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Everything You wanted To Know About PFOs and Decompression Illness, But Were Too Busy Decompressing to Ask

Though your chances of getting bent are relatively low, your risk increases five to 13-fold in the presence of a patent foramen ovale (PFO), which may enlarge and become problematic with age. Skin bends anyone? Here cardiologist and rebreather instructor Doug Ebersole explains the latest research, whether you should be tested, and what it may mean for your diving.

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by Doug Ebersole M.D.
Header image courtesy of Virginie Papadopoulou. Remaining images courtesy of Doug Ebersole.

Patent foramen ovale (PFO) is an important topic in diving as the appreciation of its relationship to decompression illness (DCI) grows within the community. More than 1200 scuba divers from around the world are affected each year by DCI. Although the incidence of DCI is relatively low, ranging from about 1 episode per 10,000 dives (0.01% per dive) to about 10 episodes per 10,000 dives (0.1% per dive), depending on the nature of the dive, the presence of a PFO is felt to increase the risk five to 13-fold (1, 2, 3). As a result, an understanding of the link between PFO and DCI, as well as various treatment options, is vitally important to divers, and the health professionals who treat them.

Diagram of the human heart showing a patent foramen ovale

Incidence and Anatomy

Autopsy specimen showing a patent foramen ovale

A PFO is an integral part of the normal fetal circulation. Normally, a portion of the blood from the inferior vena cava passes from the right atrium to the left atrium through the PFO during fetal life, bypassing the lungs. At birth, pulmonary blood flow increases greatly, increasing left atrial pressure.  The resulting atrial pressure differences compress the septum primum against the septum secundum, functionally closing the PFO.  Anatomic closure of the PFO occurs later in infancy in most people but is incomplete in approximately 25% of the population (4, 5), leaving these individuals at risk for right to left shunting. 

PFO diameters are quite variable in size ranging from 1-19 mm/0.04-0.75 in, with the average size being larger in older adults (4), suggesting PFOs may continue to enlarge during life. The cause of this is unknown, but in part may be due to known elevations in right heart pressures with aging causing the pressure difference between the left atrium and right atrium (which keeps the PFO closed) to lessen. This may result in “larger” PFOs in older adults. 

The Relationship of PFO to DCI

In 1986, it was first suggested by Wilmhurst and colleagues that a cardiac right to left shunt may be important for a paradoxical gas embolism in scuba divers (6). Subsequently, the importance of PFO for DCI in divers has been further investigated (1,7, 8, 9).  As mentioned above, the risk of DCI in sport divers is quite low but is increased by at least five-fold in the presence of a PFO (1, 2, 3).  Additionally, the average number of ischemic brain lesions as seen on MRI in experienced divers with PFO has been reported to be twice as high as in divers without PFO (11).  The etiology and clinical significance of these findings are unclear but may represent multiple subclinical paradoxical embolic events across the PFO.

The Diagnosis

Both transthoracic echo (TTE), a cardiac ultrasound performed from the chest wall, and transesophageal echo (TEE), a cardiac ultrasound performed from the esophagus, have been used for the diagnosis and assessment of PFO. TTE is considered the preferred diagnostic test of choice as it is noninvasive. However, given its better visualization of the atrial septum, TEE, while more invasive, is much more accurate than TTE and can be used if, despite a negative TTE, there is still a high index of suspicion that the patient has a PFO.  

Including a “bubble study” with the echocardiogram, either TTE or TEE, will increase the likelihood of diagnosing a PFO if it is present. This is done by connecting two syringes of saline with a small amount of air with a stopcock and then “swishing” the two syringes back and forth to “agitate” the saline, making small “microbubbles” that will be seen on ultrasound imaging. Visualization of microbubbles passing from the right to left atrium through the visualized foramen ovale is diagnostic of a PFO.  In clinical practice, the actual site of right-to-left shunting may not be convincingly visualized or recorded for technical reasons.  If the echo demonstrates microbubbles appearing in the left atrium immediately after arriving in the right atrium, then the presence of a PFO can be presumed. If bubbles appear in the left atrium more than five beats after they appear in the right atrium, then the possibility of shunting from another cardiac source (such as an anomalous pulmonary vein) or from a pulmonary source (such as a pulmonary arteriovenous malformations) must be considered.

