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Understanding Oxygen Toxicity: Part 1 – Looking Back

In this first of a two-part series, Diver Alert Network’s Reilly Fogarty examines the research that has led to our current working understanding of oxygen toxicity. He presents the history of oxygen toxicity research, our current toxicity models, the external risk factors we now understand, and what the future of this research will look like. Mind your PO2s!

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By Reilly Fogarty

Header photo courtesy of DAN

Oxygen toxicity is a controversial subject among researchers and an intimidating one for many divers. From the heyday of the “voodoo gas” debates in the early 1990s to the cursory introduction to oxygen-induced seizure evolution that most divers receive in dive courses, the manifestations of prolonged or severe hyperoxia can often seem like a mysterious source of danger. 

Although oxygen can do great harm, its appropriate use can extend divers’ limits and improve the treatment of injured divers. The limits of human exposure are tumultuous, often far greater than theorized, but occasionally–and unpredictably–far less.  

Discussions of oxygen toxicity refer primarily to two specific manifestations of symptoms: those affecting the central nervous system (CNS) and those affecting the pulmonary system. Both are correlated (by different models) to exposure to elevated partial pressure of oxygen (PO2). CNS toxicity causes symptoms such as vertigo, twitching, sensations of abnormality, visual or acoustic hallucinations, and convulsions. Pulmonary toxicity primarily results in irritation of the airway and lungs and decline in lung function that can lead to alveolar damage and, ultimately, loss of function. 

The multitude of reactions that takes place in the human body, combined with external risk factors, physiological differences, and differences in application, can make the type and severity of reactions to hyperoxia hugely variable. Combine this with a body of research that has not advanced much since 1986, a small cadre of researchers who study these effects as they pertain to diving, and an even smaller group who perform research available to the public, and efforts to get a better understanding of oxygen toxicity can become an exercise in frustration. 

Piecing together a working understanding involves recognizing where the research began, understanding oxygen toxicity (and model risk for it) now, and considering the factors that make modeling difficult and increase the risk. This article is the first in a two-part series. It will cover the history of oxygen toxicity research, our current models, the external risk factors we understand now, and what the future of this research will look like. 

Early Research

After oxygen was discovered by Carl Scheele in 1772, it took just under a century for researchers to discover that, while the gas is necessary for critical physiological functions, it can be lethal in some environments. The first recorded research on this dates back to 1865, when French physiologist Paul Bert noted that “oxygen at a certain elevation of pressure, becomes formidable, often deadly, for all animal life” (Shykoff, 2019). Just 34 years later, James Lorrain Smith was working with John Scott Haldane in Belfast, researching respiratory physiology, when he noted that oxygen at “up to 41 percent of an atmosphere” was well-tolerated by mice, but at twice that pressure mouse mortality reached 50 percent, and at three times that pressure it was uniformly fatal (Hedley-White, 2008). 

Interest in oxygen exposure up to this point was largely medical in nature. Researchers were physiologists and physicians working to understand the mechanics of oxygen metabolism and the treatment of various conditions. World War II and the advent of modern oxygen rebreathers brought the gas into the sights of the military, with both Allied and Axis forces researching the effects of oxygen on divers. Chris Lambertsen developed the Lambertsen Amphibious Respiratory Unit (LARU), a self-contained rebreather system using oxygen and a CO2 absorbent to extend the abilities of U.S. Army soldiers, and personally survived four recorded oxygen-induced seizures. 

Kenneth Donald, a British physician, began work in 1942 to investigate cases of loss of consciousness reported by British Royal Navy divers using similar devices. In approximately 2,000 trials, Donald experimented with PO2 exposures of 1.8 to 3.7 bar, noting that the dangers of oxygen toxicity were “far greater than was previously realized … making diving on pure oxygen below 25 feet of sea water a hazardous gamble” (Shykoff, 2019). While this marked the beginning of the body of research that resembles what we reference now, Donald also noted that “the variation of symptoms even in the same individual, and at times their complete absence before convulsions, constitute[d] a grave menace to the independent oxygen-diver” (Shykoff, 2019). He made note not just of the toxic nature of oxygen but also the enormous variability in symptom onset, even in the same diver from day to day. 

