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The Early Days of Technical Trimix Diving

The use of trimix, that is a breathing mix of oxygen, helium and nitrogen has become the standard for dives beyond 30-50m/100-165 ft, depending on who you dive with. However, things were much different when mixed gas technology was just emerging. Here is an in depth look at the early days of what was then called “special mix” diving including an explanation of special mix tables by Dr. RW Bill Hamilton, a report on the first 1500 trimix dives by then aquaCORPS chief Michael Menduno, and a selection of Key West Diver Consortium tables for dives to 76 m/250 ft. From the aquaCORPS archives.

Published

on

 Reports by Michael Menduno and Dr. RW Bill Hamilton

Header image: Jim King and his team including Dustin Clesi and Larry Green prepare for a dive at Diepolder 1993. Photo from the aquaCORPS archives

Stories in this section:

Is The Market Ready for Mix? aquaCORPS Editorial by Michael Menduno, 1992 

Understanding Special Tables: Some Things You Should Know by Dr. R.W. “Bill” Hamilton, 1992

The Trimix Report: A Look at the First 1500-2000 Tech Trimix Dives by Michael Menduno, 1993

Key West Consortium Trimix Tables 

Is The Market Ready For Mix?

by Michael Menduno

This editorial was originally published in aquaCORPS: The Journal for Technical Diving #4 MIX, January 1992

“The best way to predict the future is to invent it. This is the century to be pro­active about the future.” Alan C. Kay, Stanford Computer Forum 1987

There was a time not long ago that “AIR” was rarely thought of as a gas in sport diving circles, much less a technology. Like the old adage regarding a fish’s description of water, “AIR” was simply what we breathed and diving was no exception. Now it seems that the whole concept of “air technology”­—a single gas to handle all expo­sures—may be out-moded before most of us knew enough to call it nitrox. What’s going on?

Author in 1991

The self-contained diving market is in the midst of a technological revolution that is changing the way we think about diving. Similar to the PC in the world of computing, technologies and methods once limited to commercial and military use including: special mixed gases, more reliable decompression methods, diver propulsion vehicles, communications, closed circuit systems, and more, are now being rescaled and applied to sport and special interest diving, driven by the need for increased safety and performance.

Until recently, this community was viewed largely as a single, homogeneous group known as “recreational divers.” Now a more sophisticated view is emerging. To borrow several terms from the computer industry, the consumer diving market is in fact made up of many different user groups, each with its own specific diving environment and applications, and a need for appropriate tools. That’s what makes the technical revolution potentially so profound.

If the benefits of these technologies were limited to a few isolated high-tech divers, their impact would be relatively minor. Instead the tools, methods, and disciplines that are being pioneered today are applicable to a wide range of diving—from recreation to professional—and hold great promise for improving diver safety. The result is a significant opportunity for growth, and with it the need for caution and responsibility.

Conceived in the early 1900s, and developed and used in industrial and military operations over the last fifty years, mix technology offers the capability of reliably supporting divers to depths beyond 2000 fsw. At the same time, if used improperly or irresponsibly, it can be deadly. And that is cause for great concern. Though mix has been a long proven standard among its industrial and military counterparts, the sport diving industry has found challenge enough in teaching divers not to “run out of air,” let alone to manage alternative breathing mixtures. Mix technology may be ready for the market; the real question is, “Is the market ready for mix?”

Like the personal computer of ten years ago, special mix is being successfully utilized today by only a small number of experienced divers representing the vanguard of the field. We still have a long way to go before this technology can be reliably employed by the masses. Safety is the primary concern. Training and education, infor­mation, and a consistent set of operating guidelines are sorely needed at both industry and consumer levels, and there is a good deal of supporting infrastructure that is simply not in place. 

aquaCORPS N4, MIX JAN 1992

With foresight, work, and the willingness to tackle the many hard issues that remain, mix technology, like the PC, will become accessible, reliable and easy to use, to the benefit of divers at all levels. Because, if kept in the closet or ignored, it will surely become a source of harm.

Now is a time for the industry to pull together, and chart a path for the future as we transition from a “fixed mix” to a multi-mix diving environment By being proactive, we stand to usher in a new era of diving, rivaling that initiated by the introduction of open circuit scuba nearly 50 years ago, and in doing so, we can make it a safer and richer activity for generations of divers to come. That’s what this issue of aquaCorps is all about.—M2


  • Buddy Dive Bonaire

Understanding Special Tables: Some Things You Should Know 

by Dr. R.W. “Bill” Hamilton 

This article was originally published in aquaCORPS: The Journal for Technical Diving #4 MIX, January, 1992

Ever since the invention of a reliable means to supply compressed gas to divers, diving operations have been limited by the availability of appropriate decompression tables and methods. Now, with increased interest in alternative breathing mixes, advanced decompression methods, and exposures outside the range of standard air tables, special application tables are becoming an important tool in the high tech diver’s arsenal and are growing in use. 

RW Bill Hamilton

Conceptually, special tables are no different than their public domain counterparts. Each represents a decompression procedure designed to bring a diver back to the surface without incident after carrying out a planned exposure. Where public domain tables span an entire range of dives—for example 0-58 msw/0-190 fsw with bottom times ranging from five to several hundred minutes depending on the depth—and typically involve a single gas mix—usually air—special tables are typically calculated for a specific operation involving a narrow range of exposures, multiple gas mixes, decompression procedures, and custom J-factors (“Jesus” factors added for conservatism). 

Another important difference: Though most public tables have been tested on a significant number of dives, this is not always the case with special tables for which limited diving data may exist—a reason to proceed with caution. Anyone with a computer and computational algorithm can generate a table. The real issue is what the numbers mean. As Dr. Bill Hamilton points out in the article below, special tables are only as good as the experience and judgement behind them. For that reason, decompression engineering is still as much of an art as it is a science. 

“No matter what computational algorithm is used to produce a decompression table, it’s basis is still empirical. What works, works.” —RWH

Currently, special tables are being used by a small minority of technical divers, and there are only a few suppliers. For good reason—planning and conducting dives involving special tables requires a degree of skill, sophistication, and understanding that does not yet exist in the community as a whole. Training, experience, and a supporting infrastructure are required, and the potential for injury is no small matter. For that reason, many professionals in the field remain leery of producing tables for just anyone, particularly in light of the current legal climate in the industry. As special mix diving becomes more accessible, the need for special tables will continue to grow, and with it, the number of reliable suppliers. Until that time, it’s wise to proceed with caution and to work with qualified, experienced individuals who know what they’re doing. As Dr. Hamilton explains, there is a lot more involved than simply cutting a table.—Michael Menduno

John Crea, Submariner Research mixing it up with Dr. Bill at Tek.93

Many of you have asked about the preparation of special application tables. This is a somewhat sensitive subject that raises many issues, and I think it worthwhile to discuss. One issue is whether professional diving physiologists and decompression consultants should support the extreme exposure diving that some people want to do. Another is whether a calculated table is reliable enough to keep the user out of trouble. 

