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Calculated Confusion: Can O2 Get You High?

Anyone who’s dived to 30 meters on nitrox (we don’t do air here!) is familiar with what Jacques Cousteau eloquently termed “rapture of the deep”—the result of breathing high partial pressures of nitrogen. But what about the oxygen? As you may know, there are some convincing arguments that oxygen should be considered narcotic at depth as well, but there are also many qualifiers, anecdotes (ever feel narc’d on your 20-foot O2 stop?), and a broad swath of research and nuance that casts some doubt. Diver Alert Network’s Reilly Fogarty teases out what we know and what we don’t. Mind your ENDs!

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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. 

Gas Solubility Coeficients of gases in water and oils - used by the Myer-Overton hypothesis to infer the narcotic potential of breathing gases, but can also be used to deduce tissue gas loading in disolved gas Haldanean decompression models
Fig 1: Solubility of gases in water and oil (Dueker)

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. 

Photo by David Rhea, 2004.

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.

Narcotic Nuances

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: 

Meyer-Overton

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. 

Environmental Concerns

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. 

Photo from the GUE archives.

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.

Best Practices

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.

Works Cited:

1. Scuba Diving in Safety & Health by Chris W Dueker, MD

2. Diffusion Coefficients for Gases in Biological Fluids and Tissues

3. DOES OXYGEN CONTRIBUTE TO THE NARCOTIC ACTION OF HYPERBARIC AIR?

4. Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis

5. THE CORRELATION BETWEEN CRITICAL ANAESTHETIC DOSE AND MELTING TEMPERATURES IN SYNTHETIC MEMBRANES


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.

Diving Safety

Why Do Divers Run Out Of Gas?

Not surprising, the answer is more complicated than simply, they neglected to look at their gauges. Here Aussie diving medical researcher and former editor of DAN’s Annual Diving Report, Peter Buzzacott dives into several deep datasets including DAN’s Incident Reporting System (DIRS) and nearly four decades of cave diving incident data, to tease out some insights on gas emergencies and get a handle on the risks. Don’t stop those S-drills!

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by Peter Buzzacott

See companion story for a guestimate of the risk: What is the Risk of Running Out of Gas?

Next year it will be 30 years since I first learned to dive. At the time, I had no idea that diving would occupy such a large part of my life. I distinctly remember kneeling on the sandy bottom end of the Great Barrier Reef, sharing a regulator with my buddy, and seeing sunlight rippling down through crystal clear water. On one of these “confined water” dives we had to swim horizontally for 10 m/30 ft holding our regulators out of our mouths and blowing a steady stream of bubbles. This wasn’t as easy as it sounds and we had to ration our bubbles to make it the whole way. Then, on an open water dive, the instructor took turns holding us with one hand and gripping a rope with the other while we took a breath, took the second stage out of our mouths, and then went for the surface, breathing out all the way. Up, down, up, down, the instructor went, with each student—one at a time. 

Courtesy PADI Worldwide. Copyright 2021, used with permission.

Most of today’s recreational dive courses do not include buddy breathing, they teach gas sharing with an alternate air source (AAS). Even before COVID-19, the buddy breathing skill had disappeared from most recreational training programs. The controlled emergency swimming ascent (CESA) has also disappeared from some programs. 

When I became an instructor, I made many hundreds of these but, now that I think about it, I don’t recall ever seeing anyone actually make one for real after running out of gas. These days everyone dives with two second stage regulators. In technical diving, we even dive with at least two cylinders; so, I wonder, do technical divers run out of gas and, if they do, then why?

What Do The Experts Say?

Some years ago, I asked a panel of 27 diving experts a similar question regarding recreational divers in general.1 The panel consisted of nine diving/hyperbaric doctors who had treated hundreds of injured divers; nine expert dive guides, most of whom were instructors; and nine expert recreational divers who had dived all over the world and written hundreds of feature articles for dive magazines. 