The U.S. Navy Experimental Diving Unit (NEDU), among other groups in the United States and elsewhere, worked to expand that understanding with multiple decades-long studies. These studies looked at CNS toxicity in: immersed subjects with a PO2 of less than 1.8 from 1947 to 1986; pulmonary toxicity (immersed, with a PO2 of 1.3 to 1.6 bar, and dry from 1.6 to 2 bar) from 2000 to 2015; and whole-body effects of long exposures at a PO2 of 1.3 from 2008 until this year. 

The Duke Center for Hyperbaric Medicine and Environmental Physiology, the University of Pennsylvania, and numerous other groups have performed concurrent studies on similar topics, with the trend being a focus on understanding how and why divers experience oxygen toxicity symptoms and what the safe limits of oxygen exposure are. Those limits have markedly decreased from their initial proposals, with Butler and Thalmann proposing a limit of 240 minutes on oxygen at or above 25 ft/8 m and 80 minutes at 30 ft/9 m, to the modern recommendation of no greater than 45 minutes at a PO2 of 1.6 (the PO2 of pure oxygen at 20 ft/6 m). 

Between 1935 and 1986, dozens of studies were performed looking at oxygen toxicity in various facets, with exposures both mild and moderate, in chambers both wet and dry. After 1986, these original hyperbaric studies almost universally ended, and the bulk of research we have to work with comes from before 1986. For the most part, research after this time has been extrapolated from previously recorded data, and, until very recently, lack of funding and industry direction coupled with risk and logistical concerns have hampered original studies from expanding our understanding of oxygen toxicity. 

Primary Toxicity Models

What we’re left with are three primary models to predict the effects of both CNS and pulmonary oxygen toxicity. Two models originate in papers published by researchers working out of the Naval Medical Research Institute in Bethesda, Maryland, in 1995 (Harabin et al., 1993, 1995), and one in 2003 from the Israel Naval Medical Institute in Haifa (Arieli, 2003). The Harabin papers propose two models, one of which fits the risk of oxygen toxicity to an exponential model that links the risk of symptom development to partial pressure, time of exposure, and depth (Harabin et al., 1993). The other uses an autocatalytic model to perform a similar risk estimate on a model that includes periodic exposure decreases (time spent at a lower PO2). The Arieli model focuses on many of the same variables but attempts to add the effects of metabolic rate and CO2 to the risk prediction. Each of these three models appears to fit the raw data well but fails when compared to data sets in which external factors were controlled.  


Comparison of predicted and recorded oxygen toxicity incidents by proposed model (Shykoff, 2019).

The culmination of all this work and modeling is that we now have a reasonable understanding of a few things. First, CNS toxicity is rare at low PO2, so modeling is difficult but risk is similarly low. Second, most current models overestimate risk above a PO2 of 1.7 (Shykoff, 2019). This does not mean that high partial pressures of oxygen are without risk (experience has shown that they do pose significant risk), but the models cannot accurately predict that risk. Finally, although we cannot directly estimate risk based on the data we currently have, most applications should limit PO2 to less than 1.7 bar (Shykoff, 2019).  

NOAA Oxygen Exposure Limits  (NOAA Diving Manual, 2001).

For the majority of divers, the National Oceanic and Atmospheric Administration’s (NOAA) oxygen exposure recommendations remain a conservative and well-respected choice for consideration of limitations. The research we do have appears to show that these exposure limits are safe in the majority of applications, and despite the controversy over risk modeling and variability in symptom evolution, planning dives using relatively conservative exposures such as those found in the NOAA table provides some measure of safety. 

The crux of the issue in understanding oxygen toxicity appears to be the lack of a definitive mechanism for the contributing factors that play into risk predictions. There is an enormous variability of response to hyperoxia among individuals–even between the same individuals on different days. There are multiple potential pathways for injury and distinct differences between moderate and high PO2 exposures, and the extent of injuries and changes in the body are both difficult to measure and not yet fully understood. 

