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Did You Analyze Your Gas?

Italian tech instructor Eugenio Mongelli, founder of TEMC DE-OX, which makes digital gas analyzers, shares a story of a combat diver who went hypoxic on an oxygen rebreather and why it always pays to analyze your breathing gas; even when you’re filling your cylinder with 100% oxygen!



A Case of Hypoxia on an Oxygen Rebreather

by Eugenio Mongelli

In 2019, I was a member of an international diver team that was testing the most up-to-date military tactical equipment, including special dry suits, rebreathers, hi-tech ballistic protection, helmets, night vision systems, military Advanced Diving Vehicles (ADVs also known as diver propulsion vehicles), and many other real toys you see in action movies. 

One afternoon, a military diver decided to dive a light configuration with a fully closed oxygen rebreather, dry suit, dive computer, and standard accessories. The pre-dive check was smooth and successful, the oxygen cylinders fully filled, the soda lime filters newly replaced with new sofnolime. The atmosphere was of a fun, relaxed dive as when diving with a group of friends. 

The lake was calm, water temperature was 15ºC, and crystal-clear visibility—very good ambient conditions for that kind of dive. I was supporting the team by providing assistance. We began the dive with no problems, the pre-dive check was successful, and we performed a correct lung circuit purge for keeping air out. All the divers were maintaining a steady breathing rate, the rebreather lungs were regularly expanding, the oxygen was being added manually, and max depth was kept at 5 m/16 ft with the help of a line connected to a buoy. 

After twenty minutes, one team member started demonstrating signs of discomfort, as if he was in distress. He suddenly emerged and then lost consciousness. His face was blue, and he was not breathing. We immediately brought him to the lake shore removing his drysuit (actually cut it open)  and started CPR. Thankfully,we were able to revive him. One characteristic of the full oxygen rebreather is that if a diver has an accident diving with it, they tend to either die, or they survive with no permanent damage. There’s nothing in between. Full stop.

This diver luckily survived.  He was flown by helicopter to the nearest hospital for a medical screening where he spent the night. He was back with us the next day, smiling. But after the accident, we wanted to understand what caused the problem. I surmised it most likely was a caustic cocktail that had occurred because water entered the soda lime filter, affecting his lungs and causing loss of consciousness. There was no other logical reason, as the diver was trained in using oxygen rebreathers,which requires special attention.

Eugenio Mongelli at the event

The team felt some urgency to investigate in order to determine the cause of the incident and took the following steps:  First, we opened the rebreather and removed the canister, but we saw no sign of water flooding inside. The soda lime was dry, as were  the breathing lung and the mouthpiece with corrugates. Then, we speculated that the incident could have been caused by carbon dioxide poisoning, but the soda lime had not  been used before and had recently been replaced. Next, we wondered if it  might have been carbon monoxide poisoning, and because of this concern, we connected the oxygen cylinder to a DE-OX multigas analyzer. That’s when we discovered the frightening reality:  no carbon monoxide, no carbon dioxide, no helium, but only 80% oxygen. It seemed impossible. 

How could decanting certified pure oxygen from a big cylinder to a smaller cylinder lead to only 80% oxygen? The oxygen sensor must be at fault, we thought, so I replaced the analyzers with two other oxygen analyzers. The verdict was the same: 80% oxygen. The conclusion was that our diver had suffered from hypoxia when the partial pressure of oxygen went below the necessary amount for life support.  When he emerged, the oxygen partial pressure went even lower, causing him to lose consciousness.

If that was the cause, we were determined to discover how that could have happened. We tested the big oxygen cylinder, and it was 100% oxygen. Then I proposed that it could have been the booster used for pumping oxygen into the small cylinder. As the booster was driven by low-pressure compressed air, it could have resulted in a mixing of air with oxygen if compressed air leaked into the oxygen pumping chamber. Though unable to be certain, after checking all other options, we concluded that this was the most likely possibility.

TEMC DE-OX Easy O2 Analyser

How could a diver go hypoxic by breathing a nitrox gas mix with 80% oxygen? It’s a result of the nature of an oxygen rebreather, which has no O2 sensors. Oxygen is added to the loop by means of an add-valve that can be operated manually or automatically based on hydrostatic pressure. So, at a constant depth, the valve would add nitrox 80 in proportion to the amount of 100% O2 consumed by the diver and the CO2 consumed by the sorb. As a result, the FO2 and therefore the PO2 in the counterlung and loop would steadily decrease (the loop would gradually fill with nitrogen) until the gas became hypoxic. 

After experiencing this event, I definitely keep pushing all divers to analyze their gas cylinder, regardless of whatever dive and gas mix they are going to use. Gas analysis is not sexy, but it can save your life! 

Ed. Note: GUE Standard Operating Procedures: A diver should personally analyze & label all their tanks on the day of the dive.

Eugenio Mongelli was born in Rome, Italy, in 1965 and has been deeply attracted by the sea, diving, and scuba gear technology since childhood. Having learned to scuba dive in 1982, he first dived a closed oxygen rebreather in 1984 and then began using electronic closed-circuit rebreathers from the mid-1990s. Mongelli became a diving instructor in 1991 and is currently a free diving and scuba diving master instructor, and a rebreather and trimix instructor with FIPSAS, CMAS and TDI. 

