Increasing Safety Margin

[Akimbo]

Myself and several friends are contemplating a trip to Truk Lagoon, where the availability of a nearby shore-based treatment chamber is not reliable. Oxygen is available but in limited $upply.

Alert Diver | Bent in Chuuk

What is the current thinking of the hyperbaric medical community on this scenario? To expand our safety margin we are considering completing the normal dive profile with open circuit pure O2 stops from 20'. Then start a reped dive after a 5-10 minute surface interval on a pure oxygen rebreather, full face mask, and hardwire diver-diver-surface communications at 20'. That decompression/hang-off time would be a third of the bottom time of the previous dive and would not be credited to the decompression schedule for any other (deep) dives that day — a prophylactic treatment of sorts.

A related question is how much less effective would staying on deck and breathing pure O2 (closed circuit) be in comparison? I suppose you could inexpensively and safely be on O2 during most of your surface interval (except for eating and using the head) in either case.

 

[Duke Dive Medicine]

Hi Akimbo,

A couple of notes. First, even though it's a minor point in the article, we need to differentiate between skin marbling and type I skin rash. Skin marbling is otherwise known as cutis marmorata and is often accompanied by more serious DCS symptoms. As happened here, cutis marmorata is sometimes misinterpreted as "Type I" or minor DCS. So-called "Type I DCS" rash looks like hives and is usually reddened.

Second, this diver made two fairly aggressive dives, so his would probably be considered a case of "explained" DCS. If you want to lower your risk of DCS, I'd say making your dive profiles more conservative and not pushing the tables should be your first priority.

Post-dive prophylactic O2 is an interesting idea. Gene Hobbs and I spend some time talking about your question yesterday. Gene participated in some research in the early 90's that was led by Wayne Gerth of the Naval Experimental Diving Unit. If you go to Rubicon Foundation, click on "research repository", and search for "surface interval oxygen", you'll find several items that will make interesting reading. The short story is that they developed some procedures by which they could shorten surface intervals by using prophylactic O2, and not increase the theoretical risk of DCS.

The other side of the coin is this: I think prophylactic post-dive O2 could be detrimental if you've just made a dive that's going to cause you to develop DCS (which you won't necessarily know). I'll say up front that I don't have any experimental data to back this up, but I think I'm pretty well-grounded empirically. If you complete a normal dive and do not develop DCS, 100% surface O2 will speed up elimination of dissolved inert gas. However, let's say for example that you have a PFO. You complete an aggressive dive and, unbeknownst to you, you arterialize a clinically significant volume of inert gas bubbles when you pull yourself up the ladder. You go on your planned post-dive O2 right away, and the O2 masks (or partially masks) the progressing symptoms of DCS by providing a modicum of anti-inflammatory effect and an increased pressure gradient. It's been proven numerous times that a diver with DCS who becomes asymptomatic after surface O2 is at high risk of symptom recurrence once that O2 is discontinued; this would apply, by extension, to the present scenario. You go off your surface O2, put on your gear and jump in for your second dive. Now, not only do you have DCS, you're about to increase the amount of inert gas that's dissolved in your body.

Your CCR idea would certainly eliminate more inert gas than simply following a decompression profile or using surface O2, and using a FFM and hard-wired comms would increase the safety margin. However, I think that the same priciple applies here, though arguably to a lesser extent. Also, you'd need to be more mindful of your O2 clock, especially if you're using this procedure over multiple days.

 

[Akimbo]

... A couple of notes. First, even though it's a minor point in the article, we need to differentiate between skin marbling and type I skin rash...

My purpose in referencing the article was only the fact that chamber availability is unreliable. My commercial diving experience has led me to avoid decompression dives when a chamber was not onboard or at least available within a few hours.

... Gene participated in some research in the early 90's that was led by Wayne Gerth of the Naval Experimental Diving Unit. If you go to Rubicon Foundation, click on "research repository", and search for "surface interval oxygen", you'll find several items that will make interesting reading. The short story is that they developed some procedures by which they could shorten surface intervals by using prophylactic O2, and not increase the theoretical risk of DCS. ...

Very interesting, thanks. In the case of Surface Interval Oxygen Repetitive Dive Tables, would calling it prophylactic O2 be incorrect since it was used as part of the decompression procedure?

This concept occurred to me primarily because of experience using a proprietary commercial Sur-D-O2 table (Surface Decompression using Oxygen). It shortened the decompression penalties of reped dives pretty dramatically. The unconfirmed rumor going around the gas shack was that the extended time on Sur-D-O2 made us clean when the seal broke (about the same residual Nitrogen as the start of the day) -- reped dives had longer decompression but that was the rumor. We experienced zero hits over about 150 dives... not statistically conclusive but interesting. The idea of turning the approach around a little for a prophylactic measure was not much of a stretch, but certainly justified asking those who are better informed.