Interested in the factors that play into oxygen toxicity risk and what the future of this research holds? We’ll cover that and more in the second part of this article in next month’s edition of InDepth.

Additional Resources:

  1. Shykoff, B. (2019). Oxygen Toxicity: Existing models, existing data. Presented during EUBS 2019 proceedings.
  2. Hedley-Whyte, John. (2008). Pulmonary Oxygen Toxicity: Investigation and Mentoring. The Ulster Medical Journal 77(1): 39-42.
  3. Harabin, A. L., Survanshi, S. S., & Homer, L. D. (1995, May). A model for predicting central nervous system oxygen toxicity from hyperbaric oxygen exposures in humans
  4. Harabin, A. L., Survanshi, S. S. (1993). A statistical analysis of recent naval experimental diving unit (NEDU) single-depth human exposures to 100% oxygen at pressure. Retrieved from https://apps.dtic.mil/dtic/tr/fulltext/u2/a273488.pdf
  5. Arieli, R. (2003, June). Model of CNS O2 toxicity in complex dives with varied metabolic rates and inspired CO2 levels
  6. NOAA Diving Manual. (2001). 

Two Fun (Math) Things:

CALCULATOR FOR ESTIMATING THE RISK OF PULMONARY OXYGEN TOXICITY by Dr. Barbara Shykoff

The Theoretical Diver: Calculating Oxygen CNS toxicity


Reilly Fogarty is a team leader for risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.

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Part Three: Bubble-wise, pound-foolish. Are deep stops dangerous?

In part three, of this four-part series on the history and development of GUE’s decompression protocols, GUE founder and president, Jarrod Jablonski begins by asking the question, “Are Deep Stops Dangerous?” He then goes on to discuss the early rise and experience with bubble models VPM, VPM-B and RGBM in the late 1990s, and the subsequent impact on the tech community, when the U.S. Navy Experimental Diving Unit (NEDU) released their deep stop study in July 2011. Their unambiguous conclusions? The redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives! Additional work followed calling deep stops into question. Jablonski assess what these all mean for decompression procedures, and presents GUE’s current approach. Feel free to DIVE IN and share your thoughts.

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by Jarrod Jablonski

Header photo from the GUE Archives. WKPP dives in Wakulla.

Have you read Part One and Part Two?

Through the 1990s and early 2000s, technical diving was becoming progressively more popular, and the exploits of explorers were being reported around the world. There were numerous reports of greatly adjusted decompression profiles, many of them attributed to the benefit of deep stops. During this period, any mention of dissolved gas algorithms was tantamount to talking about a flat earth. It seemed self-evident to so many people that controlling bubbles just made sense. How could all of those technical divers be talking about adjusting their decompression profiles if there wasn’t something to this deep stop/bubble control concept? Were they safely reducing decompression time, just getting lucky, or perhaps exaggerating their success, whether intentionally or not? 

Questions like this encouraged decompression enthusiasts Erik Baker and Erik Maiken to work with researchers like David Yount to refine Yount’s Varying Permeability Model (VPM), extending the early concepts to support bubble management during repetitive, mixed-gas decompression diving. During one planning session, Baker demonstrated to me that VPM mirrored the type of shortened decompression that was eerily similar to the schedules our team had evolved organically. Over time, the early enthusiasm for this new model gave way to more realistic appraisals. Short dives that formed the base of “typical” tech diving–around 75 m/250 ft for about 30 minutes–were resulting in very short VPM decompressions. In fact, the output looked troublesome, causing Global Underwater Explorers (GUE) to delay implementation of VPM for more than a year. When GUE did include VPM, guidance was codified in GUE’s Standards and Procedures document and required that all profiles continue to refer to the original Buhlmann as the reference standard. That requirement remains to this day.