Of note, while the injection of “agitated saline” is routinely done via an arm vein due to convenience, it has been shown that using a femoral vein in the leg is more accurate (12-13).  

Transesophageal echocardiogram image.  LA = left atrium, IAS = interatrial septum, IVC=inferior vena cava, EV=Eustachian valve, RA=right atrium, RAA=right atrial appendage, SVC=superior vena cave

Therapeutic Options

No specific guidelines exist for PFO closure in people who have decompression illness, but the options are to stop scuba diving, decrease the depth and/or time of dives to limit the inert gas load, or undergo percutaneous PFO closure.  Some divers decide that they have many other interests and diving is not that important to them.  These divers will frequently give up the sport.  

Other divers who enjoy the sport but dive infrequently often opt for diving “conservatively” to limit their bubble-load. This might involve no-decompression diving, limiting depths to less than 30m/100ft, diving nitrox on air profiles, making prolonged safety stops (greater than the recommended 3-5 min) at approximately 4-6m/15-20 ft at the end of their dives, and limiting the number of dives per day to one or two. Tech divers could also opt to dive more conservatively depending on their risk tolerance.  

People who make their living through scuba diving—instructors and divemasters, for example—and tech divers who enjoy more aggressive types of diving such as deep wrecks, cave diving, rebreather diving, and mixed gas diving often elect percutaneous closure of the PFO.  This also holds true for divers who have had recurrent “unexpected” DCI events despite diving conservatively as defined above. 

The types of decompression illness that appear to be associated with PFO include cerebral (stroke-like symptoms), spinal (paralysis or urinary retention), cutaneous (skin bends), and inner ear (vertigo).  DCI manifested by joint pain is felt NOT to be associated and, therefore, should not prompt evaluation for PFO.



Treatment Results

A recent study reported the results of conservative diving practices after an episode of DCI (14).  Eighteen divers in this study had a right-to-left shunt, nine were small and nine were large.  Mean follow-up was 5.3 years (range 0-11 years).  Four of these divers had undergone PFO closure and had no episodes of DCI in follow-up.  The absolute risk of suffering DCI before examination for the remaining 14 divers with right-to-left shunt and no closure was 23.5 DCI events per 10,000 dives for those with a small shunt compared to 71.6 events/10,000 for those with a large shunt.  

After following the recommendations for conservative diving practices, the DCI risk at follow-up fell to 6.0 per 10,000 dives in the small shunt group and zero in divers with the large shunt.  The major limitation to this study is its small sample size, but the results suggest a need for more studies of conservative diving practices for divers with right to left shunts.

When DCI has occurred, especially after so called “undeserved” cases of DCI, divers are often encouraged to seek screening for a shunt and some diving medical societies classify these divers as ineligible to return to diving (15).   There are also several diving medical specialists who recommend that divers with a history of DCI and a positive right-to-left shunt, undergo closure if it turned out to be a PFO, even though there is no clear evidence to indicate that this intervention reduces the risk of DCI or neurologic events (16-19).

However, in a 2011 study of 83 scuba divers with a history of DCI and a follow-up of 5.3 years, 28 divers had no PFO, 25 had a PFO closure, and 30 continued diving with a PFO without closure (20).   At the beginning of the study, there were no significant differences between the groups in the number of dives, dive profiles, diving depth, or cumulative dives to more than 40 meters of salt water (msw).  

Amplatzer Cribriform Occluder

After follow-up, while there were no differences between the groups with respect to minor DCI events, the risk for major DCI was significantly higher in the divers with PFO and no closure than in divers with PFO and closure or divers without PFO.  Although this offers new evidence that PFO closure reduces the risk for major DCI, the authors do not recommend closure in all divers with a history of DCI but rather recommend further studies to confirm these results. 

A recent Divers Alert Network (DAN) funded study from our institution (21) also suggested selected divers with recurrent decompression illness may benefit from PFO closure. Seventy-seven patients with recurrent decompression illness and documented patent foramen ovale were enrolled. Please note this was not a randomized trial. Patients themselves decided whether to have PFO closure or to dive conservatively after the PFO diagnosis was made. This obviously imparts some bias into the trial.  Fifteen patients were excluded for various reasons, leaving 62 patients who were followed prospectively for 5-6 years. 

