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by Reilly Fogarty
Header image by Sean Romanowski
Hyperbarics is a tricky field to study — gas laws sometimes behave like suggestions, the effects of high-pressure gases in the body are highly subjective, and decades of research often end in what amounts to an educated guess. Our understanding of narcotic gases is a great example of this: we know a little about increasing partial pressures of nitrogen decreasing our motor function and cognitive ability, and a bit about some gases like helium ameliorating those effects, but not much else. The specific mechanisms of action, variability through human anatomy or interactions with other gases, and the range of those effects at various depths are the stuff of theory rather than proven science. There are some convincing arguments for the treatment of oxygen as a narcotic gas, but the qualifications are many, and a broad swath of research and nuance casts everything we think we know into doubt. Here’s what we know so far, as well as some best-practice recommendations. Understanding the narcotic effects of oxygen is by no means a clear-cut situation.
A Primer on Narcosis
Before discussing narcosis, it’s important to cover what we know already. Narcotic gases (any gases that can cause narcosis, including nitrogen, argon, etc.) have a wide range of effects, all affected by depth. The general consensus is that these gases likely interfere with the release or uptake of neurotransmitters in the body or alter the postsynaptic response from those transmitters. Greater partial pressures of some gases increase this effect, which is why we see increased narcosis as we descend on a gas containing nitrogen. In short, much like the gases used for surgical anesthesia, common diving gases can interfere with the communication pathways in our body.
The effects of these gases are understood by the Meyer-Overton rule, a holdover from anesthesia research in the early 1900s. *Updated: The rule predicts that the anesthetic potential of a gas is directly related to its lipid solubility (i.e., a gas that can be absorbed effectively by fatty tissue will be more narcotic than one that cannot) and ranks gases by that solubility. Helium exhibits extremely low lipid solubility and correspondingly little narcotic potential according to this rule, and this holds true to experience. WAS: The rule predicts that the anesthetic potential of a gas is inversely related to its lipid solubility (i.e., a gas that can be absorbed effectively by fatty tissue will be less narcotic than one that cannot) and ranks gases by that solubility. Helium exhibits extremely high lipid solubility and correspondingly little narcotic potential according to this rule, and this holds true to experience. The effects of oxygen, however, appear to be significantly more complex.
Note: These units are permeability coefficients. A larger number represents a greater energy required to pass the same quantity of gas through a membrane or lipid tissue, indicating decreased solubility. Gases with smaller permeability coefficients (helium, for example) are more soluble and can permeate barriers more easily while gases with a larger permeability coefficient (like oxygen) are less soluble and require more energy to pass through a barrier. More information on gas solubility and permeability in specific tissues can be found here.
Working solely from the Meyer-Overton rule, it would appear that oxygen should cause significant narcosis — it has twice the lipid solubility of nitrogen and thirty-eight times that of helium. Comparing just the lipid solubility of nitrogen and oxygen, it appears that saturation with oxygen would not only cause narcosis but would also result in stronger symptoms than those caused by nitrogen. The aptly named, Does Oxygen Contribute to the Narcotic Action of Hyperbaric Air?. a paper by hyperbaric researchers from 1990, attempted to confirm just that hypothesis. Researchers compared motor skills and mental performance with participants exposed to air and normoxic nitrogen and oxygen mixtures at 6, 8.5, and 11 bars ambient pressure. They found impairment of up to 40 percent at the highest pressures of all gases, but participants exhibited the same impairment on oxygen as gases with higher partial pressures of nitrogen. Their conclusion indicated that oxygen did not ameliorate mild narcosis and should, therefore, have some narcotic properties.
A Case for Oxygen
A similar paper from a little more than a decade before found the same results. A rise in the partial pressure of oxygen to 1.65 ATA gave similar narcotic effects as a rise in the partial pressure of nitrogen to 6.3 ATA, or an end-tidal pressure of CO2 or 10mmHg. Again, these researchers came to the conclusion that while the specific contributions to narcosis from oxygen could not be exactly measured, it did appear to contribute to the narcosis of divers.
There’s an argument for the sake of safety here too. Oxygen may be narcotic, so by calculating our equivalent narcotic depth (END), by including oxygen in the calculations as we would nitrogen (more on how to do that later), we give ourselves an extra margin of error. If oxygen is, in fact, narcotic, we’ve planned for its effects at depth, and if it isn’t, then the worst thing that happens is we have a little less narcosis than expected.