Problems with VPM became more frequent, and decompression sickness (DCS) was being reported somewhat regularly, even among die-hard enthusiasts. Ironically, a calculation error relating to Boyle’s law was discovered, and VPM was adjusted and re-released as VPM-B; although most favored calling it VPM since it was intended as a replacement. VPM was included in a variety of decompression programs and competed with Bruce Wienke’s Reduced Gradient Bubble Model (RGBM). Over time, enthusiasm for the new bubble models eased somewhat with many divers picking and choosing depending upon a given dive. People still tended to believe in the idea of bubble models, albeit with a more cautious view of the application. 

WKPP dives in Wakulla. Photo from the GUE Archives.

Extrapolating theoretical bubble dynamics into real-world application is complex but also deeply intriguing. It also encourages divers to ask if such a paradigm shift might illuminate a deeper truth about the mechanisms at work. Physiologist Brian Hills (1934–2006) became deeply intrigued by the idea, being at least partly inspired by observing pearl divers’ successful decompression in one-third the time presented in commonly accepted U.S. Navy tables. This reduction in time was similar to the claims of some technical divers who also believed the result was influenced by their control of developing bubbles. Both the pearl and tech diver “results” require a great deal of context, which we will save for a more detailed review. These results may well foreground both a flawed process and a unique insight.  

Hills commented that: 

“Haldane’s calculation method did not say the same thing as the equations he used to formulate diving tables. Haldane and subsequent Naval tables were based upon the axiom that the bends-free diver must be bubble-free. This is demonstrated qualitatively by the diver who develops a case of the bends during ascent. Now knowing that he has bubbles, you would move him deeper as a treatment. On the other hand, if those bubbles had not become manifest as the bends, you would continue to take him shallower, assuming that he was bubble-free.”

Given the complexity, the early difficulty of modeling bubbles was probably to be expected. There are numerous variables involved in developing an effective bubble model. We might speak about micro bubbles that grow from seeds, and where we strive to limit a bubble’s critical radius and the critical volume of allowable bubbles. In making these assessments, modelers must work from lab experiments which strive to determine and then extrapolate what actually happens in the body. Even if they manage to get all the particulars correct, they still remain unclear about how a given bubble may or may not impact a diver. For example, where does the bubble go, and how does this create symptoms? Are the impacts from bubbles mostly or exclusively related to where they come to rest, i.e., when they stop and block blood flow and/or impinge upon a nerve and cause pain? Or, do bubbles cause problems by their presence, signaling the body’s immune response and resulting in collateral symptoms? Even a perfect model of bubble development might fail to develop consistent and useful decompression tables.

The uncertainty revolving around bubble models was nothing new in the technical diving world. Recall that many divers had been regularly modifying their own profiles for years with little certainty. Most divers seemed to believe bubble models had value, albeit more carefully structured than early assessments might have prompted. 

Planning World Record Dives

The shot that rang across the technical diving community

In July 2011, the Navy Experimental Diving Unit (NEDU) released a deep stop study with the unambiguous conclusion that “REDISTRIBUTION OF DECOMPRESSION STOP TIME FROM SHALLOW TO DEEP STOPS INCREASES INCIDENCE OF DECOMPRESSION SICKNESS IN AIR DECOMPRESSION DIVES.” 

There it was, in black and white for all to read. Deep stops not only did not help but they also actually INCREASED the risk of decompression problems. Lest one imagine the issue settled, the protests began almost immediately. The NEDU study did not model the type of decompression used by technical divers, most notably by forgoing oxygen-rich mixes as part of the decompression. Others complained about the use of air, an abomination in some tech circles, and enough for some divers to immediately discount the study. Still others disliked the ascent profile, which placed 44 minutes of decompression between 21 m/70 ft and 15 m/50 ft alone. Some argued that even a conservative use of gradient factors with deep stops would only result in 13 minutes over the same range, leading some to argue that such on-gassing would naturally outstrip any value to deep stops that are excessively long.

Jarrod Jablonski and George Irvine at the last stop in a Wakulla Springs habitat. Photo from the GUE archives.

