Low oxygen levels are not fun, neither on land nor underwater. We need oxygen to survive, plain and simple, so we have to do whatever it takes to have it running in our bloodstream. In an interview with Tokushikai Canada, Sheena McNally, Canada's deepest woman, tapped into the uneasy world of hypoxia. What is hypoxia? What is the difference between a blackout and an LMC? Why do they occur? Find out. 


"We have a couple of terms for hypoxia, which is low oxygen, so I guess the most serious one would be a blackout, which is a loss of consciousness due to low oxygen. Typically that would happen at the end of the dive when the oxygen level is the lowest and, if it's going to happen, it would be on the surface, as the diver releases the first breath that they were holding throughout the dive, and then they start doing their recovery breathing.

Unfortunately for us, there's a delay in the time it takes for that oxygen to get into our lungs, into the alveoli, into the bloodstream, and then, you know, to the heart and then to the brain. So in that time, if we're close to our edge we can have cerebral hypoxia, which’s a blackout due to low oxygen. The body's not out of oxygen, we're not at zero, the figure that they teach in the AIDA courses is that we're probably around 50%, but I think it would vary individual to individual. But what's happening is the body is prioritizing oxygen use.

Being conscious takes a lot of oxygen, like looking at things and listening to things and thinking, and we don't need to be conscious to be alive, so I picture it as the body going into like a computer sleep mode type of thing. It's not desirable by any stretch of the imagination, but I think it's important to understand that you're not actually out; it's your body making a strong effort to actually preserve life. During freediving competitions, you will see people experience this and you will also see them get rescued in a very quick and calm manner; they'll be breathing again in a couple of seconds.


The more common, but I would say the less serious type of hypoxia, is an LMC, which stands for “loss of motor control”. In this form of hypoxia, the diver remains conscious, or you could say semi-conscious. The oxygen level is low, but it's not quite low enough for a full loss of consciousness. So your normal cellular functions are going to be disrupted because there isn't quite enough oxygen and what you see if you're watching someone have a loss of motor control is like an interrupted motion. A lot of like sort of shaking motion, usually in the head, the chin, maybe some wide unfocused eyes. Or it could be more subtle as well, sometimes if they're grabbing high on the comp line, maybe just the hand is shaking, or maybe just a foot twitches underwater.

Τhat would be, I'm gonna use the term, less serious, not to say that it's not serious, but sort of easier to manage from a safety point of view, since the diver is still conscious.

On A Personal Note


Ιn 2019 I had my first proper experience with hypoxia and it was totally my fault. Ιt was a four-day long competition which isn't normal, usually, there's kind of like two days and then a break. Βut because it was a short comp there was no break, it was kind of on the athlete to take a break, if they thought they needed one. Ι was doing dives that were quite close to my limit.

In the first two days, I did a record in free immersion for Canada and also another one with bifins, and I did that one in quite a strong current. Then I made a lot of mistakes after I stayed out in the water for seven or eight hours, watching my friends compete those were about 60 divers, so it was quite a long day. I had this water bottle, which holds one liter, which is not enough for all these hours in the sun, I can tell you that definitively no, and I didn't have food as well.

When you're in the water and it's hot - you're in the Caribbean - it feels great. You don't realize that you're getting sunburned you know, you just don't realize what's happening until you get out of the water. When I got out of the water, I was on the taxi home and I realized my face was lobster red, I was dehydrated, I was starving and I was like “oh this isn't good”, but because I was stubborn and because previously I'd been able to get away with quite a bit, in terms of how I treated myself the day before a dive, I was just thinking “okay, as long as you rehydrate and have a good sleep and have a good meal, everything will be fine”. Well, it wasn't fine! On top of that, the dive that I was trying the next day was my personal best, so mentally there's mental stress, and it was going to be a continental record, so I really wanted it.

So I dived and at the end of it, I had my first proper LMC. It was a big one, it lasted about 17 seconds, which is quite big for an LMC. So I disqualified myself during that, but I got stubborn and I was like “oh you know that was just a one-off thing and I'm sure if I fix my mistakes I can dive tomorrow” which was a mistake. I’d heard over and over that, if you are hypoxic, your body and your mind have kind of like almost like muscle memory for that. It's like a neural pathway; if you create that pathway and then you use it, it's easy to then just sort of use it again soon.

So what I should have done would be either just don't dive the next day or pull back, maybe take 10 meters off and just do a dive for fun. But I was stubborn, I was like “I’m going to try the same dive again” and the result, which was absolutely no mystery, was a super small surface blackout; the first one and actually - knock on wood - the only one I've ever had.

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