At the time, I suspected divers mostly ran out of gas because they didn’t pay attention to their gauge. But, to my surprise, the experts suggested about 20 reasons, such as diving deeper than usual, diving in a current, not wanting to end the dive for their buddy, using a smaller tank than their buddy, being underweighted, and many others, all of which sounded plausible. 

I sent the whole list of potential causes back to the group and asked them to rank, in their opinion, the five most likely causes. Then I gave five points to everyone’s most likely potential cause, four points to the second most likely, and so on. I added up all of the points and then ranked all the causes according to the total score. Then I sent this ranked list back to the group for one last review and asked them to consider the “weight of opinion” from the group as a whole, and to reconsider their top five reasons. 

As an expert panel, the group moved toward consensus. Just as I’d suspected, failing to monitor the gauge was the number one proposed potential cause of running out of gas, followed by inexperience, overexertion, inadequate training, and poor dive planning. Other than perhaps an unexpected current or underweighting leading to overexertion, the proposed reasons leaned toward human factors rather than the other two types of factors in the classic diving injury causal triad—those being environmental factors and equipment factors (Figure 1).2,3,4

Figure 1: The classic diving injury causal factors triad 2,3,4

The process I’d followed to gather expert consensus of opinion is called a “Delphi” process, which was originally developed by International Business Machines Corporation (IBM) to make forecasts on matters about which there was considerable uncertainty i.e. where there is little data. Opinions aren’t solid evidence; however, they can point towards a direction worth investigating. 

Next, I visited Divers Alert Network(DAN) as an intern and worked on an analysis of diving fatalities within a subset of technical divers—cave divers. More on that later, but while there, I had the opportunity to examine a large dataset of recorded dives from Project Dive Exploration, headed by Drs. Richard Vann and Petar Denoble. 

The dataset we had at that time revealed over 50,000 dives recorded by more than 5,000 recreational divers, (including an unknown number of technical divers). We examined these data in two ways. First, to control for environmental and equipment factors, and to focus on demographic (or human) factors, we counted each diver just once and compared those divers who had reported running out of gas, (during any recorded dive in that dataset), with divers who had not run out of gas. Surprisingly (to me), having run out of gas was more common than expected among older females (males were more likely to report other problems, like rapid ascent). 



Next, to control for the human factors, we looked at just the dives made by divers who had made both at least one dive where they ran out of gas, and at least one dive where they did not run out of gas. I wanted to know what it was about those dives that might have caused the divers to run out of gas. Well, it turned out the out-of-gas dives were deeper, shorter (probably because they were deeper), often made from a live-aboard or charter boat, and involved a higher perceived workload.5 Hmmm… Perhaps overexertion was a factor after all.

After returning to Western Australia to undertake a PhD researching this, I spent the next few years recording 1,000 recreational dive profiles made by 500 divers. I recorded their start and end pressures, tank size, and noted factors such as current, how they felt their workload was (resting/light, moderate, or severe/exhausting), how many dive experiences they had, and what previous dive training they had completed. For the analysis, dives made by divers who exited with <50 bar/725 psi of pressure (needle in the red zone, n=183) were compared with other dives recorded at the same time at the same dive site (n=510) by divers who exited with >50 bar/725 psi pressure remaining (needle not in the red zone). 

Ending a dive low on gas was correlated with younger males with a longer break since their last dive, fewer lifetime dives, at deeper depth, and a higher rate of gas consumption (adjusted to an equivalent surface air consumption (SAC) rate, for comparison between dives made at different depths). Perhaps more tellingly, compared with 1% of the dives with >50 bar/725 psi at the exit, 11% of the low-on-gas divers reported being surprised at the end of the dive by how low their remaining gas pressure was.6 A more detailed analysis of the average workload associated with recreational diving, using this same dataset, identified that higher perceived SAC rate was not associated with sex but was associated with older age, lower dive certification, fewer years of diving, higher perceived workload, and other factors.7 

Technically Out of Gas

Returning to the topic of technical diving, a colleague and I re-examined the DAN cave diving fatality reports collection that I had worked with as an intern, and this time we concentrated on the previous 30 years of data: 1985-2015. Dividing it into two equal halves which we referred to as the “early” and “late” groups, reading each report carefully, and using a reliable cave diving fatality factors flow-chart previously developed,5 we classified factors associated with each cave diving fatality and then compared the two groups. 