The researchers conducting the NEDU study are exceptionally bright, capable, and well-informed experts. They had excellent reasons for the choices they made, and these have been well defended in various media, perhaps most eloquently in online discussion forums by Dr. David Doolette and Dr. Simon Mitchell , two giants in the fields of hyperbaric research and treatment, respectively. Both men are among the world’s foremost experts in their respective fields, and anyone of reasonable sense would carefully consider an opinion that challenges their conclusions. 

My intent here is not to argue for or against the NEDU study, or even deep stops in general. I will leave that discussion for later, and the final determination belongs to those for whom it is relevant. For now, I hope to summarize the particulars and leave the reader to review the substantial body of information available. What we can say now is that it is unlikely anyone study could convince those that perceive years of success with a given approach. For some, the NEDU study was missing many critical details. One can certainly argue that deep stop efficacy should be independent of these details, but technical divers care less about that aspect than they care more about using deep stops in conjunction with their normal practices. The NEDU study argues, many would say compellingly, that deep stops are not beneficial. We know that because a larger share of divers in the study developed problems while using deep stops than not using these stops. In fact, the research is more compelling given the number and severity of DCS cases. Moreover, there are additional studies that support the NEDU conclusion, while there have not been any studies that support the value of deep stops. 

The NEDU study appears to be empirically rational and logistically consistent, though deeply unsatisfying, at least for some. With so many differences between the NEDU study and typical technical profiles, resistance was to be expected. Change can be unsettling, and no doubt some of the resistance can be explained by this discomfort, something known as cognitive dissonance among those that like to label such things. Even a casual review of the discussion forums illustrates the emotional attachment we have to long-standing ideas: Some are gleeful about the news, enjoying the chance to deride those that followed this path, which is an understandable backlash to the uber-confidence of some deep stop advocates. Others are angry, blaming others for duping them, apparently absent a sense of personal responsibility. Between the growing anti-deep movement and the declining pro-deep camps resides a mostly cautious base, with some patient experts helping to channel the discussions. Despite a few unhelpful personal insults, we can be broadly impressed with the ways in which technical divers are processing this new information. Ideally, we would learn from the largely unsubstantiated rush into deep stops enacting a measured exit strategy, especially while managing a few other pesky details relating to the use of deep stops. For example, how should a person convinced by the science against deep stops treat fellow divers? How about the dive buddy relating anxiety over the change? What about those experiencing DCS symptoms in deeper water? The onset of these symptoms can include pain, numbness, and neurological problems. There is a known risk in deeper and longer dives, and the frequency is enough to encourage a standard requiring that surface-supplied dives in excess of 91 m/300 ft be conducted as saturation-only in the U.S. Navy, among others. [Ed. note: U.S.Coast Guard regulations require that commercial diving jobs deeper than 91m/300 ft be conducted using saturation diving. Meanwhile, many clients such as BP and Shell Global mandate saturation diving below as little 37m/120 ft]. 

George Irvine before a Wakulla Springs exploration dive. Photo from the GUE archives.

It is problematic to suggest that divers experiencing DCS symptoms at depth ascend. Meanwhile, some of these divers have established protocols, including some version of deep stops, that they believe help manage the problems they are confronting. Put simply, what do you say while in the water and managing a diver that reports decompression problems in deep water during the ascent? Do you tell them to have faith in the balance of currently developing research? More broadly, how should individuals, teams, and organizations manage the variety of competing strategies within their community? In considering this problem, we find unexpected complexity, even reaching beyond the relative simplicity of the deep stop vs. no-deep-stop debate.

In this video, GUE President Jarrod Jablonski discusses historical and practical decompression aspects with GUE Explorers Mario Arena and Richard Lundgren.

Are we in a post-deep-stop world?

