Abyss Technical Article Library

Argon Useage

Argon has been used mostly experimentally as a diluent during decompression. However it is highly soluble and is very narcotic compared to nitrogen and helium. Argon's narcotic potency is about twice that of air. The premise for considering argon was due to its slower diffusion rate into tissue compared to nitrogen and helium. One current decompression model (Buhlmann) suggests the diffusion of an inert gas into tissue is inversely related to the square root of the molecular weight of the gas. For example if you compare this ratio for helium with the ratio for nitrogen you would find the helium ratio is 2.65 times the nitrogen ratio. Similarly if you compare nitrogen to argon, you would find the nitrogen ratio is about 1.19 times the argon ratio. What does this mean?

For example, if you look at the 4th compartment (tissue group)of the Buhlmann ZH-L12, 12 compartment decompression model you will find the "half-time" for helium to be 7 minutes and the half-time for nitrogen to be 18.5 minutes. The helium time is 2.65 times faster than the nitrogen time. Amazing! You could extrapolate this information and conclude the half-time for argon for this compartment would be 22 minutes. According to the Buhlmann model the increase in the compartment gas tension (pressure) is a function of the ambient pressure, the time of exposure, and the half-time for the gas being considered. Thus for an equal time of exposure and ambient pressure the gas with the highest half-time would on-gas slower, and off-gas slower. This of course only considers the exponential-exponential hypothesis (exponential on-gas and exponential off-gas).

Buhlmann simplified his theory by suggesting the total inert gas pressure, the sum of the inert gas partial pressures in the tissue, determines the tolerated ambient pressure during decompression. This sum of the inert gas pressures is subtracted and multiplied respectively by B¸hlmann's "a & b" coefficients to derive the tolerated ambient pressure (similar to "M" values in the Haldane based models). The tolerated ambient pressure is the decompression ceiling; if you ascend above the ceiling, decompression sickness may ensue. Stopping at the ceiling (decompression stop) will allow more gas to off-gas and the ceiling (tolerated ambient pressure) will move shallower until you can exit the water.

Since argon diffuses into tissue slower, the replacement of nitrogen/helium with argon during decompression would create a situation where the argon is on-gasing slower than the nitrogen and/or helium is off-gasing. This mechanism is referred to as counter-diffusion. This can occur to the degree where the total inert gas pressure in the tissue is less than the surrounding ambient pressure (fraction of inert gas in breathing mixture times the ambient pressure). This process works to shorten the decompression time. The same situation exists during gas switching from high helium content bottom mixes to air. Your decompression obligation can be significantly reduced by switching to air as deep as safely practical because the helium (2.65 times faster than nitrogen) is off-gasing rapidly while nitrogen is on-gasing more slowly.

Digressing a bit, its should be noted the reverse process of switching from a heavier inert gas to a lighter inert gas (e.g. nitrogen to helium)can create a "super-saturated" condition, where the tissue inert gas pressure is greater than the ambient inert gas pressure, even with no change in depth from the switch! This is the basis of lengthy and often misguided discussions on isobaric inert gas counterdiffusion.

To cut to the chase, using argon may shorten your decompression somewhat; however the amount of reduction may not be considered significant due to the relatively small difference in diffusion half-times between argon and nitrogen. Other considerations such as its high solubilities in fats and aqueous fluids may imply additional concerns which have not been brought to light, with risks that may not be warranted considering the degree of benefit.

TECH NOTE!!

Counterdiffusion is also a problem if a gas switch is made from argon to helium (animal experiments by D'Aoust gave near fatal results - see Bennett and Elliott, 3rd Edition).

The research that has been conducted on argon as a diving gas has generally not been for decompression but for other purposes such as studying inert gas narcosis, respiration, or counterdiffusion.

Little research has been conducted on this gas in terms of providing safe and reliable decompression.

Deep Stops

The Importance of Deep Safety Stops: Rethinking Ascent Patterns From Decompression Dives.

By Richard L. Pyle.

Before I begin, let's make something perfectly clear: I am a fish-nerd (i.e., an ichthyologist). For the purposes of this commentary, that means two things. First, it means that I have spent a lot of time underwater. Second, although I am I biologist and understand quite a bit about animal physiology, I am not an expert in decompression physiology. Keep these two things in mind when you read what I have to say.

Back before the concept of "technical diving" existed, I used to do more dives to depths of 180-220 fsw than I care to remember. Because of the tremendous sample size of dives, I eventually began to notice a few patterns. Quite frequently after these dives, I would feel some level of fatigue or malaise. It was clear that these post-dive symptoms had more to do with inert-gas loading than with physical exertion or thermal exposure, because the symptoms would generally be much more severe after spending less than an hour in the water for a 200-foot dive than they would after spending 4 to 6 hours at much shallower depths.

The interesting thing was that these symptoms were not terribly consistent. Sometimes I hardly felt any symptoms at all. At other times I would be so sleepy after a dive that I would find it difficult to stay awake on the drive home. I tried to correlate the severity of symptoms with a wide variety of factors, such as the magnitude of the exposure, the amount of extra time I spent on the 10-foot decompression stop, the strength of the current, the clarity of the water, water temperature, how much sleep I had the night before, level of dehydration ...you name it...but none of these obvious factors seemed to have anything to do with it. Finally I figured out what it was - fish! Yup, that's right...on dives when I collected fish, I had hardly any post-dive fatigue. On dives when I did not catch anything, the symptoms would tend to be quite strong. I was actually quite amazed by how consistent this correlation was.

The problem, though, was that it didn't make any sense. Why would these symptoms have anything to do with catching fish? In fact, I would expect more severe symptoms after fish-collecting dives because my level of exertion while on the bottom during those dives tended to be greater (chasing fish isn't always easy). There was one other difference, though. You see, most fishes have a gas-filled internal organ called a "swimbladder" - basically a fish buoyancy compensator. If a fish is brought straight to the surface from 200 feet, its swimbladder would expand to about seven times its original size and crush the other organs. Because I generally wanted to keep the fishes I collected alive, I would need to stop at some point during the ascent and temporarily insert a hypodermic needle into their swimbladders, venting off the excess gas. Typically, the depth at which I needed to do this was much deeper than my first required decompression stop. For example, on an average 200-foot dive, my first decompression stop would usually be somewhere in the neighborhood of 50 feet, but the depth I needed to stop for the fish would be around 125 feet. So, whenever I collected fish, my ascent profile would include an extra 2-3 minute stop much deeper than my first "required" decompression stop. Unfortunately, this didn't make any sense either. When you think only in terms of dissolved gas tensions in blood and tissues (as virtually all decompression algorithms in use today do), you would expect more decompression problems with the included deep stops because more time is spent at a greater depth.

As someone who tends to have more faith in what actually happens in the real world than what should happen according to the theoretical world, I decided to start including the deep stops on all of my decompression dives, whether or not I collected fish. Guess what? My symptoms of fatigue virtually disappeared altogether! It was nothing short of amazing! I mean I actually started getting some work done during the afternoons and evenings of days when I did a morning deep dive. I started telling people about my amazing discovery, but was invariably met with skepticism, and sometimes stern lectures from "experts" about how this must be wrong. "Obviously," they would tell me, "you should get out of deep water as quickly as possible to minimize additional gas loading." Not being a person who enjoys confrontation, I kept quiet about my practice of including these "deep decompression stops". As the years passed, I became more and more convinced of the value of these deep stops for reducing the probability of DCI. In all cases where I had some sort of post-dive symptoms, ranging from fatigue to shoulder pain to quadriplegia in one case, it was on a dive where I omitted the deep decompression stops.

As a scientist by profession, I feel a need to understand mechanisms underlying observed phenomena. Consequently, I was always bothered by the apparent paradox of my decompression profiles. Then I saw a presentation by Dr. David Yount at the 1989 meeting of the American Academy of Underwater Sciences (AAUS). For those of you who don't know who he is, Dr. Yount is a professor of physics at the University of Hawaii, and one of the creators of the "Varying-Permeability Model" (VPM) of decompression calculation. This model takes into account the presence of "micronuclei" (gas-phase bubbles in blood and tissues) and factors that cause these bubbles to grow or shrink during decompression. The upshot is that the VPM calls for initial decompression stops that are much deeper than those suggested by neo-Haldanian (i.e., "compartment-based") decompression models. It finally started to make sense to me. (For a good overview of the VPM, read Chapter 6 of Best Publishing's Hyperbaric Medicine and Physiology; Yount, 1988.)

Since you already know I am not an expert in diving physiology, let me explain what I believe is going on in terms that educated divers should be able to understand. First, most readers should be aware that intravascular bubbles are routinely detected after the majority of dives - even "no decompression" dives. The bubbles are there - they just don't always lead to DCI symptoms. Now; most deep decompression dives conducted by "technical" divers (as opposed to commercial or military divers) are very-much sub- saturation dives. In other words, they have relatively short bottom-times (I would consider 2 hours at 300 feet a "short" bottom time in this context). Depending on the depth and duration of the dive, and the mixtures used, there is usually a relatively long ascent "stretch" (or "pull") between the bottom and the first decompression stop as calculated by any theoretical compartment-based model. The shorter the bottom time, the greater this ascent stretch is. Conventional mentality holds that you should "get the hell out of deep water" as quickly as possible to minimize additional gas loading. Many people even believe that you should use faster ascent rates during the deeper portions of the ascent. The point is, divers are routinely making ascents with relatively dramatic drops in ambient pressure in relatively short periods of time - just so they can "get the hell out of deep water".

This, I believe, is where the problem is. Maybe it has to do with the time required for blood to pass all the way through a typical diver's circulatory system. Perhaps it has to do with tiny bubbles being formed as blood passes through valves in the heart, and growing large due to gas diffusion from the surrounding blood. Whatever the physiological basis, I believe that bubbles are being formed and/or are encouraged to grow in size during the initial non-stop ascent from depth. I've learned a lot about bubble physics over the last year, more than I want to relate here - I'll leave that for someone who really understands the subject. For now, suffice it to say that whether or not a bubble will shrink or grow depends on many complex factors, including the size of the bubble at any given moment. Smaller bubbles are more apt to shrink during decompression; larger bubbles are more apt to grow and possibly lead to DCI. Thus, to minimize the probability of DCI, it is important to keep the size of the bubbles small. Relatively rapid ascents from deep water to the first required decompression stop do not help to keep bubbles small! By slowing the initial ascent to the first decompression stop, (e.g., by the inclusion of one or more deep decompression stops), perhaps the bubbles are kept small enough that they continue to shrink during the remainder of the decompression stops.

If there is any truth to this, I suspect that the enormous variability in incidence of DCI has more to do with the pattern of ascent from the bottom to the first decompression stop, than it has to do with the remainder of the decompression profile. DCI is an extraordinarily complex phenomenon - more complex than even the most advanced diving physiologists have been able to elucidate. The unfortunate thing is that we will likely never understand it entirely, largely because our bodies are incredibly chaotic environments, and that level of chaos will hinder any attempts to make predictions about how to avoid DCI. But I think that we, as sub-saturation decompression divers, can significantly reduce the probability of getting bent if we alter the way we make our initial ascent from depth.

Some of you may now be thinking "But he said he's not an expert in diving physiology - why should I believe him?" If you are thinking this, then good - that's exactly what I want you to think because you shouldn't trust just me. So, why don't you dig up your September '95 issue of DeepTech (Issue 3) and read Bruce Weinke's article? I know it covers some pretty sophisticated stuff, but you should keep re- reading it until you do understand it. Why don't you call up aquaCorps and order audio tape number 9 ("Bubble Decompression Strategies") from the tek.95 conference, and listen to Eric Maiken explain a few things about gas physics that you probably didn't know before. While you're at it, why don't you order the audio tape from the "Understanding Trimix Tables" session at the recent tek.96 conference? You can listen to Andre Galerne (arguably the "father of trimix") talk about how the incidence of DCI was reduced dramatically when they included an extra deep decompression stop over and above what was required by the tables. On the same tape you can listen to Jean-Pierre Imbert of COMEX (the French commercial diving operation which conducts some of the world's deepest dives) talk about a whole new way of looking at decompression profiles which includes initial stops that are much deeper than what most tables call for. Why don't you ask George Irvine what he meant when he said he includes "three or four short deep stops into the plan prior to using the first stop recommended by each of the [decompression] programs" in the January, '96 issue of DeepTech (Issue 4)? If that's not enough, then check out Dr. Peter Bennett's editorial in the January/February 1996 Alert Diver magazine; he's talking about basically the same thing in the context of recreational diving. If you really want to read an eye-opening article, see if you can find the report on the habits of diving fishermen in the Torres Strait by LeMessurier and Hills (listed in the References section at the end of this article). The lists goes on and on. The point is, I don't seem to be the only one advocating deep decompression stops.

