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Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

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by Associate Professor David J. Doolette

Gradient factors are mechanisms which modify the decompression stops prescribed by the Buhlmann ZH-L16 decompression algorithm. ZH-L16 is a “gas content” algorithm, which tracks the uptake and elimination of inert gas in notional tissue compartments and schedules decompression stops to not exceed specified maximum permissible inert gas partial pressures in the compartments. When such maximum permissible inert gas partial pressures are specified for decompression stop depths, they are referred to as M-values.

Gradient factors (GF) modify M-values (and consequently allowed gas supersaturation) to a fraction of the difference between ambient pressure and the original M-value. Thus, GF 80 modifies the M-value to 80% of the difference between ambient pressure and the original M-value. Typical proprietary implementations of the GF method require the diver to select two gradient factors: GF low modifies the M-values for the deepest decompression stop, and GF high modifies the M-value for surfacing (often designated as GF low/high, e.g. GF 20/80). The algorithm then interpolates a series of modified M-values in between these two user-specified points. If the GF low is set less than 100%, this forces deeper stops to limit supersaturation in the fast tissues early in the ascent, and setting the GF high to less than 100% will produce longer, shallower stops to reduce supersaturation in the slower tissues in the latter phase of the ascent

In contrast to gas content decompression algorithms, bubble decompression algorithms (VPM-B is one such algorithm familiar to GUE divers) characteristically prescribe deeper decompression stops. In simple terms, bubble decompression algorithms favor deeper stops to limit supersaturation and thereby bubble formation early in the decompression, whereas traditional gas content decompression algorithms favor a more rapid ascent to maximize the inspired–tissue gradient of inert gas partial pressures to maximize tissue inert gas washout.

New Findings on Deep Stops

Deep stops came to the attention of early technical divers in the form of empirical “Pyle stops,” a practice serendipitously developed by ichthyologist and technical diving pioneer Richard Pyle, arising from a requirement to vent the swim bladders of fish specimens collected at great depth before arriving at his first decompression stop. There followed a strong trend toward the adoption of bubble algorithms, and also for the use of gradient factors to force gas content algorithms to impose deep stops (for instance, using GF low values of 30% or less). Based largely on supportive anecdotes, a widespread belief emerged among technical divers that deep-stop decompression schedules are more efficient than shallow-stop schedules. Efficiency, in this context, means that a schedule of the same or even shorter duration has a lower risk of DCS than some alternative schedule.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops. Most studies have used venous gas emboli (bubbles) as an indicator of comparative risk of decompression sickness (DCS). Blatteau and colleagues compared dives using French Navy air and trimix decompression tables (relatively shallow stop schedules) to experimental schedules with added deep stops and longer total decompression time (similar to Pyle stops). Despite longer total decompression time, the deep stops schedules resulted in either the same or more VGE than the shallow stops schedules, and some cases of DCS.1

Photo courtesy of GUE Archives.

Spisni and colleagues compared trimix dives conducted using a deep stops schedule (ZH-L16 with GF 30/85) to an even deeper stops schedule with longer total decompression time (a UDT version of ratio deco) and found no difference in VGE.2 An as-yet-unpublished study compared trimix dives using a DCAP shallow stops schedule to a ZH-L16 GF 20/80 deep stops schedule with similar total decompression time, and the deep stops schedule resulted in significantly more VGE.3 A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives.4

What To Do About Gradient Factors

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops. Fraedrich validated dive computer algorithms by comparing them to well-tested U.S. Navy decompression schedules, including the schedules from the deep stop study outlined above. For that dive, ZH-L16 with a GF low >55% (e.g. GF 55/70) produced a first decompression stop between 70 and 40 fsw.5 Tyler Coen at Shearwater Research Inc. noted that GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths. To understand this requires delving a little further into M-values.

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops.

M-values are typically a linear function of stop depth. In older algorithms such as ZH-L16, the M-value generating functions have a slope greater than one (in ZH-L16, the slopes are the reciprocals of the “b” parameters), resulting in increasing supersaturation allowed with increasing stop depth. In more modern algorithms developed by the U.S. Navy since the 1980s, including the one used to produce the shallow stops schedule in the study outlined above, the slope of the M-value generating functions are generally equal to one, so that the same level of supersaturation is allowed at all stop depths. This results in modestly deeper stops than older algorithms, but still relatively shallow stops compared to bubble models.

