Is Oxygen Narcosis A Thing?
There’s been a long running debate as to whether oxygen is a narcotic diving gas. Training agencies CMAS, GUE, PADI and PSAI include O2 in their equivalent narcotic depth (END) calculations. Others like BSAC, IANTD, NAUI and TDI do not. The problem has been reliably measuring gas narcosis. Enter medical researcher and tech instructor Xavier Vrijdag, who has developed a novel EEG algorithm that can detect the subtle effects of gas narcosis on the brain. His results promise to lead to a deeper understanding of gas narcosis. Hint: Nitrox divers can breathe easy; it’s less narcotic than air!
By Xavier Vrijdag PhD. Header image: Drs. Vrijdag and Hanna van Wart preparing participant in front of the hyperbaric chamber. The EEG electrodes are placed inside the cap and two more electrodes are placed under the eyes. Photo by Payal Razdan.
🎶🎶 Predive Clicklist: Love Is Like Oxygen by Sweet (1978)
🎶 Love is Like Oxygen,
You get too much, you get too high,
Not enough and you’re gonna die.
Love gets you high 🎶
To sustain life, every breathing gas mixture for divers must include oxygen. As we descend, invariably, we will breathe oxygen at higher partial pressures than air at the surface. Oxygen is mixed with other gasses, like nitrogen and helium, to create suitable mixes for specific depths. One of the boundaries of going deeper is the experience of gas narcosis.
The effects of gas narcosis increase with a diver’s depth because the ambient pressure and the partial pressure of the individual gasses in the breathing mix increase. Gas narcosis has traditionally been linked to nitrogen when breathing air. Narcosis causes behavioral changes and impaired cognitive abilities. Substituting helium wholly or partly for nitrogen ameliorates the narcosis effects. Similarly, it has been argued that replacing some nitrogen with oxygen reduces a gas mixture’s narcotic potency.
One tool to quantify gas narcosis in different breathing mixtures is equivalent narcotic depth (END). In this method, the narcotic effect of a certain breathing gas mixture, like trimix, is compared to breathing air. With the partial pressures for oxygen, nitrogen, and helium in the gas mixture and the intended dive depth, one can calculate the depth at which air would produce a similar narcotic effect. A long-standing debate within the dive industry is whether oxygen should be included in the ‘is narcotic’ or ‘is not narcotic’ portion of this calculation (See: Calculated Confusion: Can O2 Get You High? by Reilly Fogarty)
One of the most used approximations of the narcotic potency of gasses is the Meyer-Overton correlation. The Meyer-Overton correlation describes a relationship between the narcotic potency of a gas and its solubility in oil; the more soluble, the more narcotic. Oxygen has a higher solubility in oil than nitrogen. Therefore one could argue it is more narcotic than nitrogen. However, oxygen is special since we metabolize it.
Nitrogen is a non-metabolic gas, meaning that the inhaled partial pressure of nitrogen equilibrates with the partial pressure of nitrogen in brain tissue after a short wash-in period. In contrast, oxygen is metabolically consumed, thus lowering the partial pressure of oxygen considerably below the inspired pressure. Therefore, despite its greater solubility in oil, one could argue it might be less narcotic than nitrogen.
In addition, exceptions to the Meyer-Overton correlation exist; some gases that are very soluble in oil should be highly narcotic but turn out not to be. In anesthetic research, we have since learned that for a gas to be narcotic, its molecule needs to be able to bind to receptors on the neurons to cause its narcotic effect. Certain molecules have a shape that does not allow them to bind to these receptors. For oxygen, it is unknown if and how it binds to neuronal receptors. The point here is that just because oxygen is more oil-soluble than nitrogen, that does not automatically mean it is more narcotic than nitrogen.
Measuring the cognitive effects of nitrogen narcosis is most often done with psychometric tests, like math questions, motor tasks or memory queries. Each of these skills is affected at different partial pressures of nitrogen and is impaired at different depths breathing air. The book, Psychological and Behavioral Aspects of Diving by Nevo and Breitstein summarizes the scientific literature quite well. Higher cognitive functions, like reasoning and memory are impaired in the shallow range (~ 5% at 3 ATA (20 m/66 ft), 30% at 6 ATA (50m/164 ft)).