The baseline demographics which included age, gender, years diving, total number of dives, and number of dives per year were very similar in the two groups as was the number of divers who stopped diving or dived less after suffering decompression illness.  A greater proportion of divers in the “PFO Closure” group had “large” PFOs.  The follow up in the PFO closure group was six years and in the Conservative group was 5.5 years.  

The 42 subjects in the PFO closure group had an incidence of decompression illness of 12.9 episodes per 10,000 dives prior to PFO closure and then had a statistically significant (p<0.05) reduction to 2.7 episodes per 10,000 dives after PFO closure. The 20 participants in the Conservative group had an incidence of decompression illness of 13.4 episodes per 10,000 dives. After 5.5 years of diving conservatively without PFO closure, the incidence of decompression sickness was 3.4 episodes per 10,000 dives, but this did not meet statistical significance given the small number of subjects. 

Percutaneous PFO Closure

The closure procedure for a patent foramen ovale is relatively painless and is done nonsurgically using a needle stick into a femoral vein.  Imaging during the procedure is done with a combination of fluoroscopy and ultrasound imaging, either TEE or intracardiac echo.  The most common device in use in the United States is the Amplatzer PFO Occluder [see photo above].  This is a wire mesh made out of nickel and a titanium alloy. The device is filled with securely sewn polyester fabric to help close the defect.  It is deployed through a small catheter which has been placed across the PFO. The procedure takes about an hour and patients are usually discharged home the same day or the following morning.  

Diagram of  PFO closure procedure

Conclusions and Recommendations

The South Pacific Underwater Medicine Society (SPUMS), the United Kingdom Sports Diving Medical Committee (UKSDMC), and the Undersea and Hyperbaric Medical Society (UHMS) have all weighed in with formal recommendations on patent foramen ovale and decompression illness.  Their recommendations are:

  • A routine screening for PFO at time of dive medical fitness assessment is not necessary
  • Consideration of investigating for PFO should be for divers with:
    • History of DCI with cerebral, spinal, cutaneous or inner ear symptoms
    • Current or past history of migraine with aura
    • History of cryptogenic stroke
    • History of PFO or ASD in first-degree relative
  • If screening is performed: 
    • It should be performed in centers well practiced in the procedure
    • Transthoracic echo (TTE) with agitated saline is the preferred first test
    • Provocative maneuvers (Valsalva, sniff) should be performed
    • In the case of positive tests: A large shunt or unprovoked shunt is associated with certain forms of DCI (cerebral, spinal, inner ear, and cutaneous). Small shunts are associated with a lower but poorly defined risk of DCI 
  • If a PFO is demonstrated, options include:
    • Stop diving
    • Dive more conservatively
    • Close the PFO
  • The diver should not return to diving after PFO closure until satisfactory closure has been confirmed 

My final thoughts:

Should all divers be screened for a PFO?

No. There is approximately a five-fold increased relative-risk of DCI in patients with PFO, but the absolute risk is still quite small

Should all divers with DCI be screened for a PFO?

No. Twenty-five percent of the population has a PFO so one would expect a similar percentage of divers with DCI to have a PFO.  Not all scuba dives have the same risk of DCI. To paraphrase James Carville’s famous quote from the first Clinton presidential campaign, “It’s the bubbles, stupid”. The issue with decompression sickness is the inert gas “bubble load”, not the PFO. However, episodes of DCI in “low-risk” dives (especially neurologic, inner ear, or “skin bends” events) or multiple “undeserved” DCI events should prompt investigation for PFO.

Should all divers with DCI and PFO have a PFO closure?

No. Options for divers with PFO and DCI include discontinuing diving, instituting more conservative diving practices, or PFO closure.  Recommendations should be made on a case-by-case basis based on the DCI event(s), the type of diving being performed by the diver involved, and the risks of PFO closure.  


What does the header image (above) depict?