With this information in mind, I set my GF low to roughly counteract the ZH-L16 “b” parameters (I have been using Shearwater dive computers with ZH-L16 GF in conjunction with my tried and true decompression tables for about three years). In ZH-L16, the average of “b” parameters is 0.83. I choose my GF low to be about 83% of the GF high, for instance GF 70/85. Although the algebra is not exact, this roughly counteracts the slope of the “b” values. This approach allows me to believe I have chosen my GF rationally, is not so large a GF low as I am unable to convince my buddies to use it, and satisfies my preference to follow a relatively shallow stops schedule.

This article was prepared by Assoc. Professor Doolette in his personal capacity. The opinions expressed in this article are the author’s own and do not reflect the view of the Department of the Navy or the United States government.

Header image: Joakim Hjelm

1. Blatteau JE, Hugon M, Gardette B. Deeps stops during decompression from 50 to 100 msw didn’t reduce bubble formation in man. In: Bennett PB, Wienke BR, Mitchell SJ, editors. Decompression and the deep stop. Undersea and Hyperbaric Medical Society Workshop; 2008 Jun 24-25; Salt Lake City (UT). Durham (NC): Undersea and Hyperbaric Medical Society; 2009. p. 195-206.

2. Spisni E, Marabotti C, De FL, Valerii MC, Cavazza E, Brambilla S et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. Diving Hyperb Med 2017;47:9-16.

3. Gennser M. Use of bubble detection to develop trimix tables for Swedish mine-clearance divers and evaluating trimix decompressions. Presented at: Ultrasound 2015 – International meeting on ultrasound for diving research; 2015 Aug 25-26; Karlskrona (Sweden).

4. Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. 53 p. Report No.: NEDU TR 11-06.

5. Fraedrich D. Validation of algorithms used in commercial off-the-shelf dive computers. Diving Hyperb Med 2018;48:252-8.


Additional Resources:

PADI recently published an excellent post, “Evolving Thought on Deep Decompression Stops,” by John Adsit, on the subject of Deep Stops.

Alert Diver magazine published a profile and interview with Doolette in the Fall of 2016.

The Math behind the ZH-L16 Model: Bühlmann established, by means of many hyperbaric chamber experiments with volunteers, how much supersaturation the individual tissue compartments can tolerate without injury. He expressed the relationship through the following equation:

pamb. tol. = (pt. i.g. – a) ·b

or

pt. tol. i.g. = (pamb / b) + a

pamb. tol. – the ambient pressure tolerated by the tissue

pt. i.g. – the pressure of the inert gas in the tissue

pt. tol. i.g. – tolerated (excess)pressure of the inert gases in the tissues

pamb – current ambient pressure

a, b – parameters of the model ZH-L16 for each tissue. “a” depends on the measure unit of pressure used, while “b”  represents the steepness of the relationship between the ambient pressure pamb. and the pressure of inert gas in the tissue pt. i.g. The first equation shows which lower ambient pressure pamb. tol. will still be tolerated at the actual pressure of inert gas in the tissues pt. i.g. The lower equation shows which level of supersaturation pt. tol. i.g. can be tolerated at a given ambient pressure pamb for a given tissue.


Dr. David Doolette began scuba diving in 1979 and was introduced to the sinkholes and caves of Australia in 1984. Around this time, he alternated between studying for his B.Sc. (Hons.) and working as a dive instructor, when he developed an interest in diving physiology. He planned and conducted some of the first technical dives in Australia in 1993. Since being awarded his Ph.D. in 1995, he has conducted full time research into decompression physiology, first at the University of Adelaide, and since 2005 at the U.S. Navy Experimental Diving Unit in Panama City, Florida.