When I said hyperbarics was a tricky field to study, I meant it, and not just because of the complexity of the issues involved. Understanding the effect of oxygen in the body is an incredibly nuanced balancing act that involves attempting to apply our limited understandings of oxygen metabolism, neurotransmitter function, metabolic dysfunction, inflammatory responses and more, all in the application of something that in the end will be almost entirely subjective. There are a few notable issues with the presentation of oxygen as a narcotic gas, and they’re easiest to work through in pieces:
Rules were made to be broken and Meyer-Overton is no exception, despite holding mostly true for more than a century. Not only does it lack a specific mechanism of narcotic action, but there are some explicit exceptions to the rule. It should be noted that even these exceptions are the source of some controversy, but it’s widely believed that several anesthetic gases work in exception to the Meyer-Overton rule, specifically anesthetics with long alkane chains in their structure. Some of these gases exhibit dramatically lower potency than would be expected based on their lipid solubility, and we have no way to know whether oxygen is one of these exceptions to the rule or just another narcotic gas.
The environment we’re concerned about, primarily deep open- or closed-circuit diving has a long list of restrictions for the application of oxygen. As divers, we carefully plan our exposures to keep oxygen in a narrow range of partial pressures while diving. This careful control of the PO2 of our gas means that we’ll never see a PO2 greater than 1.6. While a ride in a hyperbaric chamber may exceed that threshold, it’s unlikely to see in the water and brings to light another question — if oxygen is narcotic, at what partial pressure do you see the effects? No study available on the subject is able to define either the PO2 at which oxygen begins to have a narcotic effect or to even strongly correlate pressure and narcosis on oxygen alone.
Compounding this confusion is the fact that oxygen is a gas that we constantly metabolize. Even if we were to breathe similar amounts of nitrogen and oxygen during a dive, the metabolic processes required to keep us alive and well constantly consume some of that oxygen. How much oxygen is consumed and at what rate is a complicated answer based on individual physiology and what a subject is doing at any given time, making it even more difficult to isolate the effects of oxygen from the metabolism of inspired gas. Until now, our understanding of narcosis has relied on our ability to estimate the partial pressure of nitrogen in our gas, but once oxygen enters the mix, a whole host of new variables become important to consider. While it’s true that we can estimate the effects of the gas based on theories and the research we do have, it’s not enough to definitively say that oxygen is a potent narcotic.
Putting It to Practice
Academic review is one thing, but putting a new concept to practice is what brings it home for most divers. Here’s how you can calculate END with oxygen included as a narcotic gas (the most common decompression planning software also offers an option for this in their calculations):
(Depth + 33) X (1 – fraction of helium) – 33
Because oxygen and nitrogen are considered equally narcotic, END can be calculated using the total of a gas minus the fraction of non-narcotic helium.
Discussions of narcotic gases rarely provide rewarding moments of discovery. What we have as divers and as an industry is a best guess that indicates that oxygen is likely narcotic, but we don’t know what the mechanism of that narcosis is, nor do we know how potent the effects of oxygen are. The issue is deeply nuanced and requires some careful consideration before arriving at a conclusion, but your mental tribulations shouldn’t ruin your next dive. As a dive safety organization, Divers Alert Network has an interest in promoting safe diving practices, and the results in this case are promising and present little additional risk. Calculating your END with oxygen as a narcotic gas is a safe and conservative practice until researchers tell us definitively that it’s non-narcotic. Aside from a slightly higher gas bill there’s no downside, but you might just be safer for it.
For more information on narcotic gases and advanced dive planning, visit DAN.org or contact the author at RFogarty@DAN.org.
Reilly Fogarty is a team leader for risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.
Fiona Sharp, You Will Be Sorely Missed
As you have likely heard, we’ve lost one of the irrepressible and much-loved characters in the tech and diving medicine community, 55-year old Australian anesthesiologist and diving physician Fiona Sharp.
By Rosemary E Lunn
Header photo by Catherine Meehan. Fiona in Sodwana Bay.
One of the colorful characters in the field of diving medicine died tragically in a rebreather diving accident on Thursday, October 17, 2019. Fifty-five-year-old, Australian diving physician and anesthetist Dr. Fiona Sharp, MBBS, FANZCA, was found unresponsive on a reef at 24 m/80 f. She had been solo diving on a rebreather, and she was discovered with her mouthpiece out. Fiona was medevaced but did not regain consciousness. The incident occurred on the last day of Fiona’s diving trip to Bonaire, located in the Leeward Antilles, Carribean Sea. It was the week after Bonaire Tek. Fiona enjoyed deep rebreather diving and was known to be a bit of a maverick.