It is an image of a heart with a PFO. Clinical bubbles were injected in the vein of the person for diagnosing the PFO, you can see that they completely fill the venous chambers (left side of the image), and because there is a PFO a few bubbles can also be seen in the arterial chambers (pointed out by the white arrows – there’s likely a lot more, if you notice the bottom of the right side is brighter compared to the rest of those chambers and that’s actually because some tiny bubbles are crossing through). Note, the “clinical bubbles” I refer to, are agitated saline contrast which are large enough that they are filtered by the lungs and don’t appear in the arterial chambers unless there is a PFO.—V. Papadopoulou

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References:

  1. Wilmshurst, PT, Byrne JC, Webb-Peploe MM. Relation between interatrial shunts and decompression sickness in divers.  Lancet. 1989;334:1302-1306.
  2. Torti SR, Billinger M, Schwerzmann M. Risk of decompression illness among 230 divers in relation to the presence and size of patent foramen ovale. Eur Heart J 2004;25:1014-1020.
  3. Bove AA. Risk of decompression sickness with patent foramen ovale. Undersea Hyperb Med 1998;25:175-8.
  4. Hagan PT, Scholz DG, Edwards WD. Incidence and size of patent foramen ovale during the first 10 decades of life: an autopsy study of 965 normal hearts. Mayo Clin Proc 1984;59:17-20.
  5. Kerut EK, Norfleet WT, Plotnick GD, Giles TD. Patent foramen ovale: a review of associated conditions and the impact of physiological size. J Am Coll Cardiol 2001;38 (3): 613-623.
  6. Wilmhurst PK, Ellis BG, Jenkins BS. Paradoxical gas embolism in a scuba diver with an atrial septal defect. Br Med J (Clin Res Ed) 1986;293:1277.
  7. Moon RE, Camporesi EM, Kisslo JA. Patent foramen ovale and decompression sickness in divers. Lancet 1989;1:513-14.
  8. Germonpre P, Dendale P, Unger P, et al. Patent foramen ovale and decompression sickness in sport divers. J Appl Physiol 1998;84:1622-6.
  9. Germonpre P, Hastir F, Dendale P, et al. Evidence for increasing patency of the patent foramen ovale in divers. Am J Cardiol 2005;95;912-15.
  10. Gempp E, Blattearu J, Stephant E, et al. Relation between right-to-left shunts and spinal cord decompression sickness in divers. Int J Sports Med 2009;30:150-3.
  11. Schwerzmann M, Seiler C, LippE, et al.  Relation between directly detected patent foramen ovale and ischemic brain lesions in sport divers.  Ann Intern Med 2001:134:21-4.
  12. Schuchlenz HW, Weihs W, Hackl E, Rehak P. A large Eustachian valve is a confounder of contrast but not of color Doppler transesophageal echocardiography in detecting a right-to-left shunt across a patent foramen ovale.  Int J Cardiol 2006;109:375-80.
  13. Gin KG, Huckell VF, Pollick C. Femoral vein delivery of contrast medium enhances transthoracic echocardiographic detection of patent foramen ovale. J Am Coll Cardiol 1993;22:1994-2000.
  14. Klingmann, C, Rathmann N, Hausmann D, et al.  Lower risk of decompression sickness after recommendation of conservative decompression practices in divers with and without vascular right-to-left shunt.  Diving and Hyperbaric Medicine 2012;42(3):146-150.
  15. [Swiss Underwater and Hyperbaric Medical Society. Empfehlungen 2007. Der Schwiezerischen Gesellschaft Fur Unterwasser-und Hyperbarmedizin Zum Tauchen Mit Einem Offenen Foramen Ovale][cited 2012 June11]. Available from: http://www.suhms.org/downloads/SUHMS%20PFO%20Flyer%20d.pdf(German
  16. Scott P, Wilson N, Veldtman G. Fracture of a GORE HELEX septal occluder following PFO closure in a diver. Catheter Cardiovasc Interv 2009;73:828-31.
  17. Wahl A, Praz F, Stinimann J, Windecker S, Seiler C, Nedeltchev K, et al. Safety and feasibility of percutaneous closure of patent foramen ovale without intra-procedural echocardiography in 825 patients. Swiss Med Wkly. 2008:138:567-72.
  18. Saguner AM, Wahl A, Praz F, et al. Figulla PFO occluder versus Amplatzer PFO occluder for percutaneous closure of patent foramen ovale. Catheter Cardiovasc Interv 2011;77:709-14.
  19. Furlan AJ, Reisman M, Massaro J, et al. Closure or medical therapy for cryptogenic stroke with patent foramen ovale. N Engl J Med. 2012;366:991-9.
  20. Billinger M, Zbinden R, Mordasini R, et al. Patent foramen ovale closure in recreational divers: effect on decompression illness and ischaemic brain lesions during long-term follow-up. Heart. 2011;97:1932-7.
  21. Anderson G, Ebersole D, Covington D, Denoble PJ. The effectiveness of risk mitigation interventions in divers with persistent (patent) foramen ovale. Diving Hyperb Med 2019 Jun 30;49(2):80-8

