DCS Diving Oxygen Rebreathers

[Trace Malinowski]

In researching decompression sickness in shallow water, it seems the military has had enough cases of divers presenting with DCS to give one pause. Speculation seems to be that a number of these incidents were generated by lots of descents and ascents while working during a single dive or a series of repetitive dives.

I can't seem to find the magic search engine keyboards that might lead me to any information on DCS related to the use of 100% oxygen rebreathers. One would think it extremely unlikely, but given the military pushing the oxygen toxicity limits especially in some earlier units, I would think that nitrogen tissue pressure from the surface being driven into tissues at the start of a dive + cellular damage from the inflammatory effects of oxygen + a series of some pretty aggressive ascents and descents would yield a hyperbaric injury that would lean toward a DCS hit rather than an over-expansion injury like AGE.

Anyone have any of the sort of information, I'm seeking (if it exists)?

 

[Whitrzac]

I havent done the math, but I would bet my lunch $$ that the pn2 from the risidual in the loop at depth is still less than what it is at the surface.

 

[Trace Malinowski]

I havent done the math, but I would bet my lunch $$ that the pn2 from the risidual in the loop at depth is still less than what it is at the surface.

I would think any cases came from pushing the operating depth of the unit beneath a ppO2 of 1.6 ATA below 20 feet, but at 20 or above DCS shouldn't occur, or am I missing something?

 

[211ratsbud]

Trace, personally if I was going to forward this question to anyone with the highest likelihood of having experienced anything along these lines I would pass you on to Roger Garcia @ FIU. Himself and Henry taught our chamber supervisor course and had about 50 years plus of actual navy diving medical and supervising experience combined. Do you mind if I reach out to them?

 

[Trace Malinowski]

Trace, personally if I was going to forward this question to anyone with the highest likelihood of having experienced anything along these lines I would pass you on to Roger Garcia @ FIU. Himself and Henry taught our chamber supervisor course and had about 50 years plus of actual navy diving medical and supervising experience combined. Do you mind if I reach out to them?

Please do!

 

[Duke Dive Medicine]

In researching decompression sickness in shallow water, it seems the military has had enough cases of divers presenting with DCS to give one pause. Speculation seems to be that a number of these incidents were generated by lots of descents and ascents while working during a single dive or a series of repetitive dives.

I can't seem to find the magic search engine keyboards that might lead me to any information on DCS related to the use of 100% oxygen rebreathers. One would think it extremely unlikely, but given the military pushing the oxygen toxicity limits especially in some earlier units, I would think that nitrogen tissue pressure from the surface being driven into tissues at the start of a dive + cellular damage from the inflammatory effects of oxygen + a series of some pretty aggressive ascents and descents would yield a hyperbaric injury that would lean toward a DCS hit rather than an over-expansion injury like AGE.

Anyone have any of the sort of information, I'm seeking (if it exists)?

Hi Trace,

If a 100% O2 rig is purged properly then the N2 pressure gradient will be from the tissues to the breathing loop. The diver would not breathe N2 at partial pressures greater than those in air at sea level so there's no mechanism for DCS there.

Best regards,
DDM

 

[jale]

Diving pure oxy rb in a recreational context often, I flush the loop at surface, at the beginning of the dive and like every10-15 minutes during the dive to make sure to remove N2 and Co2 as much as possible.
I suppose military divers don't have the luxury of doing that as they have situational and time constraints which may force them to not do flushings.
But still DCS on oxy rb...

 

[ginti]

Trace, personally if I was going to forward this question to anyone with the highest likelihood of having experienced anything along these lines I would pass you on to Roger Garcia @ FIU. Himself and Henry taught our chamber supervisor course and had about 50 years plus of actual navy diving medical and supervising experience combined. Do you mind if I reach out to them?

Hi DanaHunt, did you forward this question to FIU? If yes, did you get any answer different from what duke dive medicine explained?

Thanks

 

[John C. Ratliff]

Well, let’s do some thinking, like a “thought experiment.” Decompression sickness (DCS) is caused by inert gas or gases that, upon ascent from depth, cause the gas to form bubbles in the diver’s tissues. The key here is “inert,” as oxygen is not inert, it is used by the body. Nitrogen, hydrogen, helium and argon (I think) have all been used to modify decompression profiles or solve other problems (like nitrogen narcosis). Also, each of these gases will not form bubbles in water that is less than two atmospheres absolute pressure (33 feet or 10 meters). With an oxygen rebreather, not only is there no inert gas, but any residual nitrogen gas within the body is in the very low percentages (Probably parts per million). Also the depth limitation on pure oxygen is 25 feet (about 8 meters). So listen to Duke Dive Medicine when they say, “…there’s no mechanism for DCS there.”

By the way, when I went through the U.S. Naval School for Underwater Swimmers School in Key West, Florida in 1967, we were put into a chamber and given an “Oxygen Tolerance Test,” by taking us down to 66 feet pressure, putting on a mask with pure oxygen, and told to breathe it for the next 15 minutes. A couple had problems, but I did not.

SeaRat

 

[Dr Simon Mitchell]

Hello

Decompression sickness occurs because bubbles form from dissolved nitrogen (or other inert gases) in tissues that become supersaturated during decompression. Bubble formation from dissolved oxygen is extremely unlikely in plausible human hyperbaric or diving scenarios. This is because supersaturation of a tissue with dissolved oxygen is also very unlikely for several reasons.

In theory, O2 in tissues won't behave like an inert gas, that is, steadily washing in so that the dissolved pressure of oxygen (PO2) rises to eventually equilibrate with the arterial PO2, because of oxygen metabolism in the tissue. I would have to think about this a little more, but this probably could not happen unless the arterial PO2 exceeded that required to provide tissue oxygen metabolic requirements from dissolved oxygen alone without a need for oxygen on hemoglobin (somewhere in excess of 2000 mmHg, >2.5 ATA). After that, I suppose oxygen could start to behave like an inert gas in terms of accumulation in tissue. However, divers obviously won't use anything like such high PO2s in diving.

We do start to get close to those sort of arterial pressures of oxygen in a hyperbaric chamber and it is possible that some oxygen could start to accumulate in tissues But even so, I think it would take a very fast, if not explosive, decompression to precipitate bubble formation from dissolved oxygen whereas hyperbaric chamber decompressions are very controlled and slow. During a controlled decompression, oxygen washout as inspired pressure falls would be markedly accelerated by tissue metabolism of oxygen (and dissolved oxygen in the tissue would be the first used, before any extraction from blood). Under these circumstances tissue oxygen bubble formation would be almost impossible.

Interestingly, the famous Ken Donald claimed to have produced "oxygen bends" in goats (see attached paper). However, there are a couple of salient feature of the Donald study which I think are consistent with what I have said above. The animals were breathing 3.6 atmospheres of oxygen over 60 minutes, so some degree of oxygen behaving like an inert gas would have been at play at that pressure. The decompression was very fast (150' to surface in 120 seconds after 60 min at depth). It is also notable that the goats breathed some nitrogen at an equivalent PN2 as air at of 50' for 60 minutes which is not an inconsequential nitrogen load in human diving terms (although I have not looked at any decompression tables for goats recently!!). Anyway, the bottom line is that Donald may indeed have demonstrated oxygen bends (the quick recovery he saw after symptoms developed is consistent with that) but only under circumstances that are not relevant to diving (including oxygen rebreathers) or hyperbaric chamber exposures in the modern context.

Simon M