Undercurrent asked Bret Gilliam to write the article I suggested. But, he sent me this reply and asked me to post it:
I am not a participant in chat room and forums on the internet usually... but was asked by the publisher of Undercurrent to offer some perspective and professional insight into the apparent misconceptions that are being bandied about on this forum. I'll try to be concise and take them in order from the July 31st stream:
1. I am the guy that set the world depth record on air with scuba in February 1990. That was to 452 feet and I broke it again in 1993 when I went to 475 feet. This is not diving for anyone but highly experienced trained professionals of that era. (In those days, helium was not widely available in remote areas so you either worked on air... or didn't do the dive. Today helium is widely available and the preferred equipment would be a closed circuit rebreather, not open circuit scuba.) Yes, the PO2 is high. But you have to have a fundamental understanding of oxygen physiology to grasp that it is "dose" not partial pressure alone that defines the risk. Brief exposures to high PO2 can be accommodated by some individuals. It just means that your time at that exposure will be severely limited. A far bigger problem for some is the issue of inert gas narcosis. More people have been overcome by this than oxygen toxicity. In fact, to my knowledge, there are only six persons alive today that have ever been deeper than 400 feet on air and survived: Jim Lockwood, Tom Mount, Jim Bowden, Joe Odom, Dan Manion and myself. Also, nothing in diving is "safe". Safe literally means "without risk". What we did was calculated risk within our own ranges of comfort and we had no issues. But none of us would call it "safe" and we stridently urge others not to attempt similar dives. Most people belong above 200 feet and should stay there.
2. Another person posted about Recompression Treatment Tables and further confused the issues. Recompression outcome depends on pressure and high PO2 to get the best possible out-gassing gradient from inert gas bubbles to reduce occlusions in the blood circulation. Patients are at rest and thusly high PO2s are more easily tolerated. If an episode of tremor or seizure were to result, we simply remove the oxygen BIBS mask and stabilize the patient on air. Then the oxygen treatment is resumed. Standard PO2s for Table 5 and Table 6 are 2.8 ATA on pure oxygen at 60 feet. In some Table 6A treatments, PO2s in excess of 4.0 are used by switching to a high oxygen percentage nitrox mix at 165 feet. In extreme cases, 100% oxygen has been used briefly even at 165 feet as "shock therapy" to unresponsive patients for brief periods. This marks a PO2 of 6.0 ATA. It has saved at least three patients that I know of who would have died otherwise. (I am a credentialed Recompression Supervisor and have operated chambers worldwide since 1971... running primary treatments on over 200 patients and consulting on nearly 100 others.)
3. Finally, someone raised the mistaken issue that they are at risk for pulmonary oxygen toxicity in normal open circuit scuba diving. This is virtually medically and mathematically impossible. You are referring to what is known as the "Lorraine Smith Effect" and this involves long term, low dose oxygen effects. These are unattainable except in chambers, saturation habitats, or on exceptionally prolonged dives with rebreathers (six hours or more for a single dive). For open circuit divers, if you simply remain within no-decompression limits, you will not even approach the limits for the crucial central nervous system limits as described by "Paul Bert Effects". Your no-decompression limits for a dive will always be less than half of the single dive limits for CNS oxygen toxicity. And the "Lorraine Smith" effects are largely inconsequential anyway (slight loss of vital capacity, some breathing effects such as unproductive cough, etc.) and these are simply impossible to attain on open circuit equipment at all. There is no need to take a day off from nitrox diving to "let your lungs out-gas". It's simply an urban myth from less informed people that like to opine on a subject they lack the expertise to discuss. Incidentally, I was on the faculty of the DAN Nitrox Conference in 2000 wherein the consensus recommendation by all in attendance (medical, military, science, technical, and traditional sport diving applications) agreed that a PO2 of 1.6 ATA was fine. There has never been a case of a diver having an oxygen incident at 1.6 PO2 as long as they stayed within the "dose" time limit of 45 minutes for a single dive.
If you don't have a full grasp of oxygen physiology, don't feel bad. Even most physicians don't get it either. Undercurrent asked me to prepare a full length article on the subject that will be published in an upcoming issue. Hope this helps the dialogue. If anyone needs a specific question answered, email me directly at: email@example.com
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