Dive Deeper:

InDepth: No Fault DCI? The Story of My Wife’s PFO (12.2019)

InDepth: Undergoing PFO Surgery as a Team: Deana & Bert’s Excellent Adventure  (12.2020)

InDepth: Uncovering the Link Between PFO and Inner Ear DCS (5.2019)

European Heart Journal: European position paper on the management of patients with patent foramen ovale. Part II – Decompression sickness, migraine, arterial deoxygenation syndromes and select high-risk clinical conditions (JAN 2021)


Dr. Douglas Ebersole, MD is a cardiologist specializing in coronary and structural heart interventions at the Watson Clinic LLP in Lakeland, Florida.  He is also an avid technical, cave, and rebreather diver and instructor.  He can be reached at debersole@watsonclinic.com.

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Diving Safety

When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving

Rebreather technology has enabled cave explorers to extend their underwater envelope significantly deeper and longer. As a result, a few teams are pushing beyond the practical limits of open circuit bailout and so have turned to bailout rebreathers. But they are not without challenges, as Tim Blömeke, who dives into the latest research and field experience, explains.

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by Tim Blömeke. Lead image: KUR divers Bob Beckner and Derek Ferguson in the 124m/407 ft deep Mount Doom chamber in Weeki Wachee Spring, Florida, courtesy of Kirill Egorov.

Dual rebreathers are becoming a thing among the elite of extended-range cave diving. Yet the “Blueprint for Survival” for this type of equipment configuration has yet to be written, and practitioners are faced with difficult trade-offs between competing design goals—like fitness for purpose, logistical feasibility, simplicity, reliability, and ease of use, all of which interact with the peculiarities of human nature. A new research paper proposes a pathway for risk assessment. 

The introduction of rebreathers has considerably extended the range of exploration in cave diving. This is true especially for deeper dives, where open circuit technology faces the combined challenges of greater required gas volumes and higher required helium content, which make such dives both difficult to execute logistically due to the sheer number of cylinders involved, and prohibitively expensive due to the amount of helium in each of these cylinders.

By conserving the metabolically inert components of the breathing gas (most notably the helium), the use of closed circuit rebreathers (CCR) eliminates a good chunk of this problem, but not all of it: Traditional CCR diving procedures require that each diver have enough open-circuit bailout gas available to safely end the dive in the event of a rebreather failure. 

Granted, the amount of bailout gas required for a CCR dive is only a fraction of what would be needed to perform the same dive on open circuit, and if all goes well, the bailout gas will never be breathed by anyone and can be reused for future dives. However, bleeding-edge explorers being who they are and doing what they do, after having used their CCRs to push the range of operations a few miles deeper into the cave systems, they began to encounter an issue very similar to the one that prompted the switch to CCR in the first place: cost and logistics. 

As a real-world example, bailing out from a long-distance cave penetration of 7,500 meters at an average diver propulsion vehicle (DPV) travel speed of 40 m/min takes 187 minutes. Assuming a mean ambient pressure of 6 ATA (50 m depth) and a respiratory minute volume (RMV) of 14 l/min, the amount of bailout gas (not including decompression) required to reach the entrance would be 15,708 liters, or more than seven AL80 cylinders filled to 200 bar. This RMV is likely not conservative enough, given the extreme distance and the possibility of a hypercapnic event being the cause of the bailout so, in practice, a safety margin of at about 50% would be added, giving a total of 10-11 AL80 bailout cylinders. 