In contrast, more basic skills like gross motor tasks are only impaired at far higher pressure. Quantification of the subtle effects of inert gas narcosis is challenging. Psychometric tests can be affected by learning effects, participant motivation or boredom. Objective neurophysiological measurements like the critical flicker fusion frequency (CFFF) and quantitative electroencephalogram (qEEG) analysis could, in principle, be used to overcome these issues.
During the CFFF test, the participant looks at a small light oscillating rapidly on and off (flickering). The fusion frequency is obtained by increasing the flicker frequency until the participant perceives a change from flicker to fusion (no further flickering) or the other way around. An increase in CFFF equates to an increase in alertness, whereas a decrease in CFFF is associated with a decrease in alertness, for instance, caused by gas narcosis. Some studies have shown a decrease in CFFF during air-breathing dives at 4 ATA.
Hyperbaric inert gas narcosis is generally considered a manifestation of depressed brain activity when measured with EEG at large depths. Continuous measurement of the EEG results in a considerable amount of data, and analysis is usually performed afterward. This requires much training and time. qEEG has the potential for real-time analysis of the continuous EEG, giving the user an easy-to-interpret result about the narcotic effect. The subtle changes of gas narcosis at typical sport diving depths would otherwise be hard to detect in the EEG. Such analyses would allow comparison between divers and successive dives.
Measuring hyperbaric oxygen and nitrogen’s comparative effects on the brain is difficult. Ideally, the comparison should occur at equivalent inspired pressures of the two gasses. However, nitrogen narcosis symptoms typically present at depths greater than 30m/100 ft (4 ATA); a pressure at which oxygen-breathing would carry the risk of cerebral oxygen toxicity. A pressure of 18 m/60 ft (2.8 ATA) is considered the maximum safe inspired oxygen pressure in hyperbaric oxygen treatments with resting patients in a dry hyperbaric chamber. [WARNING: A PO2 of 2.8 ATA is incompatible with diving!] It follows that the measurement method to detect gas narcosis needs to be sensitive enough to catch the subtle effects of gas narcosis in the more limited and safe inspired pressure range of oxygen.
In a research program funded by the US Navy Office of Naval Research, I studied the subtle narcotic effects of nitrogen and oxygen together with Dr. Hanna van Waart, Professor Jamie Sleigh, and Professor Simon Mitchell.
Measuring The Subtle Effects Of Nitrogen Narcosis
In the nitrogen narcosis study, twelve technical divers breathed air and 21% / 79% oxygen-helium (‘heliox’) at 18 m/60 ft and 50 m/164 ft (2.8 and 6 ATA, respectively) inside a hyperbaric chamber while we recorded the electrical signals of the brain (EEG), performance on a mathematics test and the CFFF. Breathing air at 50m/164 ft (6 ATA) is typically cited as causing sleepiness, euphoria, overconfidence, idea fixation, and impaired reasoning, memory, calculus, and judgment. However, these symptoms may be alleviated by enhanced concentration in motivated divers.
The CFFF did not change during the air-breathing hyperbaric exposure to 6 ATA compared to surface recordings. Hence, the CFFF did not detect the nitrogen narcosis effect at this depth. We also reviewed the CFFF diving literature and found a complicated and often contradictory picture. Various studies during air breathing conducted at 4 ATA have suggested that nitrogen narcosis is the cause of a reduction in CFFF at this depth. However, this result does not extrapolate to greater depth, even though it is known that cognitive performance is further reduced with increased depth. Several studies—including our experiments—performed at 6 ATA while breathing air did not show impairment, but instead showed either no change or an increase in CFFF. Hence, in our study, CFFF failed to detect or quantify a narcotic effect known to be present at 6 ATA.
On the contrary, we successfully developed a quantitative EEG metric to measure narcosis produced by nitrogen at hyperbaric pressures. The functional connectivity metric is based on the so-called mutual information analysis, and was summarized using the global efficiency network measure. Mutual information measures the information that is similar between two signals, calculated between all EEG channels. Higher mutual information means that the signals are more alike.
Global efficiency measures how well connected a network of channels is. High global efficiency based on mutual information means that more signals are similar. Normal cognitive function requires regional specificity, which might be lost (that is, signals become more similar) during impairment.
