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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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Risk-Takers, Thrill-Seekers, Sensation-Seekers, and … You?

It’s likely that many in our community no longer think of tech diving as a risky activity, or perhaps even appreciate how important taking risks may be to one’s personal health—let alone that of our species. Fortunately, InDEPTH’s copy editing manager Pat Jablonski dived deep into the origins, meaning, and benefits of regularly taking risks, and even offers a thrill-seeking quiz for your edgy edification. What have you got to lose?

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by Pat Jablonski. Title photo courtesy of Katelyn Compton Escott.

“Life without risk is not worth living.” – Charles Lindbergh

What defines a risk? What is involved in taking a risk?

Difficult questions to answer, because something that feels risky to one person might be yawn-worthy to another. Risk taking, unscientifically, is something you do that gets your blood up, raises your heartbeat, awakens your senses, and makes you hyper-aware of your surroundings.

Surely we can agree that the Covid pandemic has added an unexpected level of risk to everyday life. Add poor drivers, mass shootings, contentious politics, global climate change, and many are left believing that meeting each day is risky enough. But that’s not true for people who identify as risk-takers or thrill-seekers.

“Everyone has a ‘risk muscle’. You keep in shape by trying new things. If you don’t, it atrophies. Make a point of using it once a day.” – Roger Von Tech

There are many activities that go to the trouble of defining the level of risk involved with a specific activity, and while that’s not the purpose of this article, you should know that scuba diving ranks fairly high on the risky behavior scale–higher than skydiving and rappelling. And, cave/wreck diving or freediving isn’t on any risk scale we could locate. We can assume it’s up there—near or at the top.

Fock A. Analysis of recreational closed-circuit rebreather deaths 1998–2010 Diving and Hyperbaric Medicine. 2013 Volume 43, No. 2. With the caveat that they are “best guess numbers,” Fock concluded that rebreather diving is likely 5-10x as risky as open circuit scuba diving, accounting for about 4-5 deaths per 100,000 dives, compared to about 0.4 to 0.5 deaths per 100k dives for open circuit scuba. This makes rebreather diving more risky than skydiving at .99/100k, but far less risky than base-jumping at 43 deaths/100k. The current belief is that rebreather diving has gotten safer.

Divers are a fairly small niche group for many reasons. One of them could involve the degree of danger associated with the sport. Answer this: Do dry land people ever ask you why you would want to take such a chance with your life in order to go where you weren’t meant to go? 

It’s a reasonable question, albeit a hard one to answer.

Photo courtesy of Glen Kwan

“A life without risk is a life unlived, my friend.” – Big Time Rush

Kevin Costner’s Waterworld aside, humans have (yet) to be born with gills or webbed toes. Still, there you are. You’ve spent unmentionable amounts of money. You’ve carved out a whole day, or maybe weeks, away from your to-do list. You’re suited up and look like an alien. You’re on a quest to explore the aquatic world where you’re able to breathe only with a cumbersome apparatus. You’re planning to explore inner space! You’re going to delve into that amazing realm that’s off limits to most people. 

You may look all matter-of-fact, cool as a cucumber, another day at the office, but it’s a thrill, isn’t it? Inside, you’re a kid with butterflies in your tummy who’s getting away with something big and exciting. Okay, it’s true–you and your team are highly trained, your equipment is top-notch, every box is checked off, and you are behaving responsibly. However, you’d have to be in a coma to not realize that what you’re about to do is taking a risk. Who doesn’t know that people have died doing what you’re doing? Answer honestly: How much more exhilarating is the experience when you know it’s not a walk in the park? Our own Michael Menduno admitted that “the feeling of being more alive lasted for days” after a dive.

So, you’re a diver. Does that mean you’re a risk taker? A thrill seeker? A sensation seeker?

Let’s dive into that subject, first by taking a little quiz, shall we? 

Photo courtesy of SJ Alice Bennett

From A Death Wish to Life Is Precious

In the past, too many mental health professionals treated risky behavior like a disease in need of a cure, focusing on the negative side of risk, even using government funding to address risky behavior and stamp it out. 

Before that, Sigmund Freud might have even believed that thrill seekers had a death wish; in fact, it’s what was believed for many years. 

Modern-day science doesn’t support either theory.

“Only those who will risk going too far can possibly find out how far one can go.” – TS Elliot

For our purposes, we’re focusing on the positive aspects of taking chances, pushing boundaries, and seeking experiences that make life feel . . . more alive. Richer. Fuller. We want to examine what goes into the psyche of a person (like you?) who is enthusiastically willing to engage in an activity already identified as dangerous, possibly even by the people who are engaging in it, and hear what some experts on the subject have to say about such people.

Photo courtesy of Jen MacKinnon

The University of Michigan’s Daniel Kruger proposes that taking chances is a fundamental part of human nature going all the way back to our ancient ancestors—prehistoric humans who had to constantly put their safety on the line in their fight for survival. Think fighting off a wooly mammoth with a stick. Kruger believes we have consequently retained many of those same instincts today, and he believes that it’s a good thing. 

This writer, who is related to a major risk-taker, has always believed that heart-quickening experiences are essential for a well-lived life. I’m convinced and have long proposed that those pulse-pounding moments are often accompanied by a deepened understanding of and appreciation for one’s life—perhaps all life. And I’m happy to report that current science confirms that belief.

“If you are not willing to risk the unusual, you will have to settle for the ordinary.” – Jim Rohn

Dr. Kruger is one of the scientists who proposes that taking risks means “seeking that moment when life feels most precious.