During the late 1990s and early 2000s, deep stops experienced great popularity, but almost as quickly as they appeared, they become a black sheep in many circles. That a community can so quickly embrace and then reject an idea is, in many ways, a positive feature of rational humans. True science, when done well, represents the best of this ideal because it takes almost nothing for certain. One develops a hypothesis, tests rigorously, and informs upon that hypothesis. Other researchers hopefully pursue a similar effort, and, over time, we gain confidence in a given idea or we do not. Even longstanding ideas are not technically settled, even though the overwhelming weight of evidence supports that hypothesis. 

When considering details regarding decompression or deep stops or any of the variety of the semi-common modifications in the technical diving community, we should maintain some balance in our view. While the rush into deep stops exemplifies the desire of the technical diving community to push past historical barriers, the enthusiasm was likely too hasty, given the lack of evidence and clarity in execution. This kind of initiative, for better or for worse, defines our species. Now some are pushing to accelerate ascents from depth. While this may well be the correct approach, we should manage the transition with a bit more foresight than was previously demonstrated. There are still many unanswered questions in the search to better understand decompression problems.

For the moment, it can be said that deep stops likely do not represent a clear value in accelerating one’s decompression and that they may actually present problems. It is obvious that the deep stop profiles, such as the one tested at NEDU, are not useful and can be dangerous. There are many compelling arguments that these results are directly correlated to the lack of utility in deep stops themselves, and other studies support this view. These individuals are sometimes frustrated by what they perceive as an outdated and unsupported view of evolving decompression science. At the same time, a research study that tests deep stops in a way that appears to be totally removed from their practical use is bound to elicit suspicion. Regardless of our personal conviction for or against an idea like deep stops, we should take the experience of our peers into consideration.

This backdrop of uncertainty requires some accommodation on two primary fronts. First, those engaged in technical dives must weigh the available evidence and make an informed decision about the best way forward. Second, one should respect the experience and choices of those with whom they choose to dive. There are no certainties in decompression, and the divers actually in the water doing the decompression maintain the ultimate responsibility for an associated plan. For GUE, and others, these aspects require careful balancing. 

GUE was founded and is managed by leading explorers, regularly conducting aggressive diving projects where lengthy exposure can become a notable liability. Changing weather, thermal problems, or other developments can force a diver to get out of the water as quickly as possible. Meanwhile, GUE is a training organization and maintains the need to establish a conservative approach in support of new technical divers. These new divers must determine, through experience, their individual susceptibility to decompression sickness. All divers should begin this process slowly, adjusting toward more aggressive profiles only if it makes sense based upon need and experience. However, in most cases, it will not make sense for divers to manipulate their decompression to be more aggressive. 

Jarrod Jablonski after a dive in Wakulla Springs during the 1990s. Photo from the GUE archives.

Evaluation of what constitutes an aggressive profile is a big part of what gradient factor methodology hoped to illuminate. To what extent that goal was realized remains an open question, but the use of gradient factors remains extremely common for both deep-stop and anti-deep-stop groups. Those favoring a move away from deep stops favor more aggressive ascents with higher gradients. Meanwhile, some divers resist rapid adjustment to what they perceive has been working. GUE policies regarding gradient factor strive to balance these factors while leaving the ultimate decision in the hands of experienced teams. It is very reasonable to act with consideration to prevailing research but we should also remember that most of the details remain unclear, leaving each diver with a burden to determine the best course of action. I would like to assert that these choices look more significant than they are in most cases, as I will detail in later sections. For now, we might ask if using low gradients can be dangerous , which is related but somewhat different from the removal of deep stops.

A deep stop profile may or may not be less efficient in terms of ascent time, but should its inclusion be strongly resisted? How about during an ascent where divers are experiencing or have experienced problems? If you feel greatly disadvantaged in terms of efficiency, then I would like to create some context. A typical dive to 45m/150 ft for 30 minutes while using a gradient of 20/85 produces a total of two minutes more decompression as compared to a 60/85 profile. Of course, the “problem” with low gradients becomes much more relevant with deeper depths and/or longer bottom times. In this case, adding deeper stops might result in a growing disparity between the total decompression times though this largely depends upon the model and safety factors utilized. Yet, these longer profiles are not conducted by new tech divers or students, and modifications to long and deep profiles ultimately rest with experienced divers making these choices for themselves. 