In the late (more recent) group, the proportion of cave divers who were trained in cave diving had significantly improved, perhaps due to increased awareness of the need for proper cave diver training before entering a flooded cave. The majority of the 67 trained cave divers in our dataset were diving with two cylinders on their back (doubles), and the late group was diving further into the cave than the early group. Of the 67 trained cave divers, 41 (62%) had run out of gas. Looking at the five “golden rules” of cave diving, the “rule of thirds” was the most common (n=20) rule that was suspected to have been broken by the trained cave divers: the most lethal.9

So, it would seem that some technical divers do run out of gas, though thankfully that appears rare. We should bear in mind that cave divers may differ from other types of technical divers in their procedures, demography, and equipment; their environment (by definition) certainly differs from that of wreck divers. 

Currently, I know of no ongoing research into out-of-gas incidents among technical divers, other than the current Diving Incident Reporting System, hosted by DAN. An analysis of the first 500 reported incidents recently examined every incident—recreational and/or technical—during which the diver ran out of gas.10 The sample (n=38) was divided into two groups: those who made a controlled ascent (e.g. on a buddy’s donated regulator) and those who made rapid ascent (e.g. a bolt to the surface). 

Among divers who reported having run out of gas, but survived to report the incident, 57% of the rapid ascents resulted in a reported injury. Among the 24 controlled ascents, just 29% reported an injury.10

Among divers who reported having run out of gas, but survived to report the incident, 57% of the rapid ascents resulted in a reported injury. Among the 24 controlled ascents, just 29% reported an injury.10 This modern finding is in line with the statistics reported 27 years ago by Dr. Chris Acott when he analyzed more than 1,000 diving incident reports. Examining 189 out-of-gas incident reports, Dr. Acott found 89 made a rapid ascent, and 58% of those reported an injury. Among the 79 controlled ascents, only 6% reported an injury.11 

Table 1 shows the total number of dive incidents in each category, after adding both studies together. It seems to me that, while we have moved on from buddy-breathing and the controlled emergency swimming ascent, in the last 30 years the problem of running out of gas has not gone away. 

No Injury
(row %)
Injury
(row %)
Total
(col %)
Non-rapid ascent 91 (88)12 (12)103 (50)
Rapid ascent43 (42)60 (58)103 (50)
Total134 (65)72 (35)206 (100)
Table 1: Injuries among 206 out-of-gas dive incidents by ascent rate10,11

In conclusion, the evidence confirms what we all know: running out of gas is associated with diving injuries and fatalities. It appears that the level of correlation of demography information (like age and sex) with out-of-gas incidents may depend upon the study design, the pool of divers studied, and/or the specific potential causes of running out of gas being investigated. For example, in one study, older females were more likely to self-report out of gas problems; in another study, young males’ remaining gas was measured and observed to be low. In yet another study, SAC rate increased when perceived workload increased, regardless of sex. 

Therefore, I’d suggest it is prudent to consider everyone potentially at risk of running out of gas and, in order to mitigate this risk, both recreational and technical divers should be proficient in gas planning and monitoring their remaining gas, regardless of age and/or sex. 

[Ed.note—Most agencies today require some level of proficiency in managing emergency out of gas scenarios. For example, GUE requires divers at all levels to train regularly for this eventuality. This training also emphasizes gas management strategies like “minimum gas reserves” and the related “one third” rule to ensure divers always have enough supply to share gas aka buddy breathe from any point in the dive, and all the way to the surface. Violation of these strategies risks insufficient gas in all environments.]

Influencers

The influence of workload is interesting, and technical divers who perceive an elevated workload may well remember that this has been associated with both higher rates of gas consumption and unexpectedly running low on gas. So, when detecting a current or perceiving an elevated workload, I recommend keeping a closer-than-usual eye on the remaining gas and, if a current is suspected before the dive, then plan for an elevated SAC rate. 