The required amount of bailout gas became too large to be carried on the person of the diver, so that cylinders again needed to be staged in a series of set-up dives. Preparations for extended range exploration dives became ever more involved, and logistics became just as difficult to manage as those of old-school open circuit dives–even more so, arguably, due to the considerably greater distance of the staging points from the cave exit. As happens so often, overcoming one obstacle resulted in the discovery of others further down the road.

Corroded stage cylinders.

New safety concerns started to appear as well: For large-scale exploration projects, bailout cylinders needed to remain in a cave system for months at a time, sustaining severe corrosion damage at the tank neck and tank valve interface in the process due to the galvanic reaction between the chrome-plated brass valve and the aluminum cylinder. This isn’t merely a hypothetical concern: On many occasions, the corrosion was so severe that the integrity of the seal was compromised, and explorers found their previously staged bailout cylinders empty when checking them on their way into the cave. While this can be counteracted by installing a magnesium anode on the cylinder (magnesium is lower in the Galvanic series than aluminum and replaces the latter in the reaction), explorers found that the countermeasure only mitigates the issue but does not eliminate it. Long story short, for extreme extended-range dives, open circuit bailout was becoming ever more impractical and problematic.

Enter The  Bailout Rebreather

As a solution to these problems, some explorers began to do away with open circuit bailout altogether and carry a redundant rebreather system—a closed circuit rebreather, or a semi-closed rebreather (SCR) instead. While this practice has gained significant traction recently, the concept itself isn’t new. In his book Into the Unknown, famed Welsh explorer Martyn Farr reports that his German colleague, pioneering cave diver Jochen Hasenmayer, had experimented with a dual unit he dubbed the Speleo-Twin Rebreather (STR-80) as early as 1981. 

Bill Stone flying FRED outside the Wakulla Springs decompression habitat. Photo courtesy of the US Deep Caving Team

In 1987, Dr. Bill Stone delivered a proof of concept by spending 24 hours underwater on a dual CCR, he dubbed “Failsafe Rebreather for Exploration Diving” (FRED), during his visionary Wakulla Springs Project 1987. However, it appears that the first person to utilize redundant rebreathers in actual exploration was Olivier Isler from Switzerland. On August 12, 1990, he first used a triple RI2000 semi-closed unit in his crossing of the Emergence du Ressel (Doux de Coly, France), covering a distance of 1850 m/6070 ft at a maximum depth of 81 m/266 ft. The following year, Isler went on to push through the 4000 m/2.5 mi penetration barrier for the first time. More than a decade later, in 2002, Reinhard Buchaly and Michael Waldbrenner pushed the exploration of the Doux de Coly farther using dual RB80s, which were originally designed by Buchaly and continue to be produced to this day by Halcyon.

Bill Stone spent 24 hours underwater on the fully redundant Cis-Lunar Mk 1, “FRED” during the at Wakulla Springs Project 1987. Here is Stone in the Wakulla deco habitat. Photo courtesy of the US Deep Caving Team

The decision to replace open-circuit bailout with a rebreather is as obvious as it is bold: Obvious because it replicates the successful solution to a past problem and restores the ability of a diver to carry all the gas they need on their person. Bold because… well. Put yourself in the drysuit boots of a cave diver, hours and hours away from the surface, who just survived an assassination attempt by a complex piece of life support equipment. All technical aspects aside, wouldn’t it be reassuring to fall back on a less complex piece of life support equipment whose proper functioning can be ascertained reliably within a few seconds? 

Olivier Isler sporting his triple redundant RI2000 SCR at Doux de Coly. Photo courtesy of O. Isler

Expressed in numbers, a paper by Andrew Fock, Analysis of recreational closed-circuit rebreather deaths 1998-2010, published in 2013, analyzed dive accident statistics for the period from 1998 to 2010 and found that CCR diving is associated with an increase in the risk of death by a factor of up to ten compared with open circuit diving. That ratio essentially applied to CCR dives, which used open circuit bailout. Rebreather technology and diving practices certainly have improved since the time under investigation, but the fact still remains that the complexity of the equipment adds to the overall risk.

With this in mind, taking a closer look at and trying to define the specific risks and benefits of replacing open-circuit bailout with a redundant SCR or CCR seems a reasonable idea. And this is precisely what a team of authors headed by Derek B. Covington did in a recent (March 2022) research paper, asking the question, “Is more complex safer in the case of bailout rebreathers for extended range cave diving?” 

