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NEDU Deep Stop Summary

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The NEDU stop study remains the most detailed deep stop research done to date. It may well remain this way for some years to come and conceivably even indefinitely. This is because such research is expensive and because the issue appears largely settled in the minds of those with the budgets needed to explore this topic. Other studies are being developed but likely with a much smaller budget and fewer controls. Moreover, the kind of study tech divers would like to see may well fail to identify enough difference between dissolved gas and deep stop profiles to be meaningful. This latter problem is a good place to start our summary of the current research since it also helps contextualize some of the decisions made by NEDU researchers.

In order to be useful a study should demonstrate some difference between the things being measured. All things being equal one might as well stay with the long-used and widely successful dissolved gas models if deep stops and bubble models appear “similar” in outcomes. This means that a study should use a provocative decompression in order to develop some decompression sickness by which to measure a difference in models. In this case, the NEDU study dived US Navy divers without thermal protection on air to a depth of 170 feet/52meters where they conducted work for 30 minutes before ascending over a 144 minute decompression. Divers were often shivering upon surfacing, reducing perfusion and increasing risk of DCS. Some research indicates cold of this sort would be like doubling one’s bottom time when considering the effect of reduced blood flow in cold divers. The NEDU abstract provides a nice overview of the study which can be reviewed in entirety here.

We should first acknowledge this study was a US Navy test designed to evaluate whether there was any benefit to move toward bubble-based models including deep stops. The procedures of tech divers vary considerably from those of most US Navy diving and so it was inevitable that tech divers would find such a study lacking as a useful comparison. These differences complicate evaluation of deep stop in the minds of some tech divers. The main complications relate to 1) the amount of decompression time, 2) the unusual decompression stop arrangement, 3) the breathing gases used, and 4) the temperature of the water. We shall take each of these into consideration in an attempt to outline the reasons for these choices and the primary discontent. However, readers are again encouraged to review in detail these assessments, so they can develop a more informed opinion. 

The amount of decompression and arrangement of stops derive from the US Navy algorithms selected. The total decompression time was based upon the gas content (dissolved gas), VVAL18 Thalmann algorithm which formed the baseline by which to compare a deep stop schedule as generated by the probabilistic BVM(3) bubble model. The bubble model was set to optimize a 174-minute decompression with the lowest possible risk, developing stops that would control bubbles in a way consistent with its model parameters. These aspects have aroused some disagreement in the technical community who argue the total time and associated “deep stops” are longer than reasonable and a far departure from what any tech diver might consider for decompression. However, the dissolved gas model accurately predicted and did result in relatively low incidence of DCS for “shallow stop” protocols. Meanwhile, the argument for deep stops is largely that they should limit supersaturation and bubble formation in a way that provides more benefit than the increase in gas dissolved in slow compartments that result from the stops. This study demonstrates this does not appear to be true, at least within the scope of these profiles. For a variety of reasons, most experts do not believe changing the stop distribution would have a significant effect on this failure of deep stops to work as it was hoped they might. Nonetheless, the additional time when compared to a common deep-stop, gradient approach of 20/85 resulted in 59 minutes of additional “deep stop” minutes, eliciting reasonable discontent among some.

Deep stop proponents also took issue with the use of air diving though most made less of this than the previous discussion revolving around the length and distribution of stops. We don’t have a good reason to believe the value of deep stops should be negated by certain breathing mixes. If deep stops control bubbling in a useful way, they should do so independent of the gasses breathed. Some argue that the value of hyperoxic mixes in concert with deep stops might have an additive value though no evidence appears to exist that supports that contention. 

Finally, some argue that the cold experienced by divers worked in concert with the added time at depth to disadvantage divers on the deep stop profile. The argument is that these divers were ascending while following an unreasonably long, deep-stop schedule and were thus reaching critical parts of their offgassing much later in the dive when they were very cold and where perfusion was greatly reduced. Meanwhile, the argument goes, the shallow-stop divers had finished the bulk of their decompression before they became cold. Some have even argued that this experiment was more about testing thermal issues than deep stops though most experts appear unified in disagreeing with that view. The experts argue that both groups suffered from the same thermal stress and that the low but relevant DCS incidence in the shallow-stop profile support this contention. 