Are you still skeptical? Let me ask you this: Do you believe that so-called "safety stops" after so-called "no- decompression" dives are useful in reducing probability of DCI? If not, then you should take a look at the statistics compiled by Diver's Alert Network. If so, then you are already doing "deep stops" on your "no-decompression" dives. If it makes you feel better, then call the extra deep decompression stops "deep safety stops" which you do before you ascend to your first "required" decompression stop. Think about it this way: Your first "required" decompression stop is functionally equivalent to the surface on a dive that is taken to the absolute maximum limit of the "no-decompression" bottom time. Wouldn't you think that "safety stops" on "no-decompression" dives would be most important after a dive made all the way to the "no- decompression" limit?

Some of you may be thinking, "I already make safety stops on my decompression dives - I always stop 10 or 20 feet deeper than my first required stop." While this is a step in the right direction, it is not what I am talking about here. "Why not?", you ask, "I do my safety stops on no-decompression dives at 20 feet. Why shouldn't I do my deep safety stops 20 feet below my first required ceiling?" I'll tell you why - because the safety stops have to do with preventing bubble growth, and bubble growth is in part a function of a change in ambient pressure, not a function of linear feet. Suppose that, after a dive to 75 feet, you make a safety stop at 20 feet. Well, the ambient pressure at sea level is 1 ATA. The ambient pressure at 75 feet is about 3.3 ATA. The ambient pressure at your 20-foot safety stop is 1.6 ATA - which represents roughly the midpoint in pressure between 3.3 ATA and 1 ATA. Now, suppose you're on a dive to 200 feet (about 7 ATA) and your first required decompression stop is 50 feet (about 2.5 ATA). The ambient pressure midpoint between these two depths is 4.75 ATA, or a little less than 125 feet. Thus, on this dive you would want to make your deep safety stop at about 125 feet - exactly the depth I used to stop to stick a hypodermic needle in my little fishies.

But of course, the physics and physiology are much more complex than this. It may be that ambient pressure mid- points are not the ideal depth for safety-stops - in fact, I can tell you with near certainty that they are not. From what I understand of bubble-based decompression models, initial decompression stops should be a function of absolute ambient pressure changes, rather than proportional ambient pressure changes, and thus should be even deeper than the ambient pressure mid-point for most of our decompression dives. Unfortunately, I seriously doubt that decompression computers will begin incorporating bubble-based decompression algorithms, at least not in their complete form. Until then, we decompression divers need a simpler method - a rule of thumb to follow that doesn't require the processing power of an electronic computer. Perhaps the ideal method would be simply to slow down the ascent rate during the deep portion of the ascent. Unfortunately, this is rather difficult to do - especially in open water. Instead, I think you should include one or more discrete, short-duration stops to break up those long ascents. Whether or not it is physiologically correct, you should think of them as pit-stops to allow your body to "catch up" with the changing ambient pressure.

Here is my method for incorporating deep safety stops:

1) Calculate a decompression profile for the dive you wish to do, using whatever software you normally use.

2) Take the distance between the bottom portion of the dive (at the time you begin your ascent) and the first "required" decompression stop, and find the midpoint. You can use the ambient pressure midpoint if you want, but for most dives in the "technical" diving range, the linear distance midpoint will be close enough and is easier to calculate. This depth will be your first deep safety stop, and the stop should be about 2-3 minutes in duration.

3) Re-calculate the decompression profile by including the deep safety stop in the profile (most software will allow for multi-level profile calculations).

4) If the distance between your first deep safety stop and your first "required" stop is greater than 30 feet, then add a second deep safety stop at the midpoint between the first deep safety stop and the first required stop.

5) Repeat as necessary until there is less than 30 feet between your last deep safety stop and the first required safety stop.

For example, suppose you want to do a trimix dive to 300 feet, and your desktop software says that your first "required" decompression stop is 100 feet. You should recalculate the profile by adding short 2-minute) stops at 200 feet, 150 feet, and 125 feet. Of course, since your computer software assumes that you are still on-gassing during these stops, the rest of the calculated decompression time will be slightly longer than it would have been if you did not include the stops. However, in my experience and apparently in the experience of many others, the reduction in probability of DCI will far outweigh the costs of doing the extra hang time. In fact, I'd be willing to wager that the advantages of deep safety stops are so large that you could actually reduce the total decompression time (by doing shorter shallow stops) and still have a lower probability of getting bent - but until someone can provide more evidence to support that contention, you should definitely play it safe and do the extra decompression time. One final point. As anyone who reads my posts on the Internet diving forums already knows, I am a strong advocate of personal responsibility in diving. If you choose to follow my suggestions and include deep safety stops on your decompression dives, then that's swell. If you decide to continue following your computer-generated decompression profiles, that's fine too. But whatever you do, you are completely and entirely responsible for whatever happens to you underwater! You are a terrestrial mammal for crying out loud - you have no business going underwater in the first place. If you cannot accept the responsibility, then stay out of the water. If you get bent after a dive on which you have included deep safety stops by my suggested method, then it was your own fault for being stupid enough to listen to decompression advice from a fish nerd!

References:
Bennett, P.B. 1996. Rate of ascent revisited. Alert Diver, January/February 1996: 2.
Hamilton, B. and G. Irvine. 1996. A hard look at decompression software.
DeepTech, No. 4 (January 1996): 19- 23
LeMessurier, D.H. and B.A. Hills. 1965. Decompression sickness: A thermodynamic approach arising from a study of Torres Strait diving techniques. Scientific Results of Marine Biological Research. Nr. 48:
Essays in Marine Physiology, OSLO Universitetsforlaget: 54-84.
Weenie, B. 1995. The reduced gradient bubble model and phase mechanics.
DeepTech, No. 3 (September 1995): 29-37.
Yount, D.E. 1988. Chapter 6. Theoretical considerations of Safe
Decompression. In: Hyperbaric Medicine and Physiology (Y-C Lin and A.K.C. Niu, eds.), Best Publishing Co., San Pedro, pp. 69-97

Physiology and Physics of Helium

Written by Robert Palmer, European Training Director, Technical Diving International

TISSUE SOLUBILITY

Helium enters tissues rapidly, up to 2.65 times faster than nitrogen, and leaves them more rapidly also. This requires the diver to use a different decompression profile with decompression stops starting deeper than for air, with short stops at depth to cope with the rapid onset of offgassing. Helium decompressions can be reduced by the use of nitrox at shallower depths, and can be further reduced by mixing helium with nitrogen to gain the best advantages of both gases where divers are shorter than about 2 hours.

MAIN EFFECTS OF HELIUM

The best known effect of helium is its distortion of speech. The thinner gas passing across the vocal cords at atmospheric pressure produces a comical high-pitched squeak reminiscent of Donald Duck and family. In fact, any change of air density can produce a similar effect - divers at a pressure of 4 bars in a recompression chamber (the equivalent of 30m while diving on air) will produce distorted speech also. Helium's speech distortion is only relevant when through-water communications are being used, and descramblers are commercially available to translate this distorted speech (usually unsuccessfully).

There is an apparent chilling during breathing. This is again due to the thinner molecular density of the gas, which transmits heat more readily by direct conduction that does air. The gas entering the diver's lungs will be colder than air, having lost heat during the journey from the cylinder via the regulator. However, the gas leaving the lungs will not conduct heat out of the body as readily as air, there being fewer molecules to warm up. Air by comparison is denser and may feel warmer when inhaled at any given depth, but will transmit more heat out from the lungs (and thereby contribute more significantly to core heat loss) than helium mixtures.

Where helium based mixtures do contribute significantly to heat loss is when they are used as drysuit inflation gases., but in general the use of trimix or heliox in drysuit inflation is to be avoided at all times.

High pressure nervous syndrome (HPNS) is possibly the most significant limitation to the use of helium as a diving gas, though the physiological process that creates this syndrome is currently still not entirely understood.

HPNS

High Pressure Nervous Syndrome is a physical manifestation of a high pressure gas gradient across tissue compartments, possibly compounded by helium breathing. It is exacerbated by rapid pressurization to depths of over 120 meters and appears at depths of between 120 and 200 meters (400-650'), depending on the speed of descent and, to a degree, the physiology of the diver. Some divers, for reasons not fully understood, appear to be more prone to HPNS than others.

The symptoms of HPNS include muscle tremors, drowsiness, loss of appetite, nausea, dizziness, vertigo, difficulty in concentrating, and visual disturbances, such as spots or patterns breaking up the diver's field of vision. Some of these symptoms are common to several forms of gas toxicity or physiological stressors (e.g. dizziness, nausea, loss of concentration) and could be confused with nitrogen or oxygen toxicity.

In commercial diving, the effects of HPNS are reduced by slow and staged pressurization, and by adding small amounts of nitrogen to "relax" tissues. Divers are pressurized to approximately 10-11 bars (90-100 meters) and held there for several hours for tissue saturation to take place, and the gas gradient to equilibrate. Pressurization is then resumed, and the dive halted again after a further increase in pressure, for the process to repeat itself. The transit to the "bottom" may thus take many hours, far longer than is possible on open or closed circuit SCUBA, with an attendant decompression lasting several days due to the complete saturation of the divers' tissues with the inert gas mixtures involved.

To reduce the effects of HPNS, small amounts of nitrogen may be used in the mixture to "relax" different tissues compartments and so reduce certain of the side effects, notably the muscle tremors that are typically the earliest and least controllable of the effects. The tremors are postulated to be caused by differential dissolution of gases into the tissues of the myelin sheath surrounding the nerves, causing the nerves to locally spasm.

At depths of up to 120 meters HPNS is unlikely to be a problem, though in general, the greater the depth, the more the chance of the syndrome appearing. On the rare occasions that open-circuit divers have descended to greater depths, trimixes containing between 7-11% of nitrogen are thought to have contributed to the partial controlling (though not the elimination) of HPNS.

GAS DENSITY

Helium's low molecular density has other practical advantages. The thinner molecular structure of helium-based mixtures produces a better regulator performance at depth by direct comparison with air. The reduced density also makes breathing easier, and may help to flush carbon dioxide out of the lungs. Carbon dioxide has been implicated in deep water blackouts, and an increased partial pressure of CO2 is dangerous. High levels can be reached in the lungs with increasing depth, by improper breathing and increased gas density affecting regulator performance. Trimix can help reduce (but not eliminate) the problem.

OTHER INERT GASES

Helium
with the lowest lipid solubility has the lowest narcotic potential (paradoxically expressed as the highest relative narcotic potency (4.26). Xenon, which has the lowest narcotic potency figure (at 0.039) is actually an anaesthetic at atmospheric pressure, while krypton causes dizziness.

Hydrogen
Hydrogen has been used in extremely deep diving operations in excess of 500 msw (1600'), most successfully in association with helium to create hydroheliox. It has no benefit to open-circuit diving, being explosive when mixed with more than 5% oxygen. This would mean that a travel gas would be required to reach a depth at which 5% oxygen would support life, and then a flushing gas used to remove excess oxygen from the diver. At great depth, and when used with oxygen alone, hydrogen has narcotic effects more similar to similar to LSD, and has been implicated in long term psychological changes in saturation divers involved in some of the tests.

Neon
Neon has some advantages for short duration deep diving, but is too expensive to use as an open circuit breathing gas. It is a denser gas than helium and nitrogen, and diffuses more slowly into tissues than both gases, making it suitable for short deep bounce dives. However, it also emerges from tissues more slowly, and where long exposures are involves, decompressions can be excessive. Neox bends may also be more difficult to treat, involving complex recompression schedules.

Argon
Whilst almost twice as narcotic than nitrogen, it is also denser, and may have some application as a decompression gas. In theory, shallow stops could benefit from argox mixtures, reducing the amount of inert gas counterdiffusion into the tissues at depths over 9 meters, though this has had only limited testing, and cannot currently be recommended as a safe practice.

OXYGEN AND CO2 NARCOSIS

There is some evidence to suggest that varying partial pressures of oxygen may affect narcosis, with higher partial pressures producing slight anaesthetic effects similar to narcosis.

Carbon dioxide retention may be involved in this, and undoubtedly higher levels of dissolved CO2 in the blood due to CO2 retention will affect the behavior of other dissolved gases to some degree. The relevance of CO2 to technical diving is primarily to the effect of increased partial pressures of CO2 causing depth blackout in association with increased workload at depth. The best way to avoid this is by breathing properly and pacing effort during the dive.

LONG TERM EFFECTS OF DEEP DIVING

There are several potential long term effects of deep diving of which the recreational or professional trimix diver should be aware. Many of these are still postulated, and remain formally unproven, but enough evidence exists to suggest that damage may be done to the diver's body by a variety of pressure-related processes.