Dr. RW “Bill” Hamilton at tek.93

When I first began working with the technical diving community, I was approached by several divers who told me what they were planning to do, and it appeared they would make up the tables themselves if I could not provide them. Thus blackmailed, and having the humility at least to realize that some of the more experienced high tech divers could easily construct a table as well as I could, I went ahead with the project and generated a set of tables for them (with some trepidation, I might add). Fortunately, it has worked out well, and because of excellent feedback and sufficient repetition, I now am relatively comfortable with the process. Note, I do not view this business as “selling tables.” Rather, it is a professional service provided to clients that includes the generation of tables. As you will see, this involves much more. 

Generating a special or custom table typically begins by finding out something about the client. The individual must be totally comfortable with gas and pressure physics and physiology, with oxygen safety as it applies to both the gas as well as understanding its physiological effects, and gas logistics and decompression methods, including the technical and operational aspects of the type of diving they’re doing.

The important thing to understand is that a reliable decompression table is not based on a formula alone. With today’s knowledge, no matter what computational algorithm is used to produce a decompression table, its basis is still empirical; tables are based on field experience, and (Ed. note: data!) “What works, works.” Although we have sophisticated computational methods—and they are getting better—there is a fair degree of judgement involved in incorporating the experience of yesterday’s dive into tomorrow’s table. For that reason, table development is still something of an art that extends beyond the ability to solve exponential gas loading equations. 

The reason for this is the statistical nature of the process; it takes many, many dives to know anything at all about incidence rates inherent in a set of tables. For example, if an individual does five dives with no problems, this only means that at a 95% confidence level, the tables have a bends incidence [Ed.rate of decompression illness] of less than 50%. It takes literally hundreds of clean dives to be sure the incidence is less than 1%. Even a couple dozen clean dives should not be greeted with too much confidence. This is especially true if many of these are not conducted for the full time and depth. 

A page from Dr. Bill Stone’s book, ” Wakulla Springs Project 1987″. Courtesy of the US Deep Cave Diving Team

To help mitigate this situation, the relatively new technique of applying maximum likelihood analysis to decompression makes it possible to estimate the risk involved with a given table based on a set of previous dives. In addition, guidelines have been developed for the development and validation of new and revised tables, and the ongoing management of existing ones (See “Validation of decompression tables.” Proceedings of the 37th Undersea and Hyperbaric Medical Society Workshop, Schreiner HR, Hamilton RW, editors.  UHMS Publication 74(VAL)1-1-88. Bethesda, MA 1989). Under these guidelines, tables based on documented experience can be introduced at the “operational evaluation” stage if done with sufficient care. 

Figure 1: In 1989, the Undersea and Hyperbaric Medical Society (UHMS) held a workshop on the Validation of Decompression Tables chaired by Heinz Schreiner. This was the diagram that was originally proposed by Dr. Elliott and edited for final print by Drs. Hamilton and Schreiner. While confusing at first glance, represents the importance of designing decompression trials with a set plan for analysis in which each step builds on each other in a “feedback loop.”2 There exists a clear delineation between the concepts of Laboratory research and Operational/Field research which is linked by the continual nature of the loop. Emphasis is also placed on the appointment of a small subset of personnel to oversee the process as a whole in order to ensure the proper safety and integrity of the researchers, participants and the scientific process—Gene Hobbs.

The basic table computational methods used by most decompression consultants are still primarily Haldanian which uses compartments (commonly but inappropriately called “tissues”), exponential gas loading equations, and ascent constraints called M-values. Promising variations include bubble growth or bubble generation factors as well, but these too are usually fed by gas loadings. The Haldane method has its defects but is workable as long as one takes its weaknesses into account and stays sufficiently close to past experience i.e. what worked in the past. Personally, I prefer the “Haldane-Workman-Schreiner” method, which is a considerable advancement over Haldane. 

Jim King at Diepolder cutting some tables

Once a formula is decided upon, it is necessary to consider other factors involved in decompression management such as the range and variation of gas mixtures, particularly oxygen management; oxygen is the key to reliable decompression. The table should take into account the oxygen tolerance limits of the diver, both of their central nervous system (CNS) and “whole body.” The rates of travel and gas mix changes should be included in the table as instructions where they are needed, as well as the accumulated decompression time—or “decom time”—as well as the “stop time” at each stop. (For a discussion of decompression time management, see Decompression Strategies, M. Menduno, Technical Diver 2.2 , Summer 1990).

KWD’s runtime slate for the USS Wilkes Barre

For technical dives, gas switches are usually determined by the specific dive plan. These are, of course, worked out in conjunction with the dive team. Once a basic operational approach is established for a dive, it is usually a relatively simple matter to make adjustments for the many factors involved, such as extensions of depth and time and J-factors, among others. However, to consider all of these, check the process thoroughly, and be sure the user has the right instructions are not trivial matters. For that reason, I feel it’s important to label tables and the corresponding computations with a unique traceable basecase name, and I encourage others to do the same rather than just calling a table the “DCAP 250/30″ or the “Wakulla tables.” That way, the origin of any table or computation can be traced and reviewed, an otherwise time-consuming task that can result in confusion. 

Another important thing that I feel is necessary is to provide the user with instructions on how to use the tables. Although you obviously cannot tell the user everything they need to know in order to perform a sophisticated dive operation requiring special tables, it is important that creators include at least a description of every item on the table and how to use them. Finally, although it’s not something that can always be enforced, I feel it is important that clients agree to provide feedback as to the results of using a set of tables. The data we get from such feedback is valuable and allows us to continue to improve the methodology. 


  • Buddy Dive Bonaire

Generating special tables for a range where the parameters are familiar and there is sufficient available experience typically represents about a day’s work and can run anywhere from US $200-$500 and up depending on the supplier. More exotic tables, requiring the acquisition and analysis of specific data, take a good deal more time and thus can cost a great deal more (often a couple of orders of magnitude more for the development of a full set of commercial or professional tables). Note that the time it takes may bear very little relation to the value of the table, but it does have an impact on the cost. 

Sheck Exley running Dr. X

No table or decompression method is completely “safe.” In fact, I do not even use that word in conjunction with decompression. It’s wrong on several counts. First, no significant dive is free of the risk of decompression sickness. Second, DCS is not an accident; it happens and it will continue to happen as a predictable part of diving. 

