Current Research on Deep Stops for Recreational Divers?

[Kingpatzer]

I have been reading material on the development of decompression tables and current research on the subject. I came across "A deep stop during decompression from 82 fsw (25 m) significantly reduces bubbles and fast tissue gas tensions." by MARRONI et al from UHM 2004, Vol. 31, No. 2.

The paper starts out with a precursory introduction as to the reason for the research, noting that:

Experience in recreational divers has shown that 65% of treated DCS cases are neurological. They usually involve the spinal cord which has a “tissue” half time of only 12.5 minutes (4). During a 30 m (100 fsw) dive of 25 minutes, the 5 and 10 minute tissues will attain a high degree of saturation. Even though current computer models de-emphasize the importance of these tissues, these may, in fact, be controlling factors. Significantly more ascent time may therefore be required to off-gas these critical fast tissues and avoid neurological DCS. Indeed, the original Haldane table (2) for a 30 m (100 fsw)/25 minute dive required decompression stops at 9, 6 and 3 m (30, 20 and 10 fsw) for a total decompression time of 19 minutes. Yet today, with an ascent of 9 m (30 fsw)/minute and a ‘safety stop’ at 5 m (15 fsw) for 3 minutes, the recreational scuba diver is on the surface in only 6 minutes. This may be far too short for adequate desaturation of a 5 minute tissue that has attained a high degree of saturation.

This clearly has some serious implications for recreational divers interested in conservative diving practices. While the risk for DCS is relatively low in recreational divers, it still does exist.

The research involving over 1,400 divers found that divers following normal recreational protocols had bubble formation 87% of the time, and that 67% of the time these were high grade bubbles.

What becomes amazing in the research is that when looking at linear ascent rates,

This research with human divers produced two primary findings: (1) Slow ascents (3m/min) produced greater bubble grades than faster ascents (see Figure 5); and (2) the inclusion of a deep stop together with a shallow stop yielded the lowest bubble grades (see Figure 6). Therefore, contrary to popular belief, this study has indicated that a slow, linear ascent may produce significantly more bubbles than a more rapid ascent rate with a deep and shallow stop.

The final conclusion is that a an ascent rate of 10m/min with deep and shallow stops is an optimal risk reduction profile.

I'm sure there is more recent research on the relationship between ascent rates, stop levels, and bubble formation than this 2004 paper. I'm wondering if:

a) someone can point me to some of the better regarded research?
b) has this research made it's way into any recreational agency's diving instruction?
c) what is the effect of being focused on fast tissues have on gas-tensions in slower tissues? On longer deep dives including deep stops do slower tissue compartments become the controlling compartment?

Thanks in advance!

 

[Thalassamania]

Here's a PDF of the 2004 paper.

 

[Kingpatzer]

Thanks! I'm working off of a photocopy of a photocopy

 

[Thalassamania]

Very interesting paper, it makes me think that we will revisit the whole ascent rate and stop thing several times. If I needed to guess: a variable rate ascent (fast deep and slow shallow, with no stops) will prove to be best, and we will wind up approximating that with something like 120 fpm to 90 FSW, 60 FSW to 30, and 30 FSW to the surface with a short stop at 90 getting slightly longer at 60, 30, and 15. Just an educated guess.

 

[Funky556]

Everything I've been reading certainly points to that direction. Been speading a lot of time reading about different algorithms and various papers that shoot off from there. The latest stuff I read seems to praise faster ascent rates at depth with shorter deco times(at depth) and slower ascend rates with longer deco times in more shallow depths.

 

[gcbryan]

The paper quoted (I haven't read it yet...problems with Adobe) if using air is a little misleading. Sure there are deco stops required with 25 minutes at 100 fsw then and now. My dive computer has the NDL for air at 100 fsw at 16 minutes. Some tables are 19 minutes.

I think the ascent rates below 100 fsw are 60fpm in many models and 30fpm above that to the first stop in many models.

I think the problems are the exceptionally slow rates below 60 fsw where bubbles are growing simply because ongassing is continuing as in you can't have your first deco stop at a point where you are still ongassing in the leading compartment.

I've seen articles that suggest 10fpm is optimal once you get into the shallower depths which is what you get if you take 1 minute to get between each increment of 10fsw.

Bubble theory of all varieties does seem to offer an improved (if not totally accurate) explanation of what's going on. Improved as in better than simple disolved gas theory (Haldane).

Dr.Deco on this board has done research that suggests that bubbles exist in our bodies, are also generated by exercise, and that half-times aren't necessarily the same for ongassing vs offgassing depending on exercise and on hot and cold environments (differing) between the dive and what's going on after the dive...working during the dive more promotes faster ongassing than sleeping just after the dive does in promoting offgassing.