Capillary Atrophication and Aseptic Bone Necrosis

Perhaps the best known of the long term problems is Aseptic Bone Necrosis, where the destruction of capillaries within bone tissues causes local necrosis of the bone - that is, the bone tissue effectively dies and falls apart. Traditionally, the long bones (thighs, shins, arms) were most at risk, with the heads of joints at shoulder and pelvis especially at risk. At one time this was though to occur primarily in commercial saturation divers, but it has been fairly commonly recorded in recreational divers, where there is some evidence to suggest that it affects the center sections of bones rather than the ends. What causes it is not entirely known, other than it is associated with capillary Atrophication. Such Atrophication may be associated with rapid pressurization and/or depressurization, where different tissues within the bloodstream on and offgas at different rates. This means that certain of the blood's constituent tissues may at different times during descent or ascent act as effective dams within the smallest capillary beds, creating tiny local embolisms or micro-Atrophication. Though this is perhaps most crucial in bones, capillary beds also exist in other vital areas of the body such as the brain, soft tissues such as the liver, kidneys, eyes, etc. At present, alterations to capillary bed structure in these other tissues are best described as "change" rather than damage, until more research is done on both cause and effect.

Research on Aseptic Bone Necrosis shows that affects approximately 5% of divers (both recreational and commercial) to some degree or another. Deep mixed gas diving may be one contributory factor, as may rapid pressurization/ depressurization, but the increase in symptoms evinced in recreational divers who do not undertake such practices suggests that the problem still warrants further research before too many conclusions can be drawn.

BUBBLE FORMATION

Micro-bubbles forming during decompression, though not creating any formal symptoms of decompression illness, may result in long term CNS damage to the spinal cord. Postmortems in divers who have not reported any symptoms of DCI during life have still been found to have significant damage to the spinal cord and central nervous system.

Those who have had formal decompression events may have significantly greater long term problems, especially divers who have suffered multiple type II bends.

Such "invisible damage" may or may not be associated with deep diving. It is possible that now out-dated diving practices may have contributed to these (e.g. faster ascent rates) and that individual physiology may also play a part. To a degree, all life activity, above or below water, contributes to the eventual long term decay of the body, and the older we get the more damage has been picked up along the way. It is possible to overreact to physiological "possibilities", and it must also be remembered that diving of any sort has a very low incidence of long term serious physiological damage per individual diver when compared with other activities.

However, when formal damage does occur it should be treated seriously. Getting decompression illness may result in small localized damage or it may contribute to longer term damage, such as possible brain lesions, which may in turn create later problems from reduced mental or physical function to premature senility. Divers of all sorts, and mixed gas divers in particular, should be aware of current research in diving medicine, and should keep themselves up to date with changes in our knowledge and understanding of diving medicine.

LIMITS

The practical depth limits of mixed gas open circuit diving, taking into account the physiological and environmental limitations of the activity, lie within the following boundaries. Cold water (below 200C) : 75-80 meters. (240-260') Warm water (over 200C) : 100-120 meters. (330-400')

The planning and execution of safe dives to these depths requires considerable knowledge and experience beyond that of the ordinary recreational SCUBA diver, and safe diving to depths in excess of these generally requires one atmosphere systems or saturation diving techniques, with all the massive attendant expense. While the occasional dive on open or closed circuit SCUBA to depths in excess of 120 meters does take place, the individual undertaking it has usually undergone considerable preparation, training and acclimatization, and has considerable support, or is simply very stupid.

Planning Gas Mixtures

Written by Robert Palmer, European Training Director, Technical Diving International

INTERMEDIATE MIXES

Deep trimix dives require the use of multiple gas mixtures, and generally use a nitrox travel gas containing between 21 and 36 percent oxygen for use during the period from leaving surface to changing to bottom mix. There are two reasons for this. The first, and physiologically most important, is that many "bottom mixes" (as the trimix is often called) do not contain sufficient oxygen to be capable of supporting consciousness at the surface. Any mixture containing less than 16% oxygen falls into this category. Additionally, oxygen lean mixtures do not provide the best decompression schedules, and one or more oxygen rich mixtures are usually employed to ensure an acceptable decompression regime.

Travel Mixes
Travel mixes usually contain between 21 and 40% oxygen, with a nitrogen balance. Richer nitrox mixtures do not allow a convenient depth range, and weaker mixes offer no advantage. The depth at which the switch to trimix is undertaken depends on the PPO2 of the travel gas. A diver on EAN40 must switch by 30m (95'), at which point the PPO2 is 1.6 bars, whilst a diver on air can switch at depths of up to 60m (200'), at which point the PPO2 is at its maximum recommended limit, and the narcotic effect of the gas is becoming cause for concern. The richer travel mixtures allow a more effective decompression, and limit ongassing of nitrogen to a small degree during the initial descent. The weaker mixtures allow some room for safer bailout from bottom mix at greater depths, though offer a less effective decompression schedule than their richer counterparts. In most cases, a compromise is reached, and EAN32 or 36 are used for travel gases, allowing some room for deep bailout, and a slightly improved decompression schedule.

Decompression Gas
The gases used for the final stages of decompression usually contain oxygen percentages in excess of 50%. Pure oxygen can be used, but only for the final stops above six meters, unless a full face mask is used or special supervisory practices are employed. We do not recommend that pure oxygen is used below 6 meters (20') for in water decompression unless in an emergency. Mixtures containing 60 to 80% oxygen are most commonly employed, ensuring that the inert gas gradient is appropriately steep without creating the danger of oxygen toxicity in the final stages of the dive, when CNS percentage loading is at its greatest. The richer the mixture, then the better is the offgassing at the shallowest stops. However, weaker mixtures such as EAN50 or 60 allow the offgassing process to begin more effectively at slightly deeper stops, and allow a deeper changeover to the decompression mixture if the travel mixture is unavailable for any reason (e.g. equipment failure). This may be even more appropriate where very weak oxygen mixtures are used for the bottom gas (e.g. less than 12% oxygen). The choice of both travel and decompression gases should take into account possible bailout scenarios and the final CNS percentage dose, as well as the most appropriate decompression schedules.

SELECTING A TRIMIX

Before choosing your gas mixtures for a mixed gas dive, you need to know You should not exceed a PPO2 of 1.4 bars.

Each gas mixture should maintain a PPO2 of 1.4 bars or less for the working portion of the dive. EAN mixtures reaching 1.6 bars PPO2 can be used during the decompression phase as long as the total oxygen CNS percentage does not exceed 100% at the end of the dive. Your required Equivalent Narcotic Depth (END). Once you have your required END, the gas mixture to be used can be selected in association with the 1.4 bar oxygen limit. The depth at which you will operate, and the time you wish to spend there. This will govern both the final selection of bottom mix and the amount of gas that must be carried to maintain a credible reserve.

Finding Your Equivalent Narcotic Depth.
Your END is the comparative depth on air at which you are entirely in control of narcosis, even under stress. You find the partial pressure of nitrogen at this depth by using the Dalton's Law formula : PPN2 = FN2 x P. For example, an END of 35 meters would give a PPN2 of : 0.79 x 4.5 = 3.55 bar.

THE "IDEAL MIX" CONCEPT

Custom mixes can be blended to generate the best trimix for a particular depth/duration. This is a mix which optimizes both oxygen and nitrogen levels to control the effects of both gases. Most trimix divers use a PPO2 of 1.4 bars and an END of 40 -50 meters.

How to Find the "Ideal Mix"
Use Dalton's Law formulae to first find the appropriate oxygen and nitrogen percentages, knowing your depth and the partial pressure of nitrogen of your required Equivalent Narcotic Depth. The balance is helium.

BLENDING TRIMIX

Firstly, do be aware that the actual blending of gases should only be done by divers or technicians who have successfully completed a recognized training course in that discipline. Not all EAN facilities or filling stations will encounter a need to blend trimix. Trimix, as its name suggests, is a blend of three gases, in this case oxygen, nitrogen and helium. Trimix is used to dive to depths greater than those encountered in recreational sport diving, most usually in the 40-100m range (130-330 feet). The use of helium reduces both the effects of oxygen toxicity and nitrogen narcosis, but requires considerable additional training and experience on the part of the diver, well in excess of that required for the safe use on EAN mixtures.

There are several ways in which trimix may be produced. A diver may prefer to use one of the standard, or may select to use what is referred to as an "ideal mix", where the partial pressures (and thus percentages) of oxygen and nitrogen are preselected according to the precise depth of the dive. Alternatively, a third option of "heliair" exists, where the diver may accept a slightly higher than normal equivalent narcotic depth to enable the simplest form of production of trimix by adding air to helium.

SELECTING A TRIMIX

A diver planning a trimix dive will generally select a trimix that gives an oxygen partial pressure of 1.4 bar (ata) and a nitrogen partial pressure of 4 to 5 bar (ata) at the deepest point of the dive. This allows a maximum oxygen exposure of up to 150 minutes, and an equivalent narcotic depth (END) of 40 meters (130') to 50 meters (160'), which means that the diver would suffer no more nitrogen narcosis during the dive than on air at those depths. Where a specific rather than a generic mix is used, this would be termed the Ideal Mix for that dive.To produce the appropriate trimix, all the blender needs to know is the depth of the planned dive and the partial pressures of oxygen and nitrogen required. The depth of the dive will give the absolute pressure in bar or atmospheres, and the proportions of the constituent gases can be worked out and related to the pressure of the cylinders to be used.

REAL AND IDEAL GAS LAWS.

The actual compressibility of gas is related to several things. The main ones are :
The molecular density of the gas.
The temperature of the gas.
The pressure to which the gas is being compressed.

Most mixing calculations work with what are termed Ideal Gas Laws. These assume that all gases have the same molecular density and react in the same way to temperature and pressure. Thus if temperature is kept constant, mixing oxygen, nitrogen and helium would be a simple and predictable process.

In reality, though oxygen and nitrogen have similar molecular densities, helium is considerably less dense than either. Real Gas Laws take such variations in compressibility and molecular density into account. Helium, being the least dense of the three gases, is the most compressible at higher pressures. The higher the pressure, the greater the variation in actual gas percentage content of the mixture. For example, if 100 bar of oxygen and 100 bar of helium were compressed together at the same temperature, the resulting mix would contain about 54% oxygen and 46% helium. We tend therefore, when dealing with trimix percentages, to round oxygen down to the nearest whole percent and helium up.

In actual fact, minor variations in compressibility, temperature, gauge accuracy and analyzer accuracy during the actual blending process all tend to even the process out. To compute for Real Gas Laws that take into account the variations in compressibility due to molecular density and the temperature generated by an actual on site mixing system requires equations that can only reliably be done by computer, and a very slow blending process to keep temperatures constant. While such computer programs exist, and are available to the Gas Blender, the final variation in analyzed trimix blended using Ideal rather than Real Gas Laws is, in real life, virtually indistinguishable from blends made from Real Gas Laws. The difference is far more in the quality and skill of the actual blender.

BLENDING TRIMIX IN THE CYLINDER

There is a standard procedure for blending trimix, taking into account the different properties (and costs) of the component gases. It is usual to place helium (the most expensive gas) in first. Oxygen (if required) is then added, both gases being decanted slowly, with intervals in between to allow temperature to be as constant and stable as possible. Once both helium and oxygen are in, it is possible to analyze the resulting mix to ensure that the correct percentage of oxygen (as a proportion of the 2 gases) is present.

The table at the end of this chapter is used for blending trimix into empty cylinders. It assumes that 1 bar of existing gas is already in the cylinder. The gas makeup of the 1 bar of gas in the cylinder (as long as it is air or trimix) will not materially affect the final balance of the new mixture. (Remember, in a 210 bar / 3000 psi cylinder, 1 bar of gas = 0.5% and 1 psi = 0.03% of the gas in the cylinder.) A the maximum depth given for the above mixtures, the PPO2 is approx. 1.4 bar and the Equivalent Narcotic Depth is approximately 40m / 130'.

Once this has been done, and the mixture is analyzed as acceptable, air should be added vigorously to encourage mixing. The tank should be laid on its side during this process, as the less dense helium will tend to layer out unless agitated. Often the tank is rolled from side to side while being filled, or immediately following filling, to ensure mixing is adequate. The cylinder should be allowed to stand till cool, at which point it should be analyzed for oxygen percentage. If the mix is too rich, more air can be added. Too low, and oxygen can be added with a booster pump if required.

Once mixed, the gases will stay mixed due to the constant movement of the gas molecules ("Brownian Motion"). It is still good practice to re-analyze trimix immediately before the dive to ensure the original analysis was correct.

If the oxygen percentage is within 1 percent, it is assumed the nitrogen and helium percentages are also accurate. In practice, a percentage variation of 3-4 percent in these inert gases will not compromise decompression requirements or nitrogen toxicity, and it is unlikely that, if the oxygen is within 1% of calculated value, that the inert gas percentages will be much adrift.