The dive team should plan for it, and in doing so can reduce the consequences to as near zero as possible. When DCS does occur, it can and should be treated promptly and adequately and, if this is done, the chance of residual injury is quite small. 

First, no significant dive is free of the risk of decompression sickness. Second, DCS is not an accident; it happens and it will continue to happen as a predictable part of diving. The dive team should plan for it, and in doing so can reduce the consequences to as near zero as possible. 

The real accidents that take divers’ lives are due to operational problems like running out of gas, getting lost or entangled, not pushing a dive computer, or running over a table. In my opinion, this is where some current technical diving practices seem to be operationally inadequate. Technical dives are operations. As such, they should have a leader and/or safety manager, planning, cooperation, and the predetermined ability to cope when any step falls short. 

This is an example of a special table calculated for a 45 minute exposure at a maximum working depth of 76 msw/250 fsw, using a trimix 17/50 (17% O2, 50% He, balance N2) bottom mix  and an intermediate mix of enriched air (EANx36: 36% O2, Balance N2) and oxygen for decompression. The table was generated using Hamilton Research’s DCAP computational program with the Tonawanda II algorithm and the ascent-limiting matrix designated 11F6, without added “conservatism factors.” Rates and gas switches are given in the Comments section, but a more detailed set of instructions should accompany a table of this complexity. Note the individual stop times and cumulative decompression times are included. The diver starts the clock upon leaving the bottom and leaves each stop at the time in the “DECOM TIME” column, and thus doesn’t have to worry about individual stop times. This is an easier way to manage a decompression than timing stops, it involves fewer chances for error, and this method enables a dive to be “reconstructed” at any time. For a dive with a planned bottom time, RUNNING TIME, which starts on leaving the surface, is preferred by some technical divers. See Decompression Strategies, Technical Diver 2.2 for a discussion. Note the cumulative oxygen dosage, measured in oxygen tolerance units (OTUs) is also displayed. 

This means building in redundancy throughout: in equipment, extra gas for contingencies, extra oxygen on board, and ensuring that other team members, especially the boat captain, know what’s going on and what to do if things don’t go as planned. Having understanding and agreements before the fact can go a long way to allay the consequences of a glitch in the operation. 

Technical diving can also be risky. The divers assume this risk, fully and personally, but this does not justify taking unnecessary risks. Tricks like diving on air to 89 m/290 ft are selfish and irresponsible. The diver who does not get away with this may be the only one to die but will not be the only one to suffer. Accidents reflect on all divers and can set progress back in a variety of ways. If you want to keep pushing the envelope, fine, but do it intelligently with care and responsibility. 

The decision to use special tables depends on the specific operational plan for the dive, whether special breathing mixtures or accelerated decompression methods are to be used, and whether or not a general purpose table is applicable. Whatever your decision, make sure that the tables you’re planning to use were generated by a reputable supplier and are, in fact, appropriate for the dives you’re planning to do. It goes without saying that you should have the necessary experience, training, and equipment required for the operation in question. No matter how reliable, no table can make up for lack of judgement or the inability of the diver to carry it out. 

Dive Deeper:

InDepth: Dr. Bill Talks Mix: The late great decompression physiologist, Dr. R. W. “Bill” Hamilton played a unique role in the early development and emergence of technical diving. Listen to “Dr. Bill” and cave diver nurse anesthetist John Crea discuss “special mix” diving at the TEK.95 conference (1995).

aquaCORPS #1 UnderPressure: “Call It High Tech Diving,” by RW Bill Hamilton, 1990

X-Ray: In Memoriam: Joel Silverstein’s The Prince of Gases: RW “Bill Hamilton (1930-2011) in X-Ray magazine.

The Trimix Report 

by Michael Menduno. 

This article was originally published in aquaCORPS: The Journal for Technical Diving #6 COMPUTING, June 1993

The mixing station at Key West Diver, Key West, FL. One of the first tech mixing panels in the world. Photo aquaCORPS archives.

Though trimix is being used successfully by small groups of divers in the U.K., Mexico, France, and Switzerland, most of the current work is being done in America. To date, conservatively 1,500-2,000 open circuit trimix dives have been conducted in the U.S. since 1987 by divers from the wreck, cave, and scientific diving communities. Depths range from about 46-267 msw/150-870 fsw, though the majority of these dives have been conducted in the 61-107 msw/200-350 fsw range with typical bottom times of 20-30 min­utes, ranging up to 60  minutes. Approximately 100 dives have been conduct­ed in the 92-153 msw/300-500 fsw range and only a handful in excess of 153 msw/500 fsw. These have been conducted by several leading cave explorers including Sheck Exley, US; Jim King, US; Claude Toulomdjian, France; and Jochen Hassenmayer, Switzerland.

Technical diving pioneer Capt. Billy Deans, Key West Diver. Photo by Joel Silverstein

Dr. R.W. Bill Hamilton of Hamilton Research Ltd, Tarrytown, NY, has been responsible for developing decompression procedures for a large number of these dives using his Diving Computational Analysis Program (DCAP) computational algorithm. Schedules from John Crea, Submariner Research Ltd., Bainbridge, GA; Randy Bohrer, Underwater Applications Corp, Nashua, NH; Kevin Gurr, Aquatronics, Redding, UK; and other sources were  used as well. To date, the decompression expe­rience has been good. In four reported inci­dents, two were “suspected” decompression illness (DCI) and were success­fully treated with surface oxygen following the dive. Another possible DCI incident appears to have involved minor pain-only symptoms that were resolved without chamber treatment, an option that was refused by the diver. A final, apparently mild, DCI incident occurred following a 20 minute, 92 msw/300 fsw exposure and was treated successfully with a USN Table 5.