“We are shattered.” Bruce and Lynn Partridge, Shearwater Research.
The disturbing news of Fiona’s death rocketed around the world in a few hours. Many people from the diving medicine and technical diving communities expressed their dismay and distress at Fiona’s death. She was gregarious, fun-loving, irrepressible, and generous. Fiona was a friendly colleague and we had dived together a few times. I wrote a heartfelt tribute about Fiona’s bulldozer attitude to life and diving, and this was published by X-Ray Mag. It includes a myriad of voices from around the globe and amply illustrates just how well-loved she was by her colleagues and friends.
“Fiona was an individual. She did what she wanted. She did what she loved. She was very much her own person, and drove us mad at times. Fiona was down to earth, had a massive heart, a huge personality, and was very dear to all of us. She will be greatly missed.”Dr. Catherine Meehan
Fiona Sharp was born in May, 1964, in Perth, Australia, and she was bright! She attended Mercedes College Perth, where she was “Dux” in her graduating year. [Dux: from Latin for ‘leader,’ the term that is now used in Australia and New Zealand to indicate the highest-ranking student in a specific achievement).
After leaving high school, Fiona studied medicine at the University of Western Australia where she graduated in 1989 as a Bachelor of Medicine and Bachelor of Surgery (MBBS UWA). After serving an internship in Perth in 1989, followed by a year as a junior Resident in Sydney, Fiona moved to England where she gained her Diploma of Anaesthesia (DA) in 1992, whilst working as a Senior House Officer in Anaesthetics in Southend, Essex.
Fiona then returned to Australia and commenced specialist Anaesthetics training. In 2000, she flew once again to the UK where she spent five years practicing diving medicine at DDRC Healthcare (Diving Diseases and Research) in Plymouth. During this period, she was awarded a Fellow of the Australian and New Zealand College of Anaesthetists (FANZCA) Fellowship in 2004.
At the time of her death, Fiona Sharp was working at the Fiona Stanley Hyperbaric Medicine Unit (FSHHMU) in Perth, Western Australia. She had been in post since it opened in November 2014. Prior to that, she was employed at the HMU at Fremantle Hospital from 2007 to November 2014. (The department then relocated to the newly built department at FSH).
Whilst writing this, I spoke to Fiona’s family and asked: “why medicine?” They responded,
“If you are really smart at school, you are expected to be an architect, a lawyer, or a doctor. Medicine appealed to her; however, it was possible that she could have become a vet because the family did have a lot of animals.”Fiona’s family
“She was nuts, about everything. But especially about diving.”Dirk Peterson
Fiona got into diving in her late teens after trying skydiving and scuba diving. Water prevailed and she learned to dive when she was 18 years old. She was a PADI Divemaster, cave certified and qualified to dive the Inspiration, Evolution, SF2, Drager Dolphin, Mark VI Poseidon, and JJ-CCR rebreathers.
You cannot ever say that diving was her hobby. It was her all-encompassing great passion. Fiona recently told her older sister that she felt happiest when she was underwater. It was therefore natural that she would take an active interest in diving medicine, and she became a fixture at all the major diving medical or tech conferences. SPUMS, UHMS, EUBS, HTNA, as well as EUROTEK, OzTek, Rebreather Forum 3 and other diving industry events. These helped keep her current and educated in this niche sector.
“Fiona loved the diving, diving medicine, and the camaraderie around the bar. She was regularly first up and last to bed. Most often, Fiona could be heard well before she was seen on land and underwater!! She was well-loved by her colleagues at these events and, as many have said, the SPUMS Conference won’t be the same without her. I think she attended at least 17.”Dr. Neil Banham, Fiona Stanley Hospital
Fiona’s first South Pacific Underwater Medicine Society (SPUMS) Conference was at Layang, Layang island, in Malaysia in 1999, a venue well suited to her type of diving because it was deep. It was at a later SPUMS, in 2008, that Fiona’s diving would change. Dr. Catherine Meehan takes up the story.
“I met Fiona at a SPUMS meeting about 20 years ago. In 2008, I chartered the ‘Golden Dawn’ liveaboard. Ten of us flew into Alotau in Papua. New Guinea. and we sailed and dived our way across to Kimbe Bay, West New Britain, to join the SPUMS annual scientific conference. There was a rebreather on board and Fiona had a guided rebreather dive. She enjoyed it so much that she dived with it for the rest of the week. I believe this was one of her earliest experiences rebreather diving, and I think that she embarked on her passion for diving rebreathers shortly afterward.”