Jarrod Jablonski and Bill Main in 1992. Photo from the GUE archives.

As an organization founded by explorers and with wide-ranging expeditions conducted annually, GUE has always provided notable latitude to experienced divers but has also guided new divers toward conservative decompression exposure. Our experience over 30+ years demonstrates that a diversity of decompression profiles can be “successful.” Yet, we should always push to better define what success looks like. That question,  along with some of the more aggressive experiments in our community, highlight an interesting, if not blasphemous, possibility: Are we making progress toward understanding the underlying issues guiding decompression, or are we merely accumulating data? If we are making progress, do we appreciate the nuances enough to properly contextualize the outcomes? 

It is clear that advances in decompression knowledge have been significant, and that most individuals can dive with relative assurance that they will not become injured. I do not intend to suggest otherwise. However, I would like to ask if we are certain enough that we should push others toward our own beliefs. Are the details too vague for there to be the best solution that works for everyone? I hope you will join us for the final, part four of our series where we explore some unique, often under-discussed aspects of decompression development. 

Note: I hope the reader is able to appreciate my intent in this writing. I am not pushing any agenda, save the idea that open dialogue and respect for the experiences and reports of others is an important part of evolving practices. GUE is strongly committed to standard practices, although an often unappreciated aspect of this commitment is the understanding that some adjustment is natural. The idea is not to create a rigid, unthinking policy but a set of common tools, useful in large part because of their standardization within a community. Those standards can and do evolve, although they should not be changed carelessly unless a meaningful value is established. 

Tell us what you think. Should the industry immediately abandon all forms of deep stops? How hard should we push resistant dive buddies? How should we manage those experiencing problems during ascent but finding resolution with the inclusion of deep stops? We welcome your thoughts and want to hear about your experiences. 

Additional Resources

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.


Jarrod is an avid explorer, researcher, author, and instructor who teaches and dives in oceans and caves around the world. Trained as a geologist, Jarrod is the founder and president of GUE and CEO of Halcyon and Extreme Exposure while remaining active in conservation, exploration, and filming projects worldwide. His explorations regularly place him in the most remote locations in the world, including numerous world record cave dives with total immersions near 30 hours. Jarrod is also an author with dozens of publications, including three books.

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Planning World Record Dives

Bubble models and the deep stops that resulted were very popular through the early 2000’s though problems with these models became more and more evident with divers reporting a growing number of problems with decompression sickness. Early imprecision in the development of VPM even required development of a replacement algorithm, originally known as VPM-B. Meanwhile, it appeared progressively more likely that whatever benefit might be obtained from use of deep stops required more precision than had become the norm. This was the backdrop the author confronted when the second of my long-time dive buddies’ retired. Our Woodville Karst Plain Project (WKPP) team would restart exploration activity after a long pause due to unfavorable conditions. 

The new exploration dives would be conducted with Casey Mckinlay and we set out to reconsider all aspects in preparation for a new round of world record cave dives. We were expecting dives of at least 12 hours at depths up to 90 m/300 ft and in-water times of nearly 30 hours. During previous dives, I had seen a modest frequency of low-level symptoms among our small group. For example, pain during ascent that resolved, reappeared and resolved again and did not persist at the surface. Another complaint related to minor pain in the joints, described by some as being like very fine crushed glass. And of course, there had been some incidents over the many years of aggressive exploration and these coming dives would be longer those ever done before. Should we use bubble or dissolved gas models? What about deep stops? 

