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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.

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The Thought Process Behind GUE’s CCR Configuration

GUE is known for taking its own holistic approach to gear configuration. Here GUE board member and Instructor Trainer Richard Lundgren explains the reasoning behind its unique closed-circuit rebreather configuration. It’s all about the gas!

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By Richard Lundgren
Header photo by Ortwin Khan

Numerous incidents over the years have resulted in tragic and fatal outcomes due to inefficient and insufficient bailout procedures and systems. At the present time, there are no community standards that detail:

  • How much bailout gas volume should be reserved
  • How to store and access the bailout gas 
  • How to chose bailout gas properties

Accordingly, Global Underwater Explorers (GUE) created a standardized bailout system consistent with GUE’s holistic gear configuration, Standard Operating Procedures(SOP), and diver training system. The system was designed holistically; consequently, the value and usefulness of the system are jeopardized if any of its components are removed.  

Bailout Gas Reserve Volumes

The volume of gas needed to sustain a diver while bailing from a rebreather is difficult to assess, as many different factors impacts the result— including respiratory rate, depth and time, CO2 levels, and stress levels. These are but a few of the variables. All reserve gas calculations may be appropriate under ideal conditions and circumstances, but they should be regarded as estimates, or predictions at best.

The gas volume needed for two divers to safely ascend to the first gas switch is referred to as Minimum Gas (MG) for scuba divers. The gas volume needed for one rebreather diver to ascend on open-circuit during duress is referred to as Bailout Minimum Gas (BMG). The BMG is calculated using the following variables:

Consumption (C): GUE recommends using a surface consumption rate (SCR) of 20 liters per minute, or 0.75 f3 if imperial is used.

Average Pressure (AvP or average ATA): The average pressure between the target depth (max depth) to the first available gas source or the surface (min depth)

Time (T):  The ascent rate should be according to the decompression profile (variable ascent rate). However, in order to simplify and increase conservatism, the ascent rate used in the BMG formula is set to 3 meters/10 ft per minute. Any decompression time required before the gas switch (first available gas source) must be added to the total time. One minute should be added for the adverse event (the bailout) and one minute additionally for performing the gas switch.

BMG = C x AvP x T

Note that Bailout Minimum Gas reserves are estimations and may not be sufficient! Even though catastrophic failures are unlikely, other factors like hypercapnia (CO2 poisoning) and stress warrants a cautious approach. 

Decompression bailout gas volumes are calculated based on the diver’s actual need (based on their decompression table/algorithm), and no additional reserve is added. 

It should be noted that GUE does not endorse the use of “team bailout,” i.e. when one diver carries bottom gas bailout and another diver carries decompression gas based on only one diver’s need. A separation or an equipment failure would quickly render a system like this useless.

Common Tech Community Rebreather Configuration

  • Backmount rebreather (note side mount rebreathers are gaining in popularity)
  • Typically, three-liter oxygen and a three-liter diluent cylinder on board (each hold 712 l/25 f3) 
  • Bailout gas in one or more stage bottles which could be connected to an integrated Bailout Valve (BOV).
Divers on the AP Diving Inspiration rebreather in typical backmount configuration. Photo by Martin Parker.
Cave diver in the DiveSoft Liberty sidemount rebreather. Photo courtesy of Marissa Eckert.

Containment and Access

Rather than carry bailout minimum gas (BMG) in a stage bottle, which is typical in the rebreather diving community, GUE has designed its bailout system as a redundant open-circuit system consisting of two 7-liter, 232 bar cylinders (57 f3 each) that are integrated into the rebreather frame, and called the “D7” system, i.e. D for doubles, 7 for seven liter. Note that GUE has standardized the JJ-CCR closed-circuit rebreather for training and operations.

Photo by Kirill Egorov.

These cylinders, each with individual valves, are linked together using a flexible manifold. This system holds up to 3250 liters of gas (114 f3), of which only about 10% is used by the rebreather as diluent. Hence, close to 3000 liters (106 f3) is reserved for a bailout situation. This gives a tremendous capacity and flexibility in a relatively small form factor for dives requiring additional gas reserves (when direct ascent is not possible or desirable). 