HELIUM SAFETY

Pure helium should never be breathed during the blending process, or at any other time. Irreversible asphyxiation may occur as a result of the rapid diffusion of the gas into the lung tissues, essentially blocking the passage of oxygen once the helium source is removed. Diving grade helium generally contains about 2% oxygen (which also allows it to be breathed from directly at saturation depths in emergency) and this helps maintain a conduit for oxygen. Be especially aware of children using trimix or helium cylinders to fill balloons unsupervised, or using it to make funny voices. Balloon grade helium contains between 20-30% oxygen to prevent asphyxiation, and this makes it unsuitable for diving purposes. Do not allow diving grade, or any other grade, of helium to be used for balloon filling or for play.

TRIMIX CYLINDERS

Trimix cylinders need not be oxygen service unless pure oxygen is used in the blending process. Where the cylinder is simply used for heliair (helium/air mixtures) or where helium is blended with the appropriate EAN mix produced from a continuous blending system, an ordinary air cylinder will suffice. If oxygen is used in the blending process, the cylinder should be in oxygen service. In either case, the cylinder should be labeled TRIMIX in large letters, and have a label on it indicating the relative percentages of the mixtures in the cylinder and its maximum depth. Several training agencies market Trimix stickers for cylinders, usually in a red color.

Dive and Decompression Planning

Written by Robert Palmer, European Training Director, Technical Diving International

TRIMIX OPERATIONS
Trimix dives are formal operations. Treat them as such, and plan them meticulously, or don't do them. The exposures you place yourself in are dangerous and extreme, and the room for error is virtually non-existent. You may die at any time on any technical dive. And YOU don't have to weep at your grave.

TEAMS
Select the appropriate team size (this may even be a team of one if redundancy levels are appropriate and the environment so dictates). Don't make the team too big, or too small. Make the team experienced, well-equipped, well-prepared and psychologically and physically stable. If boat diving, the skipper is part of the team. Brief him/her thoroughly. He or she may well not be familiar with some of the practices concerned, and may not therefore respond appropriately to incidents.

BEFORE THE DIVE
Review all plans. Check all equipment. Set all lines and decompression materials. Spend some time in mental preparation. Don't dive till everyone and everything is ready. It really isn't worth adding more stress than is necessary to a deep mixed gas dive. Do make sure that more than enough time is allowed before the dive for preparation.

ENTERING THE WATER
Carry all your gases in a logical order, with the richest mixture on the right. Know where they are, and which regulator goes with which gauge. Once having entered the water, make sure your equipment is still in the same place it was before. Never simply assume it will be where it was before you jumped in and rearranged it!. Check you can read all gauges and reach all valves and regulators and their essential equipment. Check you can reach your cylinder vales in an emergency.

Acclimatize at the surface or at 6 meters if the surface is choppy. Take time to orientate yourself to boat or shore, currents and bottom. Thorough buddy check - leaks and gear. Take a definite time check before starting your actual descent, and note your start time on your slate.

DESCENT
Stick as closely as possible to planned descent times - extra minutes on descent can mean extra hours on decompression, or lost minutes on the bottom. Control your buoyancy AT ALL TIMES. If you lose control of your buoyancy it can affect both your descent rate and your breathing rate, and add a source of stress to the dive. Be prepared for gas switches before you need to make them. Make them on the move if possible, and practice to ensure a smooth and effective switch. Stow used regulators properly, and remember to turn off travel gases at depth to avoid toxicity problems. Slow your descent with your BC or drysuit just before you reach the bottom, and hover just above it, to maintain visibility.

ON THE BOTTOM
Plan your dive to the Rule of Thirds. Be precise. Take everything slowly - don't rush. Maintain proper buoyancy. Keep stress to a minimum. Always be aware of your location. Don't exceed planned depths or times for any reason.

ASCENT
Stick to the set ascent times for your dive. The ascent period is actually your fist phase of decompression, and these times are planned into your schedule. Travel at 20 meters/minute (60'/minute) to the first stop or the nitrox switch, whichever comes first. This reverses the gas gradient more effectively. From that point, travel at 10 meters/minute (30'/minute) between stops - this controls the ascent and reduces bubble formation during offgassing. If a gas switch and a decompression stop coincide, take a moment to adjust your buoyancy before making the switch. A loss of buoyancy supervision can result in an unplanned ascent or descent of several meters if you are not careful.

DECOMPRESSION STRATEGIES
Avoid free-floating decompressions unless currents or tides dictate it. If the currents are weak, use decompression stations hung below the support vessel, with enough room at each station for the number of divers it is designed to support. Use standby divers to monitor the progress of the decompressing divers at a regular interval, and establish some method of emergency communication (e.g., slate and line) that allows rapid communication with the surface at all times. Emergency decompression cylinders should be in the water close to the divers, and these should have been placed, along with the decompression station, before the start of the dive. Wherever you can, stay as a group. If you must free-float, then a group of divers linked together is a manageable unit at all times. Several pairs of individuals floating off in subtly different directions over a one or two hour period is a recipe for disaster. Do ensure a support craft (small inflatable) travels above the group, containing back-up cylinders and a standby diver. Make the whole decompression as trauma free as possible.

ORAL REHYDRATION
It is possible to supply decompressing divers with fluids, and rehydration should be a priority on long decompressions, especially in tropical countries. Any form of collapsible container is suitable, and if proprietary brands are not available, then the lining of cardboard wine containers will do nicely. Isotonic sport drinks such as Sport Lucozade or Gatorade can be obtained in collapsible card containers, and straws can be used effectively underwater. Remember that it takes about 6 hours to properly orally rehydrate a dehydrated diver, and a fairly continuous supply of fluid, even water, is better than vast amounts before and after a dive.

DECOMPRESSION HINTS
Full face masks may be used to maintain warmth and may reduce the potential for CNS oxygen toxicity, though proper oxygen management is more effective. It is quite possible, with prior practice, to change to a FFM underwater, and the real value of doing so is that of oral communication and warmth. Be prepared for a FFM to use much more gas than predicted by open-circuit RMV calculations, and practice beforehand to establish required gas volumes. On long decompressions, it may be more appropriate to run either pure oxygen or EAN 60-80 from the surface on a long umbilical hose, either to a second stage regulator or a FFM. The length of hose required will not materially affect the delivery of gas to a depth of 6 meters from a modern regulator. This allows an uninterrupted supply of gas without too many unwieldy cylinders taking up space in the water. If this is done, however, a bailout cylinder should be kept on standby in the water to cover any interruption in the supply.

PURE OXYGEN
Undertaking the shallowest stops on pure oxygen offers the most effective offgassing gradient. However, on a long decompression the 6 meter stop may deliver over 50% of the maximum recommended CNS percentage limits, and it is recommended that pure oxygen not be used at depths exceeding 3 meters. Even a medium swell may provide pulses of increased partial pressure, and the whole toxicity tracking process becomes less predictable. Even in an emergency, it is unsafe to switch to pure oxygen below 10 meters, even if this requires breathing the bottom mix to an extremely shallow depth. As long as the PPO2 of the inspired gas is above 19%, symptoms of hypoxia are unlikely to occur. The use of slightly weaker nitrox decompression mixtures (e.g.. 50-80% oxygen) offers some extra breathing depth in such an emergency, and is to be recommended. This does not preclude the use of pure oxygen for the 3 meter stop as a safety measure. Pure oxygen should obviously be available on the surface for emergency use, and an adapter which allows an oxygen clean SCUBA regulator to be fitted directly to a large oxygen cylinder, medical and non-medical, is a distinct advantage.

SURFACING
The last few meters actually cover the greatest pressure change, and are perhaps one of the most crucial point of the ascent. It is no coincidence that most cases of decompression illness occur just after the diver surfaces. Anything that can be done at this time to reduce the potential for bubble formation should be done. Some things you can do to help this process are :
1. Take two minutes to ascend the last 6 meters.
2. Rest on the surface for 5 minutes
3. Dekit in the water - with assistance!
4. Rest on the boat / shore for at least 10 minutes before any action. Then rest some more.
5. Try and avoid undue effort at all stages of the surfacing, getting out of the water, and dekitting process. Remember that helium bubbles are present, and more will offgas very readily if you give them the slightest provocation!

In-Water Recompression

In-water Recompression as an Emergency Field Treatment of Decompression Illness

Richard L. Pyle and David A. Youngblood

Abstract

In-Water Recompression (IWR) is defined as the practice of treating divers suffering from Decompression Sickness (DCS) by recompression underwater after the onset of DCS symptoms. The practice of IWR has been strongly discouraged by many authors, recompression chamber operators, and diving physicians. Much of the opposition to IWR is founded in the theoretical risks associated with placing a person suffering from DCS into the uncontrolled underwater environment. Evidence from available reports of attempted IWR indicates an overwhelming majority of cases in which the condition of DCS victims improved after attempted IWR. At least three formal methods of IWR have been published. All of them prescribe breathing 100% oxygen for prolonged periods of time at a depth of 30 feet (9meters), supplied via a full face mask. Many factors must be considered when determining whether IWR should be implemented in response to the onset of DCS. The efficacy of IWR and the ideal methodology employed cannot be fully determined without more careful analysis of case histories.

Introduction

There are many controversial topics within the emerging field of "technical" diving. This is not surprising, considering that technical diving activities are often high-risk in nature and extend beyond widely accepted "recreational" diving guidelines. Furthermore, many aspects of technical diving involve systems and procedures which have not yet been entirely validated by controlled experimentation or by extensive quantitative data. Seldom disputed, however, is the fact that many technical divers are conducting dives to depths well in excess of 130 feet for bottom times which result in extensive decompression obligations, and that these more extreme dive profiles result in an increased potential for suffering from Decompression Sickness (DCS).

Although technical diving involves sophisticated equipment and procedures designed to reduce the risk of sustaining DCS from these more extreme exposures, the risk nevertheless remains significant. Along with this increased potential for DCS comes an increased need for many "technical" divers to be aware of, and be prepared for, the appropriate implementation of emergency procedures in response to DCS. In the words of Michael Menduno (1993), "The solution for the technical community is to expect and plan for DCS and be prepared to deal with it".

There is almost universal agreement on the practice of administering oxygen to divers exhibiting symptoms of DCS. This practice is strongly supported both by theoretical models of dissolved-gas physiology, and by empirical evidence from actual DCS cases. The answer to the question of how best to treat the afflicted diver beyond the administration of oxygen, however, is not as widely agreed upon. Perhaps the most controversial topic in this area is that of In-Water Recompression (IWR); the practice of treating a diver suffering from DCS by placing them back underwater after the onset of DCS symptoms, using the pressure exerted by water at depth as a means of recompression.

At one extreme of this controversy is conventional conviction: divers showing signs of DCS should never, under any circumstances, be placed back in the water. As pointed out by Gilliam and Von Maire (1992, p. 231), "Ask any hyperbaric expert or chamber supervisor their feelings on in-the-water recompression and you will get an almost universal recommendation against such a practice." Most diving instruction manuals condemn IWR, and the Divers Alert Network (DAN) Underwater Diving Accident & Oxygen First Aid Manual states in italicized print that "In-water recompression should never be attempted" (Divers Alert Network, 1992, p. 7).

On the other hand, IWR for treatment of DCS is a reality in many fields of diving professionals. Abalone divers in Australia (Edmonds, et al., 1991; Edmonds, 1993) and diving fishermen in Hawaii (Farm et al., 1986; Hayashi, 1989; Pyle, 1993) have relied on IWR for the treatment of DCS on repeated occasions. Many of these individuals walking around today might be dead or confined to a wheelchair had they not re-entered the water immediately after noticing symptoms of DCS.

At the root of the controversy surrounding this topic is a clash between theory and practice.

IWR in Theory

There are many important reasons why the practice of IWR has been so adamantly discouraged. The idea of placing a person who is suffering from a potentially debilitating disorder into the harsh and uncontrollable underwater environment appears to border on lunacy. Hazards on many levels are increased with immersion, and the possibility of worsening the afflicted diver's condition is substantial.

The most often cited risk of attempted IWR is the danger of adding more nitrogen to already saturated tissues. Using air or Enriched Air Nitrox (EAN) as a breathing gas during attempted IWR may lead to an increased loading of dissolved nitrogen, causing a bad situation to become worse. Furthermore, the elevated inspired partial pressure of nitrogen while breathing such mixtures at depth leads to a reduced nitrogen gradient across alveolar membranes, slowing the rate at which dissolved nitrogen is eliminated from the blood (relative to breathing the same gas at the surface).

The underwater environment is not very conducive to the treatment of a diver suffering from DCS. Perhaps the most obvious concern is the risk of drowning. Depending on the severity of the DCS symptoms, the afflicted diver may not be able to keep a regulator securely in his or her mouth. Even if the diver is functioning nearly perfectly, the risk of drowning while underwater far exceeds the risk of drowning while resting in a boat. Another complicating factor is that communications are extremely limited underwater. Therefore, monitoring and evaluating the condition of the afflicted diver (while they are performing IWR) can be very difficult.