The novel EEG method successfully differentiated between breathing air at the surface and air at 50m/164 ft (6 ATA) with a 35% increase in the functional connectivity metric. To our surprise, we could even detect a trend of a 19% increase at the much lower pressure of 18 m/60 ft (2.8 ATA). This indicates a dose-response between EEG functional connectivity and nitrogen narcosis. No significant change in the metric was found during the hyperbaric heliox exposures, which was expected because helium is known to be a non-narcotic gas. This lack of an effect of heliox confirmed that nitrogen exposure and not the environmental pressure change was responsible for the changes we saw in EEG functional connectivity.
Another implication of the increased functional connectivity metric during air breathing at 18 m/60 ft (2.8 ATA) is that it should be sensitive enough to measure the narcotic effects of oxygen at 18m/60 ft inside a hyperbaric chamber, if such effects exist. As stated above, we can safely breathe oxygen at 18 m/60 ft inside the safety of a hyperbaric chamber, where there is no risk of drowning, compared to being submerged. If oxygen were to have similar narcotic effects as nitrogen, we expect that functional connectivity would change in a similar way during oxygen exposure as it did in the air-breathing experiment at 18 m/60 ft.
Measuring Oxygen Narcosis
In the oxygen narcosis study, twelve technical divers breathed 100% oxygen at the surface, 4 m/13 ft and 18 m/60 ft (1, 1.4, and 2.8 ATA, respectively) inside a hyperbaric chamber while we recorded the EEG. Four meters/13 ft (1.4 ATA) was chosen as it approximates the upper limit of oxygen exposures accepted in diving, while 18m/60 ft (2.8 ATA) is the maximum exposure to oxygen inside a hyperbaric chamber with a minimal risk of oxygen toxicity.
The EEG functional connectivity metric did not increase while breathing hyperbaric oxygen. This contrasts with the 19% increase we reported above in participants breathing air at 18m/60 ft (2.8 ATA). This suggests that oxygen is not producing the same changes in brain electrical activity seen during hyperbaric air breathing.
It is only now, with the development of this novel EEG analysis algorithm, that we have been able to investigate the subtle effects of hyperbaric oxygen in more detail, and based on our results we hypothesize that oxygen probably does not bind to the same neuronal receptors and hence does not cause similar narcotic effects to those induced by nitrogen. For nitrox divers, this probably means that the increased oxygen slightly reduces the narcotic potency of the breathing gas.
OzTek presentation: Measuring gas narcosis in divers
British Journal of Sports Medicine: Towards gas narcosis monitoring in compressed gas diving (PhD Academy Award). (2022)
Scientific Reports: EEG functional connectivity is sensitive for nitrogen narcosis at 608 kPa. (2022)
Physiological Reports: Does hyperbaric oxygen cause narcosis or hyperexcitability? A quantitative EEG analysis. (2022)
Diving and Hyperbaric Medicine Journal: Investigating critical flicker fusion frequency for monitoring gas narcosis in divers. (2020)
InDEPTH: Calculated Confusion: Can O2 Get You High? by Reilly Fogarty (2019)
Alert Diver.Eu: Rapture of the Tech: Depth, Narcosis and Training Agencies by Michael Menduno (2020)
Alert Diver.Eu: Measuring Inert Gas Narcosis by Michael Menduno (2020)
Xavier Vrijdag is a diving medical researcher at the University of Auckland investigating the effects of gas narcosis in divers on the brain. He has a master’s degree in Technical Medicine for the University of Twente, the Netherlands, where he developed an algorithm to quantify cerebral arterial gas embolism in the hyperbaric environment. In 2022, he completed his doctoral thesis on the effects of nitrogen, oxygen, helium and carbon dioxide under pressure.
Xavier has worked as a researcher and technical physician at the department of Diving and Hyperbaric Medicine of the Academic Medical Centre, Amsterdam, the Netherlands. Before coming to New Zealand, he worked as a hyperbaric technician and researcher at Deep Dive Dubai. He is an instructor for medical courses of DAN Europe, holds certifications from seven training agencies, and is an SSI divemaster instructor, normoxic trimix instructor, cave instructor and freediving level 2 instructor, and is a GUE fundamentals instructor.