The following advantages were considered when designing the bailout system:

  • The D7 system is consistent with existing open-circuit systems utilized by GUE divers. A bailout system that is familiar to the user will not increase stress levels, which is important. A GUE diver will rely on previous experience and procedures when most needed.
  • The system contains the gas volumes needed according to the GUE BMG calculations as well as the diluent needed for a wide range of dive missions.
  • The system is fully redundant and has the capacity to isolate failing components, like a set of open-circuit doubles and still allowing full access to the gas.
  • The overall weight of the system is less, compared to a standard system with an AL11 liter (aluminum 80 f3) bailout cylinder. In addition, it contains 800-900 liters/20-32 f3 more gas available for a bailout situation compared to the AL11 liter system. Weight has been traded for gas.
  • The system does not occupy the position of a stage bottle which allows for additional stages or decompression bottles to be added.
  • If the ISO valves on each side were closed, the flex manifold can be removed and the cylinders transported individually while still full.

Bailout gas can be accessed quickly by a bailout valve (BOV), which is typically configured as a separate open-circuit regulator worn on a necklace, consistent with GUE’s open-circuit configuration. However, some GUE divers use an integrated BOV. After evaluation of the situation, while breathing open-circuit from the BOV, the user can transition to a high-performance regulator worn on a long hose if the situation calls for it.

The long hose is carried under the loop when diving the rebreather. The chances of having to donate to another GUE rebreather diver is low, as both carry redundant bailout. Still, GUE maintains that the capacity to donate gas must be present. The process is more likely to involve a handover of the long hose rather than a donation. 

Photo by Jesper Kjøller.

Still, if needed, such a donation is made possible by either removing the loop temporarily or by simply donating the long hose from under the loop. 

Bailout decompression gasses are carried in decompression stage bottles. If more than three bottles are needed, the bottles that are to be used at the shallowest depths are carried on a stage leash (i.e. a short lease that clips to your side D-ring to carry multiple stage bottles). Maintaining bottle-rotation techniques and capacity through regular practice is important and challenging, as this skill is rarely used with the rebreather.

Bailout Gas Properties

The choice of bailout gas is extremely important, as survival may well depend on it. It is not only the volume that is important, the individual gas properties will decide if the bailout gas will be optimal or not. As the D7 system contains both the diluent and bailout gas, both gasses share the same characteristic. The following gas characteristics must be considered when choosing gas:

Density

The equivalent (air) gas density depth should not exceed 30 meters/100 ft or 5.1 grams/liter. This is consistent with the latest research by Gavin Anthony and Simon Mitchell that recommends that divers maintain maximum gas density ideally below 5.2 g/l, equivalent to air at 31 m/102 ft, and a hard maximum of 6.2 g/l, the equivalent to air at 39 m/128 ft. You can find a simple gas density calculator here.

Ventilation is impaired when diving, due to several factors which increase the work of breathing (WOB); when diving rebreathers, the impairment is even more so. High gas density, for example, when diving gas containing no or low fractions of helium, significantly decreases a diver’s ventilation capacity and increases the risk of dynamic airway compression. CO2 washout from blood depends on ventilation capacity and can be hindered if a high-density gas is used. The impact of density is very important, and the risk of using dense gases is not to be neglected. Note that this effect is not limited to deep diving. Using a dense gas as shallow as 30 meters/100 ft reduces a diver’s ventilation capacity by a staggering 50%.

Narcosis

The (air) equivalent narcotic depth should also not exceed 30 m/100 ft, or PN2=3.16. Rebreathers and emergency situations are complex enough without further being aided by narcosis.

Oxygen Toxicity

The PO2 should be limited to allow for long exposures. GUE operating standards call for a maximum PO2 for bottom gases of 1.2 atm, a PO2 of 1.4 for deep decompression gases, and a PO2 of 1.6 for shallow decompression gases. GUE recommends using the next deeper GUE standard bottom gas for diluent/bailout when diving a rebreather in combination with GUE standard decompression gases.

Bailout gasses are not chosen in order to give the shortest possible decompression obligation. They are chosen in order to give the best odds of surviving a potentially life-threatening situation. 

Two GUE CCR divers in California. Photo by Karim Hamza.

In Summary

GUE’s D7 bailout system is flexible and contains the rebreather’s diluent as well as bailout gas reserves needed for a range of different missions. The familiarity the system, along with the knowledge that they are carrying ample gas reserves, gives GUE divers peace of mind. Choosing gases with properties that will aid a diver in duress while dealing with an emergency completes the system.

GUE did not prioritize the ease of climbing boat ladders or reducing decompression by a few minutes. These are more appropriately addressed with sessions at the gym, combined with finding aquatic comfort. Nothing prevents a complete removal of the entire system at the surface if an easy exit is needed.


Founder of Scandinavia’s Baltic Sea Divers and Ocean Discovery diving groups, and a member of GUE’s Board of Directors and GUE’s Technical Administrator, Richard Lundgren 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 outside 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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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.