Have Some Control of a CNS Hit
When you did your Enriched Air 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 subside within a few minutes. The convulsion 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 molded by one of the equipment manufacturers either in fashionable black or in yellow or even home made using two half hitches.
Being the Alpha male that I am, I like to be in control of what I am doing at any particular time. So if I were to get 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.
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. 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 then any of the other traditional methods. It’s also then easier for your buddy when assisting you. I have even done this with my bottom gas bail out cylinder for my rebreather dives.