125 feet air dive DCS risk?

[Diver0001]

The discussion very much has to do with the bottom gas that the victim was breathing along with hypothermic stress, especially now that there is a history of unexplained DCS despite the seemingly NDL multi-level repetitive profiles starting at around 36m depth: Obviously, given the victim's physiology & deco stress susceptibility, the "algorithm could not compensate" for the high FN2 load of 79% nitrogen and the 4 deg C water temp

For better conservative deco strategy and practice even within NDL, the switch to 100% Oxygen at 6m depth with a slow controlled 1 meter/min ascent maximizes the partial pressure gradient for off-gassing supersaturated blood & tissues or pathogenic DCS causing gas bubbles, and also prevents new uptake of inerts -which is why it doesn't make smart deco strategy sense in general to use Air backgas for a "10min safety stop at 5m" in this instance. More effective resolution of surfacing slow tissue supersaturation and potential bubble formation can be accelerated by breathing 100% O2, or even Nitrox50, instead of 21% O2 and 79% Nitrogen (Air), given the additional stress of hypothermic cold water exposure conditions.

I think for this particular diver having had this bend and wanting to avoid a repeat that Kevin's approach makes sense. There are obviously equipment and possibly training concerns that would make it infeasible for most divers but if I were the one who got that hit, I may take this advice.

I may also stop making two dives a day in the winter, extend safety stops as a matter of course and make longer surface intervals generally. There may also be other things the diver can do. We don't know him but he may be a smoker in a poor state of physical fitness, so he may have options open that we don't know about.

R..

 

[RyanT]

Kevin and Diver0001 provide good information. I've made hundreds of clean dives with similar profiles to the OP, often 3 or 4 per day. My dives were warmer and I was in my 20s and bulletproof (or so I thought), but there is nothing in the profiles that set off alarm bells (assuming the last 3 meter instant ascent was a computer artifact). It could certainly be individual susceptibility, PFO, and/or cold, but clearly being more conservative and adding high O2 deco mixes can only help.

 

[RainPilot]

Ok I must have missed something. The OP spoke of a PADI AOW diver and novice divemaster and everyone's advice is to use high O2 deco mixes?

I looked at the profiles , runtimes, the RGBM computer set on medium conservatism and based my comments on the assumption that we were talking about minimal experience Rec divers here (i.e. Less than 100 dives)

In that case, I still contend that the gas mix is the least of their worries, unless they didn't set the correct mix on their dive computer.

 

[Diver0001]

Ok I must have missed something. The OP spoke of a PADI AOW diver and novice divemaster and everyone's advice is to use high O2 deco mixes?

I looked at the profiles , runtimes, the RGBM computer set on medium conservatism and based my comments on the assumption that we were talking about minimal experience Rec divers here (i.e. Less than 100 dives)

In that case, I still contend that the gas mix is the least of their worries, unless they didn't set the correct mix on their dive computer.

You didn't miss anything. Kevin was speculating that using O2 at the safety stop could help with offgassing, although in usual Kevin style he formulated it in such a way that the idea sounded much more impressive than it is. Nevertheless I agreed with that assessment because if you got a weird hit like this you would want to build in as much of a safety margin as you can. That said, I also commented that there would be concerns for equipment and training that could make it infeasible and mentioned a couple of more pragmatic ideas that I had as well.

As for all the talk about gasses, I agree that these dives should have been feasible on air. Even vplanner shows them as being within NDL's.until you put it on +4. That someone got bent from these two dives is actually quite surprising.

R..

 

[RyanT]

Sorry if it sounded like I was suggesting the OP, just grab a bottle of O2 and jump in. Obviously everyone should seek the appropriate training and experience before embarking on that kind of diving. I was merely suggesting that it could be one avenue of increased conservatism for the OP in the future. It's just rather puzzling that the OP got hit on this profile, even cold and on air. Given what seems to be an increased DCS susceptibility, additional future conservatism seems like a good idea.