With regard to operational safety, three fatali­ties, one “near miss,” and a “blow-up” result­ing in decompression illness occurred during trimix diving operations in the US in the last two years, although these had little to do with  the gas mix itself. The first fatality was due to a freak water reversal in an  underwater cave near Tallahassee, Florida, that resulted in two divers being trapped during a deep mix dive. One member of the team survived. The second fatality occurred on the Andrea Doria, when a diver who had separated from his partner during a wreck penetration run, ran out of gas after exiting, and drowned. Several weeks later, another fatality, when a diver got lost in the wreck after getting sepa­rated from the team’s mainline during a trimix operation and drowned. In another incident, related to mix, a diver suffered a CNS convul­sion during in-water oxygen decompression following a deep mix dive when he lost buoy­ancy control, drifted down, and “dozed off” at 11 msw/35 fsw (PO2=2.1). The diver, a para­medic, had had only two hours of sleep the night before the dive. Managing  to inflate his BCD before convulsing, he floated to the surface, and was rescued by an excellent surface sup­port team and resuscitated. A final incident was due to operational problems during a wreck dive—the diver missed the “down line” on descent, got lost, ran out of gas, and was forced to “blow up” to the surface omitting decompres­sion. He was rescued, evacuated to a chamber and treated success­fully.1

1See “Safety First, An Analysis of Recent Diving Accidents,” by Michael Menduno, technicalDIVER 3.2, 1992, pg.23

Methods, Procedures and Training

Habitat from Wakulla Springs Project 1987. Photo courtesy of the USDCT

From a technological perspective, a focus of many of these pioneer­ing dives has been to develop reliable operational procedures including gas mixes, reliable decompression methods, stag­ing, and adequate diver support. Two groups, led by Capt. Billy Deans of Key West Diver Inc., Key West, Florida; Jim King of Deep Breathing Systems, Sevierville, Tennessee; and several other individuals, including Sheck Exley, have been responsible for much of this effort in the U.S. Their work draws on the earlier work of the Wakulla Project conceived and led by Dr. Bill Stone (“The Wakulla Springs Project,” edited by Dr.William C. Stone, U.S. Deep Cave Diving team , Derwood , MD.1989); the Sullivan Connection project organized by Bill Gavin, Bill Main, and Parker Turner; and Sheck Exley’s pioneering dives. Open water work uti­lizing heliox mixes has also been conducted by Ken Clayton, Washington D.C.; Gary Gentile, Philadelphia, Pennsylvania; and Steve Gatto, Atco, New Jersey.

Key West Diver has developed many of the field methods and procedures for open water operations and has served as a training center for the majority of other technical diving operators and many users in the US and abroad (Presently there are about ten operators/user groups regularly conducting trimix diving operations and offering training—see below). In parallel, Deep Breathing Systems has been responsible for developing many of the operational procedures and equipment for deeper exploration work. In addition, with the help of Canada’s Defence and Civil Institute of Environmental Medicine (DCIEM) Toronto, Canada, Deep Breathing Systems has spearheaded the testing and validation of the decompression methods and tables generated by the DCAP algorithm using Doppler monitoring and other tests designed to measure decompression stress.

WKPP founders, Bill Gavin, Parker Turner, Bill Main and Lamar English. Photo from the GUE archives.

On the training side, the International Association of Nitrox and Technical Divers (IANTD), Miami, Florida, led by Tom Mount, has pioneered the development of user and instructor trimix training programs and cur­rently has a roster of over 20 trimix instructors (see the tek.GUIDE, aquaCorps Journal #5, 1993, for facility and instructor listings). IANTD has made significant progress over the last year in developing training standards, texts, and materials with the help of Dr. Lee Somers, University of Michigan, Ann Arbor, Michigan, and is currently the only company offering formal trimix certification.

[Ed. note—American Nitrox Divers Inc (ANDI), Professional Scuba Instructors Intl (PSAI) Technical Diving International (TDI) followed within two years.]

Explorer Olivier Isler with his redundant semi-closed rebreather in Doux de Coly, France. Photo courtesy of O. Isler
Sheck Exley with Jim Bowden prepping for a deep dive at Zacaton. Photo courtesy of Jim Bowden

IANTD has report­edly just obtained instructor insurance to cover its activities, though rates are not yet available. Island Scuba Center, headquarters of ANDI, has been conducting trimix dives for some time and plans to launch an extended range pro­gram later this year. In addition, there is at least one University trimix diving program at Florida State University that was developed by Gregg Stanton. The British are also involved through IANTD UK, Somerset, England, headed by Rob Palmer, though their efforts to date have focused primarily on enriched air nitrox and some of the early development work on closed circuit rebreathers. There is also some interest in Australia, spearheaded by Rob Cason—an IANTD director—of Fun Dive Centre, Sidney, NSW, and others.

Operating Guidelines 

Lou Sarlo, co-founder of the Gas Station, New Jersey. Photo from the aquaCORPS archives.

Though rudimentary training standards exist, there is as yet no written set of operating guidelines for self-contained trimix diving. However, an emerging set of “community” operational guidelines is developing with some success. (See “Blueprint for Survival Revisited”). With regard to deep div­ing, the emerging “community consensus” is that dives beyond about 62 msw/200 fsw should be conducted on a helium-based mix.

Today, most U.S. dives conducted in the 55-92 msw/180-300 fsw range have standardized around the use of trimix 14-17/X, (14-17% FO2 depending on the depth, FHe=25-60%, maximum working PO2= 1.4-1.45 atm) with an “equivalent narcot­ic depth” (END) ranging from 26-54 msw/85-175 fsw depending on the exposure and application. Though “lean” trimixes with a helium content of 25% or less (an END of 54 m/175 f at 76 msw/250 fsw)—sometimes referred to as “poor person’s mix” or heliair (helium-diluted air) on the basis of their relatively low cost and mixing ease—are some­times used to reduce the risk of O2 toxicity and “take the edge off narcosis,” the general thinking today is to eliminate as much narco­sis as is economically feasible. 

As a result, mixes with at least 50% helium (an END of 26-36 msw/85-118 fsw at depths of 76-92 msw/250-300 fsw) are gaining in popularity and will likely become a community standard. Decompressions for these operations utilize a single enriched air nitrox (EAN) intermediate mix, typically an EAN 32-36, which is breathed beginning at the first or early decompression stops—typical­ly in the 33-40 msw/110-130 fsw range—fol­lowed by oxygen breathing at the 3 and 6 msw/10 and 20 fsw stops. Based on the success of these procedures, a set of provisionally-standardized trimix decompression schedules for dives to 76 msw/250 fsw, known as the “Key West Consortium Tables” (see below), were pre­pared by Key West Diver and Hamilton Research Ltd, to assist in standardizing opera­tions and managing access to tables. 

Polly Tapson’s Lusitania expedition team. Photo from the aquaCORPS archives
Kevin Denlay preparing to dive the USS Atlanta. Photo courtesy of K. Denlay

These tables are unique in that they allow a “variable percentage of helium,” and have been adopted by the majority of technical diving operators in the US, including Enchanted Diver, Floral Park, NY; The Gas Station, Gloucester, NJ; Ocean Odyssey, Santa Cruz, CA; and Scuba Adventures, San Bernardino, CA. Schedules uti­lizing air as an intermediate gas were also developed by Underwater Applications Corp. and used with success where access to sufficient onboard enriched air nitrox supplies is a problem.