Using a qualitative approach, the authors discuss the reasoning behind bailout rebreather use, its history, different configurations and the various advantages and disadvantages and, finally, the additional potential for human error created by increasing the complexity of the equipment.

Olivier Isler at a deco stop in Doux de Coly

Bailout SCR vs. Dual CCRs

In terms of configurations, there are two main choices for a bailout rebreather: SCR or CCR. With an SCR, the diver still has to carry bailout gas. However, an SCR (such as the side-mounted Halcyon RBK) extends the use of this gas by a factor somewhere between four and ten, thereby drastically reducing the number of cylinders needed while being only the size of a single AL80 cylinder itself. Other advantages of a bailout SCR are that its relative simplicity and lack of sensors or other electronics make it much easier to set up, maintain, and use than a secondary CCR.

These advantages, however, do not come without downsides. With an SCR, the diver does not have the option of adding oxygen into the loop, and the actual oxygen content of the gas breathed is always somewhat lower than the oxygen content of the gas in the cylinders carried. How much lower exactly depends on the portion of the gas vented into the water on each operating cycle of the unit—or the rate of fresh gas supply into the unit—as well as the metabolic needs of the diver. 

Therein lies the crux: For normal operation, the amount of oxygen consumed by the diver, and thus the resulting effective composition of the breathing gas, can be calculated quite reliably. In a bailout scenario, however, it isn’t unlikely for the metabolic needs of the diver to be increased due to higher workload. Without sensors to measure PO2, the precise composition of the breathing gas in the SCR loop becomes unknown, creating a risk of hypoxia, with all the potential consequences that come with it. This risk is unique to SCRs and not present when diving open circuit (where the cylinder sticker tells us what we’re breathing) or while on a CCR (where sensors tell us what we’re breathing).

The other approach is to go for a redundant CCR, as Stone envisioned back in 1987. While seemingly the “purest” in concept—replacing like with like—and optimizing redundancy, the added complexity is significant. Everybody who owns a CCR (especially an eCCR) knows that these machines need lots of love to remain in good working condition. Now multiply that by two: twice the number of sensors, two scrubbers, two sets of primary electronics, two sets of secondary electronics … and that’s just out of the water. 

Some open water explorers are also turning to dual rebreathers. Here is RAID president Paul Toomer in blue water wearing a back mount Divesoft Liberty with a sidemount Liberty bailout. Photo courtesy of Kristof Goovaerts.

To have the redundant system available to them at all times during the dive, divers now need to manage the contents of two breathing loops instead of one. Furthermore, in order to be able to provide assistance in the event of a problem, divers working in a team need to be aware of the failure modes of and emergency procedures for not only their own units, but also the units used by their teammates. Unless everybody on the team is using the same machines for primary and bailout, this considerably adds to the training requirements, as well as to the complexity of the decision-making tree in an emergency situation. Nevertheless, by maximally reducing the required amount of gas to be carried by each diver, a redundant CCR theoretically provides the greatest degree of independence and offers the greatest potential range of exploration.

Approaches to Risk Assessment

To date, the use of dual rebreathers is still too rare for a quantitative, empirical assessment of its safety to be practical, and there is no systematic process in place for collecting data on dual-rebreather dives. “It’s really almost impossible to put a number on it,” said researcher and explorer Andy Pitkin, who co-authored the study. “I think there are only a small number of divers in the world who really need a bailout rebreather, but there are probably quite a few who use them because the idea appeals to them more than using OC bailout. Of course, there is no hard dividing line between the two groups. Where does logistical difficulty become impossible? That’s a very subjective judgment.”

The diversity of configurations and procedures used is another obstacle to objective study. “Are we using identical primary and bailout rebreathers, or is one unit specifically designed as a backup? If the latter, should the bailout unit be another CCR or an SCR? If the former, what are the diving procedures? Does the diver switch between loops at regular intervals, analogous to the procedures for independent doubles or sidemount diving? This would arguably add to task loading. Do the units have separate DSVs or a single, shared one, like that used by Richard Harris and Craig Challen of the Wet Mules? If the diver doesn’t alternate between units, then what other procedures are in place to ensure that both loops remain breathable at all times, especially during depth changes? If using dual CCRs, then what is the approach to ensuring redundancy of the diluent and oxygen supplies?”


