DCS Diving Oxygen Rebreathers

[Angelo Farina]

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?

There was no OVP in the Cressi 57b we were using...
Nor there was any manual discharge valve. Only way to eliminate the excess gas was exhaling through the nose, if wearing a normal mouthpiece and mask.
If wearing a full face mask (standard equipment for Comsubin) you had to use the three way manual valve on the corrugated hose for exhaling out of the circuit.
Here a Cressi 57B:

 

[Angelo Farina]

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.

There is a diffuser (or, better, a silencer) only on the Nitrox semi closed circuit version.
But again this is classified, I cannot say more...

 

[cerich]

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.

Taravana: Fact or Falacy?

The condition, known as “taravana” amongst pearl divers of the Tuamotus (a chain of islands and atolls in French Polynesia), has increasingly become accepted as a unique form of DCI

www.dansa.org

 

[cerich]

There is a diffuser (or, better, a silencer) only on the Nitrox semi closed circuit version.
But again this is classified, I cannot say more...

IYup, no need for a diffuser on a pure O2 unit CCR if you know how to use to not make bubbles when ascending.

 

[Akimbo]

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.

Taravana: Fact or Falacy?

The condition, known as “taravana” amongst pearl divers of the Tuamotus (a chain of islands and atolls in French Polynesia), has increasingly become accepted as a unique form of DCI

www.dansa.org

Interesting and thanks. I wonder if Florida is seeing an in increase in the form of DCS?

Ping: @MAKO Spearguns @Duke Dive Medicine
Have you come across this? I wonder if any of the freediving computer algorithms are taking dive profiles like this into account?

 

[Trace Malinowski]

The military DCS cases in shallow water were attributed to multiple ascents and descents, and exposures closer to the NDL of shallow dives. The equipment worn was not recorded but given the profiles, we can probably assume the divers were involved in training dives and shallow water construction. I can't even count the number of ascents and descents I made in a day of portable cofferdam installation.

 

[newman_diver]

The military DCS cases in shallow water were attributed to multiple ascents and descents, and exposures closer to the NDL of shallow dives. The equipment worn was not recorded but given the profiles, we can probably assume the divers were involved in training dives and shallow water construction. I can't even count the number of ascents and descents I made in a day of portable cofferdam installation.

That was what I was getting at. The cases are likely across all equipment configurations, which are many other than O2 rebreathers.

 

[Duke Dive Medicine]

Interesting and thanks. I wonder if Florida is seeing an in increase in the form of DCS?

Ping: @MAKO Spearguns @Duke Dive Medicine
Have you come across this? I wonder if any of the freediving computer algorithms are taking dive profiles like this into account?

Thanks for the add. Fascinating phenomenon and some interesting theories behind it. I don't recall ever having seen a case of it though our physicians would almost certainly treat it as DCS if other causes of symptoms were ruled out.

Best regards,
DDM

 

[Dr Simon Mitchell]

Interesting and thanks. I wonder if Florida is seeing an in increase in the form of DCS?

Ping: @MAKO Spearguns @Duke Dive Medicine
Have you come across this? I wonder if any of the freediving computer algorithms are taking dive profiles like this into account?

Hello,

Decompression sickness (DCS) in breath hold divers is definitely possible. DHM published a review article on 'Taravana' by one of the world's experts last year. It is freely available here.

DCS-like symptoms are starting to be seen more often in extreme breath hold divers doing deep repetitive dives. These cases may have a strange presentation and there are may be some clinical differences when compared to the typical cases seen in compressed gas diving. However, we have not seen enough of them to be sure yet.

As DDM said, most diving physicians would recognise that DCS-like symptoms after extreme freediving may indeed be DCS, and most would offer recompression treatment, especially if the symptoms were non-mild and neurological in nature. DHM is publishing a case representing exactly such a conundrum faced by diving physicians in Malta recently in the September issue (Diacono E et al.)

Simon M

 

[MAKO Spearguns]

Interesting and thanks. I wonder if Florida is seeing an in increase in the form of DCS?

Ping: @MAKO Spearguns @Duke Dive Medicine
Have you come across this? I wonder if any of the freediving computer algorithms are taking dive profiles like this into account?

I don't have any personal knowledge about this, other than to say that it is my understanding the DCS in freediving is absolutely a "thing". It is extremely rare, basically unheard of for typical recreational (free)divers. It is also my understanding that "fast" tissues are most susceptible to problems with this type of diving, so that might be related to the types of symptoms that can occur.

For the super elite divers it is something to consider and it is my understanding that some training agencies have put out some guidance with respect to recommended surface intervals between deep dives. I don't want to say anything that implies that I am a good source for this topic, but if you are planning on doing a bunch of dives to over 80 feet in short succession it is something to consider.

As far as I know, the dive computers that operate in freedive mode, are basically acting like a depth gauge and timer (for dive and surface interval), so they do not include a decompression modeling/algorithm component.