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Central Nervous System - Oxygen Toxicity

Dr Jules Eden, dive medicine specialist and founder of e-med, answers divers' questions - as published in Sport Diver magazine:
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Q - I've just done my Nitrox course and learned lots about gases and partial pressures, but would like to know how they worked out what is a safe partial pressure of oxygen to dive with?

A - Let’s recap on some elementary physics. Dalton’s Law of partial pressures is the one that’s relevant here. Air is approximately 79% nitrogen and 21% oxygen, give or take; so at the surface (0m, or 1 bar), the partial pressure of nitrogen is 0.79 bar, and of oxygen 0.21 bar. At 60m (or 7 bar), we’re getting a nitrogen partial pressure of 0.79 x 7 = 5.53 bar, and an oxygen partial pressure of 1.47 bar. To check, 5.53 + 1.47 = 7. Awesomely simple. The precise point at which oxygen becomes toxic will vary between individuals (and also in the same individual at different times), but some guide to its rough whereabouts is provided by the research conducted in the 1930’s and ‘40’s by various gung-ho Naval recruits. The Medical Officer to the Admiralty Experimental Diving Unit, a chap called Kenneth Donald, conducted experiments on 2000 Naval “volunteers”, and concluded that 7.6m for 2 hours was the safe limit for pure oxygen diving without risk of fitting. Later work collating all the available human data set the threshold for convulsions at 1.7 bar, and the level for any symptom of CNS toxicity at 1.3 bar. Hence most training organisations will recommend maximum operating depths (MOD’s) giving oxygen partial pressures within the 1.4 – 1.6 bar range, depending on exertion and time spent at depth.

Q - This is not a diving-related question, but I am a diver, so forgive me for picking your brains but I hope you can help. I'm diabetic and due to start treatment in a hyperbaric chamber for an ulcer on my heel that just won't heal. The doctors treating me have mentioned fits as a possible side effect of the treatment, which naturally worries me slightly! How common is this, and what increases the risk of it happening? Are there any early warning signs of oxygen toxicity, or ways to reduce it? Obviously I want to try anything that will help my foot but don't want to become epileptic in the process.

A - I think I can reassure you that the risk is very, very low. Convulsions are the most dramatic end-result of central nervous system oxygen toxicity, and thankfully “doing the funky chicken” is a rarity in modern day diving and hyperbaric practice. Often a seizure will be preceded by other signs such as muscle twitches or spasms (often in the lips), anxiety, irritability, nausea or (my favourite) “a sense of impending doom” (think Private Frazer, the dour wild-eyed Scotsman in Dad’s Army). At this stage, further toxicity can be avoided simply by removing or reducing the inspired oxygen. During hyperbaric treatment (for divers and non-divers alike), “air breaks” are used for precisely this reason – 5 minute periods of breathing normal air will reduce the body’s oxygen content to non-toxic levels. Clearly longer exposures to higher oxygen partial pressures will increase risk, as will exercise. Drugs that mimic the sympathetic nervous system are thought to enhance toxicity, so for this reason Sudafed and similar medications are not advisable when diving near the oxygen limit. Finally, there’s no evidence that an oxygen-related convulsion leads to epilepsy; much more evidence exists that hyperbaric oxygen will help your ulcer, so I’d encourage you to crack on with the treatment. Good luck.

Q - What are the effects of oxygen on the eye? I am asking because my boyfriend is diving on high partial pressures of oxygen (1.4 - 1.6 ATA) on a regular basis and he is complaining of some difficulties with focussing. He says his vision blurs for a few hours after his dives, but because it always returns to normal he has not been to see an eye doctor. I am worried that if he keeps this up he may cause permanent damage to his eyes. Is this possible?

A - There are lots of potential causes of visual blurring after diving. Many are to do with contact lenses – gas bubbles underneath them, dryness, they’ve scooted up under the eyelids or been otherwise misplaced etc. So if your boyfriend is a lens wearer then try some lube first. Other possibilities include corneal scratches, sunburn of the eye, irritation from anti-fogging mask solutions, or anti-seasickness medication side effects. Rarely one sees visual blurring in decompression sickness, but usually with other symptoms as well. However, high partial pressures of oxygen can indeed affect vision, as you allude to in your query. One of the commoner side effects of treating patients with hyperbaric oxygen is a temporary deformation of the lens, that causes a degree of short-sightedness. This is of the order of 0.25 dioptres per week and is progressive, but usually reverses when treatment is stopped. Most cases return to normal within 6 weeks but it can take up to 12 months. It’s extremely unusual in recreational diving, but has been reported. I would expect the same return of vision once exposure to hyperoxic gas mixtures ceases, so I doubt he will cause permanent damage to his eyes, but nevertheless would suggest he stops this type of diving until he is back to normal. With very long hyperbaric oxygen inhalation, visual blurring can progress to field constriction, loss of vision and hallucinations, and eyelid twitching often heralds a full-blown seizure. Luckily removal of the oxygen will stop the fit, but if this occurs underwater, you can imagine the consequences are potentially fatal.

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