Besides work, Xavier is an avid cave and wreck diver and photographer, as well as being part of a CaveSAR/LandSAR cave rescue team in New Zealand.
Confessions of a Chamber Diver
Interview by Tim Blömeke. Images courtesy William Howell unless otherwise noted. Special thanks to Duke consulting professor and biomedical engineer Dr. Rachel Lance who suggested this story . All subjects provided express written consent for the use of their images and names for this article.
Forty-one year old computer scientist and cave diver Mike Winters, has been a frequent lab rat, err test subject at Duke Center for Hyperbaric Medicine & Environmental Physiology, and has participated in most of their hyperbaric research studies over the last 15 years. The low profile, former special operations soldier, now a software researcher and endurance athlete, says he has always been interested in advancing hyperbaric science and medicine, as well as learning the limits and capabilities of his own body in various extreme situations.
Mike, you must be one of the most seasoned chamber divers out there. How does one get a gig like this?
I first got into chamber diving when I was an undergraduate university student. I was heavily involved in our SCUBA leadership program – helping to teach classes, maintain equipment, etc. It just so happened that my diving instructor, a the head of the University’s Scuba program, was a longtime friend of a man named Gene Hobbs.
At the time, Gene was in charge of the Duke Hyperbaric Chamber. I don’t recall his exact title or responsibilities, but he mentioned that the chamber was doing a research study and looking for candidates, and suggested I go try to participate. He also ran the Rubicon Foundation Research repository at the time and had an Explorer’s Club flag. Great guy, endless fount of knowledge.
As a broke college student who was also very interested in the science behind it, I was extremely interested (this was maybe in 2007-2008). Then I got on the Duke chamber’s mailing list and have been getting invited to participate in studies ever since.
Anyone can contact their local hyperbaric chamber and inquire about current or future studies. For that matter, universities and hospitals are often running various research studies on a range of subject areas.
Do you log your chamber dives? How many have you done?
I’ve logged some of them, but I unfortunately have not been very organized or fastidious with my regular diving logs or my chamber dives. I did contact the research coordinator at Duke and got a list of most (All? I’m not sure) of the studies I’ve participated in over the years. See: “List of Studies” below.
Most of those involved multiple chamber dives and/or multiple days at the chamber. One of them involved living in the chamber for about three days. I would guesstimate I have about three dozen dives in the chamber at this point.
How would you describe the experience to someone who has never been in a hyperbaric chamber in their life?
If I had to pick just one word, I’d say interesting or perhaps surreal. It’s hard to describe the entire experience, and each study has been a bit different. Before you even enter the chamber area, you walk down a hallway lined with display cases full of old diving gear, photos of astronauts and researchers who have been here or been involved with the chamber over the decades. It’s an interesting walk through history.
Seeing the chamber control room area kind of feels like a smaller version of the Apollo 13 movie—it’s an area of maybe 20’ x 20’ [6m x 6m] with large banks of controls. Some of them are video monitors, but mostly it’s just large dials and gauges—a lot of old, manual analog equipment which sits next to the chambers themselves. It’s all in one very large room, with a second story “loft” area built above parts of it.
How old is the chamber?
I believe the chamber was originally built back in the 1960s or so, and has done some extreme dives. They even had three people live at ~2,250 ft/686 m deep for a month or so back in the late nineteen seventies and early eighties. Duke Hospital is also quite old; walking into it gives you a sense of history.
At no point does it feel rickety or unsafe; you can just tell that it has been here for a while. It was built and designed in a different era, and it was built to last. It’s well used—the paint is fading in some places or scratched and peeling in others, and there are things built into the walls of the chamber that feel decades out of date but are still functional.
The staff are all incredibly knowledgeable and very friendly. These are some of the people who have literally written the books on hyperbaric medicine, and they will joke around with you and tell stories. It’s a very friendly and casual atmosphere. At the same time, it’s immediately obvious that these people are very, very good at what they do.
Please describe a chamber dive for us.
Doing a chamber dive is very different from a real dive, namely that you are (or can be) in a dry chamber most of the time, depending on the dive and chamber configuration. It can be fairly loud at times, as the pumps are pressurizing the large steel room you’re in. Sound carries and echoes well there.
