Deep Stops Increases DCS

[David Doolette]

But, that still leaves our tech wannabe/light tech/pushing the NDL diver out in the cold. How does this affect them? I remember how Deep stops really made a difference for me when I first incorporated them into my diving. These divers usually have plenty of gas to handle deep stops or even a short incidental deco stop.But, for the most part, they are ascending within NDL: no stops required. Hell, I fit into this group quite often when I'm ocean diving. Is there a reason why I shouldn't a deep stop in this case? By deep stop, I'm meaning half of my max depth for for a minute or two. Should I change that to a third, now? Should I discontinue them? To clarify, I do 5 minute safety stops, not 3. I think the extra two minutes are important when you're riding the NDL with a PDC.

As Simon points out, the sub-no-stop deep safety stops are a different thing, and the experimental data are conflicting. The disadvantageous aspect of a deep stop is continued uptake of gas into, or slowed washout of gas from, slower tissues (slower than the ones hovering the deep stop). If the total dive time, bottom time plus ascent plus any safety stops, is less than the no-stop limit, the continued gas uptake is not problematic. Under these circumstances, the deep safety stop may or may not be beneficial.

 

[leadduck]

A lot of decompression research was done to support high altitude aircraft in WW2. Iirc, that is where the dehydration/DCS connection was made.

That reminds me of one aspect that I'm also wondering about in connection with deep stops: accelerated deco and oxygen bubbles. Up to pPO2<0.2atm, almost all oxygen in blood is chemically bound to hemoglobin and does not contribute to bubbles; I guess pilots ascending quickly are concerned only with nitrogen but not oxygen for DCS, and that may be a reason why decompression research focused on inert gases.

Divers however use up to pPO2=1.6atm during accelerated EAN decompression and can have a lot of oxygen dissolved in tissues that may bubble. Many decompression models however track only inert gas loads. That's OK for air and trimix where oxygen contributes little, but it underestimates DCS risk for divers who do accelerated deco with EAN50. There are a few publications that show how remarkably bad our models are at predicting DCS risk for accelerated decompression.

Now, in connection to deep stops, I assume this is the worst-case scenario: calculate a deep stop ascent schedule that gets you up really slow between 70fsw and 20fsw, using EAN50 and oxygen for accelerated deco, with a model that doesn't track tissue pPO2. This way you get the maximum oxygen tissue load, but the model doesn't track it and recommends too short shallow stops.

 

[cerich]

in my personal recreational diving I haven't ever really done the "deep stop at half of deepest depth", I tend to do a min or two at 30 safety stop, then another 3 min at 15. My thought have been that 30 is "most likely" off gassing most tissues while still under enough pressure to keep it from fizzing (unscientific and not even an accurate description really, because fizzing would be seriously, seriously bad), then 15. But then again I am also a biog fan of taking a half day break every third day of multiple dives.

 

[lowviz]

Since the compartments are a fiction and do not represent particular organs or tissue we can't really compare between them. So if 1 to 3 are off gassing but 4 to 16 on gassing is that a net on gas or off gas? ...//...

I see the compartments as modeling equations. An exponential equation models any given fast/slow tissue rather well but once adjusted, it will not predict other real human tissues very well at all. So how many equations/compartments do you want to combine when fitting your model to the whole human body? (rhetorical question)

It is always valuable to start with the least complicated model and see how it fails. That is the intent of me entering this off-and-on interesting discussion. Deep stops seem to be the bone of contention. I'm struggling with 'exactly' what that nomenclature is.

I think a concept that comes close to what you are looking for is called a "profile dependent intermediate stop" (PDIS) in Uwatec/Scubapro computers. It is the depth to which you have to ascend before the medium-fast compartments switch from on-gassing to off-gassing. That's about the halftime that is commonly associated with the sensitive spinal chord and I find it an interesting idea. ..//...

You obviously understand my confusion. You nailed it. Thank you.

Exactly that. So if everybody is willing to more or less agree as to where the medium-fast equations are the ones to be dealt with first, then a signpost has been placed from which one can determine deep/intermediate/shallow stops. I would see that as valuable for the non-expert in this field. It would allow me to better follow the discussion.

... Bubble models insert deep stops to limit the growth of their imaginary bubbles and may create stops at a depth where medium compartments are still on-gassing; whereas PDIS tells you the depth to which you have to ascend at least to get into the realm of off-gassing. ...

Nicely stated, I'm sure that this is assumed by the experts. It isn't obvious to everyone who is following this discussion.

...I don't use it like a mandatory stop, it just tells me: get up quickly to this depth, and from then on ascend slowly.

That makes sense.

I consider my question to have been answered, quite well in fact. Please continue, sorry if this was a "trying to catch-up with the discussion" hijack...

 

[victorzamora]

So how many equations/compartments do you want to combine when fitting your model to the whole human body? (rhetorical question)

I know this was rhetorical, and as you go on to state later, the idea would be "the minimum number such that there's no loss in safety." The current Buhlmann model most people use has 16 unique compartments. I believe there are variants with 8- and 12-compartment models.

It is always valuable to start with the least complicated model and see how it fails.