For deeper dives in 92-153 msw/300-500 fsw, ENDs are generally set at 46 msw/150 fsw or less, and decompression procedures typically involve air as an intermediate gas beginning as deep as 67-70 msw/220-230 fsw, followed by one or more enriched air interme­diate mixes. For example, a typical procedure is to use an EAN 30 for stops beginning at 46 msw/150 fsw with a switch to EAN 46-50 at the 24-21 msw/80-70 fsw stops. This would be followed by oxygen decompression at 6-9 msw/20-30 fsw. (Ed. note—the community consen­sus is to limit oxygen use to 6 msw/20 fsw or less.)  Doppler testing following these dives has suggested the use of air as a deep intermediate gas reduces decompression stress significantly, though the jolt of “instant” narcosis and cumulative oxygen tol­erance must be managed with care.

Tom Mount and Gary Taylor doing some garage blending. Photo courtesy of G. Taylor.
Jim King and Capt Billy Deans decompressing after diving the USS Saufley at 130m/425 f. Photo by Dan Burton.
Sheck Exley and Nuno Gomes at Busmansgat in 1992. Photo courtesy of Nuno Gomes.
Dr. Ann Kristovich after her record dive at Zacaton. Photo courtesy of Jim Bowden.

In response to the desire to maximize dive time at remote sites—for example, during offshore wreck diving operations—provisional repetitive trimix decompression procedures were prepared by Hamilton Research Ltd. utilizing conventional gas loading analysis. This method is criticized by some experts, but there is no accepted computational algorithm for repetitive diving. Though the potential for being able to employ repetitive diving seems promising and will probably grow as trimix use becomes more widespread within the community, these tables have received only minor usage to date. More data is needed in order to refine the computational method. Work on altitude tables has also been done by Submariner Research Ltd. and Underwater Applications Corporation for specific applications. 

The next boom in decompression management tools will likely come from the growth of “desktop decompression software”—computational algorithms running on a personal computer being pioneered by Corey Bergren of Cybertronix, Knoxville, Tennessee; Sheck Exley, Dr. X, Live Oak, Florida; Kevin Gurr, Aquatronics, Redding, England; and Dan Nafe, Miami, Florida.  


  • Buddy Dive Bonaire

The Operations Approach 

Sheck Exley preparing for a dive at Mante. Courtesy of Mary Ellen Eckhoff and Brian Udoff. 1989

To date, the field experience with trimix has shown that a rigorous “operations approach,” including thorough planning, proper equipment, diver support, compe­tent execution, and the ability to respond promptly to and deal effectively with emergencies, is required to conduct these dives safely. Deep mix dives are by their nature com­plex because of the gas, decompression, and equipment management involved. Much can potentially go wrong. Simply breathing a mix is the easy part. The general consensus seems to be that the community still has a long way to go in improving operational procedures. In this respect, a lot can no doubt be learned from the commercial diving community.

Over the next five years, trimix diving—supported by an “operations approach” and more in depth training—is expected to grow and become more accessible because of its safety advantages. As a result, trim­ix/heliox will likely replace the use of air for open circuit deep diving operations beyond about 55 msw/180 fsw— that is, until reliable, low-cost, closed circuit systems find their way to market. Take another breath!

Key West Consortium Trimix Tables 

Trimix, or as it then was called, “special mix diving,” required tables or a program to generate them. There were no dive computers at the time that calculated deco for mixes other than nitrox. A group of technical divers including Billy Deans, Lou Sarlo, Bart Malone and Jennifer Hunt pitched in to purchase a set of trimix tables from Bill Hamilton for diving depths of 190-250 fsw/58-76 m with varying bottom times, and with helium fractions from 17%-50% They were known as the “Key West Consortium Tables.” Here is a selection for dives to 250 fsw/76 msw with 10-45 minute bottom times and a set of reset tables as well.—M2.

Dive Deeper:

Letter from Capt. Billy Deans to Bart Malone, The Gas Station re: The Use of KWD Consortium Repet Tables


Michael Menduno/M2 is InDepth’s editor-in-chief and an award-winning journalist and technologist who has written about diving and diving technology for more than 30 years. He coined the term “technical diving.” His magazine “aquaCORPS: The Journal for Technical Diving” (1990-1996) helped usher tech diving into mainstream sports diving, and he produced the first tek.Conferences and Rebreather Forums 1.0 & 2.0. In addition to InDepth, Menduno serves as an editor/reporter for DAN Europe’s Alert Diver magazine, a contributing editor for X-Ray mag, and writes for DeeperBlue.com. He is on the board of the Historical Diving Society (USA), and a member of the Rebreather Training Council

Dr. R.W. Bill Hamilton (1930-2011) is a diving physiologist and principal of Hamilton Research Ltd. with over 20 years of decompression management experience in the hyperbaric and aerospace industries.


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DCS

Between the Devil and the Deep and the DCS—My Own

His new book, Between The Devil and The Deep, which was released last summer, delved into the ordeal of British cave instructor Martin Robson, who suffered a life-threatening, deep-water onslaught of DCS at Blue Lake, Russia. But, just after the book was released, Mark Cowan suffered his own debilitating, inner ear hit while diving the SS Wisconsin in Lake Michigan that has left him reeling for answers. Here is the intrepid diver journalist’s first-hand account.

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by Mark Cowan

Mark Cowan in the emergency room in Kenosha, WI, in the hours immediately after the incident, unable to see properly, breathing oxygen and awaiting transfer to the hyperbaric chamber, future uncertain.

I STARED straight into the empty chamber where the sickest of divers went, eyes hopeful as the world spun anti-clockwise around me. I sat in a wheelchair at St Luke’s Medical Center in Milwaukee shortly before midnight on August 7, and listened as Dr. Gerald Godfrey said the treatment for my complex case of decompression sickness would be relatively straightforward.

My world was spinning, literally and metaphorically. Vertiginous feelings had tipped my life on end after I had completed an uneventful summer afternoon dive in Lake Michigan, Wisconsin, eight hours earlier. I was unable to walk unaided, my balance had disappeared, and I couldn’t focus on anything because the horizontal and vertical hold on my vision had vanished.

Earlier that evening, doctors in a hospital emergency room wired me up to a heart monitor and administered fluids, oxygen, and an antihistamine (for nausea) to stabilise me. Then, they transferred me to St. Luke’s—home to one of the earliest hyperbaric medicine programs in the US—for treatment.

Three doctors were waiting for me when I arrived at the hyperbaric department at 11 pm. Three doctors? I guessed they were there because they had not seen anyone like me before; it turns out they had rarely seen divers with decompression sickness.