The Wet Mules dual Megalodon CCR rebreather connected at the Bail Out Valve (BOV). Photo courtesy of Richard Harris.

The number of open questions and the range of possible, viable answers seem endless. Similar to the situation in the early days of cave diving, the book on bailout rebreathers has yet to be written. While many of the timeless principles from Sheck Exley’s famous booklet, Basic Cave Diving: A Blueprint for Survival continue to apply accordingly, there is no broad consensus yet on best practices, no SOP Manual, no standardized configuration, no published training standards for dual rebreather diving by any training agency. People are still working things out for themselves or their teams.

In consideration of these difficulties, and as a starting point for a discussion, the authors of Is more complex safer… propose a generalized approach to assessing the risks of dual-rebreather diving. Rather than delving into the minutiae of the failure modes of each individual diver’s equipment setup and diving procedures, they outline a method for identifying potential error-producing conditions (i.e., opportunities for human operators to make mistakes) based on a theoretical model originating in risk assessment for nuclear power plants: the WITH/TWIN model (Table 1). The acronym WITH stands for Workplace Design, Individual Capabilities, Task Design, Human Nature. TWIN refers to the same items (Task, Workplace, Individual, Nature).

The underlying idea of this approach is to move beyond merely looking at “human error” prima facie—oh, the diver failed to pack his scrubber properly? How could they! They neglected to monitor their PO2? Pay more attention!—and instead, analyze the conditions that are conducive to such errors. For the purposes of the model, a diver’s equipment configuration is part of their Workplace, their training and fitness belong in Individual Capabilities, the mission, including not only managing one’s gear but also navigation, linework, photography/videography, and surveying fall under Task. 

All these aspects interact with Human Nature. We get stressed when things get exciting, we get complacent when things go smoothly. We are prone to false assumptions, we are terrible at intuitive probability assessment, and our ability to pay attention falls off rapidly once the number of items that need our attention increases significantly beyond the number of voices in our heads. Much like running a nuclear power plant, excellence in cave diving isn’t achieved by sporadic strokes of genius but instead by consistently avoiding mistakes, and an important aspect of the design of equipment and procedures for either is to compensate for the inherent weaknesses of the human mind.

In the words of the study’s authors: 

“Divers and explorers need to consider not just the technical aspects of operating the dual CCR as an equipment-based system, but also the socio-technical aspects and error-producing conditions that adding additional complicated equipment has to the wider system, especially when it comes to training for and executing abnormal operations when workout levels will be high and awareness will be reduced. Nonetheless, as the use of this configuration grows, the risks and benefits will become clearer to investigators and divers alike.”

It will be exciting to observe the future development of dual-rebreather diving as it matures and see where the consensus for best practices will end up… stay tuned and stay safe!

References:

Diving and Hyperbaric Medicine: Is more complex safer in the case of bail-out rebreathers for extended range cave diving? Derek B Covington, Charlotte Sadler, Anthony Bielawski, Gareth Lock, Andrew Pitkin

Fock AW. Analysis of recreational closed-circuit rebreather deaths 1998-2010. Diving Hyperb Med. 2013;43(2):78-85.

NSS-CDS (free download): Basic Cave Diving: A Blueprint for Survival by Sheck Exley

Dive Deeper

InDEPTH: The RB80 Semi-closed Rebreather: A Successful Exploration Tool by David Rhea

Halcyon: Using The RB80 As A Side-mounted Bailout Rebreather by Andy Pitkin, Karst Underwater Research (2018)I

InDepth: Rebreather Holiday Shopping Guide (2020)

aquaCORPS Pioneer Interviews: Stoned: Interview With Dr. William Stone (1994) by Michael Menduno

InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties by Michael Menduno and Nuno Gomes

Deep Tech: Victory At Last (1998) by John Simenon: Olivier Isler is setting penetration records with a triple-redundant semi-closed rebreather


Tim Blömeke is a cave, wreck and CCR diver who teaches in Taiwan and the Philippines. You can reach him on Facebook: facebook.com/semantics, and Instagram: www.instagram.com/timblmk/

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