Rapid pressure changes also alter the temperature drastically: a fast descent can get quite warm, while a drop in pressure can get quite chilly. You’re also able to notice a difference in your voice because of the denser gas you’re breathing. Anyone who has used a full-face mask while diving may have experienced this before, but for most divers this would be a new sensation. For me personally, I sometimes feel nitrogen narcosis more acutely in a chamber than I do on a real dive—not so much the mental aspects, but the warmth and dizzying effect of it.
On dives where I’ve had to exercise in a wet chamber, that’s also a bit unique, as they have custom built a special underwater bicycle. In some configurations, you sit in it similar to a recumbent bicycle. In others, you lie down face first. On some dives I used a normal SCUBA regulator, on others, a full-face mask. Some dives have required the flying of a simulated aircraft or submarine while listening to radio commands and juggling other tasks, like managing fuel leaks in the craft. Each brings its own physical and mental challenges.
In addition to the time spent with the actual diving inside the hyperbaric chamber, there usually are some additional medical portions; for example, ultrasounds and ECGs. Many of the studies I’ve participated in required blood draws, arterial catheters, or a Swan-Ganz catheter, which is a small balloon that is inserted into an artery at the wrist or elbow and then run all the way into the heart, where it can be inflated to lock it into position, allowing researchers to draw blood from directly inside the heart.
Many studies have also involved physical fitness tests, or a VO2 max test, which basically makes you exercise at increasingly high levels until you are unable to perform or endure the pain any longer.
Going back to your first ride, what was most unexpected to you from a subjective, sensory standpoint?
It’s been so long since my first chamber dive (15-ish years?), and I’ve done so many different studies at this point that I actually don’t recall my first experience. At the moment, I’d say the most unexpected thing was the way narcosis tends to feel so much more pronounced in a dry chamber than on a real dive. I’ve never really felt narked on a real dive, even when diving moderately deep or exercising hard at depth, but I’ve felt it on a number of different chamber dives, even while resting.
In broad strokes, could you describe some types of experiments that you took part in?
Some of them were cognitive tests – breathing various different gasses (several involved high CO2 content) and seeing what sort of cognitive decline they cause. This is measured with increasingly complex cognitive tasks and hand-eye coordination tasks. This is really important for rebreather divers.
Others were trying to determine the correlation between exhaled CO2 and the CO2 levels directly in the heart. Again, important for rebreather divers from a monitoring perspective, if you can put a CO2 sensor in the exhalation part of the loop.
Some of the studies involved diving at altitude, or flying after diving, and trying to develop safer ways to do that. Some studies were testing new medications, or new applications for existing medications, and how they affected the body in various ways. One involved seeing if breathing low concentrations of carbon monoxide could increase or stimulate muscle growth and repair. Another involved seeing if a three-day ketogenic diet would enhance resistance to oxygen toxicity (which for me was a massive change, for others it had less of an effect). My most recent study was helping them to build an AI/ML system to detect bubbles in ultrasound videos.
Among these studies, which are the ones that stand out to you and why?
Some dives have required breathing high concentrations of CO2, which is not a very fun experience. I had one dive where I essentially blacked out, but was still conscious and trying to do the tasks. I can distinctly remember the change in taste when switching over to the mix with the elevated CO2 content, and thinking to myself “you only have to do this for five minutes; just tough it out”.
As a military veteran who served in a special operations unit, and former marathoner and ultramarathoner, I’m used to pushing through pain and discomfort. That thought was the last thing I remember. A few minutes later, the chamber attendant was yelling in my ear that I could come off the gas now, and I believe she even had to take the regulator out of my mouth. I’m not sure. That five-minute window of time is completely lost to me.
However, the scary thing is that I was still exercising—underwater, at about 165 ft (50 m) of depth if I recall correctly—and trying to pilot the sub, basically a flight simulator video game, designed by NASA, I believe. I am sure I was doing a horrible job of it. I was conscious and active, but the brain just wasn’t there anymore; the lights were on but nobody was home, which gives me a very healthy respect for some of the dangers of rebreather diving. They told me I had the highest CO2 tolerance of anyone they’d ever seen, which is not a good thing, as it means I can get myself into trouble by working too hard on a deep dive, or by “enduring” a rebreather with a scrubber failure.