There are no good “bends” to get, but if there were, mine was not that kind. I had a case of pure or isolated inner ear decompression sickness. The manifestation of inner ear DCS was, in medical terms, incompletely understood and infrequently seen in divers. The small number of studies concerning the issue offered a perplexing series of possible causes.

Examining me, the doctors spoke of neurological deficit and labyrinthine dysfunction. They used names like Sharpened Romberg—a test to measure my balance—and they also used other gestures of their own design to expose the worst in me. Dr. Godfrey, my primary doctor, asked me to follow his index finger with my eyes. The further his finger moved to my left, the more my vision seemed to skip back and forth. He described this condition as “Left-beating nystagmus of the third degree.” “Nystagmus” sounded so disconcerting. (Nystagmus means abnormal eye movement.)

A video had been taken on my cell phone earlier that day by an ER doctor to help me understand what was happening with my vision. Watching the video some days later, I saw my pupils involuntarily bounce back and forth like a rubber ball.

While inner ear DCS is unusual, the therapy was the same as if I had been suffering from joint pain—a US Navy Recompression Treatment Table 6. The procedure was—as accepted medical wisdom goes—the gold standard for DCS, and five hours spent putting me back together again didn’t seem so bad.

Shortly before midnight, I was wheeled into the chamber, and the medic accompanying me closed the door behind us. I leaned back in the chair as the incoming gas hissed into my ears. When the chamber reached the depth of 18 m/60 ft, the medic placed a plastic hood over my head and opened the valve to let oxygen flush into my lungs. I breathed slowly and deeply and waited for my vision to stabilise. 

After 20 minutes, the medic removed the hood, held her hand in front of my face and asked, “Can you follow my finger with your eyes?” As she moved her hand to my left my pupils beat back and forth.

Things will be better next time, I told myself. 

Twenty minutes later, my balance had improved slightly, but the nystagmus was still beating. Next time? The medic called Dr. Godfrey and he watched through the porthole as my pupils still bounced back and forth. 

Recovery was not going well. I had been in the chamber breathing high concentrations of oxygen for an hour under pressure, and right then I couldn’t see any improvement in my condition. 

The author exploring the forecastle deck of the wreck of the SS Wisconsin, off the coast of Kenosha, WI. Image taken by Robert Personen.

The Dive

Earlier that afternoon, as the dive boat powered back to the port and I felt the waves of Lake Michigan wallow inside my head, I didn’t immediately suspect that anything was really wrong. My first thought was that I had early signs of motion sickness. My second thought was I needed to get some air. My third—if I had one—was that this was a sour end to a great day of diving.

Friends and I had spent the afternoon exploring the shipwreck of the SS Wisconsin, 40 m/130 ft down off the coast of Kenosha, Wisconsin. Once we got back to port, we had plans for dinner. There were stories to share and another dive to organise. Apparently, someone had found a way into the engine room. 

The dive had gone as expected. Underwater, I had examined historic cars—including a Hudson Super Six automobile in the stern cargo hold—and I pushed further into the forward hold than I had been before. There, amid the debris, I saw the cargo of radiators stacked on pushcarts alongside furniture, ladies’ shoes, stoves, and rolls of hoses. That was a small slice of American Midwest history, right there in front of me.

The author on the ladder of the dive boat exiting the water after surfacing from the dive. Image taken by Jitka Hanakova.

After 39 minutes on the bottom, I began my ascent. The 17 minutes of decompression I had amassed was nothing onerous. After completing all my deco stops, and with my two computers clear, I surfaced and climbed up the ladder back onto the boat, happy. The skipper had taken a photograph of me minutes after I exited the water. There I was, sat on the dive bench, still wearing my rebreather, with a smile on my face.  

Mark Cowan, second right, with friends taken five minutes after surfacing following the dive. Image taken by Jitka Hanakova.

But, as I sat there smiling, trouble was already bubbling inside my head. I couldn’t feel it, but that trouble was working from the inside out. An hour after the photograph was taken, it boiled over. 

The first thing to go was control of my stomach. As the boat arrived in Kenosha Harbor and approached the quayside, I hung over the side and vomited. 

The next thing I lost was my balance. Stepping off the boat, I set off toward my car. The ground pitched and pulled in odd directions beneath my feet like a carnival funhouse floor trick. I see-sawed across the car park, and my head felt fuzzy, intoxicated. To anyone watching, I probably looked like a drunk, too. 

Then, my sight went haywire. Suddenly, I couldn’t tell left from right, up from down. 

I dropped to the ground beside my vehicle.

“I think I’m in trouble,” I said to my buddy Robert Personen as he carried his bail-out cylinders from the boat. “Can I have your O2?”

As much as I didn’t want to admit it, I knew I was suffering every diver’s worst fear. I’d even written a book, Between the Devil and the Deep, about decompression sickness, published just three weeks earlier, and all the research I had done left me with no doubt about what was happening to me, and nothing about the situation was good. 

In the chilly afternoon hours of August 7, I sat on the rain-soaked asphalt, propped against my car, sucking oxygen from the tank like my very life—and everything I had devoted almost 20 years to—depended on it. As the deep yanked at my sensory system like a gremlin inside my head, I had only one thought: What’s going to happen to me?

Between the Devil and the Deep Redux

WHEN I collaborated with Martin Robson to write about his battle with decompression sickness for Between the Devil and the Deep, I had no idea what it took to overcome a potentially life-changing injury. I’d managed to get through more than four decades without spending a single night in a hospital. It’s not that I hadn’t fallen out of trees as a child, or tumbled down a flight of 13 concrete steps that scarred my back, or sliced my hand open deep enough to see the bone, or cracked my head open on a garden step. It’s just that I seemed to have a durability that kept me on my feet.

So, on Monday morning, when I was wheeled into the private hospital room assigned to me by people who seemed to suspect a lack of response to treatment, I was defeated by hope and expectation. I’d fooled myself into thinking one treatment would put me back together again, but the ground beneath my feet still felt like it was trying to shake me down, and my vision remained unstable. I felt overcome with passivity as a nurse ordered breakfast for me, as a porter pushed me in a wheelchair between hospital departments for tests, and as I slept through my MRI scan. 

The author in his hospital room in Milwaukee, WI, on the morning of August 8, an hour after exiting the chamber following the completion of his first treatment.
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I was wheeled back in the chamber on Monday afternoon for a second treatment using US Navy Treatment Table 9. Table 9 was introduced in 1999 to provide a dosing protocol for cases of incomplete resolution of DCS. I was taken down to the equivalent of 13.5 m/45 ft and told I would be given three 30-minute sessions of oxygen breathing.

