Have Some Control of a CNS Hit

By Pat Voorma, PADI Course Director and TecRec IT.

When you did your Enriched Air Nitrox Course (EANx) you would have read about Central Nervous System Toxicity and Pulmonary Oxygen Toxicity. Your instructor would have gone to some lengths explaining to you the signs and symptoms of these two real dangers of increased exposure to high partial pressures of Oxygen.

The most common taught acronym for CNS Toxicity is VENTID or even ConVENTID. The early warning signs are Visual Disturbances, Ear Ringing, Nausea, Twitching, Irritability and Dizziness and then a Convulsion. However some people have had no warning and have had a convulsion. The convulsion will be much like an epileptic convulsion and will hopefully subside within a few minutes. The convulsion in its self  is not really harmful but the fact that we are underwater with a regulator as our only means of getting sufficient breathing gas means that once we convulse we are unable to retain the regulator in our mouth and will inhale water and drown.

If you are lucky enough to get some of these early warning symptoms then the only real remedy is to switch to a gas with a lower PPO2 and ascend to drop the PPO2 even further.

When diving Open Circuit Technical diving you would traditionally attach your back up regulator around your neck with a necklace. The most common being those moulded by one of the equipment manufacturers either in fashionable black or in yellow or even home made using two half hitches.

necklace 2necklace 3necklaces

As a Technical diver I like to be in control of what I am doing at any particular time during the dive. Our training and drills have ensured that we have left nothing to chance. This starts from the initial planning of the dive using the A Good Divers Main objective Is To Live acronym right through to the S Drills and bubble and descent checks at the start of the dive to the NOTOX drills at the gas changes. So if I were to feel the onset of CNS Toxicity I would like to be able to do more than just go onto back gas and ascend and hope for the best


This is when I came up with a simple solution. Take a piece of bungee and cut it the correct length for a necklace. Tie a knot in each end. Remove the cable tie holding your secondary regulators mouth piece and place the two ends on either side of the regulator. Replace the cable tie and pull tight as per usual.


Gag 1

Gag 2Gag 3

Now you have a necklace with a difference. If you were to feel the first warning symptoms of a CNS hit you could immediately switch to your back gas necklace regulator. Put it in your mouth and pull on the two ends of the necklace. Even though it is held in place by the cable tie you will find that is actually pulls tight quite easily. Pull it tight enough so as to keep the regulator in your mouth. Should you have a convulsion you at least have a better chance of retaining the regulator in your mouth than any of the other traditional methods. It’s also easier for your buddy when assisting you as it would free up the hand that would have otherwise been used to hold the regulator in place during a rescue. I have even done this to the BOV regulator on my rebreather and on the bottom gas bail out cylinder for my rebreather dives.

Safe Diving!


%d bloggers like this: