Hello,
I am fully supportive a IWR using oxygen by divers who are properly equipped and who have appropriate training. Indeed, I have recently led an expert consensus process to modernize guidelines for management of DCS in the field, and I think it will be the first time that IWR has been medically endorsed in such guidelines. Hopefully these will be published by the end of this year. I have personally treated other divers and myself with IWR.
However...
The idea of using a Table 5 in the water is nuts. 2.8 ATA of oxygen in water is a very dangerous dose, and a Table 5 requires that it be breathed for 40 minutes. Indeed, in his Undercurrent blog Brett Gilliam even endorses the use of "extra 20 minute oxygen breathing periods at 60' if necessary". It is difficult to characterize this as anything other than gross ignorance.
To plagiarize my own reply to his Undercurrent post: I could raise various references to the literature, but the most important and revealing data come from an era when the sort of experiments required to answer the relevant questions could still be done. I refer to Donald’s seminal work in the UK during the second world war. The key findings relating to discussion of IWR were reported by Vann and Hamilton in the DAN Technical Diving Workshop Proceedings which can be downloaded here:
Technical Diving Conference Proceedings.
If you look at figure 12 of their paper on page 48 you will immediately appreciate why the advice to recompress sick divers to 60′ on pure oxygen is dangerous. The survival curve in 12(d) for exposure to 50′ of oxygen (LESS than the 60' being recommended by Gilliam) when immersed at REST shows that 50% of divers will exhibit symptoms of oxygen toxicity in LESS time than the exposure being recommended. It should be noted also that Donald’s subjects were not being recompressed after a prior oxygen exposure (which might be substantial – eg after a long constant PO2 dive), or in a physiologically compromised state. These factors without doubt will further increase risk. In his work Donald concluded that “diving on pure oxygen below 25′ is a hazardous gamble”.
Figure 15 (page 53) in the Vann and Hamilton paper is also revealing. It is based on US Navy data from working dives, and shows iso-risk curves for seizures at different inspired PO2s (on the vertical axis). It is clear that at 2.8 ATA there is virtually NO time before the risk exceeds 8% (the rightmost curve). I accept that divers undergoing in-water recompression should be resting, but this can never be guaranteed. In truth, activity during an in-water recompression in realistic conditions is likely to lie somewhere between work and rest.
I appreciate the logic in Akimbo's reference to having to choose between a dangerous but effective table and a less dangerous but less effective table. However, the difference in efficacy between 2.8 ATA of oxygen and 1.9 ATA (the latter recommended by the Australian IWR method mentioned earlier in this thread) may not be that great. The justification for that claim will appear in our guidelines paper. But the difference in safety is likely to be vast. On this basis I do not accept that a choice exists. We should simply not be using at Table 5 underwater and I am amazed that after so much adverse commentary by true experts that the Gilliam blog is still up on line.
Simon M
I am fully supportive a IWR using oxygen by divers who are properly equipped and who have appropriate training. Indeed, I have recently led an expert consensus process to modernize guidelines for management of DCS in the field, and I think it will be the first time that IWR has been medically endorsed in such guidelines. Hopefully these will be published by the end of this year. I have personally treated other divers and myself with IWR.
However...
The idea of using a Table 5 in the water is nuts. 2.8 ATA of oxygen in water is a very dangerous dose, and a Table 5 requires that it be breathed for 40 minutes. Indeed, in his Undercurrent blog Brett Gilliam even endorses the use of "extra 20 minute oxygen breathing periods at 60' if necessary". It is difficult to characterize this as anything other than gross ignorance.
To plagiarize my own reply to his Undercurrent post: I could raise various references to the literature, but the most important and revealing data come from an era when the sort of experiments required to answer the relevant questions could still be done. I refer to Donald’s seminal work in the UK during the second world war. The key findings relating to discussion of IWR were reported by Vann and Hamilton in the DAN Technical Diving Workshop Proceedings which can be downloaded here:
Technical Diving Conference Proceedings.
If you look at figure 12 of their paper on page 48 you will immediately appreciate why the advice to recompress sick divers to 60′ on pure oxygen is dangerous. The survival curve in 12(d) for exposure to 50′ of oxygen (LESS than the 60' being recommended by Gilliam) when immersed at REST shows that 50% of divers will exhibit symptoms of oxygen toxicity in LESS time than the exposure being recommended. It should be noted also that Donald’s subjects were not being recompressed after a prior oxygen exposure (which might be substantial – eg after a long constant PO2 dive), or in a physiologically compromised state. These factors without doubt will further increase risk. In his work Donald concluded that “diving on pure oxygen below 25′ is a hazardous gamble”.
Figure 15 (page 53) in the Vann and Hamilton paper is also revealing. It is based on US Navy data from working dives, and shows iso-risk curves for seizures at different inspired PO2s (on the vertical axis). It is clear that at 2.8 ATA there is virtually NO time before the risk exceeds 8% (the rightmost curve). I accept that divers undergoing in-water recompression should be resting, but this can never be guaranteed. In truth, activity during an in-water recompression in realistic conditions is likely to lie somewhere between work and rest.
I appreciate the logic in Akimbo's reference to having to choose between a dangerous but effective table and a less dangerous but less effective table. However, the difference in efficacy between 2.8 ATA of oxygen and 1.9 ATA (the latter recommended by the Australian IWR method mentioned earlier in this thread) may not be that great. The justification for that claim will appear in our guidelines paper. But the difference in safety is likely to be vast. On this basis I do not accept that a choice exists. We should simply not be using at Table 5 underwater and I am amazed that after so much adverse commentary by true experts that the Gilliam blog is still up on line.
Simon M