In almost all cases, attempts at IWR will occur in water which is colder than body temperature. Successful IWR may require several hours of down-time, and even in tropical waters with full thermal diving suits, hypothermia is a major cause for concern. Exposure to cold also results in the constriction of peripheral circulatory vessels and decreased perfusion, reducing the efficiency of nitrogen elimination (Balldin, 1973; Vann, 1982). In addition to cold, other underwater environmental factors can decrease the efficacy of IWR. Strong currents often result in excessive exertion, which may exacerbate the DCS problems. (Although exercise can increase the efficiency of decompression by increasing circulation rates and/or warming the diver [Vann, 1982], it may also enhance the formation and growth of bubbles in a near- or post-DCS situation.) Depending on the geographic location, the possibility of complications resulting from certain kinds of marine life (such as jellyfish or sharks), cannot be ignored.

Published methods of IWR prescribe breathing 100% oxygen at a depth of 30 feet (9 meters) for extended periods of time. Such high oxygen partial pressures can lead to convulsions from acute oxygen toxicity, which can easily result in drowning.

Another often overlooked disadvantage of immersion of a diver with neurological DCS symptoms is that detection of those symptoms by the diver may be hampered: the "weightless" nature of being underwater can make it difficult to assess the extent of impaired motor function, and direct contact of water on skin may affect the diver's ability to detect areas of numbness. Thus, an immersed diver may not be able to determine with certainty whether or not symptoms have disappeared, are improving, are remaining constant, or are getting worse.

The factors described above are all very serious, very real concerns about the practice of IWR. There are really only two main theoretical advantages to IWR. First and foremost, it allows for immediate recompression (reduction in size) of intravascular or other endogenous bubbles, when transport to recompression chamber facilities is delayed or when such facilities are simply unavailable. Bubbles formed as a result of DCS continue to grow for hours after their initial formation, and the risk of permanent damage to tissues increases both with bubble size and the duration of bubble-induced tissue hypoxia. Furthermore, Kunkle and Beckman (1983) illustrate that the time required for bubble resolution at a given overpressure increases logarithmically with the size of the bubble. Farm, et al. (1986, p. 8) suggest that "Immediate recompression within less than 5 minutes (i.e. when the bubbles are less than 100 micrometers in diameter) is...essential if rapid bubble dissolution is to be achieved" (italics added). If bubble size can be immediately reduced through recompression, blood circulation may be restored and permanent tissue damage may be avoided, and the time required for bubble dissolution is substantially shortened. Kunkle and Beckman, in discussing the treatment of central nervous system (CNS) DCS, write:

"Because irreversible injury to nerve tissue can occur within 10 min of the initial hypoxic insult, the necessity for immediate and aggressive treatment is obvious. Unfortunately, the time required to transport a victim to a recompression facility may be from 1 to 10 hours [Kizer, 1980]. The possibility of administering immediate recompression therapy at the accident site by returning the victim to the water must therefore be seriously considered." (p. 190)

The second advantage applies only when 100% oxygen is breathed during IWR. The increased ambient pressure allows the victim to inspire elevated partial pressures of oxygen (above those which can be achieved at the surface). This has the therapeutic effect of saturating the blood and tissues with dissolved oxygen, enhancing oxygenation of hypoxic tissues around areas of restricted blood flow.

There is also some evidence that immersion in and of itself might enhance the rate at which nitrogen is eliminated (Balldin and Lundgren, 1972); however, these effects are likely more than offset by the reduced elimination resulting from cold during most IWR attempts.

IWR in Practice

Three different methods of IWR have been published. Edmonds et al., in their first edition of Diving and Subaquatic Medicine (1976), outlined a method of IWR using surface-supplied oxygen delivered via a full face mask to the diver at a depth of 9 meters (30 feet). According to this method, the prescribed time an treated diver spends at 9 meters varies from 30-90 min depending on the severity of the symptoms, and the ascent rate is set at a steady 1 meter per 12 min (~1 ft/4 min). This method of IWR was expanded and elaborated upon in the 2nd Edition (1981), and again in the 3rd Edition (1991); and has come to be known as the "Australian Method". It has also been outlined in other publications (Knight, 1984; 1987; Gilliam and von Maier, 1992; Gilliam, 1993; Edmonds, 1993), and is presented in Appendix A of this article. [NOTE: Appendices are not included on this web page].

The U.S. Navy Diving Manual (Volume 1, revision 1, 1985) briefly outlines a method of IWR to be used in an emergency situation when 100% oxygen rebreathers are available. Gilliam (1993, p. 208) proposed that this method could "easily be adapted to full facemask diving systems or surface supplied oxygen". It involves breathing 100% oxygen at a depth of 30 feet (9 meters) for 60 min in so-called "Type I" (pain only) cases or 90 min in "Type II" (neurological symptoms) cases, followed by an additional 60 min of oxygen each at 20 feet (6 meters) and 10 feet (3 meters). This method is outlined in Gilliam (1993), and in Appendix B of this article. [NOTE: Appendices are not included on this web page].

The third method, described in Farm et al. (1986), is a modification of the Australian Method which incorporates a 10-minute descent while breathing air to a depth 30 feet (9 meters) greater than the depth at which symptoms disappear, not to exceed a maximum depth of 165 feet (50 meters). Following this brief "air-spike", the diver then ascends at a decreasing rate of ascent back to 30 feet (9 meters), where 100% oxygen is breathed for a minimum of 1 hour and thereafter until either symptoms disappear, emergency transport arrives, or the oxygen supply is exhausted. This method of IWR, developed in response to the experiences of diving fishermen in Hawaii, has come to be known as the "Hawaiian Method". This method is described in Appendix C of this article. [NOTE: Appendices are not included on this web page].

All three of these methods share the requirement of large quantities of oxygen delivered to the diver via a full face mask at 30 feet (9 meters) for extended periods, a tender diver present to monitor the condition of the treated diver, and a heavily weighted drop-line to serve as a reference for depth. Also, some form of communication (either electronic or pencil and slate) must be maintained between the treated diver, the tending diver, and the surface support crew.

Information on at least 535 cases of attempted IWR has been reported in publications. Summary data from the majority of these attempts are included in Farm et al. (1986), who present the results of their survey of diving fishermen in Hawaii. Of the 527 cases of attempted IWR reported during the survey, 462 (87.7%) involved complete resolution of symptoms. In 51 cases (9.7%), the diver had improved to the point where residual symptoms were mild enough that no further treatment was sought, and symptoms disappeared entirely within a day or two. In only 14 cases (2.7%) did symptoms persist enough after IWR that the diver sought treatment at a recompression facility. None of the divers reported that their symptoms had worsened after IWR. It is also interesting (and somewhat disturbing) to note that none of the divers included in this survey were aware of published methods of IWR (i.e. all were "winging it" - inventing the procedure for themselves as they went along), and all had used only air as a breathing gas.

Edmonds et al. (1981) document two cases of successful IWR in which divers suffering from DCS in remote locations followed the Australian Method of IWR with apparently tremendous success (both are presented below as Case #8 and #9). Overlock (1989) described six cases of DCS involving divers using decompression computers. Of these, four involved attempted IWR, three of which were apparently successful (the results from the fourth case are unclear). Two of these cases are described as Case #1 and Case #4 below. Hayashi (1989) reported two cases of attempted IWR, one of which involved the use of 100% oxygen, and the other, involving air as a breathing gas, was also described in Farm et al. (1986) and is described below as Case #2.

At present, we are aware of about twenty additional cases of attempted IWR which have not previously been reported in literature. Of these, two resulted in the death of the attempting divers (both divers were together at the time - see Case #3 below), and one resulted in an apparent aggravation of the conditions (i.e. turning a sore shoulder into permanent quadriplegia - see Case #10 below). Another case, for which we do not have details, involved a diver who apparently worsened his condition with IWR, but eventually recovered after proper treatment in a recompression chamber facility. In six other cases, the condition of the diver had remained constant or improved after attempted IWR, and further treatment in a recompression chamber was sought by most of them. In all of the remaining cases, the diver was asymptomatic after IWR, they sought no further treatment, and their symptoms did not return. Without doubt, many more attempts at IWR have occurred but have not been reported. Edmonds, et al. (1981, p. 175), in discussing the practice of the Australian Method of IWR, note that "Because of the nature of this treatment being applied in remote localities, many cases are not well documented. Twenty five cases were well supervised before this technique increased suddenly in popularity, perhaps due to the success it had achieved, and perhaps due the marketing of the [proper] equipment..." Several professional divers have privately confided to one of us (RLP) that they have used IWR to treat themselves and companions on multiple occasions, and all have reported great success in their efforts. Some continue to teach the practice to their more advanced students (although the practice was once taught on a more regular basis, it has since fallen out of widely accepted instruction protocol).

Evaluation of Case Histories

In determining the relative value of IWR as a response to DCS, it is perhaps most useful to carefully examine case histories involving attempted IWR. DCS is, by nature, a very complex, dynamic, and unpredictable disorder, and evaluation of the role of IWR as a treatment in reported cases is often difficult. Assessing the success or failure of an attempt at IWR is obscured by the fact that a positive or negative change in the victim's condition may have little or nothing to do with the IWR treatment itself. Furthermore, even the determination of whether or not a DCS victim's condition was better or worse after attempted IWR is not always clear. For example, consider the following case, first reported by Overlock (1989):

Case #1. Fiji.
Five minutes after surfacing from the fourth dive to moderate depth (75-120 feet) over a 24 hr period, a diver developed progressive arm and back weakness and pain. She returned to 60 feet for 3 min, then ascended (decompressed) over a 50-minute period (with stops at 30, 20, and 10 feet), breathing air. Tingling and pain resolved during the first 10 min of IWR. Three hours after completing IWR, she developed numbness in the right leg and foot, and reported "shocks" running down both legs, whereupon she was taken to a recompression chamber. After 3 successive U.S. Navy "Table 6" treatments, she still felt weakness and some decreased sensation. The effect of IWR on the recovery of this diver is unclear. Although the pain and weakness were resolved during IWR, more serious symptoms developed hours afterward. Perhaps numbness would never have developed had the diver been taken directly to a recompression chamber instead of re-entering the water, in which case she may have responded to treatment without residuals. On the other hand, had she not returned to the water, the initial symptoms may have progressed into paralysis during her evacuation to the chamber, and she might have ultimately suffered far more serious and debilitating residuals. Cases such as this do not contribute much insight into the efficacy of IWR. Other cases, however, provide stronger evidence suggesting that IWR has been of benefit. Consider the following case documented in Farm et al. (1986) and Hayashi (1989):

Case #2. Hawaii.
"Four fisherman divers were working in pairs at a site about 165 to 180 feet deep. Each pair alternated diving and made two dives at the site. Both divers of the second pair rapidly developed signs and symptoms of severe CNS decompression sickness upon surfacing from their second dive. The boat pilot and the other diver decided to take both victims to the U.S. Navy recompression chamber and headed for the dock some 30 minutes away [the recompression chamber was an additional hour away from the dock]. During transport, one victim refused to go and elected to undergo in-water recompression, breathing air. He took two full scuba tanks, told the boat driver to come back and pick him up after transporting the other bends victim to the chamber, and rolled over the side of the boat down to a depth of 30 to 40 feet. The boat crew returned after 2 hours to pick him up. He was asymptomatic and apparently cured of the disease. The other diver died of severe decompression sickness in the Med-Evac helicopter en route to the recompression chamber." (Hayashi, 1989, p. 157) This is just one example of many which provide compelling evidence that IWR can, in some circumstances, result in dramatic relief of serious DCS symptoms. Ironically, had this incident occurred in an area where a recompression chamber was not an option, both divers would probably have opted for IWR, and the less fortunate victim might possibly have survived the ordeal. On the other hand, attempts at IWR under inappropriate circumstances can lead to tragedy, as is clearly evident from the following case:

Case #3. Sussex, England.
Twelve experienced divers conducted an 18-minute dive on a wreck in about 215 feet. They surfaced following 38 minutes of air decompression, at which time two of the divers reported "incomplete decompression". These two divers obtained additional supplies of air and returned to the water in an apparent effort to treat DCS symptoms. They never returned to the boat, and their bodies were recovered two weeks later. The reason for their deaths remains a mystery. It is possible that they were suffering from neurological DCS symptoms, and drowned as a result of these symptoms. The tragedy of this case lies in the fact that they most likely would have survived had they not re-entered the water. The boat was equipped with 100% oxygen (surface-breathing) equipment, and the incident occurred in an area where emergency air-transport could have delivered the divers to a recompression chamber less than an hour after surfacing. The water temperature in this case was about 61-63° F (16-17° C), and the surface conditions were relatively rough (3-5 ft seas). Whether or not these divers perished as a direct result of DCS symptoms, they would, in all likelihood, have survived the incident had they not returned to the water. The main potential benefit of IWR lies in the ability to recompress the DCS victim immediately after the onset of DCS symptoms, before intravascular bubbles have a chance to grow or cause serious permanent damage. The apparent success of many reported attempts of IWR may be attributed to the immediacy of the recompression. In one case, reported by Overlock (1989), IWR began before the diver even reached the surface:

Case #4. Hawaii.
After ascending from his second 10-minute dive to 190 feet, a diver followed the decompression `ceilings' suggested by his dive computer. As he was nearing the end of his computer's suggested decompression schedule, he suddenly noticed weakness and incoordination in both arms, and numbness in his right leg. He immediately descended to a depth of 80 feet where, after 3 min, the symptoms disappeared. After a total of 8 min at 80 feet, he slowly ascended over a period of 50 min to 15 feet (his companion supplied him with fresh air tanks). He remained at this depth until his decompression computer had "cleared". He felt tired after surfacing, but was otherwise asymptomatic. In many other cases, IWR was commenced within a few minutes after surfacing, usually resulting in the elimination or substantial reduction of symptoms. In cases where DCS results from gross omission of required decompression, divers may anticipate the probable consequences, and often return immediately to depth as soon as possible in an effort to complete the required decompression. Two such cases are presented here:

Case #5. Hawaii.
While conducting a solo dive at a depth of 195 feet, a diver became entangled in lines and mesh bags. In his struggles to free himself, he extended his time at depth well beyond the intended 10 minutes, and squandered much of the air he had expected to use for decompression. Upon freeing himself, he immediately began his ascent, but was mortified to discover that the boat anchor had broken loose and was gone. Swimming down-current, he fortuitously saw the anchor dragging across the bottom, and quickly caught up with the anchor line at a depth of 60 feet. At this time, his decompression computer indicated a `ceiling' of 70 feet, and his pressure gauge showed that his scuba tank was nearly empty. He slowly ascended to the surface and quickly explained his predicament to his companion in the boat. While waiting for his companion to rig a regulator to a fresh tank of air, he began feeling symptoms of severe dizziness and had problems with his vision. Grasping the second tank under his arm, he allowed himself to sink back down, nearly losing consciousness. Upon reaching a depth of 80 feet, his clouded consciousness fully resolved, and he remained 10-15 ft below his computer's recommended `ceiling' during subsequent decompression. Although he eventually exited the water before his computer had "cleared", he did not experience any additional symptoms.

Case #6. Central Pacific.
A diver had partially completed his decompression following 15 minutes at 200 feet, when he suddenly became aware of the presence of a very large and somewhat "inquisitive" Tiger Shark. Initially, the diver maintained his composure, fearing DCS more than the threat of attack. When the shark rose above, passing between the diver and the boat, the diver reconsidered the situation and opted to abort decompression. After a rapid ascent from about 40 feet, the diver hauled himself over the bow of the 17-foot Boston Whaler (without removing his gear). Anticipating the onset of DCS, he instructed his startled companion to quickly haul up the anchor and drive the boat rapidly towards shallower water. By the time they re-anchored, the diver was experiencing increasing pain in his left shoulder. He re-entered the water and completed his decompression, emerging asymptomatic. There are many other cases in which divers must interrupt their decompression temporarily, then resume decompression within a few minutes without ever experiencing symptoms of DCS. Generally, these cases of asymptomatic `interrupted decompression' are not considered as IWR. However, one such incident which recently occurred in Australia is worth mentioning:

Case #7. Australia.
After spending 18 minutes at a depth of 220 feet, a diver experienced a serious malfunction of her Buoyancy Compensator inflation device which resulted in the rapid loss of her air supply and a sudden increase in her buoyancy. Additionally, she became momentarily entangled in a guide line, further delaying ascent, and was freed from the line with the assistance of her diving companion. As they ascended, they were met by a second team of divers just beginning their descent. Although one of the members of the second team was able to provide her with air to breathe, he was unable to deflate her over-expanded B.C., and both ascended rapidly to the surface. Within 4 minutes, she returned to a depth of 20 feet where she breathed 100% oxygen for 30 min. She then ascended to 10 feet where she completed an additional 30 min of breathing oxygen. Upon surfacing, she was taken to a nearby recompression chamber facility, breathing oxygen during the 30 min required for transport. Arriving at the facility, she noticed no obvious symptoms of DCS, but was diagnosed with mild "Type II" DCS and treated several times in the chamber. She suffered no apparent residual effects. Although no DCS symptoms developed prior to recompression, serious symptoms undoubtedly would have ensued had recompression not been immediate, given the extent of the exposure and the explosive rate of ascent. It is interesting that a modified version of the Australian Method of IWR was employed, rather than the diver descending to greater depth on air to complete the omitted decompression. Recompression depth was limited to a maximum of 20 feet due to concerns of oxygen toxicity at greater depths. The victim was monitored continuously while breathing oxygen underwater by at least two tending divers. It should be noted that successful attempts at IWR are not limited to cases which take advantage of the ability to immediately recompress the victim. Edmonds et al. (1981) report on a case where IWR yielded favorable results many hours after the initial onset of DCS:

Case #8. Northern Australia.
After a second dive to 100 feet, a diver omitted decompression due to the presence of an intimidating Tiger Shark. Within minutes of surfacing, he "developed paraesthesia, back pain, progressively increasing incoordination, and paresis of the lower limbs". After two unsuccessful attempts at air IWR, arrangements were made to transport the victim to a hospital 100 miles away. He arrived at the hospital 36 hours after the onset of symptoms, and due to adverse weather conditions, he could not be transported to the nearest recompression chamber (2,000 miles away), for an additional 12 hours. By this time, the victim was "unable to walk, having evidence of both cerebral and spinal involvement", manifested by many severe neurological ailments. The diver was returned to the water to a depth of 8 meters, where he breathed 100% oxygen for 2 hours, then decompressed according to the Australian Method of IWR. Except for small areas of hypoaesthesia on both legs, all other symptoms had remised at the end of the IWR treatment. This case suggests that in-water oxygen treatment in depths as little as 8 meters can have positive effects on DCS symptoms even after much time has elapsed. It also underscores another aspect of IWR; the fact that it may be the only treatment available in remote areas where recompression chamber facilities are many thousands of miles and several days away. For example, Edmonds et al. (1981) report on another case which occurred in the Solomon Islands. At the time, the nearest recompression chamber was 3,500 km away and prompt air transport was unavailable:

Case #9. Solomon Islands.
Fifteen minutes after a 20-min dive to 120 feet, and 8 min of decompression, a diver developed severe neurological DCS symptoms, including "respiratory distress, then numbness and paraesthesia, very severe headaches, involuntary extensor spasms, clouding of consciousness, muscular pains and weakness, pains in both knees and abdominal cramps". No significant improvement occurred after 3 hours of surface-breathing oxygen. She was returned to the water where she followed the "Australian Method" of IWR (breathing 100% oxygen at 9 meters [30 feet]). Her condition was much improved after the first 15 minutes, and after an hour she was asymptomatic, with no recurring symptoms. Although most of the reported attempts at IWR have utilized only air as a breathing gas, this practice has been strongly discouraged due to the risks of additional nitrogen loading. The concern that air-only IWR may transform an already bad situation into tragedy seems clearly validated by the following case:

Case #10. Caribbean.
A young diver experienced pain-only symptoms of DCS after an unknown dive profile. He made three successive attempts at IWR (presumably breathing air), each time worsening his condition. After the third attempt, his condition had degenerated into quadriplegia. Because of transport delays, he did not arrive at a recompression chamber until about three days after the incident. Saturation treatment yielded no improvement in his condition, and he remained permanently paralyzed. Whereas the above case illustrates an unsuccessful attempt to treat relatively mild symptoms of DCS with air-only IWR, the following case, reported by Farm et al. (1986), represents an apparently successful attempt at treating very severe symptoms with similar techniques:

Case #11. Hawaii.
Shortly after a third dive to 120-160 feet, a diver developed "uncontrollable movements of the muscles of his legs". Within a few minutes, his condition deteriorated to the point where he was paralyzed; numb from the nipple-line down and unable to move his lower extremities. He was able to hold a regulator in his mouth, so a full scuba tank was strapped to his back and he was rolled into the water to a waiting tender diver. The tender verified that the victim was able to breathe, and proceeded to drag him down to 35-40 feet. When the symptoms did not regress, the victim was pulled deeper by the tender. At 50 feet, he regained control of his legs and indicated that he was feeling much better. He was later supplied with an additional scuba tank, ascended to 25 feet for a period of time, and then finished his second tank at 15 feet. Except for feeling "a little tired" that evening, he regained full strength in his arms and legs and remained asymptomatic. Another, previously unpublished case, involved a DCS victim whose symptoms were so severe that IWR was not attempted for fear that he would drown:

Case #12. Central Pacific.
Four aquarium fish collectors ascended rapidly from their second 200 feet dive of the day, aborting essentially all decompression. All immediately began experiencing nausea and varying degrees of neurological DCS symptoms. Three of the divers returned to a depth of about 50 feet, but the fourth opted instead to stay in the boat. When the three completed their abridged attempt at IWR (after which all three felt noticeably improved), they headed for shore. Help was summoned, and additional scuba tanks and 100% oxygen were obtained and loaded into the boat. By this time, one of the divers felt only pain in his shoulders, and the other three were experiencing varying degrees of neurological DCS symptoms. The worst of these was diver who did not attempt IWR immediately after the initial onset of symptoms: he was unable to move his arms or legs and was having difficulty breathing. The other three attempted to assist him back in the water, but they eventually gave up, fearing that he might drown (due to his inability to hold the regulator in his mouth). The other three continued IWR, breathing both air and 100% oxygen at 30-40 feet, until nightfall forced them out of the water. That night, all four took turns breathed 100% oxygen on the surface while waiting for the emergency evacuation plane to arrive. The following day, the three who had attempted IWR were flown to Honolulu, where they experienced varying degrees of recovery after treatment in a recompression chamber. The one who did not attempt IWR died before the plane arrived. All of the cases described thus far have involved either 100% oxygen or air (or both) as breathing gasses during IWR. In at least one reported case, EAN was used as a breathing gas for the IWR treatment:

Case #13. Northeastern United States.
After spending 25 minutes at a maximum depth of 147 feet, a diver ascended following decompression stops required by his tables. He began feeling a tingling sensation and sharp pain in his right elbow as he arrived at his 30 feet decompression stop. He completed an additional 30 min at 10 feet beyond what was called-for by his tables, and then surfaced. His symptoms subsided somewhat after an hour of breathing 100% oxygen on the boat, but persisted enough to prompt the diver to attempt IWR. He returned to the water with an additional cylinder containing EAN-50 (50% oxygen, 50% nitrogen) and descended to 100 feet for a period of 10 minutes. He ascended to 20 feet over a 10-minute period, and remained there for 68 min. He spent an additional 5 min at 10 feet, then surfaced asymptomatic, with no recurrence of symptoms. This case illustrates another fundamental risk associated with IWR; that of acute CNS oxygen toxicity. During the deepest portion of above IWR profile, the diver was breathing an oxygen partial pressure of 2.02, considerably greater than what is considered safe. The diver was aware of the potential for acute CNS oxygen toxicity and had an additional cylinder of air with him, just in case. Furthermore, he was exposed to this excessive oxygen partial pressure for only 10 minutes.

Discussion

As stated earlier, the source of controversy surrounding the topic of in-water recompression is essentially the conflict between what is predicted by theory, and what appears to be demonstrated by practice. In reviewing the issue of IWR, several questions require attention. First and foremost, should IWR ever be attempted under any circumstances? If the answer is "yes", then under what circumstances should it be performed? Also, if the decision to perform IWR has been made, which method should be followed?

The Efficacy of IWR

From the cases described above, it should be evident that IWR has almost certainly been of benefit to some DCS victims in certain circumstances. If the selection of cases seems biased towards "successful" attempts at IWR, it is only a reflection of the numbers of actual cases on record. Whereas only one additional attempt at IWR (besides Case #3 and #10) clearly led to deterioration of the condition of a DCS victim, there are literally hundreds of additional cases where IWR was almost certainly of (sometimes great) benefit.

Opponents to the practice of IWR are usually quick to point out that DCS symptoms are often relieved, sometimes substantially, when the victim breathes 100% oxygen at the surface (the presently accepted and recommended response to DCS). Indeed, if symptoms do resolve with surface-oxygen, and recompression treatment facilities are relatively close at hand (via emergency transport), then the additional risks incurred with re-immersion seem unwarranted. The two deceased divers discussed in Case #3 would have, in all likelihood, survived their ordeal if oxygen was administered on the boat and transport to the nearby recompression chamber was effected. However, in cases where chamber facilities are not available, or when symptoms persist in spite of surface-oxygen (such as in Case #9 and #13), then recompression is clearly necessary, and IWR perhaps should be attempted.