In the end, these are not issues that can resolve through additional debate as evidenced by hundreds of posts and extensive argumentation. However, most divers and especially most experts appear convinced the NEDU study supports an argument that deep stops are actually less efficient because they do not appear to control bubbling enough to overcome the additional gas absorbed by slow tissues during the additional time at depth. The experts argue that all aspects of concern for tech divers i.e. use of air, cold water and extended stop time are not arguments in support of deep stops. Adjustments in these areas through use of shorter deep stops and hyperoxic mixes might reduce the difference but would merely be masking the lack of improved efficiency. 

The NEDU study appears reasonably convincing to most, at least with respect to a lack of compelling value in favor of deep stops, though with some complications as discussed. I will come back to some of these complications but first our review should conclude with the apparent relevance of other studies seeking to establish the value of deep stops. In 2005 a French study evaluated deep stops by measuring venous gas bubbles. We previously discussed the complexity of relying upon such measures though the technique likely remains broadly useful for considering decompression stress across a diving population. The French study suggested that none of the deep models appeared superior in venous bubble control and one was rated inferior. A Ljubkovic Study conducted in 2010 again used venous gas emboli (bubbles) to see how effectively the varying permeability model (VPM) controlled bubbles. They determined it was not particularly effective in this regard although they did not compare its success to bubbles present with other strategies. The Spisni Study in 2017 compared a ratio-deco, rule-based approach to a dissolved-gas, gradient-factor approach and concluded that adding longer and/or deeper stops was not more effective as based upon higher post-dive inflammation associated with deeper stops. 

When all these pieces are considered alongside the more compelling NEDU research, it appears that deep stops are not bringing the long-imagined benefit sought by proponents, at least not in a way that is easily qualified. Anti-inflammatory markers and venous bubbles are both imperfect markers and leave ample room to argue against these studies. Yet, our efforts should be less about resisting developing knowledge and more about learning what we can from accumulated wisdom. To this end, we can merge three of these conclusions into growing sense that deep stops do not appear to be controlling venous bubbles in a pronounced way. This adds an interesting dimension but is it important? We would be hard pressed to argue that increased venous bubbling is a positive development even while acknowledging it is a relatively common occurrence for blood leaving tissues in the process of off-gassing. 

On the one side, deep-stop advocates can argue 1) that venous gas bubbles are not a useful diagnostic measure of DCS, 2) that anti-inflammatory markers show contradictory results in various studies, and 3) that the NEDU study is not representative of technical diving profiles and therefore not an effective indictment of deep stops as commonly used. On the other side, one can argue that 1) deep stops do not appear to effectively control venous bubbles which are very problematic for some forms decompression illness and generally correlated with DCS likelihood across populations of divers, 2) that several studies hint at a weakness in deep stops with the most detailed study to date showing a clearly increased risk of DCS, and 3) no objective study to date appears to support the value of deep stops.

An objective review of the developing science does appear to support the idea that deep stops fail to provide compelling value and may, in fact be less efficient. Some find the evidence compelling, some feel swayed but promote a measured response and some remain entirely unconvinced. Is there anything else we might interpret from the trending science on deep stops? We will return to this subject in part four of our series.

Education

How Two Tech Agencies Address Isobaric Counterdiffusion

NAUI is one of the few training agencies that offers specific protocols to address Isobaric Counterdiffusion (ICD). Here NAUITEC Instructor Trainer and Examiner Daniel Millikovsky explains their approach to minimize ICD risks based on their Reduced Gradient Bubble Model (RGBM). GUE Instructor Trainer and Evaluator Richard Lundgren then explains GUE’s position on the subject. The gas diffuses both ways!

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Header image by Derk Remmers

Not A Theory — A Fact! How NAUITEC Manages Isobaric Counter Diffusion

by Daniel Millikovsky 

There is some confusion in the technical diving community as to whether we should pay attention to the physical law while planning gas switches, particularly on ascent. Here are some of the basics of this topic and how NAUI’s technical division, NAUITEC, has addressed this matter in training and diving operations since 1997. 

Fact: Isobaric counterdiffusion is a real gas transport mechanism. We need to pay attention to it in mixed gas diving.  

Fiction: Isobaric counterdiffusion is a theoretical laboratory concept and doesn’t affect divers at all.