... The other side of the coin is this: I think prophylactic post-dive O2 could be detrimental if you've just made a dive that's going to cause you to develop DCS (which you won't necessarily know). ...

Agreed, but isn't the same is true of any normal repetitive dive? The intent is to following RGBM-based schedules with air or Nitrox deep and O2 at 20' up -- with no credit for prophylactic O2.

Besides limiting boredom, the "normal" possibility of DCS was a primary reason for full face masks and comms. The downside of full face masks is succumbing to nappy time. Surface O2 would be easier and lower risk, but I am assuming would be significantly less effective than at 20'. I am personally inclined to accept these risks over the possibility of taking a hit that far from a chamber. I just don't want to use this procedure based on ill-informed intuition.

... I'll say up front that I don't have any experimental data to back this up, but I think I'm pretty well-grounded empirically. If you complete a normal dive and do not develop DCS, 100% surface O2 will speed up elimination of dissolved inert gas. However, let's say for example that you have a PFO. You complete an aggressive dive and, unbeknownst to you, you arterialize a clinically significant volume of inert gas bubbles when you pull yourself up the ladder...

This is for fun. Aggressive decompression dives are for being paid and having a chamber onboard. I encourage anyone reading this to adopt the same attitude!

... You go on your planned post-dive O2 right away, and the O2 masks (or partially masks) the progressing symptoms of DCS by providing a modicum of anti-inflammatory effect and an increased pressure gradient. It's been proven numerous times that a diver with DCS who becomes asymptomatic after surface O2 is at high risk of symptom recurrence once that O2 is discontinued; this would apply, by extension, to the present scenario. You go off your surface O2, put on your gear and jump in for your second dive. Now, not only do you have DCS, you're about to increase the amount of inert gas that's dissolved in your body. ...

Understood and agreed. I suppose it comes down to probabilities and risk analysis. Although this scenario understandably has the potential to exacerbate a hit from the initial dive, wouldn't prophylactic O2 be more likely to prevent developing symptoms in the first place? The assumption is the surface interval is part of the decompression profile, not just time spent in the water.

On the other hand, when DCS symptoms develop after a normal reped, wouldn't the lower residual Nitrogen (from prophylactic O2) be advantageous?

...Your CCR idea would certainly eliminate more inert gas than simply following a decompression profile or using surface O2, and using a FFM and hard-wired comms would increase the safety margin. However, I think that the same priciple applies here, though arguably to a lesser extent. Also, you'd need to be more mindful of your O2 clock, especially if you're using this procedure over multiple days.

Good point. Since we aren't crediting prophylactic O2, we would not take our computers with us on O2 repeds. Therefore we could not depend on computers to track our O2 clock.

Thank you very much for your informative and thought-provoking reply

 

[Duke Dive Medicine]

My purpose in referencing the article was only the fact that chamber availability is unreliable. My commercial diving experience has led me to avoid decompression dives when a chamber was not onboard or at least available within a few hours.

Roger; the point about the cutis was just to clarify for other readers.

Very interesting, thanks. In the case of Surface Interval Oxygen Repetitive Dive Tables, would calling it prophylactic O2 be incorrect since it was used as part of the decompression procedure?

I think it comes down to semantics. You could call it decompression or prophylactic O2, either way the physiologic effect is the same.

This concept occurred to me primarily because of experience using a proprietary commercial Sur-D-O2 table (Surface Decompression using Oxygen). It shortened the decompression penalties of reped dives pretty dramatically. The unconfirmed rumor going around the gas shack was that the extended time on Sur-D-O2 made us clean when the seal broke (about the same residual Nitrogen as the start of the day) — reped dives had longer decompression but that was the rumor. We experienced zero hits over about 150 dives — not statistically conclusive but interesting. The idea of turning the approach around a little for a prophylactic measure was not much of a stretch, but certainly justified asking those who are better informed.

Sur "D" O2 is just that; it's designed, as you know, to get the diver out of the water and into a more protected and controlled environment. Bringing a diver up then putting him/her into an O2 rebreather under water eliminates the advantages of that.

Agreed, but isn’t the same is true of any normal repetitive dive? The intent is to following RGBM-based schedules with air or Nitrox deep and O2 at 20' up — with no credit for prophylactic O2.

Not in the setting of a developing case of DCS. Granted that a diver may not become symptomatic on the surface interval even breathing air, but if symptoms are arising, O2 could mask them. I'll admit that this is a pretty unlikely scenario but still a consideration especially if you're doing multiple days of provocative diving (which, later on in your post you say you aren't, so take it for what it's worth).

