In-Water Recompression, Revisited

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

 

Hello Kay Dee,

I understand why you might say this, but it is a hypothetical argument only because when it comes to IWR a more aggressive protocol would never be the only option. If you are capable of a Table 5 (which you should never do) then you would also be capable of a safer protocol.

Simon M

 

Kay Dee,

An important teaching point and paradox is this: if you are in that bad a shape neurologically, you are going to be at a much higher risk of suffering from CNS O2 toxicity at 60 feet under water. There are two groups of experts here (and I'm putting Simon above myself) who are saying that an underwater Table 5 is too hazardous to attempt, which was the original point of this thread. This statement is backed by a large body of scientific work, as Simon elegantly pointed out, and is not a matter of opinion. The fact that an individual claims to have gotten away with it does not make it any less hazardous. The proof for you should be in the pudding. You treated severe, sudden-onset neurological DCS on yourself (about as bad as it gets, absent hemodynamic or respiratory decompensation) using an abbreviated Australian IWR protocol and you're reporting that you came out fine.

Best regards,
DDM

 

The increased risk of 60'/TT5 is very real and largely related to the increased risk of OxTox. My decision to go to that depth would largely depend on the the severity of Type 2 symptoms and if any relief was achieved at a shallower depth. I would also consider shortening the time at 60' to one 20 minute cycle if symptoms subside. I would not drop directly to 60' before trying to evaluate symptoms at 25-33'. I would only consider progressing to a TT5 IWR if I believed I would die or be severely injured anyway.

You basically have to resign yourself to dying
with your fins on and hope you get lucky.​

Expedition divers have to give possibilities like this a great deal of consideration, especially today with mixed gas rebreathers and exotic remote locations. I have seen enough DCS in commercial diving to appreciated how critical rapid treatment is. We always had chambers onboard and almost certainly over-treated -- diving supervisors rarely just said "that's probably just a mussel strain". I think it is fair to say everyone was at 60' on O2 within 10 minutes of reporting.

However, there is also no way of knowing how many Type 1 symptoms would soon become Type 2. I can say that everyone walked out of the chamber smiling and joking. Fortunately, I have not been one of them... yet.

 

Can you give me an example of a case where you might attempt this?

 

I understood perfectly. My point was that despite Mr. Gilliam's standing among some in the diving community, the recommendation he made in his article is hazardous and should not be attempted. The severely injured divers who are most likely to benefit from O2 at 60 feet in a hyperbaric chamber are also the ones who are most likely to be harmed by going to 60 feet on O2 in the water. You already described a near-worst-case scenario in yourself where a shallower table was beneficial.

Best regards,
DDM