Another really memorable one involved living in the chamber for three days with three other people. We were in an approximately 8 ft. (2.4 m) diameter sphere the entire time, living at high altitude, except for when we went into the adjacent chamber to do our wet dives. While there were some interesting physiological changes from living and diving at altitude, the big takeaway for me was a newfound respect for astronauts, who have to live in such a small space for an extended period of time. I’m not claustrophobic at all – I’m a cave diver and quite used to tight places, but the boredom and lack of autonomy from being confined in the chamber was interesting.
Another memorable study involved muscle biopsies. They essentially had to cut and drill into the thigh, insert a small tube about the size of your pinky finger about three inches deep, and then suction or vacuum out a small chunk of muscle. That was not particularly fun. I was numbed, so it wasn’t painful, but I could still feel the pressure of the procedure, which was a very strange experience.
Many of the studies I’ve done involved arterial catheters or Swan-Ganz lines. I’ve probably had 10-15+ of those over the years, which always shocks any doctor who discovers that. It’s not really an uncomfortable experience, although there is a bit of pain at the incision site, and it takes a few days or a few weeks to fully heal. But mostly it’s just an odd experience watching a device getting inserted into your heart on the monitors, or feeling your heart skip a beat sometimes because of the device partially blocking one of the valves or the blood flow. It feels a little “different” or “off”—you know something is there—but it doesn’t really affect you very much.
What are the most important things you learned as a test subject in scientific experiments?
I learned a lot about my own body and its limitations, which is something I’ve always been curious about. As an athlete, all the VO2 max tests I’ve done over the years have been useful benchmarks for me. I remember using some of those as training aids when I was preparing for an Ironman.
But more than that, I’ve learned so much about how my body reacts to carbon dioxide, both physically and mentally. I’ve also learned about my susceptibility to oxygen toxicity and how much it can change when being in ketosis, which makes me feel much safer and more comfortable for my real-world decompression dives.
In addition to all the above, I’ve learned more about decompression theory, hyperbaric medicine, and physics, as well as various aspects of medicine and anatomy. As a former combat medic and EMT, that part fascinates me. Most importantly perhaps, I’ve met some really great people over the years, and I consider myself fortunate for having had the opportunity to learn from them.
Is there anything you learned during your chamber dives that you’ve been able to apply in your wet diving practice?
For me, the ketogenic study showed me that I can massively increase my own oxygen tolerance. While this is valuable information, I choose not to do that diet because I’m too lazy to be very precise with it. However, knowing my own O2 limits helps me stay within a safe window during my own decompression dives.
The several studies involving CO2 probably had the biggest impact on me personally. I’ve seen how it affects my physical capacity, both in an exercise sense and a lingering fatigue sense, as well as a mental capacity. I even had the one experience where I essentially blacked out.
With all of that, I have a very healthy respect for rebreather failures, and even though I am a cave and technical diver, and some of my dives have been in the six hour range, all of my “serious” dives are open circuit. I view rebreathers as a useful tool, but not one that I need for my diving at the moment.
As a final question, what kind of diving do you do when you actually get in the water?
I’m currently a bit landlocked where I live, so most of my diving has been in a local rock quarry. But my biggest love in diving are caves. I love the mental and physical challenge, the planning and preparation, the excellence and focus that it demands, and the exploration aspect. It’s an incredible feeling to be in a place where, in the history of our species and our planet, only a small number of living things have ever been.
Mike, this has been excellent. Thank you very much for your time, and safe diving always.
You’re very welcome.
Special thanks to photographer Christopher Wilson for his help illustrating this story. Check out his amazing work at: ChristopherWilsonPhotography.com
Other InDEPTH stories by Tim:
DAN Europe blog: Highlights from the first day of Rebreather Forum 4 by Tim Blomëke
DAN Europe blog: What You Missed from Day Two of Rebreather Forum 4 by Tim Blomëke
DAN Europe blog: Insights and Breakthroughs: A Recap of Day Three at Rebreather Forum 4 by Tim Blomëke
InDEPTH: When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving by Tim Blömeke (SEP 2022)
Tim Blömeke teaches technical and recreational diving in Taiwan and the Philippines. He is also a freelance writer and translator, as well as a member of the editorial team of Alert Diver. For questions, comments, and inquiries, you can contact him via his blog page or on Instagram.