Besides limiting boredom, the “normal” possibility of DCS was a primary reason for full face masks and comms. The downside of full face masks is succumbing to nappy time. Surface O2 would be easier and lower risk, but I am assuming would be significantly less effective than at 20'. I am personally inclined to accept these risks over the possibility of taking a hit that far from a chamber. I just don’t want to use this procedure based on ill-informed intuition.

You'd be pretty unlikely to develop symptoms of DCS at 20 feet on O2 after all that deco. I was thinking the FFM and comms were in case of O2 toxicity.

Understood and agreed. I suppose it comes down to probabilities and risk analysis. Although this scenario understandably has the potential to exacerbate a hit from the initial dive, wouldn’t prophylactic O2 be more likely to prevent developing symptoms in the first place? The assumption is the surface interval is part of the decompression profile, not just time spent in the water.

Possibly; it would depend on the severity of the symptoms. Like I said, whether you call it decompression or prophylactic O2, the physiology would still be the same.

On the other hand, when DCS symptoms develop after a normal reped, wouldn’t the lower residual Nitrogen (from prophylactic O2) be advantageous?

Yes, until you discontinue the O2 and go back down. Then you're increasing the inert gas load in an already bent diver.

Thank you very much for your informative and thought-provoking reply

Thanks for the thought-provoking post! Have a great trip!

 

[Thalassamania]

A few thoughts:

1. I was under the impression that the best possible screening for a PFO was, in fact, completion of many DCS symptom free dives.

2. If the divers on the trip all meet that spec then the likelihood of DCS being masked by O2 at 20 feet on up is rather slim, no?

3. There is no chamber that one may count on there, so there are increased risks either way. I'd consider a plan to be able to do IWR if need be.

Experiences:

1. When working at some remote site, when we had plenty of oxygen, we rigged second stages at 20 feet (with a metal bar and seat belts) for decompression, we USN Standard Air tables, and just used the oxygen as a bit of a cushion. It seemed to have worked fine.

2. On more than a few occasions, when I had to fly shortly after diving, I used a couple of hours of surface breathing O2 to scub out ... two hours of surface O2 takes you from Repetitive Group N down to Repetitive Group A. That was back in the day of being able to fly in Repetitive Group D or better (we used Repetitive Group C since a new D diver was fine, but a D diver who was almost in E was not).

 

[Akimbo]

…Sur "D" O2 is just that; it's designed, as you know, to get the diver out of the water and into a more protected and controlled environment. Bringing a diver up then putting him/her into an O2 rebreather under water eliminates the advantages of that…

That is certainly one goal of Sur-D, but getting the diver on high doses of O2 is the a close second — for divers in hats and hot water suits anyway. (for others reading this) We normally allow no more than 5 minutes from last water-stop to 60' on O2 in the chamber, but that is part of the scheduled decompression profile.

You can perform a reped dive in 10 minutes following normal decompressing tables, which is “presumed” safe. It just happens that this reped would be at 20' on pure O2 but counted as part of the surface interval.

...I was thinking the FFM and comms were in case of O2 toxicity…

Full Face Masks are great for rapid onset convulsions, but really suck for vomiting — both O2 toxicity symptoms. The FFM allows conversing with the diver which helps evaluate the onset of many suspicious symptoms including excessive burping! I have seen lots of DCS in my career but have never seen, and rarely heard of, confirmed cases of O2 toxicity. I know you have seen plenty so I can only assume it is because we operate in such different environments and hyperbaric subjects.

Thanks again.

 

[Akimbo]

…
1. When working at some remote site, when we had plenty of oxygen, we rigged second stages at 20 feet (with a metal bar and seat belts) for decompression, we USN Standard Air tables, and just used the oxygen as a bit of a cushion. It seemed to have worked fine…

Same here. Did you combine the time for the 10 and 20' stops, and surfacing after or make the 10' stop on O2? We did it all at 20' and nobody took a hit. I first heard one of the hyperbaric docs talk about it when I was in the Navy.

 

[Thalassamania]

We tried to make the 20 and the 10 as specified, but you know how the seas can be ... we were often chased back down to 15 or 20 ... and we figured the O2 would compensate ... that's how you make a table, measure with a micrometer, mark with chalk, cut with an axe.

 

[Mr Carcharodon]

... two hours of surface O2 takes you from Repetitive Group N down to Repetitive Group A. ).

How did you arrive at that? All the oxygen window plans I know of seem to be very seat of the pants. Can you quantify the effect of the oxygen window?

 

[Thalassamania]

This is gonna be a bit weird, it was on a little plastic table that came attached to a FENZY I bought. I checked it out with my then housemate (Ocean Engineer, deco expert and later Director of Technology for ORCA Industries) and it worked ... try it on deco planner, it comes out right.