Determining Circumstances Appropriate for IWR

It should also be clear that identifying those circumstances under which IWR should be implemented is an exceedingly difficult task. A wide variety of variables must be taken into account, and many factors must be carefully considered. Although the decision to perform IWR should be made quickly, it should not be made in haste.

Hunt (1993) pointed out that DCS often carries with it a certain stigma. Under some circumstances, a diver suffering from the onset of DCS symptoms may be reluctant to reveal their condition to companions. Consequently, such an individual might attempt IWR so as to "fix" themselves without anyone else becoming aware of the problem. For obvious reasons, this alone is not a reasonable justification for considering IWR, and is especially dangerous because it likely results in the diver attempting IWR without the safety of an observing attendant or tender. Similarly, IWR should never be thought of as a substitute for proper treatment in a recompression chamber. IWR is not a "poor man's" treatment, and the decision to implement it should not be motivated by financial concerns. Regardless of the outcome of an IWR attempt, medical evaluation by a trained hyperbaric specialist should always be sought as soon afterward as possible.

The major factor in determining whether IWR should be implemented is the distance and time to the nearest recompression facility. In a study of more than 900 cases of DCS in U.S. Navy divers, Rivera (1963) found that 91.4% of the cases treated within fifteen minutes were successful, whereas the success rate when treatment was delayed 12-24 hours was 85.7%. A similar study on DCS cases among sport (recreational) divers showed similar results. Of 394 examined cases, 56% of divers with mild DCS symptoms achieved complete relief when treated within 6 hours, whereas only 30% were completely relieved when treatment was delayed 24 hours or more. The same study found that 39% of divers with severe symptoms were relieved when treated within 6 hours, whereas only 26% were relieved when treatment was delayed 24 hours or more (Divers Alert Network, 1988). In reviewing these numbers, Moon (1989) stressed that delay of treatment for DCS should be minimized, but also noted that response to delayed treatment is not entirely unacceptable. Knight (1987) recommends that IWR should be considered when the nearest recompression facility is more than 6 hours away. Such generalizations are difficult to make, however, as indicated by the fact that the ill-fated diver in Case #2 was less than 2 hours away from a recompression chamber.

One of the most important variables affecting the decision to attempt IWR is the mental and physical state of the diver. Certainly divers who are, for whatever reason, uncomfortable or reluctant to return to the water for IWR should not be coerced or forced to do so. The extent and severity of the DCS symptoms are also important factors. Whether or not mild DCS symptoms (i.e. pain-only) should be treated is not certain. One perspective is that such symptoms are not likely to leave the diver permanently disabled, and thus the risks associated with attempted IWR would not be worth taking. Furthermore, individuals with such symptoms are prime candidates for "making a bad situation worse" (as was demonstrated in Case #10). Conversely, the risks of submerging severely incapacitated divers might override the potential benefits of IWR when serious neurological manifestations are evident. Edmonds (1993) recommends against the practice of IWR in situations "where the patient has either epileptic convulsions or clouding of consciousness. "The death of the two divers in Case #3 might have resulted from drowning due to loss of consciousness from severe neurological symptoms. However, some evidence indicates that IWR may be of value even under these circumstances. Although the divers treated in some cases (e.g. #2, #5, and #11) might have gone unconscious underwater and drowned, the consequences of no immediate recompression may have been equally grave. Also, the diver who perished in Case #12 may have survived had he performed IWR along with his companions. The immediacy of recompression may be particularly advantageous if DCS symptoms develop soon after surfacing from a deep dive, and when these symptoms are neurological and "progressive" (sensu Francis, et al., 1993). Under such circumstances, the condition of the DCS victim can rapidly degenerate, and permanent damage may ensue in the absence of immediate recompression. However, it is also particularly critical in these circumstances to monitor the condition of the treated diver with a tender close by.

As mentioned earlier, environmental factors such as water temperature, surface conditions, hazardous marine life, and strong currents might significantly influence the feasibility of IWR. Many technical dives are conducted in relatively cold water (such as Europe, the northeastern and western coasts of the continental United States, southern Australia, and many freshwater systems), and the risk of hypothermia and decreased nitrogen elimination rates create additional complications for attempted IWR in these environments. Edmonds et al. (1981) and Edmonds (1993) have pointed out that reduced water temperature is not necessarily as great a concern as many opponents of IWR have suggested. The reasoning is that divers in these environments are usually well-equipped with thermal protection such as dry-suits, which have come into wide-spread use among technical divers. If the divers have adequate thermal protection to conduct the initial dive, then they are likely prepared to tolerate additional in-water exposure during IWR. However, Sullivan and Vrana (1992) reported on two cases of simulated IWR off Antarctica in - 1.4°C water, and concluded that "[IWR] cannot be considered sufficiently reliable in [extremely] cold waters..." protection.

Sharks and other hazardous marine life can tremendously complicate IWR efforts. In Case #5, a large Tiger Shark did appear during IWR, but did not influence the diver's ascent profile. Divers omitted required decompression in Case #6 and #8 due to the presence of large Tiger Sharks, thus leading to subsequent attempts at IWR. The risks of this threat are generally minuscule, however these cases illustrate that such problems can occur.

In addition to the factors discussed above, the availability of large quantities of 100% oxygen and the equipment needed to deliver it safely to a diver 30 feet (9 meters) underwater are also very important factors when considering an attempt at IWR. These factors are discussed in greater detail in the following section.

Methodology of IWR

Once the decision to perform IWR has been made, the next question to consider concerns methodology. The fundamental difference between the Australian Method and the Hawaiian Method of IWR is that the latter incorporates a deeper "air-spike" as an initial step in the treatment. The two methods are analogous in form, respectively, to the U.S. Navy's "Table 6" and "Table 6A" (however, the depths at which 100% oxygen is breathed is shallower, and the durations shorter for the IWR methods than for the chamber schedules).

The primary purpose for the deeper "air-spike" of the Hawaiian Method is essentially to exert a greater pressure on the diver so that the DCS bubbles are further reduced in size. In addition to restoring circulation, the extra "overpressure" may facilitate bubble resolution (Kunkle and Beckman, 1983; Farm et al., 1986). Air is used instead of oxygen because of the risk of acute CNS oxygen toxicity which results from breathing oxygen at such depths. Along with the benefits of increased bubble compression, however, come the risks of additional nitrogen absorption during this "spike".

To address the therapeutic advantages of the "spike", it is important to examine the physical effects of pressure on bubble size. Although by Boyle's Law alone there is a substantial "diminishing of returns" in terms of bubble size reduction as one descends deeper, gas phase bubbles are subject to other forces that may affect their size. Although a discussion of bubble physics is beyond the scope of this article, suffice it to say that bubble radii are reduced proportionally more with increasing depth than what would be predicted by Boyle's Law alone. Perhaps more importantly, the pressure of the gas within the bubble increases proportionally more, which leads to increased rates of bubble dissolution. However, the added risks of nitrogen loading and nitrogen narcosis increase with depth, adding potentially substantial greater risk to performing the deep spike. A depth of 165 feet was chosen by the USN (Table 6A) and Farm et al. (1986; the Hawaiian Method) as the maximum at which benefit from recompression was significant. Descent to a depth of 30 feet, the maximum depth prescribed by the Australian Method, yields a nearly 50% reduction in bubble volume, and approximately 20% decrease in bubble diameter. Descent to 165 feet further reduces the bubble volume by an additional 33%, and the diameter by an additional 25%. Thus, in the case of bubble volume, more benefit results in the first 30 feet of recompression than is gained in the next 135 feet, whereas the reduction in bubble diameter is slightly greater during the subsequent 135 feet depth than the initial 30 feet. Whether or not bubble diameter or bubble volume is more critical to the manifestation of DCS symptoms is uncertain.

The fundamental question is whether or not the additional recompression confers physiological advantages sufficiently in excess of the disadvantages associated with breathing air at depth (in an IWR situation). Obviously, this depends on the immediate diving history of the afflicted diver, and the particular circumstances involved. The practice of subjecting DCS victims to a 165 feet "spike" during chamber treatments has recently begun to "fall out of favor" among hyperbaric medical specialists. Hamilton (1993) points out that "the 6-atm recompression with air or enriched air of Table 6A is likely to be discontinued as evidence accumulates that it offers no real benefit over the 100% oxygen [treatment] of Table 6". This philosophy may also be applied to IWR treatment procedures. The possibility of substituting EAN or high-oxygen Heliox during the "spike" must also be examined. Modern technical diving operations often involve EAN for some portion of the dive, and thus EAN may be available in some DCS situations. EAN contains a percentage of oxygen which is greater than 21%, and thus may offer therapeutic advantages over air. The presence of nitrogen as a diluent in EAN allows a diver attempting IWR to recompress at a greater depth than permitted by 100% oxygen (for reasons associated with acute CNS oxygen toxicity). In at least one case (#13), EAN was used during IWR, with apparently successful results. James (1993) outlines the benefits associated with using 50/50 Heliox (50% helium, 50% oxygen) for recompression therapy. Since helium mixtures commonly incorporated into technical diving operations do not contain such high proportions of oxygen, a supply of high-oxygen Heliox would have to be maintained at the dive site specifically for the purpose of IWR. Unless closed-circuit rebreathers are available at the site, the option of using Heliox for IWR is probably unfeasible.

There are a number of safety advantages to the Australian Method over the Hawaiian Method. Since the only breathing gas of the Australian Method is oxygen, there is no risk of additional loading of nitrogen or other inert gases. Thus, if the treatment must be terminated prematurely (e.g. in response to the onset of nightfall; see Case #12), there is no risk of aggravating the DCS symptoms. Furthermore, the Australian Method may be conducted in shallow, protected areas such as lagoons or boat harbors, where sea surface and current conditions are less likely to be adverse.

We are unable at this time to entirely condemn the Hawaiian Method of IWR, for it may confer important advantages under certain circumstances. Edmonds (1993) suggests that the Australian Method of IWR is "of very little value in the cases where gross decompression staging has been omitted", presumably because such situations may require recompression to depths in excess of 30 feet (9 meters) (although see Case #7 and #8). Under such circumstances (e.g. `interrupted decompression' situations), the "spike" might be advantageous. Nevertheless, we are compelled to strongly discourage technical divers from incorporating an "air-spike" into IWR attempts, at least until additional verification of its efficacy can be established through empirical and theoretical lines of evidence.

The USN method of IWR differs from the Australian Method primarily in the recommended ascent pattern. Whereas the Australian Method advocates a slow steady (1 meter/12 min.) ascent rate, the USN Method divides the ascent into two discrete stages at 20 and 10 feet. Although at first this difference may seem trivial, it might, in fact, have important physiological ramifications. Edmonds (1993) reports that "It is a common observation that improvement continues throughout the ascent, at 12 minutes per meter. Presumably the resolution of the bubble is more rapid at this ascent rate than its expansion, due to Boyle's Law". If this is true, then divers attempting IWR according to the USN Method could conceivably suffer recurrence of symptoms immediately following ascent to the next shallower stage. The validity of this argument has yet to be verified.

Hyperbaric Oxygen

All of the published IWR methods advocate breathing an oxygen partial pressure of 1.9 atm for extended periods. Such high levels permit increased saturation of dissolved oxygen in the blood and tissues, which may help provide badly needed oxygen to areas of restricted circulation or tissue hypoxia. At such concentrations and durations, however, the risks of acute CNS oxygen toxicity are a serious consideration. Oxygen partial pressures of 1.2-1.6 atm have been suggested as the upper limit for technical diving operations. The published IWR methods have endorsed exposure to higher oxygen partial pressures because of the therapeutic advantages, and because a diver performing IWR is apt to be at rest (reducing the likelihood of an acute oxygen toxicity seizure). In at least one case (Case #7 above), the depth of in-water oxygen treatment was limited to a maximum of 20 feet (oxygen partial pressure of 1.65 atm) in an effort to avert oxygen toxicity problems. Because the consequences of convulsions resulting from acute oxygen toxicity are particularly serious underwater, all three published methods of IWR strongly recommend that a tender diver be continuously present, and that oxygen be administered via a full face mask. Although not prescribed in any of the in-water recompression methods, most recent publications discussing the use of oxygen as a decompression gas advise that the long periods of breathing pure oxygen be "buffered" by 5-minute air breaks every 20 minutes. The risk of additional nitrogen loading from these brief periods is more than offset by the reduced risk of acute oxygen toxicity problems.

Standard recompression chamber treatments commonly incorporate breathing 100% oxygen at a simulated depth of 60 feet (2.8 atm), however this should not be attempted during IWR due to changes in human metabolism when immersed in water, and to the grave consequences of an oxygen toxicity-induced convulsion underwater.