He has been a member of the Undersea Hyperbaric Medical Society since 1987, received their 2003 Oceaneering International Award, and is a member of their Diving Committee. He has also been a member of the South Pacific Underwater Medicine Society since 1990 and served as the Education Officer for five years. He is a member of the Cave Diving Association of Australia, the Australian Speleological Federation Cave Diving Group, Global Underwater Explorers, and the Woodville Karst Plain Project. He remains an avid underwater cave explorer, both near his home in Florida and abroad.


Education

Situational Awareness and Decision Making in Diving

Situational awareness is critical to diving safety, right? But how much of your mental capacity should be devoted to situational monitoring, e.g., How deep am I? How much gas do I have? Where is my buddy? Where is my boat? More importantly, how does one develop that capacity? Here GUE Instructor Trainer Guy Shockey, who is also a human factors or non-technical skills instructor, explores the nature and importance of situational awareness, and what you can do to up your game.

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By Guy Shockey
Header photo by Kirill Egorov

It is not surprising that given the nature of the activity and its heavy reliance on equipment, the majority of diving discussions focus on the “technological” side of diving which includes equipment, gases, decompression, etc. These discussions will assuredly still continue but over the last few years we have seen a renewed focus on what we refer to as “Human Factors” (HF) and their role in technical diving and diving in general. 

I for one am happy with this shift in emphasis; regardless of what equipment, gases or deco protocols you are using, HF is always a part of the equation. It strikes me as a bit odd that divers would spend hundreds and thousands of dollars trying to find the “perfect” bolt snap or retractor and ignore training the “human in the system”.  This despite the knowledge that we can learn how to be better decision makers once we are aware of just what things influence our decision making. Ultimately, it doesn’t matter what gear configuration or equipment or gases we are using if we have no ability to make good decisions while diving. It doesn’t matter a lot what “make of vehicle” I drive, if I don’t make smart decisions while driving. 

Thankfully, there has been a sea change in this attitude and today, just about every diving conference, magazine or blog has started discussing Human Factors or non-technical skills (NTS). As an active GUE instructor, I have tried to stay current with this and include HF training in all my classes in some capacity. I believe HF becomes more important as the diver progresses in their technical training, and even more so if they make the shift to CCR diving. Regardless of the level of diving though, the one common feature of all divers is, as Human Factors coach, Gareth Lock writes, “the human in the system.” It seems only logical then that it would make sense to turn our attention onto the human diver.

Brain Capacity

Human Factors includes many aspects of understanding our decision making process, however, I believe there is one aspect of HF and NTS training that is particularly relevant to every diver. The concept of “Situational Awareness” (SA) has been a buzzword for several years now, but only more recently have we started to talk about it in terms of HF and diving. Former Chief Scientist for the United States Air Force, engineer Mica Endsley has been one of the luminaries on the subject of situational awareness and defines it as “the perception of the elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future”. This is a simple yet powerful sentence and deserves more consideration.

A lot going on.
Photo courtesy of Jarrod Jablonski.

The new diver or a diver working at the limits of their capacity in a new training or diving environment has a limited amount of internal RAM (random access memory) or CPU (central processing unit) power to call on to make decisions. They are typically overwhelmed by a new environment that includes changes in sight (everything is closer), sound (it travels faster underwater), physical changes on the body (changes in drysuit or wetsuit pressure), temperature (usually colder), and the overwhelming knowledge that humans are using life support equipment to operate in a hostile environment. Within that environment we are expecting divers to also monitor depth, time, location, team, gas, etc.  Then, if there is an emergency, we also expect them to react with precision and skill to solve the problem. And finally, our expectation is that we are doing all this for fun!

In summary, what we are expecting is for our divers to maintain a high level of situational awareness while operating in a hostile environment and maintaining enough capacity to deal with emergencies.  Seems simple on paper right? 

It is readily apparent to any instructor that trying to monitor situational awareness is an overwhelming task for new divers or those divers pushing their training limits in a more advanced class. If the typical diver, who originally started diving to have fun, is using 75% of their capacity just to monitor their situational awareness (Where am I? How deep am I? How much gas do I have? Where is my buddy? Where is my boat? etc.) they only have 25% of their remaining capacity to do what they intended to do. 