From NAUI Technical Diver (textbook):

 Isobaric counterdiffusion (ICD) describes a real gas transport mechanism in the blood and tissues of divers using helium and nitrogen. It’s not just some theoretical concoction, and it has important impacts for tech diving. It was first observed in the laboratory by Kunkle and Strauss in bubble experiments, is a basic physical law, was first studied by Lambertsen and Idicula in divers, has been extensively reported in medical and physiology journals, and is accepted by the deco science community worldwide.

Isobaric means “equal pressure.” Counterdiffusion means two or more gases diffusing in opposite directions. For divers, the gases concerned are the inert gases nitrogen and helium and not metabolic gases like oxygen, carbon dioxide, water vapor, or trace gases in the atmosphere. Specifically, ICD during mixed gas diving operations concerns the two inert gases moving in opposite directions under equal ambient pressure in tissues and blood. In order to understand this, we have to consider their relative diffusion speeds. Lighter gases diffuse faster than heavier gases. In fact, helium (He) is seven times lighter than nitrogen (N2) and diffuses 2.65 times faster.

If a diver has nitrogen-loaded tissue, and if their blood is loaded with helium, this will result in greater total gas loading because helium will diffuse into tissue and blood faster than nitrogen diffuses out, resulting in increased inert gas tensions. Conversely, if a diver has helium-loaded tissues, and their blood is loaded with nitrogen, this will produce the opposite effect: Helium will off-gas faster than nitrogen on-gases, and total inert gas tensions will be lower. This last case is what we can call in decompression planning a “Good ICD,” but we need to choose the fractions of N2 wisely on ascent.

Also, Doolette and Mitchell’s study of Inner Ear Decompression Sickness (IEDCS) shows that the inner ear may not be well-modelled by common (e.g. Bühlmann) algorithms. Doolette and Mitchell propose that a switch from a helium-rich mix to a nitrogen-rich mix, as is common in technical diving when switching from trimix to nitrox on ascent, may cause a transient supersaturation of inert gas within the inner ear and result in IEDCS. They suggest that breathing-gas switches from helium-rich to nitrogen-rich mixtures should be carefully scheduled either deep (with due consideration to nitrogen narcosis) or shallow to avoid the period of maximum supersaturation resulting from the decompression. Switches should also be made during breathing of the largest inspired oxygen partial pressure that can be safely tolerated with due consideration to oxygen toxicity. 

In the case of dry suits filled with light gases while breathing heavier gases, the skin lesions resulting are a surface effect, and the symptomatology is termed “subcutaneous ICD.” Bubbles resulting from heavy-to-light breathing gas switches are called “deep-tissue ICD,” obviously not a surface-skin phenomenon. The bottom line is simple: don’t fill your exposure suits with a lighter gas than you are breathing and avoid heavy-to-light gas switches on a deco line. In both cases, the risk of bubbling increases with exposure time.

More simply, light to heavy gas procedures reduces gas loading, while heavy to light procedures increases gas loading. Note, however, that none of these counter transport issues come into play when diving a closed circuit rebreather.

The NAUITEC Way

ICD is not scientific theory, it is fact. Understanding and avoiding ICD  is the way to reduce bubble formation and an increased risk of DCS, and to allow for a more efficient decompression practice in the long term.

Deep trimix dives require a high helium and low nitrogen mix [Note that NAUITEC mandates an equivalent narcotic depth (END) of 30 m, similar to Global Underwater Explorers (GUE)]. NAUITEC takes a hierarchical approach to trimix decompression based on risk reduction. 

In its preferred “Zero Order Rule” (zero risk from ICD), NAUITEC recommends that divers not switch from helium to nitrogen (nitrox) breathing mixtures upon ascent. Instead, divers decompress on their bottom gas (trimix) until reaching their 6 m/20 ft stop, and then decompress on pure oxygen (O2). This reduces task loading and minimizes switch changes. 

If the diver wants to reduce their deco obligation and/or add a deep deco gas, they would switch to an intermediate deco mix, specifically a “hyperoxic” trimix, also called helitrox or triox, with an oxygen fraction greater than 23.5%. In practice this is accomplished by replacing the helium with oxygen and keeping the fraction of N2 the same, or ideally less. This avoids a N2 slam from ICD. Note that it is recommended that NAUI divers always maintain an equivalent air depth (END) of no more than 30 m/100 f.