This should not be news for you diving adventurers out there.

Nature vs. Nurture: Born That Way or Learned To Love Adventure

Another scientist, Marvin Zuckerberg, affirms the theory that risk taking is in our DNA. “Certain people have high sensation-seeking personalities that demand challenges and seek out environments that most people’s brains are geared to avoid.” I’ll go out on a limb and say that underwater caves or shipwrecks would qualify as environments most would avoid.

Dr. Cynthia Thompson, the researcher behind a 2014 study from the University of British Columbia, was early to look at the genetic factors that might make a person predisposed to participating in extreme sports, ones that are typically defined as activities where death is a real possibility. The results of her study revealed that risk-takers shared a similar genetic constitution, a genetic variant that influences how powerful feelings are during intense situations.

Photo courtesy of Steve Boisvert

Most scientists agree that personality is a complicated mix of genetic and environmental influences. The “nature vs. nurture” dilemma is alive and well. Dr. Thompson concluded that people who engaged in so-called high-risk sports were not impulsive at all, not reckless either. Instead, “they’re highly skilled masters of their discipline who take a very thoughtful approach to their sports.”

A study conducted in 2019 examined human boundaries, people who pushed them to their limits and beyond, and what made those people tick. Zuckerman labeled such people “sensation seekers” and defined them as “people who chase novel, complex, and intense sensations, who love experience for its own sake, and who may take risks to pursue those experiences.” Is that you?

“History is full of risk-takers. In fact, you could say that risk-takers are the ones who get to make history.” – Daniel Kruger

Other experts posit an alternate theory—one proposing that modern society in the age of seatbelts, guardrails, child-proof caps, safety precautions, laws, rules, and regulations has dulled the sense of survival. In other words, life has flattened out and no longer feels exciting, or risky. So, is one of the reasons we seek excitement because of boredom? 

Maslow’s Theory of Self-actualization

I don’t honestly know who was the first proponent of risk-taking being a positive thing, but the work of Abraham Maslow, the founder of humanistic psychology, was one of the first. Maslow became one of the most influential psychologists of the twentieth century, and he developed a theory of human motivation that advocates for “peak experiences.” Peak experiences are not attained without risk.

“One can choose to go back toward safety or forward toward growth. Growth must be chosen again and again; fear must be overcome again and again” – A Maslow

He proposed that, in addition to meeting basic needs, all humans from birth seek fulfillment in terms of what he called self-actualization—finding their purpose/being authentic. Self-actualization involves peak experiences—those life-altering moments that take us outside ourselves, make us feel one with nature, and allow us to experience a sense of wonder and awe. Maslow also believed that those who were able to have such peak experiences tended to seek them out rather than waiting for the next random occurrence. Hence the anticipation of the next dive?

“Do one thing every day that scares you.” – Anonymous

Photo courtesy of Adam Haydock

Out of Your Comfort Zone Into A World of Wonder

Psychologist Eric Brymer from Queenstown University of Technology in Brisbane, Australia, has spent years studying extreme athletes and has this to say: “They’re actually extremely well-prepared, careful, intelligent, and thoughtful athletes with high levels of self-awareness and a deep knowledge of the environment and of the activity.”

Recent research backs up what some extreme sports athletes have been saying for years, even if only to themselves.

“What participants get from extreme sports is deeply transformational, a sense of connecting with a deep sense of self and being authentic, a powerful relationship with the natural world, a sense of freedom,” says Brymer. “They get a strong sense of living life to its fullest as if touching their full potential.”

Brymer’s comments mirror what Maslow, the founder of humanistic psychology, said back in the 1940s.

We’re not advocating for taking stupid chances (such as diving without proper training, or necessary precautions) and we don’t believe anyone reading this article does that. We simply intended to focus on the scientific evidence that supports adventurers—people who get a thrill from an activity that offers—as a bonus; a chance to feel awakened from the mundane and thrust into a world of wonder. 

Risk-takers and sensation- or thrill-seekers chase unique experiences. Often, those experiences bring awareness of important issues or increase essential knowledge about the planet we share. Many people overanalyze and dither when faced with an unfamiliar situation; they shy away from unsettling circumstances. Risk-takers face the unknown and trust themselves to prevail. Learning to scuba dive, for example, pushes people out of their comfort zone, takes them into a realm foreign and mysterious. Diving forces divers to pay complete attention to a task, to focus with laser-like precision in order to conquer misgivings, and to attain a skill that few others have. Confidence comes with accomplishment. Leadership emerges. Fear is overcome. 

Sensation-seekers see potential stressors as challenges to be met rather than threats that might defeat them. With action, resilience develops. High sensation-seekers report lower perceived stress, more positive emotions, and greater life satisfaction. Engaging in extreme activities brings them peace. 

What does it bring you?

Dive Deeper

Bandolier: Risk of dying and sporting activities

National Geographic: What Makes Risk Takers Tempt Fate? Recent research suggests that genetic, environmental, and personal factors can make people take on risky—even potentially fatal—challenges.

Healthday: Taking Risks By Chris Woolston HealthDay Reporter


Pat Jablonski heads up the copy edit team for InDEPTH. She is a blogger, a writer of stories, a retired tutor, English writing teacher, and therapist. She’s a friend, a wife, a proud mother and grandmother. She is also a native of Florida, having spent most of her life in Palm Beach County. She has a B.A. in English from FAU in Boca Raton and an M.S.W. from Barry University in Miami. She learned to swim in the ocean, a place she thinks of as home, but she doesn’t dive.

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