Catherine and Fiona would regularly dive together, at least two or three times a year, all over the world.
“We did a lot of conferences and diving together. We were most recently in South Africa diving Sodwana Bay. She was dressed in her vibrant orange drysuit so it was easy to see where she was, doing her own thing. It was tough cold water diving, but she was very hardy and didn’t miss a dive, even when her suit leaked. It is a good lasting memory of her.”
“It was like she had been shot out of a cannon when she entered a room.”Joanna Mikutowicz, DiveTech
Fiona Sharp never did anything by halves, and this is amply demonstrated by a classic Fiona story that her older sister Donna regaled to me.
“Many years ago Fiona rang me up and said, ‘I have got two tickets to the rugby game on Friday night, do you want to come with me?’ I thought, ‘Why not?’ One of our kids plays rugby at school. I rocked up at the game and she said to me, ‘Here is your ticket. Don’t worry about paying. And by the way, these tickets are not just for tonight’s game. They are season tickets’.”
She goes on:
“We ended up supporting the Western Force, a professional Perth-based rugby team, for the next twelve years and watched them play Super Rugby against New Zealand and South Africa. What I found ironic was that I went to nearly every game. I think Fiona missed more games than any of us because she was away diving so much.”
“Fiona Sharp drew no quarters when it came to life and diving. I only met her a few times, but she left an aircraft carrier shipwreck-sized impression on my psyche and we remained in contact.”Laura James, Environmental Campaigner, Underwater Cinematographer
Many divers have been generous with their Fiona stories. Todd and Tiffany Winn of Silent O Solutions reached out to me with another classic Fiona tale and said I could share it. When Fiona decided you would be friends, the recipient really didn’t stand a chance.
“Fiona’s reputation as ‘difficult’ preceded her, and our first encounter with her was memorable, to say the least. It was in San Diego for an in-water recompression symposium and training event in 2014. She exceeded my expectations. I believe she only told me I was completely mistaken two or three times. I conceded two of three and agreed to disagree on the third. She had an uncanny ability to defuse my ire with a wry smirk. As she had already decided we’d be friends, I had little say in the matter.”Tiffany Winn
“Tiff liked Fiona immediately and loved her unflinching honesty and authenticity. Fiona threatened to visit us on Maui on one of her transits across the globe, but unlike nearly everyone else she called and texted for a month straight, ironing out the details, and sure enough, one day, showed up. We loaded up our little boat for its maiden voyage, and Tiff and Fifi had a girl’s day rebreathering all by themselves. We had a beautiful day and a fabulous sunset. We will remember her fondly and often, and will miss her dearly.”Todd Winn
It is only right that I leave the last words to her family. I was told that Fiona had wanted to climb Mount Everest, but she suffered so badly with altitude sickness, that she just about made it to base camp and no further. Fiona was always willing to take a risk, and push herself. Apparently she competed in triathlons in her early 20’s, and she liked challenges. Everyone who came into contact with her soon found out she had a very dry sense of humor.
“Fiona didn’t like cheap champagne or wine. It had to be good quality and lots of it, and she always brought home two bottles of whiskey from every trip for her father.”Donna Sharp
Fiona was close to her family. She is survived by her mother, three sisters, a brother, and 18 cousins. She was an “oh so very proud” aunt to 13 nieces and nephews. Typically big-hearted and kind, Fiona had planned to take a nephew to Antarctica this November. You mad as a box of frogs lady, you will be missed.
The author is very grateful for the assistance of the Sharp family and Dr. Neil Banham, Director of Hyperbaric Medicine at Fiona Stanley Hospital in writing this tribute.
Dive industry fixer, Rosemary E Lunn (“Roz”) is the Business Development Director at The Underwater Marketing Company. This British firm specializes in providing marketing, communications, social media and event management for the “tecreational” and technical diving industry. Rosemary is a PADI IDC Staff Instructor, BSAC Advanced Instructor, Trimix, and CCR diver. Before moving into the public relations field, she worked as a professional recreational instructor, safety diver, and underwater model underwater and appeared on the History Channel and National Geographic documentaries. She established TEKDiveUSA and organized Rebreather Forum 3 on behalf of AAUS, DAN, and PADI. In 2008, Rosemary co-founded EUROTEK, the European advanced and technical diving biennial conference. She is a respected and prolific diving author, an SSI Platinum Diver, an Associate Member of the Women Divers Hall of Fame, and sits on the SITA Board (Scuba Industry Trade Association).
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