This is what we recommend and practice, and we believe it offers less risk than switching from a trimix bottom gas to an enriched air nitrox (EAN) 50, (i.e. 50% O2, 50% N2) at 21 m/70 ft, which is a common community practice. The bottom line here is that in-gassing gradients for nitrogen have been minimized by avoiding isobaric switch. THERE MUST BE A HIGH BENEFIT TO RISK RATIO to deviate from Zero Order Rule! 

The additional rules present increased risk. The First Order Rule: No switches from helium to nitrogen breathing mixes deeper than 30 m/100 ft. The Second Order Rule mandates no switches from helium to nitrogen mixes deeper than 21 m/70 ft.

The last rule seems to be common in technical diving, but it has certainly not been formally tested. Just say no when the risks outweigh the benefits. Many times, the benefit of a gas switch does not outweigh the risk. Risk reduction is always the primary goal. 


GUE On Isobaric Counterdiffusion

By Richard Lundgren

GUE does not dispute Isobaric Counterdiffusion (ICD) as it’s a natural part of how we achieve decompression efficiency, i.e. maximizing the gradient between the different inert gases in a diver’s tissues and what is being respired. This is sometimes referred to as the positive ICD effect. 

The flip side of the coin, the negative ICD effect, involves a potential increased risk for decompression illness (DCI), most commonly subclinical manifestations affecting the inner ear and causing inner ear decompression sickness (IEDCS). 

Although the exact mechanics are not known, one potential aggravating factor could well be ICD when the gradient resulting from a switch from a helium to a nitrox mix is too large. This is sometimes called a “nitrogen slam.” This occurs when a gas with slow diffusivity is transported into a tissue more rapidly than a higher-diffusing gas is transported out, like when switching from bottom gas, for example a Trimix 15/55 (15% O2, 55% helium, balance N2) to a nitrox decompression gas like Nitrox 50 (50% O2, 50% N2) at 21m/70 ft. This can result in supersaturating of some tissues and consequently, bubble formation. 

Photo by Derk Remmers.

Based on ICD theory alone, one could draw the conclusion that any gas switch not containing helium after a trimix/heliox dive would be provocative and increase the decompression stress. This is where academics need to be tuned to the application and empirical evidence. 

The practice of “getting off the mix early and deep,” which led some divers to switch to air at great depth in order to maximize the off gassing of helium, was a common early practice in the tech community. It was a practice that most likely resulted in elevated risk of not only DCI, but also inert gas narcosis and the problems it can engender. This practice, as most people likely know, was not subscribed to by GUE. 

On the contrary, GUE was the first organization to call for helium-enriched gases when diving deeper than 30 m/100 ft, both for bottom gas and decompression gas. We were also early advocates for switching from helium-based bottom gas to nitrox 50 under special circumstances. 

However, it should be made very clear though, that among the very active GUE dive community, we have seen no indications or significant statistics implying that the DCI risk or occurrence is elevated when switching to Nitrox 50 as the first deco gas after a 72m/250ft dive breathing Trimix 15/55. For deeper dives, additional deco gases are used. All of these contain helium.

Another possible issue could occur when divers switch to their helium-based back gas briefly after decompressing on Nitrox 50 but before switching to pure oxygen, and/or taking an oxygen break during their 6 m/20 ft O2 decompression. However, based on thousands of decompression dives in the GUE community, these gas breaks have not been reported to cause problems. Note that these switches occur at shallow depths, and therefore reduced pressure gradients.

Superficial ICD, i.e. when the body is surrounded by a less dense gas compared to what’s being respired is more of a theoretical problem for divers, as we don’t use helium mixes to inflate our dry suits for the obvious reasons of thermal conductivity.

Interestingly, the concerns over ICD may at first glance seem irrelevant to rebreather (CCR) divers, assuming that their diluent remains the same throughout the dive. But remember most CCR divers rely on open circuit bailout, which may require gas switches.

Additional Resources:

Counterculture and Counterdiffusion: Isobaric Counterdiffusion in the Real World  

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

Not A Theory — A Fact! References: 

NAUI Technical Diver, National Association of Underwater Instructors, 2000. 