Inside the chamber, there was nothing to do but read or watch television and I couldn’t do either. So, I closed my eyes, and my mind wandered. I thought about the SS Wisconsin and how much more there was to explore. I thought of everything I enjoyed about being underwater. I thought of the dive of that day. I accepted what had happened—as much as I could—but I still couldn’t quite believe where I was. 

At the end of my first treatment, I had talked through the dive with the doctors. Scrolling through the log on my dive computer, I looked for a catastrophic event that could have explained the severity of my injury. I wanted to find a mistake—something I had overlooked underwater, something I could point the finger of blame at and say: Cowan, you idiot, look what you did!” I wanted to find something that said I didn’t have an undiagnosed medical condition that could change my life forever. There was nothing there.

During my third dose of oxygen, my vision suddenly stabilised. There was no warning, no gradual resolution; just an absolute change one would get from flicking off the power switch. One minute, I couldn’t focus, and the next, I could. Dennis Quaid appeared on the TV screen at the end of the chamber. I couldn’t hear him over the hiss of oxygen coming into my hood, but I could read the subtitles. The movie was Flight of the Phoenix and I tuned in just as Quaid was making his escape from the Gobi Desert on a makeshift aircraft rebuilt from the wreckage of his crashed plane.

The positive development inspired a new attitude toward my treatment, one I copied from Martin Robson. There is a passage in our book that resonated with me. It reads, “Like everything he did in life, Robson dealt with the here and now, what was in front of him. There was no point in trying to tackle the whole thing at once. After being ambushed underwater, he’d focused on what needed to be done to survive. One step at a time. Make it through one day at a time and figure out how to survive the next day when it was time.”  

As I surfaced at the end of my second chamber treatment, I insisted on walking out of the chamber unaided. 

Back in my room, I retrieved a notebook from the table beside my bed and, just like Robson, I wrote down everything I could remember about my condition. Then, just like Robson, I began to exercise. I paced the floor of the room. I repeated the test with my eyes closed. I tried walking heel-to-toe, as drivers must do during a roadside sobriety check, and I wrote down the results in the notebook (16 steps, wobbly). I stood on my left leg and timed how long I could hold my balance (20 seconds). I switched to my left leg (16 seconds). I switched back and tried to balance with my eyes closed (extremely difficult). I did it all again an hour later and noted the results. I assessed the stability of my vision and wrote that down: Looking to the right, stable, peripheral vision to the left blurred. 

That evening, my wife Alison arrived at the hospital with fresh clothes and my toothbrush. She had been in the UK to celebrate her mother’s 80th birthday and was at the airport hotel on her way home when a friend called her.

“Mark’s okay, but he is in the emergency room,” he told her. “He had a problem after a dive.” 

“Is he conscious?” she asked him.

“Yes.” 

Alison couldn’t sleep after that and had an anxious wait as she flew back into Chicago. For 18 hours, she had no idea what was happening to me, but she knew it probably wasn’t good. When Alison finally walked into the room, I tried to give the impression I was okay, but I couldn’t fool her. I looked hurt, tired, and vulnerable. 

Seeing her was the best part of my day, though. For the first time, I could think about something else as she told me about the surprise trip to see family and how she sat there just holding her mother’s hand after three years apart. Then we talked about where she could get some dinner, and we talked about health insurance. Eventually, we talked about the incident. 

“I’m not going to ask if you plan to stop diving,” she said.

I appreciated her concern, but right then I didn’t know if I had a say in that.

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Facing the Worst

I WAS discharged from the hospital after four days and five chamber treatments totalling more than 15 hours. I left feeling better than I had when I arrived, but I was not fully healed. The nystagmus had disappeared in my right eye; in my left eye it was “very slight.” Doctors decided there would be no more chamber treatments for me. My condition had plateaued. There was nothing more my doctors could do. Recovering, whatever that looked like, was up to me.

I went home. During the day, I sat on the sofa and watched television until Alison finished work. There was not much else I could do because going for a walk along the lakefront near my home strained my eyes and my head. I felt like I had just emerged from a bar after a heavy night; my body moved at one speed while my head lagged a second behind. The world seemed to race past me, but I was in limbo.

I had left the hospital with one number ringing in my ears. It was not the number on the insurance bill. That did hurt, but not as much as the figure on page 20 of the International Consensus Standards for Commercial Diving and Underwater Operations the doctors handed to me. More precisely, the number on the sixth row of table 2.4.10, “Return to Duty after Diving Related Incidents.” According to the table, I had suffered a “Neurological injury needing several treatment tables to resolve,” and that meant I had to wait four to six months before a return to diving. Beneath the table was a kicker: “Persistent neurological deficits following diving-related incidents are generally disqualifying.”

The details contained in the table sat heavily on my mind and made me angry as I stumbled about at home. It felt good to be angry, to vent, to cry, because if I wasn’t angry I would have to face my fears: the fear that the neurological deficit might be persistent, the fear that bubble in my inner ear was going to be the last word on my diving, that the cornerstone of my identity could be taken from me. 

“I’ll be okay, I’m always okay,” I said to my wife each time I went diving. Well, I couldn’t say that now, because I wasn’t okay, and I was scared. Scared of what I may have done to my life; scared because I was too young to have put limits on myself. I was physically fitter than I had ever been, I was racing triathlons, and I was excited by the possibilities of exploration offered by my rebreather. I had big plans for the next decade. Would I now not get a chance to dive the WindiateKamloops, or the Norman? To dive caves again? To test myself in another triathlon?

What a gnawing sense of waste. What a shocking spell of self-pity.

The feelings came from the same place—the scarcity of information. My doctors had turned to the standards for commercial divers because there was little research relating to the recreational diving world to assist them. There was little in the medical literature on inner ear bends at all.

Inner Ear DCS?

Physician Andrew H. Smith was arguably the first to describe the symptoms of inner ear DCS in 1873 when he noted both extreme deafness and vestibular problems in caisson workers building the Brooklyn Bridge. After that, however, the condition was infrequently recorded as a discrete clinical entity. It wasn’t until the 1990s that the problem was reported in sport divers breathing compressed air. 

Even then, studies suggested inner ear decompression sickness (IEDCS) is a low-incidence event. According to one report, IEDCS accounted for approximately 0.005% of cases. Another estimated the rate at close to 3%. One report found most victims were injured following dives which had pushed no-decompression limits, omitted decompression stops, or violated ascent rates. Another found the potential for isolated events to occur randomly during otherwise uneventful deep technical dives that had gone according to plan. 

