Oxygen and Rebreathers
Oxygen is a poison yet rebreathers give you more. Lots more. The effect manifests itself in pulmonary toxicity and in CNS toxicity, where your body goes into an uncontrollable fit.
Understanding tolerance to Oxygen is important but is not yet understood by science. It can vary enormously between different people. It can vary enormously in the same person on different days. It depends on what you eat, how hydrated you are, your vitamin and supplement consumption and factors we don’t even know. This is why divers still take CNS hits when diving within supposedly safe limits.
But there is more. Much more.
Oxygen is a pulmonary vasodilator and a periheral vasoconstrictor. So more oxygen opens the blood vessels in your lungs and closes them around your body. On the surface this is great because your lungs get more oxygen in (and CO2 out) but that oxygen isn’t wasted by being sent to where there is already enough.
Increase the PPO2 though and this mechanism causes problems. The lung blood vessels open excessively which inflames the lung tissue. Fluid then builds up in the alveolei reducing their capacity for gas exchange. Meanwhile the blood vessels round your body close down during the dive reducing your capacity to offgas from the surrounding tissues.
These problems build up steadily during the dive and only reverse very slowly. So you need to keep your PPO2 low, the opposite of what a rebreather does. We still need to deco at high PPO2 because the advantage is so great, but even then it is good to cycle to a lower PPO2 to prevent the buildup of these symptoms. These gas breaks are exactly what a rebreather doesn’t give you.
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3 Responses to “Oxygen and Rebreathers”
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Luke Sanders
Said this on January 18th, 2007 at 9:02am:I’d like to see your evidence behind this – it sounds an awful lot like scaremongering to me, and I have very extensive knowledge of how the body works. I think you’re referring to pulmonary oxygen toxicity when you talk about the lungs filling with fluid but you’re wrong how this works. This is caused by long term exposure to levels of oxygen below that seen in technical diving (perhaps 0.7 bar) and we’re talking many hours of exposure. It’s caused by the production of oxygen free radicals in excess of what can be scavenged by the protective cells in the lungs. You’re looking at 12-16 hours of constant exposure to cause this, though, and it’s not really a problem seen in diving. Maybe if you get potted (hence the air breaks in the pot), but not so much in the water.
Stewiebouy
Said this on January 22nd, 2007 at 12:34pm:Are you recommending that we keep the PPO of our nitrox mixes down, if so has there been any research into what is considered an acceptable risk for ppo levels. I have experienced a feeling of agitation and discomfort whilst diving a wreck that I had on previous occasions dived on air, the PPO was 1.6 at a depth of 28 meters. Conditions were good with no other reason for apprehension etc. Could it be possible that the oxygen ppo had somthing to do with it? It was quite a long dive but the feeling evaporated at my fist deep stop, I have never had this feeling since but tend to stick to 1.4 ppo or less
cloudboy55
Said this on January 27th, 2007 at 2:22pm:Perhaps this article should be entitled “The importance of air breaks during decompression”. I don’t really see any reason that using a rebreather requires one to use a a PPo2 – I could take open circuit air breaks as easily on a rebreather as on open circuit. Conversely, I could plan an open circuit dive that includes no air breaks and mimick the effect you’re referring to.
cloudboy55