Minimizing freediving risk part 1: shallow water blackout

Minimizing freediving risk part 1: shallow water blackout

Disclaimer: this article is not a substitute for safety training, and is not meant to teach you how to dive alone. Always dive with a buddy, and train with a suitable instructor.

What prompted me to write this was an anecdotal shallow water blackout story. Shallow water blackout is the result of insufficient oxygen delivered to the brain. If this happens without a diving buddy it will nearly inevitably lead to death. The chances of a diver surviving with a trained buddy and good safety protocol is close to 100%.  The anecdotal evidence of freediving blackouts is nearly always the same: 1) the freediver does not see it coming, 2) it can occur after a dive to any depth, 3) it can occur after a dive of any duration. Shallow water blackouts are common with other breath hold activities too, such as synchronized swimming and underwater hockey and have in some cases led to death. Always remember that if your airways are submerged, you can drown. Two inches or twenty meters of water feel the same to an unconscious person.

Minimizing the risk of a shallow water blackout

Hyperventilation

Hyperventilation used to be common practice in freediving in order to reduce the urge to breathe during freediving. Hyperventilating has the effect of removing CO2 from the body. Removing CO2 from the body has several effects: 1) it will take you longer to become hypercapnic. This means that it will take you longer to accumulate enough CO­­2 to feel the urge to breathe in the form of contractions and so on. 2) It makes your blood more alkaline. This has the scary effect that hemoglobin, the red blood cells that transport oxygen, are more prone to hold on to their oxygen. Think of this for a second. A freediver hyperventilates to not feel an urge to breathe, causing them to think everything is ok, while their blood does not deliver oxygen to the necessary tissues as effectively anymore. Sounds like a recipe for shallow water blackout, right? Hyperventilating is therefore a big ‘no’. Some courses do however teach their students to do a specific amount of ‘purging’ breaths, decreasing CO2 while still being safe. Although there is nothing wrong with this method if applied conservatively, it does open the door to abuse. Think of the little voice in your head that says ‘well, I can just purge a little harder and my dive will be more relaxed..’, or ‘If I just do 10 purging breaths instead of 8, it is probably still ok…’. I would personally argue that if you want to create the biggest safety margin, you stay away from any form of hyperventilation, including purging.

an example:

exerpt from the description:

“After consulting with some freediving instructors, I have realized that my breathe up wasn’t optimal and that instead of purging (which I thought I was doing), I was hyperventilating on my breathe up.”

Subconscious hyperventilation

Subconscious hyperventilation can be a big problem too. Here it helps to dive in only the best conditions; where you do not need to move at all during your breathe-up. Some studies indicate that exertion may cause hyperventilation, your body prepares anticipates a build up of CO2 and starts preventative measures (see here, and here for some open access material). If a diver is unaware of this, and even worse, adds some conscious hyperventilation or purging breaths into the mix, the risk of suffering shallow water blackout becomes very high even at shallow short dives.

Exhaustion

Although I am unsure of the exact mechanisms, there is anecdotal evidence that exhaustion places a diver or swimmer at increased risk of blackout (see here). The data here does not so much apply to freedivers, as the statistical database may not be big enough to find such a correlation. However, all freedivers know that apnea capabilities change from day to day depending on mood, mindset, tiredness and so forth. It is definitely in your best interest to be well rested and in the right mindset before diving. Exhaustion may also occur during the dive session, for example if you need to do a long surface swim, or kayak paddle before diving, or if there is a lot of current or swell at the dive site.

Depth and the low oxygen zone

Partial pressure changes at depth make deep dives more dangerous. Here is a quick recap on how this works: gases react with your body in accordance with their partial pressure, not their absolute concentration. Let’s consider a 30 m dive. At 30 meters, the oxygen in your lungs will react at 4 times the rate (because you are at 4 atmosphere). This means that if you are at 30 meters until you feel like you have 16 % oxygen left, you have an absolute amount of oxygen left of 16% / 4 = 4%. When you ascend the partial pressure will become lower and the chemical balance between the oxygen in your blood and the oxygen in your lungs start to change. In the low O2 zone oxygen will move back from your blood into your lungs. Exactly where this happens depends on a set of variables like the O2 consumption of the diver and the depth. Since most blackouts happen from 0 – 10 m (for the reasons described above) we consider 0 – 10 m the anecdotal low O2 zone. But how do you know what depth you can dive to on a specific day? You don’t, until you hit the limit, which you don’t want to. In a recreational session where you do not have a line or if you dive without a buddy (or with a buddy in compromised conditions such as poor visibility), you will need to take baby steps toward a maximum depth that still contains a large safety margin.

Carbon dioxide accumulation

CO­2 accumulation can be as large a risk as a CO­2 deficit. The difference is that CO2 accumulation comes from an external CO2 source. There is no confirmed story of a blackout in which CO2 accumulation played an integral role, but because it can potentially cause blackout I have chosen to include it. CO2 accumulation can be caused by standing in traffic, diving next to a powerboat with a faulty engine or even diving next to a big idling boat with the wind in the wrong direction. If you are breathing in exhaust fumes you are likely breathing in air that has a lower O2­ concentration and a higher CO2 concentration than you want to. In my case, I was concerned about this when I was diving in the Maldives with only a surface safety and I could occasionally ‘taste’ the garbage burning facility through my snorkel. Too much pre-dive CO2 does the same as an accumulation of CO2 during the dive: it kicks O2 off the red blood cell. This is good when you are diving (generate more CO­2 and you release more O2) but pre-dive it means you start with less O2 upon descent. Even worse is that you may not notice it at all.

shallow water blackout
It keeps coming back to the delicate balance between oxygen and carbon dioxide.

Entanglement

Entanglement is a huge hazard when freediving without a buddy, or when diving in poor visibility. Of course it does not directly increase your chances of a shallow water blackout, but it does so indirectly. If you can get loose, it will have increased your dive time and commonly increase your heart rate and oxygen consumption. I have been entangled once and did not really enjoy the experience (saved by my diving buddy!). The risk increases when spearfishing around uneven bottoms or in kelp forests. You can partially mitigate the risk by making sure that anything that can get tangled can either be cut (wear a knife) or ditched easily. Make sure that you can ditch your speargun, and can reach your knife easily. I like wearing a knife on my upper leg, so that I can reach it with either hand.

In part 2 we will get into the details of safety protocols, when they work and when they don’t. Let me stress again that this article is not meant to teach you how to dive alone. Instead it is written in order to create an awareness of risk in freediving, and an idea of how to mitigate that risk while freediving. Always dive with a buddy. 

 

 

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Jaap

Jaap is a geologist by trade and a freediver by passion. Jaap wrote the book Longer and Deeper in 2018. His book teaches how to train for freediving and spearfishing on land.

This Post Has 2 Comments

    1. Jaap

      Thanks Rick 🙂

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