Wienke BR, O’Leary T. “Ins and Outs of Mixed Gas Counter Diffusion.” Tech Corner. Sources 3Q 2004: 45-47

Wienke B.R. & O’Leary T.R. Isobaric Counterdiffusion, Fact And Fiction. Advanced Diver Magazine

Technical Diving in Depth, B.R. Wienke

Lambertsen C. J., Bornmann R. C., Kent M. B. (eds). Isobaric Inert Gas Counterdiffusion. 22nd Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 54WS(IC)1-11-82. Bethesda: Undersea and Hyperbaric Medical Society; 1979; 182 pages. 

Doolette, David J., Mitchell, Simon J. (June 2003). “Biophysical basis for inner ear decompression sickness.” Journal of Applied Physiology, 94(6): 2145–50. 


Daniel Millikovsky is a lifetime NAUI member (NAUI# 30750). He’s been a NAUI instructor exclusively for 22 years, a Course Director for 20 years, and in 2016, became a Course Director Trainer and Representative in Argentina. Daniel is a very active NAUI Technical Instructor Examiner (#30750L) for several courses including OC and CCR mixed gas diving. He has also been a member of the NAUI Training Committee since 2020. He owns Argentina Diving, a NAUI Premier, Pro Development, and Technical Training Center based in Buenos Aires, Argentina.

Daniel began diving in 1993 as a CMAS diver and then continued with his NAUI career, becoming an instructor in 1998. He opened his first NAUI Pro Scuba Center (DIVECOR) in Cordoba, Argentina. Daniel is enthusiastic about teaching and training and is a sought after presenter at numerous international dive conferences and shows. He can be reached at info@argentinadiving.com, website: www.argentinadiving.com.


Richard Lundgren is the founder of Scandinavia’s Baltic Sea Divers and Ocean Discovery diving groups, and is a GUE Instructor Trainer, an Instructor Examiner, and a member of its Board of Directors. He has participated in numerous underwater expeditions worldwide and is one of Europe’s most experienced trimix divers. With more than 4000 dives to his credit, Richard Lundgren was a member of the GUE expeditions to dive the Britannic (sister ship of the ill-fated Titanic) in 1997 and 1999, and has been involved in numerous projects to explore mines and caves in Sweden, Norway, and Finland. In 1997, in arctic conditions, he performed the longest cave dive ever carried out in Scandinavia. Richard’s other exploration work has included the 1999 filming of the famous submarine, M1, for the BBC; the side scan sonar surveys of the Spanish gold galleons off Florida’s Key West in 2000; and the search for the Admiral’s Fleet, an ongoing project that has already led to the discovery of more than 40 virgin wrecks perfectly preserved in the cold waters of the Swedish Baltic Sea.

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Education

Classical decompression algorithms limit hypothetical tissue gas contents and prescribe decompression schedules with most of the total stop time (TST) allocated to shallow decompression stops. More recent bubble-model-based algorithms limit hypothetical bubble profusion and size and prescribe decompressions with TST skewed toward deeper stops. A large man-trial compared the efficiency of these approaches. Divers wearing swimsuits and tshirts, breathing surface-supplied air via MK 20 UBA, and immersed in 86 °F water were compressed at 57 fsw/min to 170 fsw for a 30 minute bottom time during which they performed 130 watt cycle ergometer work. They were then decompressed at 30 fsw/min with stops prescribed by one of two schedules. The shallow stops schedule, with a first stop at 40 fsw and 174 minutes TST, was prescribed by the, deterministic, gas content, VVAL18 Thalmann Algorithm. The deep stops schedule, with a first stop at 70 fsw, was the optimum distribution of 174 minutes TST according to the probabilistic BVM(3) bubble model. Decompression sickness (DCS) incidence following these schedules was compared. The trial was terminated after the midpoint interim analysis, when the DCS incidence of the deep stops dive profile (11 DCS/198 dives) was significantly higher than that of the shallow stops dive profile (3/192, p=0.030, one-sided Fisher Exact). On review, one deep stops DCS was excluded, but the result remained significant (p=0.047). Most DCS was mild, late onset, Type I, but two cases involved rapidly progressing CNS manifestations. Results indicate that slower tissue gas washout or continued gas uptake offsets the benefits of reduced bubble growth at deep stops.