Alert diver article on Deep Stops

[miked]

I don't know if this is old news, but I just got my winter 2010 issue of Alert Diver, and skimmed through the article on deep stops and recreational diving. I'll need to look at the article more carefully, but if I read it correctly, the panelists (?) were dismissive of the concept (for rec. diving within "NDLs").
I have been using the "half depth stop" for the past few years, and thought that they were an upcoming part of safer diving.
Did I miss a memo ?

Any thoughts on the article?
Thanks,
Mike

 

[Bubbletrubble]

I think the panelists were just citing "evidence-based" dive research. The majority opinion was that there is insufficient evidence that no-deco recreational dives (within NDLs) benefit from "half depth" stops. Only a few studies have been published on this (the ones I'm aware of have used Doppler-measured precordial venous bubbling as assessment), and the dive profiles investigated don't cover the entire spectrum of no-deco diving open to recreational divers. I'm more inclined to accept that stance than the lone dissenting opinion by Peter Bennett. On some level, I think Bennett confuses deco theory (critical gas supersaturation in theoretical fast tissues) with empirical research on human beings. He would have been better off citing increased precordial venous bubbling, stating its likely relevance to DCS, and letting recreational divers judge for themselves what to do. I was very surprised by his answer. I agree completely with what was said by Mitchell, Southerland, Gutvik, and Doolette.

At issue here is that there have been a high number of dives (without deep stops) conducted within no-deco recreational parameters that have not resulted in DCS. That's a large data set. The data set (using gold standard DCS as endpoint) for recreational divers doing "half depth" stops is not nearly as large. Recommendations to rec divers should necessarily be conservative. Until sufficient evidence exists that deep stops significantly decrease the likelihood of DCS in divers doing these kinds of dives, it makes sense to recommend maintaining the status quo (not exceeding ascent rate guidelines, doing an extended shallow stop, etc.).

By doing deep stops on no-deco recreational dives, you're kind of turning yourself into a guinea pig. Admittedly, there's probably a very small chance that you are increasing DCS likelihood by adding in deep stops...so long as your computer doesn't trip into deco mode during the deep stop. But none of us know for sure. We'll all just keep an eye out for more research on the subject.

 

[TSandM]

I was very interested in Doolette's viewpoint in this article, and surprised by it. GUE works very closely with those guys (Doolette is prominently features in their "Mysterious Malady" DVD) and they recommend slowing the ascent at 1/2 the maximal depth.

The biggest problem is that DCS is SO rare in recreational dives that are executed within the existing guidelines, that trying to prove that a change in procedure is beneficial will take a very large n.

 

[halocline]

I was a little surprised by the article as well, although it is typical for changes in official agency recommendations for recreational diving safety procedures to change quite slowly.

When I read it, I was guessing that maybe the pressure gradients in recreational diving are not high enough to warrant off-gassing at what is still a relatively deep position, and it might be more effective off-gassing to simply head straight for a shallower stop, maybe substituting an extra minute of safety stop for the deep stop. I guess one question if you alter an existing recreational dive plan to do a one minute stop is, where does that one minute comes from? Do you leave early one minute, or do you arrive at your safety stop one minute later?

 

[fnfalman]

I'm not a deep diving/decompression diving expert, but I must question what exactly does a deep stop do? That's multi-level diving to me. It's one thing to do deco stops when you do deco diving but when you do NDL diving, a deep stop is just another multi-level dive.

What's the difference between diving down to the bottom of a reef wall at 120-ft and spend some times there, then went up to 90-ft and spend some times there, then go up to 50-ft and spend some times there? That's multilevel diving.

Going to see a wreck at 120-ft and spend some times there, then go up to 60-ft and stay there for a bit for this "deep stop"...what's the difference? Multi-level diving is multi-level diving. Safety stop is safety stop. There's a good reason why safety stops are being conducted at less than 20ft depth.

 

[Sas]

I went to a DAN Safety seminar yesterday (agenda) and deep stops came up. One of the speakers said that for recreational NDL diving less than 30m a deep stop of at least 2.5mins at half your depth can be be beneficial in reducing bubbling. Another speaker said that the benefits in technical diving were not established. I'm interested to read further research myself about it.

 

[TMHeimer]

I'm not a deep diving/decompression diving expert, but I must question what exactly does a deep stop do? That's multi-level diving to me. It's one thing to do deco stops when you do deco diving but when you do NDL diving, a deep stop is just another multi-level dive.

Certainly makes sense to me. Another way of putting it is say you dive 30 mins. at 70 feet. You do the 3 min. shallow safety stop even though it's not required. If you dive to 120 feet and do a deep stop at 70 you are doing a safety stop at a depth from which you should do a safety stop.

 

[SkimFisher]

I'm not a deep diving/decompression diving expert, but I must question what exactly does a deep stop do? That's multi-level diving to me. It's one thing to do deco stops when you do deco diving but when you do NDL diving, a deep stop is just another multi-level dive.

What's the difference between diving down to the bottom of a reef wall at 120-ft and spend some times there, then went up to 90-ft and spend some times there, then go up to 50-ft and spend some times there? That's multilevel diving.

Going to see a wreck at 120-ft and spend some times there, then go up to 60-ft and stay there for a bit for this "deep stop"...what's the difference? Multi-level diving is multi-level diving. Safety stop is safety stop. There's a good reason why safety stops are being conducted at less than 20ft depth.

I am no expert either, but I think the theory is that as soon as you leave max depth you begin off-gassing the nitrogen you absorbed at that depth. Hence the "credit" NDL time you get when ascending. The pause at half-depth I think is there to allow the body a little extra time to off-gas the nitrogen absorbed from the 60-120 and then proceed to the safety stop instead of heading straight there.

It seems like sound reasoning to me - but again, not an expert.

What exactly is the argument AGAINST a deepstop? All I've read so far is that it's not needed/no point/etc etc. But what's the harm in it?

I did a dive yesterday to 120 and then piddled around at 70ft for a bit. What's the harm in me staying at 60 ft for 3 minutes and heading to my SS vs. me spending 20 minutes at 60ft and heading for my safety stop? It seems like six one way and a half-dozen the other...

So long as I'm within the NDL it doesn't seem like it should matter. So again, what's the HARM in doing a deepstop?

 

[muddiver]

Part of the agument aginst deep stops my be the full-program that includes ascent to the half-depth stop at 60 ft/min. Most of the scuba agencies see fast ascents (>30 ft/min) as the primary cause of DCS incidents.

That along with the known fact that when you deal with the term "agency" or established "organization" there is always a major resistance to change. Lack of qualified studies does not help either.

 

[Blackwood]

I'm not a deep diving/decompression diving expert, but I must question what exactly does a deep stop do? That's multi-level diving to me. It's one thing to do deco stops when you do deco diving but when you do NDL diving, a deep stop is just another multi-level dive.

For one, I don't think most people would call a 1-minute stop to be a "level."

But in any case I think you are a victim of flawed terminology' in specific: "ND(L)".

Why "can" spend 20 minutes at 100 feet, 10 at 50, 10 at 30 and 10 at 10 while you can't spend all that time at 100 feet?

Because, on those higher 'levels' at least some of your tissues are offgassing (i.e. decompressing).