The influence of training/certification consistently appears to be associated with the risk of running out of gas, as does having made fewer lifetime dives. Highly trained and experienced divers might bear this in mind when diving with buddies who are newer to our sport. Offer them opportunities to gain experience and recommend additional training when they are ready. We were all inexperienced once.

Technology has improved in recent years; for example, tank pressure transponders are more reliable today than ever before. It is possible that in the future these resources, coupled with audible alarms, may prove to be highly effective at preventing technical divers from running out of gas. Until we know how effective such alarms are at preventing out-of-gas dives, our best course of action is to dive within the limits of our training and experience, and to keep an eye on our remaining gas. 

See companion story for an estimate of the risk: What is the Risk of Running Out of Gas?

Do you think that it could it happen to you?

References

1. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.

2.Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.

3. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.

4. Buzzacott P, Rosenberg M, Pikora T. Using a Delphi technique to rank potential causes of scuba diving incidents. Diving and Hyperbaric Medicine. 2009;39(1):29-32.

5. Buzzacott P, Denoble P, Dunford R, Vann R. Dive problems and risk factors for diving morbidity. Diving and Hyperbaric Medicine. 2009;39(4):205-9.

6. Buzzacott P, Rosenberg M, Heyworth J, Pikora T. Risk factors for running low on gas in recreational divers in Western Australia. Diving and Hyperbaric Medicine. 2011;41(2):85-9.

7. Buzzacott P, Pollock NW, Rosenberg M. Exercise intensity inferred from air consumption during recreational scuba diving. Diving and Hyperbaric Medicine. 2014;44(2):74-8.

8. Buzzacott P, Zeigler E, Denoble P, Vann R. American cave diving fatalities 1969-2007. International Journal of Aquatic Research and Education. 2009;3:162-77.

9. Potts L, Buzzacott P, Denoble P. Thirty years of American cave diving fatalities. Diving and Hyperbaric Medicine. 2016;46(3):150-4.

10. Buzzacott P, Bennett C, Denoble P, Gunderson J. The Diving Incident Reporting System. In: Denoble P, editor. DAN Annual Diving Report 2019 Edition: A Report on 2017 Diving Fatalities, Injuries, and Incidents. Durham (NC): Divers Alert Network; 2020. p. 49-67.

11. Acott C. Diving incidents – Errors divers make. Safe Limits: An international dive symposium; 1994; Cairns: Division of Workplace Health and Safety.

12. Buzzacott P, Schiller D, Crain J, Denoble PJ. (2018). Epidemiology of morbidity and mortality in US and Canadian recreational scuba diving. Public Health 155: 62-68. 

13. Buzzacott P. (editor) (2016). DAN Annual Diving Report 2016 Edition: A report on 2014 data on diving fatalities, injuries, and incidents. Durham, NC, Divers Alert Network

14. Buzzacott P (editor) (2017). DAN Annual Diving Report 2017 Edition: A Report on 2015 Diving Fatalities, Injuries, and Incidents. Durham (NC), Divers Alert Network.

15. Buzzacott P and Denoble PJ. (editors) (2018). DAN Annual Diving Report 2018 Edition: A report on 2016 data on diving fatalities, injuries, and incidents. Durham, NC, Divers Alert Network

16. Denoble PJ. (editor) (2019). DAN Annual Diving Report 2019 Edition: A Report on 2017 Diving Fatalities, Injuries, and Incidents. Durham (NC), Divers Alert Network.

You can add a diving incident to the DAN database by name or anonymously here: Diving Incident Reporting System (DIRS).


Dr. Peter Buzzacott MPH, PhD, FUHM, is a former PADI Master Instructor and TDI Advanced Nitrox/Decompression Procedures instructor, having issued >500 diver certifications. Today he is an active cave diver, holding various advanced cave diver certifications including advanced (hypoxic) trimix diver, and he is gradually gaining experience with CCR diving. To finance this, he conducts research into diving injuries and decompression/bubble modeling at Curtin University in Perth, Western Australia.

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