Is there data on how close to NDL undeserved hits occur?

[FishWatcher747]

I am curious to know if an analysis has ever been made on how close to NDL does the average diver get to then suffers DCS on an "undeserved" hit dive? If so what is the range and what is the shape of the curve?

Edit: Let us assume there was no rapid ascent contributing to the DCS, just a normal recreational dive following all usual training guidance.

 

[tbone1004]

Short answer is no. @Duke Dive Medicine might be able to confirm, but the data would be nearly impossible to extrapolate given the different algorithms that each computer is using, the general lack of downloaded dive profiles until the last 10 years and even then only if you have a computer that supports relatively easy log downloads. Part of the other issue is that the dive would have to be reported as a DCS incident into some sort of database for analysis and a lot of divers either won't go to a chamber and either resolve with surface O2 or perform IWR if it's mild and unless they filed the report through DAN it probably wasn't even recorded as an incident.

 

[FishWatcher747]

Short answer is no.

Thanks for the response. Maybe it would have to be broken into Type 1 and Type 2 DCS. I agree that Type 1 would be very difficult to get data on.

However, Type 2 involving chamber rides should be doable.

My understanding is the chamber people want to see the patient's computer. I would assume they then document this data in the chart.

In my short diving career I personally know 2 people with Type 2 DCS needing multiple chamber rides who had "undeserved" hits. If I know 2 people there must be many more.

Recently an experienced SBer mentioned he now dives no closer to NDL than 20 minutes or something to exercise extreme caution.

So it might be helpful for the diving community if data could be developed to say only 1% of Type 2 DCS involving chamber rides were in dives that the diver stayed 20 minutes away from NDL.

To be fair even my sample of 2 people had an added complexity that would have to be taken into account. One had a workup showing no underlying risk factor and one had a large PFO (since closed and back diving).

 

[johndiver999]

20 minutes from ndl. that would be kinda ridiculous if you are diving "deep". I would think a percentage of the ndl, at whatever the max depth is, might be reasonable.

 

[FishWatcher747]

To put it a different way can we even crudely say how much "riding the NDL" contributes to recreational undeserved Type 2 DCS?

 

[scubadada]

@FishWatcher747

All the various deco algorithms have different NDLs. What you really want to know is the exposure (depth, time) that results in an unexpected hit. DAN Project Dive Exploration was not able to do this, a very low rate of DCS. You can find the results of PDE on the DAN website.

You might enjoy looking at the SAUL probabilistic deco algorithm and dive planner, search on modern decompression. It will give you probability of DCS for a single dive on air or 32% to a dictated depth for a given time. There is a discussion of repetitive similar dives in the blog.

Of course, there are many individual factors contributing to an episode of DCS. So, you will not find your answer.

The longer your exposure at depth, the greater your risk of DCS

 

[FishWatcher747]

The longer your exposure at depth, the greater your risk of DCS

Well if this is true then there are 2 pieces of information. I have heard it said that something like 30 foot recreational dives within NDL do not lead to DCS. Correct? And the longer your exposure at depth the greater your risk of DCS.

My question is what does the curve look like?

Now square profile dives would be easier to get data from. But that is a tiny minority of recreational dives.

If one could get the downloaded graph of the dive off a computer showing depth and time it wouldn't matter what algorithm was used. Correct?

How about a data point of maximum depth of dives that led to an NDL recreational Type 2 undeserved hit? Can we say for example that 75% of dives leading to Type 2 hits in recreational NDL dives occurred in dives with a maximum depth greater than 80 feet?

 

[rsingler]

My personal belief is that, absent medical factors which change the DCS probability (like PFO, since we all bubble), it's ascent rate, particularly in the last 30 feet, which plays a much more significant yet underappreciated role.
In a way, that's what safety stops do: they significantly drop your mean ascent rate from 30 feet. Unnecessary? Not in my book. It's the right thing to do for the wrong reason. It's not just safety padding. I believe that the incidence of "undeserved hits" would have been much greater in the last 30 years had we not incorporated this practice. It was not just a "good idea"; it was a great idea! Just hard to prove before ultrasound evaluation of venous bubbling.

I try to take five minutes to get from 15ft to the surface. Hard to do. Good buoyancy practice. Practicing hovering at 3 feet in calm water makes it fun and helps the time pass.

I go past NDL all the time. That's tech diving. Crossing past NDL "0" is a bogeyman. You just need extra offgassing time, and there's a whole "science" and certification business built around that. For good reason.

 

[Ukmc]

There are other personal factors (eg obedity, dehydration, alcohol consumption, above mentioned PFO…) that contribute to this. Make it very difficilt to analyze low frequency events…

 

[lexvil]

The term “unexpected” may fit better than ”undeserved” due to the personal factors mentioned earlier.