patient with respiratory failure must be intubated with a breathing (endotracheal) tube in the windpipe (trachea) which connects to the ventilator, and to tolerate this they often need sedation and occasionally chemical paralysis to prevent them from “fighting” the ventilator, although most modern ventilators have useful support modes that synchronize the ventilator with the patient’s own respiratory efforts. An alternative to the endotracheal tube placed through the mouth, especially useful in the longer term, is to place a tube in the trachea through the front of the neck (tracheostomy), and this approach undoubtedly contributed to Ibsen’s success in Copenhagen as many polio patients had paralysis of the muscles of their throat and voicebox (larynx) and so were unable to protect their lungs from contamination by coughing. Both endotracheal tubes and tracheostomies bypass the normal humidification that occurs in the upper airways so humidification of the gas supplied to the patient is important to prevent drying out of the lining of the airways.

Current medical practice is to set tidal volumes at 6-8 ml/kg (ideal body weight) and adjust respiratory rate to achieve the desired carbon dioxide (CO2) clearance. Almost all modern ventilators can be time-cycled (most common), pressure- and volume-cycled. Patients are generally most comfortable when the partial pressure of CO2 in their blood (PaCO2) is just below normal but with severe respiratory failure it is frequently better to allow it to rise above normal as long as adequate oxygenation can be maintained rather than subjecting diseased lungs to a higher tidal volume or respiratory rate – this is called permissive hypercapnia.  Oxygenation is largely determined by the fraction of inspired oxygen (FiO2) and mean (average) airway pressure, the latter being influenced by a number of ventilator settings such as inspiratory/expiratory time ratio, positive end-expiratory pressure (PEEP) as well as patient factors such as lung compliance. For complex reasons it is common for other organ systems such as the heart and kidneys to fail in patients with respiratory failure, so for them to survive it requires a skilled team of doctors, nurses and other staff to monitor and intervene when problems occur. 

Bjorn Ibsen went on to establish the first real intensive care unit, which he called an ‘observation unit’, at the municipal hospital of Copenhagen where he was appointed chief of the anesthesiology department in 1954. He widened its scope to include any patient who needed respiratory or cardiovascular support for any reason, such as trauma, post-operative complications, sepsis and so on. One of the pioneering features was a multidisciplinary team approach that has stood the test of time and is still used today. Another was the idea that no matter what the original disease, the same principles of organ support apply. These ideas are so universally accepted now that it is hard for us to conceive of a time when they were not. 

Ventilators and the Covid-19 Virus

At the time of writing this, ventilators look like they could soon be in short supply and they cost tens of thousands of dollars. Why are they so complex and expensive? It is relatively easy to build a system of valves and sensors and an electronic control system that will function, but quite another to do so to the (justifiably) high standards of design quality, reliability, safety, ease of use and infection control that modern medicine demands. Getting any medical device approved by the US Food and Drug Administration (FDA) is very expensive and time-consuming, and similar regulatory processes exist in Europe and the rest of the world. Nevertheless, some would argue that medical device costs (like medications) are out of control, but essentially the price of sophisticated medical technology is a balance between its development costs and the price that the market will support. 

Dr. Andrew Pitkin at work.

The COVID-19 pandemic has spawned a number of projects to tackle the projected shortage of ventilators with homebuilt systems. Usefully, the Medicines and Healthcare products Regulatory Agency (MHRA) in the United Kingdom has published a set of specifications for a Rapidly Manufactured Ventilator System (RMVS), a minimum standard that such a ventilator has to meet to be useful, based on expert opinion3. These efforts are to be applauded, but use of such a ventilator would be close to a last resort, after all the ventilators in anesthesia machines in every operating room were in use. What is more, it doesn’t address the issue that patients with severe COVID-19 disease often have multiple organ involvement and it is the highly-skilled and cohesive medical and nursing teams to apply the principles that Ibsen and others pioneered that are the real limiting factor.

References

  1. Drinker P, McKhann CF. The use of a new apparatus for prolonged administration of artificial respiration. JAMA 1929; 92: 1658-61.
  2. Mapleson WW. The effect of lung characteristics on the functioning of artificial ventilators. Anaesthesia 1962;17:300-14.
  3. https://www.gov.uk/government/publications/specification-for-ventilators-to-be-used-in-uk-hospitals-during-the-coronavirus-covid-19-outbreak accessed April 17th, 2020.

Additional Resources

Karst Underwater Research


Andrew Pitkin learned to dive in 1992 in the cold murky waters of the United Kingdom and started cave and technical diving in 1994. His first exposure to exploration was in 1995 when he was one of a team of divers who were the first to reach the bottom of the Great Blue Hole of Belize at 408 fsw (123 msw). Subsequently he has been involved in numerous cave exploration projects in Belize, Mexico and Florida.
From 1996-2000 he was employed at the Royal Navy’s Institute of Naval Medicine, running a hyperbaric facility, treating decompression illness, participating in research into outcome after decompression illness, submarine escape and testing of new military underwater breathing systems. He is one of a handful of civilians to be trained by the Royal Navy as a diving medical officer.  
He moved to Florida in 2007 and is currently on the faculty of the College of Medicine at the University of Florida in Gainesville. With Karst Underwater Research he has participated in numerous underwater cave exploration and filming projects. Like many explorers, he spends much of his spare time developing and building innovative equipment for exploration purposes.

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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.  

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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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