In the broader community, a variety of deep stop strategies were used but most sought to reduce gas pressure below the maximum value (M-value) by some margin, usually by conducting the first stops around 75% of the maximum depth of the dive. One aspect that can vary greatly relates to the time spent in deep water which, in many cases, was not tracked carefully. I felt divers had become progressively sloppier about the execution of deep stops. Being able to relax after hauling large payloads of equipment encouraged divers to stay longer and not properly account for the time spent. GUE had already moved to better clarity with ascent protocols, instituting a “run time” approach with deep stops and a “stop time” approach for middle and shallow stops, effectively encouraging their divers to move efficiently in deep water and not to overstay missed time from deeper stops. In considering an approach for the upcoming WKPP dives, I proposed elimination of the deepest of our decompression stages and efficient movement to more shallow stops at 57 m/190 ft. We still engaged in what one can call deep stops, albeit with less emphasis and greater attention to detail. 

Ultimately our teams successfully conducted many dozens of very aggressive dives with numerous setup, exploration and cleanup dives toward the discovery of a connection between Wakulla Springs and the Leon Sinks cave system in 2004. The longest of these dives involved more than 12 hours at depths approaching 90m/300 feet, requiring nearly 30 hours total immersion. We had used similar procedures to those in use over many years and changes to our use of deep stops might not appear significant, but they had marked a subtle though persistent reappraisal of deep stop methodology which continues to this day.

Dr. Doolette is an active technical diver whose extensive research experience and role among the principle investigators at NEDU make his contributions to any discussion particularly relevant and useful.

Personal Profile

Dr. Mitchell is a technical diver and gracious contributor to diving conferences and public forums. His general knowledge and experience treating decompression sickness are highly valuable contributions to the industry and this discussion of deep stops.
Personal Profile

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.

US Navy Manual 14-4.19 Decompression Sickness in the Water. Decompression sickness may develop in the water during surface-supplied diving. This possibility is one of the principal reasons for limiting dives to 90m/300 fsw and allowing exceptional exposures only under emergency circumstances. The symptoms of decompression sickness may be joint pain or more serious manifestations such as numbness, loss of muscular function, or vertigo.

USN procedures for in-water onset of DCS include recompressing up to 20 fsw and lengthening stops.

USN Diving Manual, Rev 7 Change A, Ch 15-12.7.1

During the early 2000’s, GUE developed a reference standard based upon Buhlmann algorithms. The intent was to ensure divers reference profiles with the most successful history. From this base, consideration for team control and unity during ascent, as well as the potential utility of bubble control were considered. Balancing these factors resulted in a gradient factor of 20/85. These settings will not only result in deeper stops but will also account for these stops with additional decompression time.

Current research challenges the value of deep stops, suggesting they may be less efficient. GUE has been slow to adjust parameters for dives conducted during training because the relatively short profiles of students and their need to gain proficiency with a controlled ascent speaks against the value of faster ascents and/or shorter total decompression time. Balancing the experience in our community, while considering the most useful priorities for students, supports a deeper gradient than might be otherwise encouraged by developing research.

GUE protocols maintain a 20/85 reference gradient for training dives where the priority is ascent training and team refinement and where a slight increase in additional decompression time is not problematic. As divers gain experience. they are free to adjust gradients in a way that is suitable to the team while considering personal experience, team preference, mission objectives, and evolving research.

Low gradients such as 20/85 should not be confused with studies like that done at NEDU. The NEDU research greatly delayed the ascent, adding 3.4 x’s the stop time in the first three stops alone, as compared to a similar 20/85 profile. More importantly, the NEDU study did not account for the additional decompression time these low-gradient stops develop because it was testing whether the value of deep stops in controlling bubbles was enough to overcome the increased on-gassing at these deeper depths. Yet, a 20/85 profile is very different, since it will increase the decompression time as a consequence of the lower gradient at depth while actually “insisting” upon a relatively low compartment pressure upon surfacing. One can certainly argue that these deep stops are not useful or that they delayed the ascent unnecessarily, but it is difficult to argue that they are more dangerous, unless the diver ignores the resulting shallow decompression time. The problem with low gradients is mainly an issue of decompression time, with risk accruing when divers add deep stops while ignoring the consequences of that gas loading. This reduced time was the hope and advertised benefit with deep stops, but shortening shallow time is not necessary with the inclusion of deep stops.