I think it's pretty much a fact at this point that bubbles aren't created just by the decreasing pressures experienced during scuba ascents. Heat creates them in liquid where none existed but pressure doesn't. So in this regard it's known that Haldane simply isn't accurate in the underlying facts.

I agree that these studies are interesting and doppler studies also show that bubble growth is highest not at surfacing but up to 15 minutes later. Which is a good reason to really limit the exercise right after a dive to the extent possible.

My point is (if I have one) that traditional theory has always recommended fast ascents, slowing near the surface without taking into account bubble formation.

This is not realistic however that doesn't mean the opposite is correct either. It is possible to stop too deep or to ascend too slowly. The answer is recognizing the two principles which are competing...faster offgassing near the surface with controlling bubble growth by staying deep a little longer.

Most of these theories are a bit of voodoo in the first place and empirical observations are really what drive the tables (and computers) that we actually use.

The real gamblers are those pushing the boundaries where there is less empirical observation going on in the first place.

 

[Soggy_Diver]

Hi Kingpatzer,

There is a thread about a 2007 article here--I think that it is a follow-up to the article that you cited.

http://www.scubaboard.com/forums/ask-dr-decompression/234869-extend-deep-stops.html

 

[gcbryan]

Very interesting paper, it makes me think that we will revisit the whole ascent rate and stop thing several times. If I needed to guess: a variable rate ascent (fast deep and slow shallow, with no stops) will prove to be best, and we will wind up approximating that with something like 120 fpm to 90 FSW, 60 FSW to 30, and 30 FSW to the surface with a short stop at 90 getting slightly longer at 60, 30, and 15. Just an educated guess.

I'm not sure what you are saying here. In one sense you are just describing traditional gas theory...hurry up and get close to the surface and then slow way down....not accounting for bubbles at all.

Are you just talking about what to do for a NDL rec dive? Otherwise, I don't get it as it would be virtually impossible to have no stop diving for an extended deco dive (the reason we have stops in the first place).

Or are you just saying that you think research will show faster ascent rates than most use today until you get to 30 fsw?

Once one gets past the ongassing point of the leading compartment what would be the rush in getting to the surface?

Just trying to figure out your post.

Thanks..

 

[gcbryan]

...
c) what is the effect of being focused on fast tissues have on gas-tensions in slower tissues? On longer deep dives including deep stops do slower tissue compartments become the controlling compartment?
...

First of all, I'm not sure what they are talking about regarding dive computers de-deemphasizing these tissues (fast tissues). They neither emphasize nor de-emphasize them. Some have suggested (Baker I believe) that we don't need a 4 minute compartment as the fastest compartment (too fast) so Buhlmann's ZHL-16 does have an option to go to 5 minutes for a first compartment half-time but I don't get the emphasizing comment.

Change the half-times for the compartment used if that's what you want to do. Maybe that's what they are arguing for.

Anyway, regarding your question regarding controlling compartments...yes as the ascent progresses (time and depth) the leading compartment can change from faster to slower compartments. This certainly happens with longer dives.

A deep but short dive results in the very slowest compartment barely changing from surface pressures (think bone) but the fastest compartments may saturate during such a dive. Six half-times is usually considered saturated so over a 60 minute dive compartments up to 10 minutes are saturated for some depth.

As time passes and you get closer to the surface, the fastest compartment will reach a point where it can tolerate oversaturation to a point greater than the surface but let's say the next fastest compartment can't yet...now that's the leading compartment.

This process continues until all of the compartments can tolerate ambient pressures equal to or less than that of the surface.

On a longer deco dive the controlling compartment by the end of the dive will be somewhere between the fast and slow compartments. The very slowest compartments would be leading only for multiple dive/multiple day diving or really only for commercial saturation diving.

 

[Thalassamania]

I'm not sure what you are saying here. In one sense you are just describing traditional gas theory...hurry up and get close to the surface and then slow way down....not accounting for bubbles at all.

Are you just talking about what to do for a NDL rec dive? Otherwise, I don't get it as it would be virtually impossible to have no stop diving for an extended deco dive (the reason we have stops in the first place).

Or are you just saying that you think research will show faster ascent rates than most use today until you get to 30 fsw?

Once one gets past the ongassing point of the leading compartment what would be the rush in getting to the surface?

Just trying to figure out your post.

Thanks..

I suspect that the entire concept of "no stop" diving will, in time go by the way. I'd predict that we will see a system for all dives of faster ascents from deep, a deep stop, slower ascents in the shallower range with required shallow stops.