Mongelli was shocked to discover aquaCORPS magazine in the early ‘90s, which opened his mind to new horizons in diving that have become his life path. A doctor in management engineering and a former naval officer, he founded the TEMC DE-OX company in 1996 with the intent of providing digital gas analyzers specifically designed for both sport divers as well as the military and commercial diving communities. Since then, he closely follows the development of technical diving in all aspects of training, learning, designing new gear, and has been an active part of the global community.  


Undergoing PFO Surgery as a Team: Deana & Bert’s Excellent Adventure

People like to give GUE a hard time for their uncompromising focus on team diving. But a pair of divers from GUE Seattle has taken it to a new level: getting their PFOs fixed together. The team that bends together, mends together? Instructor and tech diver James D. Fraser willingly tells the tale.




by James D. Fraser
Header photo courtesy of Dr. Doug Ebersole

This is the follow-up to the story that ran in InDepth December, 2019: No Fault DCI? The Story of My Wife’s PFO

It has been a year since my wife Deana had a decompression illness (DCI) hit in Bonaire requiring her to do a Table 5 recompression profile in a hyperbaric chamber. At the time of my previous article’s publication, Deana had a Transthoracic Echocardiogram (TTE) bubble study and found out she did have a small to moderate patent foramen ovale (PFO). Two physicians offered similar options for Deana to consider when it came to her diving activities: 

  • Stop diving, as this eliminates any risk of DCI in the future.  
  • Modify her dive profiles to be more conservative: diving only once per day, diving nitrox 32 using air tables, and/or extending her decompression profiles and safety stops.
  • Have the PFO repaired, knowing it is not a guarantee, and continue diving as conservatively as possible. 

Deana had initially decided to wait on doing a PFO closure until after our daughter’s wedding in March 2020, but she realized very quickly that being “conservative” was not in her nature. Deana had already returned to diving within 12 days of her hyperbaric chamber ride. In the 46 days since her treatment, Deana had already done another 15 dives to depths of 90 feet; being conservative really was proving to not be an option for her. Diving was just too much a part of her life. 

Deana with Dr. Ebersole. Photo by D. Ebersole.

In mid-November, Deana reached out to cardiologist and tech diving instructor Dr. Doug Ebersole for a second opinion on the bubble study and his advice about her options. Dr. Ebersole gave Deana the same response as the other physicians; but, knowing Deana and her passion for diving, he suggested that she have her PFO fixed, since her plan was to continue diving. 

Deana also spent time talking to other divers who had been diagnosed with PFOs—some who had them repaired and some who had decided against it—in order to get a more complete picture from both a patient and a doctor point of view. One of the final conversations that pushed Deana to have her PFO repaired was with a coworker who was a nurse practitioner in cardiology with knowledge of PFOs and diving. Her coworker was pretty blunt, stating, “Why are you playing Russian Roulette? You have worked in cardiac and know the risks.” 

Some of these risks include Arterial Gas Embolism (AGE), Venous Gas Embolism (VGE), and cerebral embolism. That was the final “Aha” moment to tip the scale and get Deana to schedule her PFO repair, since “Russian Roulette” was exactly what Deana was doing based on her diving activities following her DCI hit.

Team Approach to Treatment 

Bert Berzicha, one of our GUE Seattle community members, also completed his TTE as a result of having had some symptoms of DCI in the past. The test confirmed the presence of a large PFO. Deana and Bert compared notes initially and discussed diving as a team on future dives using more conservative decompression profiles than other teams, allowing the other teams to get out of the water sooner. Deana, however, related what she had learned from talking with her coworker and changed her mind about diving conservatively and instead decided to get the PFO repaired. 

Bert and Deana with some of the GUE-Seattle Tech crew. Photo by Bert Berzicha.

Deana did not want to take the risk of neurological deficits that could be irreversible. Deana suggested to Bert that he come with her to have his PFO repaired at the same time. Bert continued to research the subject, looked at his work schedule, and decided doing a “team” procedure made sense. Just as a dive team shares a plan, resources, and emergency procedures, a medical procedure shares similar benefits when working as a team. 

It was time to plan a date for both of them to have the procedure. Deana and Bert both live in the Seattle area. Dr. Ebersole lives in Lakeland, Florida, so logistics included time off work, pre- and post-surgical care, flights, hotels, and transportation. Deana arranged to have her sister Jessica fly into Tampa from Dallas, prior to them arriving, so she could pick them up from the airport to make it to the hospital in time for the procedure. I was going to be in Australia on a business trip at the time, so I was not able to be there pre-surgery. I ended up reworking my return trip and flew from Canberra, AU to San Francisco, then on to Tampa, to land just an hour after their surgeries were finished and meet them back at the hotel. 