And I think the reason you received responses was due to the widely-accepted Buhlmann ZHL-16 having 16 compartments. I think this is why your question concerning the point of "net, whole-body off-gassing" is so hard to answer....and you seem to have hit on a pretty key point. At what depth should you be such that you're off-gassing more than you're on-gassing without putting undue stress on your body? You described it as an off-gassing ceiling, but I actually think it should more be thought of as a floor with max deco stress defining the ceiling...giving you a bracket.

 

[Basking Ridge Diver]

You described it as an off-gassing ceiling, but I actually think it should more be thought of as a floor with max deco stress defining the ceiling...giving you a bracket.

Which is why I believe everyone describes it as a grey area versus a bright red line. An area should fit the general populace whereas a red line would need to be grossly exaggerated to fit those in the extreme ends of the bell curve. Pushing decompression times to unacceptable limits.
I guess this is why it is a theory and not a fact - it would be great if we could get personal decompression times in the near future versus general guidelines.

 

[igor p]

I think since this thread began in response to NEDU TR 11-06, it is unequivocally about efficiency.

I want to clarify a point that has been made before on the various deep stops threads but may have gotten lost in the heated debate. I (and I think the other ‘anti-deep-stops’ crowd) am not arguing that deep stops per se are bad. I am arguing against the notion that “at the same risk level, deep stops get you out of the water faster than shallow stops” (equivalent to deep stops schedules are more efficient than shallow stops schedules). My reply to NetDoc (post #826) encapsulates my point of view: his deep stop at half depth would not allow him to shorten his shallow decompression stops and overall decompression time – but that that deep stop in combination with additional shallow decompression time is probably a good thing. Similarly, I am not trying to “kill” (not my words) VPM-B. The argument has been advanced in these threads that VPM-B (and RGBM) does produce deep stops schedules that are more efficient than shallow stops schedules, and I have argued that there is no evidence to support that contention, and my synthesis of the available evidence supports the contrary. I do believe that a VPM-B schedule is not the optimum distribution of stop time, and the true optimum is some (as yet unknown) shallower distribution of stop time. That is not equivalent to saying do not use VPM-B, it is equivalent to saying if you choose to use VPM-B (or other ‘deep stops’ algorithm), it is prudent to use a higher conservatism level rather than select a lower conservatism level in the mistaken belief that “deep stops get you out of the water faster”.

Hi David,

thanks for clarifycation. Yes, It was lost in the heated debate. Some went further from the position you are describing.

 

[victorzamora]

Which is why I believe everyone describes it as a grey area versus a bright red line. An area should fit the general populace whereas a red line would need to be grossly exaggerated to fit those in the extreme ends of the bell curve. Pushing decompression times to unacceptable limits.
I guess this is why it is a theory and not a fact - it would be great if we could get personal decompression times in the near future versus general guidelines.

Absolutely. I mean, you'll read all up and down these threads that there's some yet-unknown "perfect" plan and how that'd be the holy grail. You also can see (you know, if you're reading any of this) that there are pretty strong opinions on what we currently believe to be closest to that plan. Even that "perfect" plan would have to take some intensely specific (impossible for now) factors into account to get a line....and even then I think a solid line would be impossible. I think a fuzzy gray splotch of "sufficiently safe" is all we'll ever be able to hope for.

 

[orange_diver]

Now, in connection to deep stops, I assume this is the worst-case scenario: calculate a deep stop ascent schedule that gets you up really slow between 70fsw and 20fsw, using EAN50 and oxygen for accelerated deco, with a model that doesn't track tissue pPO2. This way you get the maximum oxygen tissue load, but the model doesn't track it and recommends too short shallow stops.

I was thinking the exact same thing as I recalled the RD dives I did/planned using EAN50 and stop distribution heavily weighted at 70' to take advantage of the "oxygen window" thought to exist back then. No wonder people were getting all sorts of hits and not really talking about it...

 

[igor p]

I was thinking the exact same thing as I recalled the RD dives I did/planned using EAN50 and stop distribution heavily weighted at 70' to take advantage of the "oxygen window" thought to exist back then. No wonder people were getting all sorts of hits and not really talking about it...

That reminds me of one aspect that I'm also wondering about in connection with deep stops: accelerated deco and oxygen bubbles. Up to pPO2<0.2atm, almost all oxygen in blood is chemically bound to hemoglobin and does not contribute to bubbles; I guess pilots ascending quickly are concerned only with nitrogen but not oxygen for DCS, and that may be a reason why decompression research focused on inert gases.

Divers however use up to pPO2=1.6atm during accelerated EAN decompression and can have a lot of oxygen dissolved in tissues that may bubble. Many decompression models however track only inert gas loads. That's OK for air and trimix where oxygen contributes little, but it underestimates DCS risk for divers who do accelerated deco with EAN50. There are a few publications that show how remarkably bad our models are at predicting DCS risk for accelerated decompression.

Now, in connection to deep stops, I assume this is the worst-case scenario: calculate a deep stop ascent schedule that gets you up really slow between 70fsw and 20fsw, using EAN50 and oxygen for accelerated deco, with a model that doesn't track tissue pPO2. This way you get the maximum oxygen tissue load, but the model doesn't track it and recommends too short shallow stops.

Where did you found this? Can not believe what I read. Never heard one could get hit (DCS) for O2..
Can you please direct me to a study that would support this? Or dokument that would talk about something like this?