 

[Into the Water]

I think for this particular diver having had this bend and wanting to avoid a repeat that Kevin's approach makes sense. There are obviously equipment and possibly training concerns that would make it infeasible for most divers but if I were the one who got that hit, I may take this advice.

I may also stop making two dives a day in the winter, extend safety stops as a matter of course and make longer surface intervals generally. There may also be other things the diver can do. We don't know him but he may be a smoker in a poor state of physical fitness, so he may have options open that we don't know about.

R..

Small remark , that diver is a woman 27 years, not smoker, not overweight and have very active life. But the night before the dive she was very tired from work and had only 3-4 hours of sleep. Don't know if it would not be too much but I will try to ask her about PFO

 

[Duke Dive Medicine]

A lot of excellent commentary here. A note on patent foramen ovale (PFO), which has been mentioned twice in this thread: PFO is associated with sudden-onset severe neurological DCS, inner ear DCS and cutis marmorata, or type II skin bends (not to be confused with itching, hive-like type I skin bends). The symptoms described in the original post are not consistent with suspected bubble shunting. Statistically the diver in question has a 25% chance of having a PFO, but if she does, it's probably not related to this particular incident of DCS.

Into the Water, it is probably fruitless to continue to speculate on the cause of this diver's DCS. There are only two scientifically supported predisposing factors for DCS: heavy work under water, and being cold on decompression. Consider those two, and also consider the preceding recommendations for reducing the probability of DCS, but also understand that there is such a thing as an "unexplained" DCS hit, that is, the diver does everything "correctly" but still gets DCS. Also understand, once again, that switching to HeO2 below 100 feet will NOT necessarily reduce the probability of DCS. You can still get bent on heliox and trimix.

Best regards,
DDM

 

[Duke Dive Medicine]

Ok, I'll bite. You keep talking about air dives, but as far as DCS, if you dive within your planned limits and ascend at the correct rates etc then the algorithm should compensate.

The interesting thing is that the algorithms don't appear to compensate the way you'd think they would. We did some probabilistic modeling of several commercial off-the-shelf decompression algorithms a few years ago and with each one, the probability of DCS increased with increasing bottom time and depth. In theory, the probability of DCS should remain constant, but that's not what we found. Not to say that the algorithms haven't been improved since then, but that's the latest as far as I know.

[abstract]DECOMPRESSION RISK EVALUATION FOR TRIMIX DIVES DERIVED FROM COMMERCIALLY AVAILABLE DESKTOP DECOMPRESSION ALGORITHMS

Best regards,
DDM

 

[Diver0001]

I'm not surprised to hear that at all, DDM. Mark Ellyatt told me in 2005 that all of the extreme deep crowd were seeing the same thing in practice.

 

[RainPilot]

The interesting thing is that the algorithms don't appear to compensate the way you'd think they would. We did some probabilistic modeling of several commercial off-the-shelf decompression algorithms a few years ago and with each one, the probability of DCS increased with increasing bottom time and depth. In theory, the probability of DCS should remain constant, but that's not what we found. Not to say that the algorithms haven't been improved since then, but that's the latest as far as I know.

[abstract]DECOMPRESSION RISK EVALUATION FOR TRIMIX DIVES DERIVED FROM COMMERCIALLY AVAILABLE DESKTOP DECOMPRESSION ALGORITHMS

Best regards,
DDM

Thanks for the link, good read.
That makes total sense to me, I imagine the original testing by Buhlmann et al was conducted in a range far shallower than the dives being conducted now. Any algorithm shaped to experimental data is going to lose a lot of utility as you move out of that range.

However for 30 ish metre dives there must be a gajillion dives performed on any number of DCs with all kinds of algorithms without people getting bent all outta shape.

I think that in this particular case, the gas mix and even the water temps to a certain degree are of secondary concern.

As @Diver0001 said, these are dives any Rec diver would be pretty happy to do on any mix with any algorithm and be quite confident of success. I really think the issue here is an individual susceptibility which means the only valid advice would be to go see a good hyperbaric physician and see what's what.