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Scuba Q&A: CO2 Narcosis and Hypercapnia

CO2 narcosis and the link with hypercapnia when scuba diving

In this scuba Q&A article, I will address the issue of CO2 narcosis and hypercapnia.  CO2 narcosis is far more commonly encounter than nitrogen narcosis. Knowing the signs and symptoms of CO2 narcosis will help you better understand, avoid and deal with the problem.

QUESTION:  I was having trouble with headaches off and on during diving, which we think might be due to CO2 retention. It improved a lot with taking very deliberate deep breaths rather than what I thought were deep breaths. I also experienced some nitrogen narcosis for the first time. 113 ft on 31% nitrox, which is normally a depth that I’ve been to multiple times before and haven’t had any issues with. I think the precipitating factor was that I had taken some Dramamine before, and wasn’t feeling anywhere near 100% before the dive. It resolved at 90 ft, and I felt totally normal at 70 ft. I think the biggest symptom of narcosis for me this time is anxiety, in addition to very fuzzy feeling head. Sometimes notice very mild symptoms when I’m below 90 ft for a while, which I definitely have done more on this trip than before because my air consumption has been good, and the limit for me is the ndl time rather than air.

ANSWER: It’s likely that the narcosis you experienced was a result of hypercapnia.

What is Co2 narcosis?

CO2 is far more narcotic (anesthetic) than nitrogen. It typically onsets much more quickly given that it’s an internal product of respiratory metabolism, rather than having to be absorbed into the body as a result of partial-pressure differential. CO2 narcosis is commonly known as ‘dark narc’ because it usually manifests through psychological symptoms of unease; building up into an inclination to active panic when severe.

CO2 narcosis can diminish quickly as you ascend – because you’re lowering the partial pressures and you’ll expire it rapidly as a natural course of respiration. The problem being that someone ascending may have already entered a stress response due to the narcosis – which leads to their respiratory function becoming less efficient (rapid, shallow breathing).

In contrast, nitrogen narcosis is much slower in onset; taking circa 15mins to become observable at a typical depth of 30m/100ft (according to studies). It is also persistent; unlike what is commonly taught, it doesn’t go away on ascent – the body has to off-gas that nitrogen over time. Again, in studies, persistent nitrogen narcosis was recorded persisting over 30mins after surfacing from 30m/100ft dives.

co2 narcosis scuba diving hypercapnia gas density

How to reduce Co2 narcosis

Reducing hypercapnic effect requires a three-pronged strategy. I’ve ranked these in order of effectiveness below:

Reduce exertion: Improving fundamental diving skills and configuring one’s equipment to be more efficient and effective reduces physical demand during a dive. Where those issues aren’t optimized, one can temporarily reduce exertion by consciously slowing down the pace of a dive and/or stopping to take a break (with deliberate respiration to help flush out CO2).

Reducing gas density: DAN have published guidelines on recommended maximum gas densities. Higher breathing gas density reduces respiratory efficiency leading to CO2 retention. According to those guidelines, a maximum recommended gas density of 5.2g/L is reached at circa 32m when breathing air. Beyond that depth helium can (should?) be added to reduce gas density in the breathing mix. Some diving agencies have embraced those recommendations, others have utterly ignored it and, thus far, declined from even teaching the issue to students. A more comprehensive explanation on gas density can be found on the DAN website: https://dan.org/alert-diver/article/performance-under-pressure/

Improving cardiovascular fitness. It’s better to deal with the causes (see points 1 & 2) rather than the symptoms. However, improved cardiovascular fitness will decrease CO2 metabolism for a given level of exertion and the corresponding respiratory demand.

It’s also worth noting that there is both individual variance in one’s susceptibility to CO2. There are published US Navy studies that show some people have a more attuned innate response to elevated dissolved CO2 (their respiratory rate/depth naturally increases quickly), whereas others physiologically respond more slowly to elevated CO2 levels; they need to consciously act to change their respiratory function. People can also physiologically acclimate to higher CO2; for instance, smokers develop more tolerance to CO2 (I’m not suggesting to start smoking!).


About the Author

andy davis technical diving philippines

Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia.  Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.

Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.

He is currently writing a series of books to be published on advanced diving topics.  Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.

Andy Davis technical diving subic bay philippines


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