Even though Deana and Bert could fly home 24 hours after the procedure, they decided to stay the weekend just in case there were any complications and to take it easy. Deana, however, had a different take on “easy.” The morning after surgery, Deana was invited by Dr. Ebersole to watch a procedure that he and his team perform called the “WATCHMAN” procedure (less than 24 hours after post-op). Then we picked up Bert and Jessica, and jumped into the truck to do a 300-mile, five-hour road trip to High Springs, FL, to take a tour of the Halcyon facility and say “Hi” to Orie Braun, Lauren Fanning, and Mark Messersmith; stop in at Global Underwater Explorers (GUE) HQ to buy some swag; and drive down to Ginnie Springs to see where Cave 1 may take place in Deana’s and my near future. Not bad a day after surgery. 

Deana and Bert after surgery. Photo by James Fraser

It was at Ginnie Springs where Bert came to Deana and stated he thought he had active bleeding. We all paused and turned pale, knowing we were not in a great location for this to be happening, but after being assessed by Deana it turned out to be post-op bruising from the surgery. This did, however, make us stop and think, “We just drove 300 miles away from the hospital we had decided to be close to in case of complications.” I am sure Gareth Lock would find a really good human factors story in there somewhere. 

Deana’s PFO adventure Timeline


  • OCT 8: 47 m/153 ft technical dive resulting in a DCI episode requiring recompression.
  • OCT 20: First dive post-chamber ride to 16 m/52 ft
  • OCT 29: TTE Bubble Study; “Deana has a small to moderate PFO”
  • NOV 17: Dr. Ebersole receives Deana’s TTE study for a second opinion 
  • NOV 21: Deanna dives now to 28 m/90 ft
  • DEC 4: Last dive before PFO repair. In the 46 days since her hyperbaric treatment Deana made 15 dives: “Conservative Not”
  • DEC 12: Deana and Bert have PFO procedure
  • DEC 13: Lakeland to High Springs road trip


  • JAN 27: First dive post closure—15 m/49 ft and spaced dives 2-3 days apart
  • FEB 15: Started doing multiple dives daily no greater 15 m/50 ft
  • MAR 8: PFO follow-up; OFFICIALLY cleared by Dr Ebersole to dive
  • MAY 7: Dives now pushing 30 m/100 ft
  • MAY 31: First Tec dive to 33 m/110 ft 

Since May, Deana has done 120 dives in 2020 with a max depth of 52 m/170ft, which she did on September 12. Deana has gone back to no-restriction diving and has completed 16 technical dives since this summer. Some of these have been assisting with photogrammetry dives.

  • 46 m/150 ft to 52 m/170 ft: 3 dives 
  • 40 m/130 ft to 46 m/150 ft: 5 dives 
  • 30 m/100 ft to 40 m/130 ft: 8 dives 

Getting Personal With PFOs

COVID-19 has prevented us from doing a dive trip this year, which is the one main test we still have yet to do: repeat the scenario that always led to her getting DCI, which was three consecutive days of recreation and tech dives, to see if she experiences any recurrence of DCI symptoms. 2021 will hopefully open up this opportunity, or by that time Deana will already be training for GUE’s Tech 2 course. In either case, Deana and Bert are both very happy to have had their PFOs repaired; both have seen improvements in their health in other areas such as endurance, no longer being easily winded, and, in Bert’s case, less headaches, which he had prior to the PFO closure. 

James, Deana and Bert. Photo courtesy of James Fraser.

To get a PFO repaired is a personal choice, and no one should ever take surgery lightly as it has its own risks. Divers with PFOs need to do their own research and consult an interventional cardiologist, such as Dr. Ebersole, who understands diving. Only then can they make an informed choice based on their own unique situation whether or not a PFO closure is right for them. This article is meant to show the process and outcome of two very experienced and ambitious divers who made the choice to have their PFO repaired and the results of that decision.  

(PFO) Free At Last! Deana after completing her first dive to 52 m/170 ft in Lake Crescent, WA diving the Warren car. Photo by Bert Berzicha

Additional Resources

Diving and Hyperbaric Medicine: The effectiveness of risk mitigation interventions in divers with persistent (patent) foramen ovale  by George Anderson, Douglas Ebersole, Derek Covington and Petar J Denoble. 2019 Jun 30.

Alert DiverPFO Study Update by Petar J Denoble

Alert Diver: Cases studies of divers who had their PFOs closed with transcatheter-applied occluders: Divers with Holes in their Hearts by Petar J Denoble 2010

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James D. Fraser is a GUE Fundamentals and Rec 1/2 Instructor, PADI MSDT, and NAUI Scuba Instructor, and has been diving in the Pacific Northwest for over 30 years. James currently lives in Seattle, WA, with his wife and dive teammate Deana Fraser. As a member of the GUE Seattle Board of Directors, James is able to share his experiences and work with Deana at growing the local diving community sharing their passion with all who are interested. James recently embraced technical diving, becoming certified as a Technical 1 diver with GUE. James and Deana have had opportunities to travel all over the world to experience their passion in amazing places such as Egypt and the Maldives. James currently works as a Cyber Security Director with a Fortune 500 Defense Contractor and has been a residential construction business owner and Emergency Medical Technician (EMT). 

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