DCS Diving Oxygen Rebreathers

[Akimbo]

In researching decompression sickness in shallow water, it seems the military has had enough cases of divers presenting with DCS to give one pause.

Are these reports publicly available? Given the 80-90 year history of pure O2 rebreathers used by combat swimmers you would think that some very unusual circumstances were introduced.

Even if Oxygen bubbles form, or even 80% Nitrox bubbles, you would think that DCS symptoms would be very short lived due to more rapid metabolic consumption of the gas volume.

I can't recall any cases of DCS in air divers operating in 20'/6m. Interesting.

 

[Akimbo]

Another though just came to mind. Were these military divers involved with submarine lockout operations? Is it possible that they were exposed to compressed air in the lockout chamber before switching to their O2 rebreathers?

 

[newman_diver]

Military divers use other equipment than O2 RB (surface supplied, nitrox RB, SCUBA) that would likely be the source for the cases in the study. There is no issue with DCS on an O2 set, only O2 toxicity (CNS and pulmonary)

 

[broncobowsher]

DCS as must understand it, dissolved inert gas in the tissues forming bubbles when pressure drops, looks to require extraordinary conditions outside the accepted operating conditions of an oxygen rebreather.

The risk of an over expansion exists. The same as you can get with regular scuba gear in a home swimming pool. I could see where some people would lump an over expansion injury in with DCS. It is an injury from a bubble of gas inside the body.

 

[Vicko]

There are freedivers who managed to get bent in just 10m depth with many, many fast ascents and considerable bottom time, so if there was a way to supersaturate the tissues with oxygen and do a stupid amount of fast ascends it could be possible.

 

[Akimbo]

There are freedivers who managed to get bent in just 10m depth with many, many fast ascents and considerable bottom time, so if there was a way to supersaturate the tissues with oxygen and do a stupid amount of fast ascends it could be possible.

I have not read anything about that. Can you point us to reports? I have read about freedivers getting bent from very deep dives competitive apnea dives.

 

[Angelo Farina]

The only cases of which I have heard (at Italian Comsubin) regards case of traumatic embolism (lung overextension), not due to bubbles of Nitrogen.
But of course this is classified military information.
Lung overextension can occur also at shallow depth, and it is particularly insidious on a closed circuit, as even if you breath, the volume of air trapped in lung+bag is limited.
I was trained and certified for ARO CC rebreathers back in the seventies, and the risk of embolism due to rapid ascent without releasing gas from the circuit was clearly explained and considered quite risky.
With an ARO, the normal recommendation of "not hold your breath while ascending" is not enough. You can being ventilating and still get a lung overexpansion...

 

[Akimbo]

With an ARO, the normal recommendation of "not hold your breath while ascending" is not enough. You can being ventilating and still get a lung overexpansion...

Is this because the diver can ascend faster than the OVP (Over Pressure Valve) can vent? I wonder if the OVP has been enlarged on newer models made for combat swimmers?

 

[broncobowsher]

I'll take a wild guess that a military version is doing a diffuser of some sort that minimizes bubbles, that may have limited flow capacity.

 

[Vicko]

I have not read anything about that. Can you point us to reports? I have read about freedivers getting bent from very deep dives competitive apnea dives.

The diver in question was a good friend of mine. After half a day and around 100 dives to 10 meters, average lenght of dive was close to 3 minutes (pulling a stuck grouper out of a hole) with short surface intervals he felt sick, some pain around his spine appeared the night after and by morning he was mostly paralyzed bellow the waist.
The doctors refused chamber treatment because no one believed you could get bent freediving (a story often heard here, we have some very good spearfisherman so it's not that rare) and only got him into the chamber after 3 days of various test and he did get some use of his legs back but still has trouble walking.
Ironically the next year the same doctor refused treatment for one of the best freedivers in the world presenting with the exact same symptoms but was thankfully overruled.

There is a weird type of sickness that happens to spearfisherman that we call taravana, I know of at least 10 people who got it and the symptoms are very reminiscent of type 2 DCS.