In the Absence of Oxygen

Perhaps one of the most critical conditions affecting the decision to perform in-water recompression is the availability of 100% oxygen, especially in a system capable of delivering it to a diver underwater. Although the risk of acute oxygen toxicity symptoms is certainly a cause for concern, the added advantages to effective decompression/recompression are tremendous. However, there will be cases of DCS which occur in situations where 100% oxygen is unavailable. Surely, in light of the theoretical disadvantages of attempting IWR using only air, such a practice would seem absurd. Indeed, all of the cases for which IWR left the divers in worse shape than when they began (e.g. Case #3 and #10), involved air as the only breathing mixture. Furthermore, the diver in case #8 did not improve after air-only IWR, and may have exacerbated his condition during his failed attempts. Nevertheless, the vast majority of the reported "successful" attempts of IWR (including Case #2, #4, #5, #6, and #11 above) were conducted using only air. Several early publications proposed methods of air-only IWR (e.g. Davis, 1962), however none are presently recognized as practical alternatives to oxygen IWR.

In two of the above cases of air-only IWR (#4 and #5), the afflicted divers followed the advice of their decompression computers in determining an air recompression/decompression profile, with apparent success. However, as pointed out by Overlock (1989), use of computers for this purpose "was never intended by the designer/manufacturer, nor would it be recommended". The reason this practice is not advisable is that the algorithms utilized by such devices for determining decompression profiles do not account for the complexities introduced by the presence of intravascular bubbles, which can dramatically affect decompression dynamics (Yount, 1988).

Edmonds et al. (1981, p. 173) sum up air IWR as follows: "In the absence of a recompression chamber, [air IWR] may be the only treatment available to prevent death or severe disability. Despite considerable criticism from authorities distant from the site, this traditional therapy is recognized by most experienced and practical divers to often be of life saving value".

Our suggestion (and an underlying message of this article), is that technical divers, who are already familiar with the use of 100% oxygen underwater as a decompression gas, should add to their equipment inventory the necessary items (such as a full face mask and large supplies of extra oxygen) to perform proper IWR procedures. Having done this, these divers avoid facing the decision to perform the risky gamble of air IWR.

Conclusions

It should be clarified at this point that the main purpose of this article is to bring forth the issue of IWR as an alternative response to DCS, and to summarize available information on the subject. We do not necessarily endorse IWR; however we see an increasing need by technical divers to become aware of the information available on this topic. Several disturbing facts have prompted us to bring this issue to light. First, based on available reports, it is clear that many people are attempting IWR without even knowing that published procedures are available. Furthermore, most reported attempts were conducted using only air. Although the practice seems to have led to a surprising number of successful cases, the advantages of using oxygen for IWR are tremendous, and cannot be denied. Thirdly, and perhaps of greatest concern, few of the individuals who successfully attempted IWR sought subsequent examination by a trained diving physician.

We feel compelled to strongly emphasize the importance of seeking a thorough medical examination after any situation where DCS symptoms have been detected. Regardless of how successful an attempted IWR procedure may be, the affected divers should arrange for transport to the nearest recompression facility as soon as possible to undergo examination by a trained hyperbaric medical specialist. The practice of IWR should never be viewed as an alternative to proper treatment in a recompression chamber. Rather, it should be viewed as a means to arrest and possibly eliminate a progressing or otherwise serious case of DCS. In most cases, in-water recompression should be used as an immediate measure to arrest or reverse serious symptoms while arrangements are being made to evacuate the victim to the nearest operating chamber facility. Without doubt, a person suffering from DCS is better-off within the warm, dry, controlled environment of a chamber, under proper medical supervision, than he or she is hanging on a rope underwater.

The information contained in this article is directed at the growing numbers of "technical" divers, who are conducting dives which expose them to elevated risk of sustaining serious DCS symptoms. These sorts of divers tend to be more experienced and better prepared and equipped to handle many of the procedures outlined by published IWR methods. As put forth by Menduno (1993, p. 58), "In-water oxygen therapy appears to be a promising, though perhaps transitional, solution to the problem of field treatment for technical divers. Though the concept will take some work to properly implement on a widespread scale, the technical community does not suffer from the same limitations as its mass market counterpart." By "transitional", Menduno was no doubt referring to the possibility that lightweight, portable recompression chambers may soon become standard technical diving equipment, and may be available on a much broader basis in the future. Selby (1993) describes one such chamber design which can be compactly stored and quickly assembled in field emergency situations. Edmonds (1993, p. 49), however, cautions that: "When hyperbaric chambers are used in remote localities, often with inadequate equipment and insufficiently trained personnel, there is an appreciable danger from both fire and explosion. There is the added difficulty in dealing with inexperienced medical personnel not ensuring an adequate face seal for the mask. These problems are not encountered in in-water treatment."

In any case, the present high cost of portable recompression chambers will prevent their widespread availability anytime soon. Furthermore, there will always be DCS incidents in situations where no recompression chambers are available nearby.

Our intention is to illustrate that the issue of IWR is far from clearly resolved. We have little doubt that staunch opponents to the practice of IWR will angrily object to even discussing the issue, on the grounds that it might lead improperly trained individuals to make a bad situation worse. But we adhere to the idea that the dissemination of information to those who may need it is of utmost importance, especially when lives may be at stake. It is indeed tragic when a person suffering a relatively minor ailment resulting from DCS attempts IWR incorrectly and leaves the water permanently paralyzed or dead. However, it is perhaps equally tragic when a DCS victim ends up suffering from permanent disabilities because of a long delay in transport to a recompression facility, when the damage might have been reduced or eliminated had IWR been administered in a timely manner. We believe that the time has come to address this issue seriously, openly, and with as much scrutiny as possible. Only through further controlled experimentation and careful analysis of reported IWR attempts will this controversial issue progress towards resolution.

In an effort to document larger numbers of IWR cases, we have begun to collect data on this topic and intend to establish a database of reported IWR attempts. If any readers have ever attempted IWR, or know of anyone who has, we would be greatly indebted if copies of this form could be filled out and mailed to Richard L. Pyle, Ichthyology, B.P. Bishop Museum, P.O. Box 19000-A, 1525 Bernice St., Honolulu, HI 96817; or sent by FAX to (808) 841-8968.

Appendix A. The "Australian Method" of Emergency In-Water Recompression.

Notes:
1. This technique may be useful in treating cases of decompression sickness in localities remote from recompression facilities. It may also be of use while suitable transport to such a centre is being arranged.
2. In planning, it should be realised that the therapy may take up to 3 hours. The risks of cold, immersion and other environmental factors should be balanced against the beneficial effects. The diver must be accompanied by an attendant.

Equipment: (The following equipment is essential before attempting this form of treatment.)
1. Full face mask with demand valve and surface supply system OR helmet with free flow.
2. Adequate supply of 100% oxygen for patient, and air for attendant.
3. Wet suit [or dry suit] for thermal protection.
4. Shot with at least 10 metres of rope ( a seat or harness may be rigged to the shot).
5. Some form of communication system between patient, attendant and surface.

Method:
1. The patient is lowered on the shot rope to 9 metres, breathing 100% oxygen.
2. Ascent is commenced after 30 minutes in mild cases, or 60 minutes in severe cases, if improvement has occurred. These times may be extended to 60 minutes and 90 minutes respectively if there is no improvement.
3. Ascent is at the rate of 1 metre every 12 minutes.
4. If symptoms recur remain at depth a further 30 minutes before continuing ascent.
5. If oxygen supply is exhausted, return to the surface, rather than breathe air.
6. After surfacing the patient should be given one hour on oxygen, one hour off, for a further 12 hours.

Table Aust 9 (RAN 82), short oxygen table
DEPTH..................ELAPSED TIME...................RATE OF ASCENT
(metres)..........Mild...............Serious
9.................0030-0100.........0100-0130
8.................0042-0112.........0112-0142
7.................0054-0124.........0124-0154......12 minutes
6.................0106-0136.........0136-0206......per metre
5.................0118-0148.........0148-0218......(4 min/ft)
4.................0130-0200.........0200-0218
3.................0142-0212.........0212-0242
2.................0154-0224.........0224-0254
1.................0206-0236.........0236-0306....From Edmonds et al. (1981), p.558.

Appendix B. The U.S. Navy Method of Emergency In-Water Recompression

If the command has 100% oxygen-rebreathers available and individuals at the dive site trained in their use, the following in-water recompression procedure may be used instead of Table 1A:
1. Put the stricken diver on the rebreather and have him purge the apparatus at least three times with oxygen.
2. Descend to a depth of 30 feet with a stand-by diver.
3. Remain at 30 feet, at rest, for 60 minutes for Type I symptoms and 90 minutes for Type II symptoms. Ascend to 20 feet after 90 minutes even if symptoms are still present.
4. Decompress to the surface by taking 60 minutes stops at 20 feet and 10 feet.
5. After surfacing, continue breathing 100% oxygen for an additional three hours.
From the U.S. Navy Diving Manual, Vol. One, Section 8.11.2, D.
NOTE: Gilliam (1993) adds that "This method can be easily adapted to full facemask diving systems or surface supplied oxygen. However, it requires a substantial amount of oxygen to be available, both for the in-water treatment and subsequent surface breathing period."

Appendix C. The "Hawaiian Method" of Emergency In-Water Recompression.
Notes:
This decompression sickness treatment table was designed for use by Hawaii's diving fishermen when afflicted with decompression sickness while diving and when more than 30 minutes away from a regular recompression treatment facility. In such an event, treatment must be initiated as soon as the signs or symptoms of decompression sickness are recognized. The urgent nature of the treatment must be recognized and acted upon immediately, inasmuch as nervous tissue of the brain or spinal cord can only be completely revived within the first 7 to 8 minutes after its oxygen supply has been stopped by the intravascular bubble emboli of decompression sickness. (Although its use by technical divers is generally discouraged, this method is presented here for the purpose of providing information to readers of these proceedings. Readers are strongly advised to obtain a copy of Farm et al. (1986) for further details concerning this treatment. Some suggested modifications to allow for more general applicability of this method and some additional comments have been added in italics.)

Equipment Required
1. An adequate supply of oxygen on board boat, i.e., a 120 cu ft capacity or greater bottle, an oxygen-clean hose at least 40-ft long plus fittings, and an oxygen-clean scuba regulator and mouth piece (NOTE: Use of full face mask with demand regulator is very strongly encouraged for administering oxygen underwater during these treatments)
2. A length of line marked to 30 ft from the waterline with seat attached upon which the victim can sit during decompression (the seat should be weighted so as to make victim and seat negatively bouyant)
3. Extra air tanks for victim and attending diver (minimum of two)
4. Anchor rope or sounding float line marked at 165 ft
5. Depth gauge and watch for use by attending diver
6. Wet suit jacket (or other adequate thermal protection) for use by victim with appropriate weights

Method
Upon recognizing symptoms or signs of decompression sickness, immediately --
1. Stop the engines (of the boat, if the boat is already moving)
2. Throw over anchor line and let out 165 feet or to bottom
3. Rig one full air tank for victim and another for attendant diver
4. Put victim in water with one attendant diver (or two if required) to take victim down anchor line (Extreme caution should be excercised in choice of attendant diver - the risk of DCI occurring in the attendant diver as a result of the IWR attempt should be very seriously considered)
5. Descend to depth of relief plus 30 fsw (not to exceed 165 fsw)
6. Keep victim at that depth for 10 minutes
7. Attending diver and victim start slow ascent with initial rate of 30 ft/minute with stops every minute for assessment of patient's condition
8. Ascent from maximum depth to oxygen breathing depth should not take less than 10 minutes. Suggested rates of ascents from 165 fsw are: 30 ft/minute x 2 minutes; 15 ft/minute x 2 minutes; 10 ft/minutes x 3 minutes; 5 ft/minutes x 3 minutes
9. If patient starts to experience recurrence of any signs or symptoms, return to 10-ft deeper stop for 5 minutes, then resume ascent
10. During deep air breathing period, crew in boat rigs oxygen breathing equipment with regulator (or preferably, full face-mask with demand regulator) attached to hose and line with seat at 30 fsw
11. Upon reaching 30 fsw victim switches to oxygen breathing
12. Victim breathes oxygen at 30 fsw for a minimum of 1 hour
13. If victim had initial symptoms of pain only, and if signs and symptoms are relieved after 1 hour of breathing oxygen, start slow ascent. If victim had signs and symptoms of CNS disease, keep victim at 30 fsw on oxygen for one or two additional 30-minute periods. When victim is completely relieved (or emergency transport arrives, or oxygen supply is exhausted), start slow ascent to surface while breathing oxygen (or air if oxygen supply is exhausted)
14. If the in-water recompression is not effective and the supply of oxygen is apparently inadequate, emergency transport to the on-shore recompression chamber should be arranged (Technical divers are strongly encouraged to begin making arrangements for emergency transport to a recompression facility as soon as DCI symptoms become evident). Recompression on oxygen at 30 fsw should be continued until the oxygen supply is exhausted or transport arrives.
15. Even if victim is asymptomatic when reaching surface, have victim breathe oxygen in boat on surface until supply is exhausted. Consult with diving medical officer upon return to shore.

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