Increasing Safety Margin

[Rhone Man]

When I was in Truk, we did a lot of relatively deep dives, but only one was a planned decompression dive. We tried to mitigate risk by doing 6 minute safety stops rather than 3 minutes. We stayed out of deco, but after a week of 4 or 5 dives a day, there is no doubt your joints are starting to ache a bit from the residual build up of nitrogen.

If I was to do it again (and I hope that I do...) I would probably pay for a bottle of pure O2 and spend ten minutes or so at the end of each day washing out at 15 feet.

Not sure what sort of profiles you have in mind, but that is my 2 PSI.

[Mr Carcharodon]

I do not have deco planner but thought it used Haldane's equation to arrive at gas tensions and scaled m-values for limits. But that does not address using high ppO2 for decompression. If there is a way to calculate it I would be very interested. J Brian offered an explanation of why it works, but other than ratio deco rules of thumb I have never seen calculations.

[Akimbo]

…If I was to do it again (and I hope that I do...) I would probably pay for a bottle of pure O2 and spend ten minutes or so at the end of each day washing out at 15 feet.

Not sure what sort of profiles you have in mind, but that is my 2 PSI.

We divers do a lot to ensure we never run out of breathable gas, for more than 2-5 minutes anyway. IMHO, a serious DCS hit is a close second or maybe even worse because I might survive… like broccoli. I am quite happy to return to the boat with 200 PSI because gas management is relatively predictable and there are lots of things I can do to mitigate failures. Not so with DCS when the nearest chamber is in Guam.

If running out of gas is unacceptable, then why isn’t getting bent with no treatment chamber within hundreds of miles across the Pacific?

[Rhone Man]

We divers do a lot to ensure we never run out of breathable gas, for more than 2-5 minutes anyway. IMHO, a serious DCS hit is a close second or maybe even worse because I might survive… like broccoli. I am quite happy to return to the boat with 200 PSI because gas management is relatively predictable and there are lots of things I can do to mitigate failures. Not so with DCS when the nearest chamber is in Guam.

If running out of gas is unacceptable, then why isn’t getting bent with no treatment chamber within hundreds of miles across the Pacific?

Slight change of topic from your OP, but I'll answer anyhow. People are allowed to make personal decisions on the level of risks they will take. If you decide that you will never dive unless there is a chamber nearby, you cut off huge segments of the world (although not Truk - it does have a chamber). You will never, for example, see the wrecks of Bikini Atol. That is your choice. Me? If I get the chance I will go, and I will plan according to the risks. I took my 72 year old father to Truk. He is old, overweight and not in great shape. So we played it very conservative, and came back safely.

This is not unique to diving. If you climb Mount Everest or K2, your chances of suffering a serious injury are not insubstantial. There is no hospital you can get to in less than 3-4 days. That is why a lot of people die on those mountains. If you want to climb then, take all appropriate precautions. And if you are still not comfortable with it - don't do the climb. I would love to look down on the roof of the world one day, but I have a wife and kids who I love. So I won't ever make that climb.

But you knew all of this already...

[Akimbo]

…People are allowed to make personal decisions on the level of risks they will take. If you decide that you will never dive unless there is a chamber nearby, you cut off huge segments of the world (although not Truk - it does have a chamber)…

I couldn’t agree more regarding personal decisions on risks. I am just amazed that one risk factor is so over emphasized in this sport while DCS is so understated. Avoiding significant decompression dives without a chamber onboard or within 2 hours has been a personal rule most of my life. In the case of Truk, they do (did?) have a chamber, but they don’t always have the people that operate it. Thus my opening statement:

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...

This article makes me question if the chamber, compressors, and O2 supplies are all actually operational.

In my view, the statistical occurrence of DCS is an acceptable risk when treatment is less than 2 hours away. This is like my rule that it is OK to leave bottom with 300 PSI in 20' of open water, but not in 200'. As risk increases, it is appropriate to compensate. Exploring methods of mitigating DCS risk was the purpose of this post. IMO, taking extraordinary precautions to avoid DCS this far from treatment is on par with observing the rule of thirds for breathing gas.

I don’t think Truk would be worth the time and expense to me without making decompression dives. I have been on lots of wrecks, virtually all in cold water, shallow and deep. My first civilian sat was on the Doria so Truk isn’t a Mount Everest or K2 for me. It would be fun and easy, but not a lifetime experience. The only reason I haven’t gone back to the Doria for fun is because none of the boats have a chamber… yet anyway: Darkwater Expeditions

[Duke Dive Medicine]

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.

That's sort of a paradox because we typically don't check a diver for a PFO unless he or she has a case of DCS that raises our suspicions. Still, about 30% of the population has one, which means that 30% of divers have one, and we don't see a 30% incidence of DCS. So, absence of DCS isn't necessarily a good way to rule out PFO.

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?

When I was talking about masking symptoms, I meant surface O2, and to a lesser extent, O2 at 20 feet on a CCR following a dive (Akimbo's sur "D" O2 without the "sur"). If a diver gets DCS at a 20 foot decompression stop, he/she has probably done something seriously wrong.

[Duke Dive Medicine]

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.

Maybe. I saw a few O2 convulsions when I taught at the (former) College of Oceaneering, all of them in the chamber (we only used compressed air for diving). Never saw an O2 hit in the Navy and I did a lot of Mk-16 diving. I've seen a few clinical patients convulse here but they were either hypoglycemic or critically ill. For what it's worth, in my own experiences, I've never seen anyone vomit from O2 toxicity. I'd be interested to hear what others' experiences have been.

[Thalassamania]

That's sort of a paradox because we typically don't check a diver for a PFO unless he or she has a case of DCS that raises our suspicions. Still, about 30% of the population has one, which means that 30% of divers have one, and we don't see a 30% incidence of DCS. So, absence of DCS isn't necessarily a good way to rule out PFO.

Wow!

When I was talking about masking symptoms, I meant surface O2, and to a lesser extent, O2 at 20 feet on a CCR following a dive (Akimbo's sur "D" O2 without the "sur"). If a diver gets DCS at a 20 foot decompression stop, he/she has probably done something seriously wrong.

Done something very, very, wrong.

[Akimbo]

... I've never seen anyone vomit from O2 toxicity. I'd be interested to hear what others' experiences have been.

Neither have I, since I have never seen a confirmed O2 hit, but any indication of nausea, twitching, or most any other symptom got the mask yanked just in case. The one exception was irritability, usually accompanied by foul language.

Interesting about CDC. Were those hits while decompressing from air sats or regular training chamber runs?

[Duke Dive Medicine]

Neither have I, since I have never seen a confirmed O2 hit, but any indication of nausea, twitching, or most any other symptom got the mask yanked just in case. The one exception was irritability, usually accompanied by foul language.

Interesting about CDC. Were those hits while decompressing from air sats or regular training chamber runs?

Divers? Foul language? Say it ain't so!

They were all at 60 feet on the mock treatment table 6A. The sat system was dormant for most of my time there, and when I left, only the bell and handling system were working. We'd do simulated bell bounce dives just to get the students the experience. The whole Wilmington facility closed down a while back, unfortunately.