The GUE philosophy is to train in such a fashion that we are able to switch this around in order to effectively dedicate 25% of our capacity to situational awareness monitoring and thus have 75% of our capacity to do what we came to do: have fun! There is an interesting yet critical corollary of this change in that when the first diver has an emergency they have only 25% of their capacity to dedicate to the problem. Contrast this to the trained GUE diver who has 75% of their capacity to help solve the emergency. 

Helper Muscle

Positioning in a team of three during a safety drill.
Photo courtesy of GUE archives.

GUE classes are intended to help build your situational awareness while also developing fundamental skills for the level of training you are doing. Hence, it is not enough to just “do an S-drill” (check to see the long hose is not encumbered); we expect you to “do an S-drill” while also being aware of your position in the water column, proximity to the line, and awareness of your team mates. 

Consider SA as a “helper muscle” that we are developing while also working on the primary muscles. A good analogy might be using dumb bells for a chest press in a gym which requires you to stabilize the weight while also pushing it upward. This is quite different from doing a similar exercise on a machine where rails or tracks keep the load stabilized while you push or pull it. We carry this forward into our upper level classes where we require an even higher level of situational awareness such as tracking gas by time, etc. 

It is for this reason that I have become more and more convinced that situational awareness is quite possibly the most important skill that a diver must develop. As Endsley wrote, “As technology has evolved, many complex, dynamic systems have been created that tax the abilities of humans to act as effective timely decision makers when operating these systems”.  As GUE has moved into the CCR training world, I believe we are seeing just how prophetic this statement from over 20 years ago actually is. 

SA is not just about “what is” but about “what will be”. In this aspect it requires the diver to first recognize the situation, then analyze what it means, and then project into the future how it will affect them. As the environment the diver is operating within continues to change, SA management becomes a complex and ever-changing exercise. Further, it stands to reason that poor SA will lead to poor decision making. 

Developing Situational Awareness

The net result is that good situational awareness will help the diver in their decision-making process. It will help free up some mental and physical capacity to enjoy their dive and even perhaps more importantly, it will provide extra resources when dealing with problems and emergencies. GUE instruction is designed to encourage growth in your building “SA”. So the next time your debriefing includes a critique of more than just the demonstrated skill, be assured that we are doing this for a reason and to make your diving more enjoyable and safer. 

Developing situational awareness will not happen without consciously working on it. One way you can do this on every dive is by making a conscious effort to anticipate your expected gas usage and then verifying it every five minutes. You can also work on anticipating the next step or waypoint in your dive and arriving there ready to perform whatever action you are expected to do. 

For example, if you are the one expected to deploy a surface marker buoy (SMB) at the 20 minute mark, then anticipate that and arrive at that time waypoint with the SMB out of your pocket and ready to deploy. If you are the one running a line from the shot line to the wreck, then arrive at the bottom of the shot line with the reel out and ready to go. These are only a few of the ways you can work on developing your situational awareness and you will find it gets easier over time.

Keep practicing and you will be able to master most any situation.
Photo courtesy of Jong Moon Lee.

I tell my students that learning how to plan and complete a dive is not unlike learning a new dance where at first you may need numbered footprints on the ground telling you what foot to place and where. Then after practicing a few times, you can remove the footprints and then soon your footwork becomes second nature and you can concentrate on smiling at your dance partner as you prepare for the next “So You Think You Can Dance” tryout. 

Make every dive an effort to develop your situational awareness. It will pay off handsomely in terms of making you a more relaxed and confident diver. Before you know it, you will be doing things subconsciously that used to require significant RAM. This will make your diving more enjoyable and you will retain lots of capacity for problem solving, and worrying about your next dance lesson.


Guy Shockey is a GUE instructor and trainer who is actively involved in mentoring the next generation of GUE divers. He started diving in 1982 in a cold mountain lake in Alberta, Canada. Since then he has logged somewhere close to 8,000 dives in most of the oceans of the world. He is a passionate technical diver with a particular interest in deeper ocean wreck diving. He is a former military officer and professional hunter with both bachelor’s and master’s degrees in political science. He is also an entrepreneur with several successful startup companies to his credit.

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