Then there was the contradictory information about the causal factors on IEDCS. “The biophysical basis for this selective vulnerability of the inner ear to DCS has not been established,” one report stated. Another report suggested the inner ear offered the potential for considerable supersaturation, and therefore possible bubble formation, during the initial phase of a conventional decompression. The environment allowed bubbles to grow until they eventually obstructed the labyrinthine artery. Since this artery was relatively small, there was a low probability for a bubble to enter it, another report indicated. Further studies, however, found a possible link between IEDCS and Patent Foramen Ovale (PFO), a hole in the heart which can allow gas bubbles to shunt from the arterial system into the venous system. 

Nothing about an inner ear decompression sickness seemed clear, which revealed that the scientific community had a lot of ideas but not much definite information. I was confused and needed to go in search of answers for myself.

Aftermath Analysis

NINE WEEKS after suffering DCS, I was strapped into a chair in a pitch-black chamber at Aurora Physical Therapy, Neurotology & Audiology in Milwaukee. A set of infrared video goggles sat heavily on my head. The chair rotated back and forth at varying speeds, and the cameras in the headset recorded my eye movement. The test examined the components of the vestibular system all the way to the brain stem. It measured my vestibulo-ocular reflex—how my eyes and vestibular system interacted— and kept my visual field in focus while moving my head.

Mark Cowan seated in the rotary chair and wearing the infrared video goggles as he competes the first of four tests to examine his vestibular system.

The rotary chair was one of several overlapping and complementary tests I had agreed to because I wanted to quantify the scale of the damage caused by the bend, and because I wanted to address the concerns of my doctor. I had fully recovered, but he was concerned about the dangers of my return to diving if my vestibular system had not fully healed (I might “suffer vertigo, lose my mouthpiece, and drown,” he said). And, he was concerned I might suffer a second bend in the fully functioning side of the vestibular system (I could suffer vertigo). He seemed particularly concerned that I would return to diving at all.

After the rotary chair had finished spinning and the results were collated, I moved to another seat and put on another pair of goggles. The video head impulse test examined the three semicircular canals in each inner ear. I was first asked to focus on a dot drawn on a sticky note placed on the wall in front of me. The audiologist stood behind me and he jerked my head in different directions. The video goggles captured my eye movement and analysed the time it took to return to the dot on the wall. I donned another set of goggles for the caloric test, which involved the blowing of hot and cold air into my ear to test for dizziness. Then electrodes were placed on my cheeks and neck and measured muscle response as loud sounds were played into my ear for something called the Vestibular Evoked Myogenic Potential Test which examined the upper and lower branches of the inner ear. 

Once the tests were completed and the results were analysed, I met with my consultant, Dr. Aaron Benson. He was surprised by the findings. I think he was expecting to see global labyrinthine defects. Instead, he told me that almost everything was normal. Tests on my vestibulo-ocular reflex found just one area of minor deficit which revealed itself when my head was jerked over my right shoulder. That finding was confirmed by one of the other tests and indicated the possible site of my bend. 

“What’s neat about you,” Dr. Benson said, “is that I can tell you exactly where [your initial insult] localises to a very specific area; it’s your right horizontal semicircular canal. That’s where your deficit was.”

“Functionally, you have demonstrated resolution of the initial insult. The question then is: ‘Why there?’ It is hard to say. It is possible you had a little nitrogen bubble right there that caused you all this mischief. That really does speak to the randomness of this.”

While the tests pinpointed the spot where I was hit, they revealed nothing of what had caused the incident. “Don’t ask too many questions,” one friend told me. “It will drive you nuts.” I couldn’t help myself, though, I needed to know. 

I sent a download of my dive to Martin Parker, managing director of AP Diving, with the hope he could spot something. “It looks like a benign dive,” he emailed back.

He asked several questions. Did I use a heated vest? No. Did I do any gas switches? No. Was I dehydrated? Not that thought. Fatigued? Possibly, I’d had a long bicycle ride the day before. When I came off the bottom, did I have to swim up or was I neutrally buoyant? Neutrally buoyant.

The possibility of a PFO was raised again and the details of another medical paper, this one from 2017, were shared with me. The study reported a total of 62 divers with DCS. In all cases, divers were tested for PFO and 29 were found to have one. The highest prevalence was found in divers with cutaneous and vestibular DCS—my bend. It was suggested I get checked for a PFO to be on the safe side.

I couldn’t help but think that if I had a PFO, I would have been aware of it before now. Over the course of almost 20 years of diving, I had completed more challenging technical dives in more difficult conditions than I was subjected to when I suffered my injury. However, I was told that a PFO might not be an issue on every dive. That was why many divers could complete many deep, long decompression dives without incident only to one day get a bend on a moderate dive.

So, now I’ve been referred to a cardiologist to have a transesophageal echocardiograph to test for a PFO, and we will see what that means for my future. If the examination does not find anything, I will continue to be at a loss to explain the incident and the cause of the bend will remain a bit of a mystery. I hate mysteries, I’m never satisfied with a mystery; there’s always a reason, I just need to find it.

You can find Mark and Martin’s book here, “Between The Devil And The Deep.”

Dive Deeper

Alert Diver: May I Bend Your Ear? (2015) by Michael Menduno

PubMed: Inner ear decompression sickness in sport compressed-air diving (2001) by Nachum Z, et al.

J Appl Physio: Biophysical basis for inner ear decompression sickness (2003) by David Doolette and Simon Mitchell

J Appl Physio: Selective vulnerability of the inner ear to decompression sickness in divers with right-to-left shunt: the role of tissue gas supersaturation (2009) by Simon Mitchell and David Doolette

DAN: PFO and Inner Ear DCS (2014) by Petar  Denoble, MD, D.Sc.

PubMed: Pathophysiology of inner ear decompression sickness: potential role of the persistent foramen ovale (2015) by Simon Mitchell and David Doolette

InDEPTH: Everything You Wanted To Know About PFOs and Decompression Illness, But Were Too Busy Decompressing to Ask (2021) by Doug Ebersole M.D. 


Mark Cowan is a journalist with over two decades’ experience in newspapers and television. He spent twelve years on the police beat covering the war on crime for a series of newspapers in Birmingham, UK, and reported on the peace-keeping operations in war-torn Kosovo while embedded with the British Army. He has worked on a number of documentaries, including the BAFTA-winning Gun Number 6 which was inspired by his original reporting on the realities of gun crime in the UK. He has been a diver for 20 years, is a PADI Master Scuba Diver Trainer, trained to use a rebreather in 2012, and learned to cave dive while researching and writing the book Between the Devil and the Deep, One Man’s Battle to Beat the Bends with co-author Martin Robson. He is an avid wreck diver and is now based in Chicago, Illinois.

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