Missed Deco Stops (long)

[BillP]

Hi Dr. Deco:

I would like to hear your thoughts on the rationale behind decompression algorithms requiring specific decompression stops and your thoughts on recommended procedures for dealing with missed decompression stops. I know why decompression stops in general, but what is the maker of the table saying when he says, "This dive requires a stop at 20ft for 3 minutes and a stop at 10ft for 23 minutes" rather than saying, "Oh, maybe stop at 20ft for awhile and if you don't mind slow down for a bit as you pass 10ft"?

The reason I ask is many recreational divers are starting to do dives requiring mandatory decompression time according to the decompression algorithm they've selected for their dives. They are diving for fun, and decompression time is usually no fun. Computer divers on vacation who accidently go into deco don't want to spend time at stops. They want to be able to skip the stops and then continue diving the rest of the day. They certainly don't want to re-enter the water to satisfy the deco obligation. To minimize the time at deco stops more technical recreational divers sometimes seem to pick the most liberal deco programs, tables, computers they can find. Then they start skipping deco time. They'll say, "One minute less deco time doesn't matter" or, "This is a way too conservative algorithm and 17 minutes deco is just too much." After skipping mandatory deco time, they want to continue subsequent dives as if nothing had happened. Safe? Why does the creator of an algorithm make deco time "mandatory" rather than just a rough suggestion to be fudged as the diver sees fit? Or do they?

For example, US Navy Table protocol. According to the US Navy Dive Tables, if a diver omits a mandatory decompression stop at 20ft or less and he can return to the water within one minute, he returns to the level of the missed stop, lengthens that stop by one minute, and completes the subsequent stops. If the surface interval is greater than one minute and the diver remains asymptomatic, the diver returns to the missed-stop level and lengthens the stop by 1.5 X (then completes the remaining stops). Alternatively the diver can be treated in a chamber with Navy Treatment Table 5 (135 minutes in the chamber). The Navy tables do not list a procedure for allowing subsequent dives if the above procedures are not followed. The Mplan tech/recreational dive planning software program seems to take a much more liberal view. According to Mplan, if you ignore a deco obligation, but re-enter the water for a subsequent dive at the same time you would have if you'd made the mandatory stop, you spend less time in deco on the second dive (presumably because you've off-gassed faster on the surface than you would have at depth). Very appealing to the recreational diver. Should it be OK for the diver to ignore the procedures for a missed deco stop and just continue diving?

I believe that you've been involved in the creation of decompression algorithms. Why didn't you make them more liberal with extended no decompression limits and shorter deco stops so they would be more fun or convenient? What goes through a decompression algorithm creator's mind when he sets a "limit"? If a recreational diver chooses to ignore the limit set by the decompression algorithm they've picked and skip mandatory deco time, what do you think should be their procedure for subsequent dives? Just continue the day's dives as if nothing had happened? Have longer NDL's and less deco time on the next dive (like Mplan) because you've off-gassed faster on the surface than you would have on the deco stop at depth? Have shortened NDL's or extended deco time on the next dive to make up for the missed stop? Or maybe stop diving for the day to prevent further uptake of nitrogen and take time to see if you develop DCS from ignoring a mandatory deco stop(s)?

TIA,

Bill

 

[Dr Deco]

Hello Bill:

You have posed a very interesting set of questions, and I will break them up into pieces for ease of answering---this is the first. These answers must be considered as my opinion only, as other barophysiologists would certainly have opinions about this, although they might not differ from these answers to any great degree. To start with, below are two quotes from lectures in my decompression physiology class.

• Because tables are expressed in numbers with several significant digits, they give an impression of precision and accuracy that does not in actually exist.
  
• Tables are a road map and not a license.

I would like to hear your thoughts on the rationale behind decompression algorithms requiring specific decompression stops…. I know why we have decompression stops in general, but what [about just] … slow down for a bit as you pass 10ft"?

While most dive computer programmers and table designers believe whole heartedly in their models, the truth is they are all dealing with what Dr. Chris Lambertsen refers to as“paper bubbles.” All algorithms attempt to express what has been empirically noted in actual dive situations and produce some type of a ”Unified Theory.” This is then called the decompression model, but it is a phenomenological model and not a mechanistic one. It is only a bookkeeping system logging the concentration of gas molecules throughout the body.

Because we see multidigit numbers, we have the impression that things are very accurately known and the final results are expressed with great precision. One comes to believe that small changes will produce an all-or-none effect , that is, “DCS or no DCS.” In actuality, there exists a condition of “subclinical DCS” that can eventually develop into “frank DCS.” Other than pain or paralysis, there do not exist any other signs or symptoms (except Doppler bubbles in some instances) to indicate that you are at the incipient “bends point.”

The table designer attempts to distill a large number of dive scenarios into the schedules based upon actual experience, often totaling thousands of dives. When the table or algorithm is ignored, essentially the diver is titrating his own schedules and creating his own NDLs and decompression stop times. It is similar to increasing speed on a highway, in many cases, and up to a point, you can get away with it. But when it fails, it often fails catastrophically. The system is such that a hard depth and time MUST be given. That is the "nature of the beast," but remember it is only a paper bubble.

It is necessary to follow some schedule because of the large time lag that exists between gas phase formation and DCS. It is therefore not possible to control decompression in real time. Gas uptake is “conservative” and is independent of the path. Gas elimination is “nonconservative” and is not independent of the path. The decompression procedures are always developed and tested with specific decompression paths. One cannot change this path in an arbitrary fashion without recalibrating the allowable supersaturations. This is basically what table testing is all about, viz, calibrating the allowable supersaturations for any given set of procedures. A variance from the procedure then requires a retest.

One will sometimes hear that decompression algorithms are based solely on theories, and that none of these have been proved or disproved. Actually, all decompression systems are based solely on EMPIRICISM. There does not exist a single ab initio (= from first principles) calculation in the world for diving tables, although there are many theories that purport to explain why the algorithm works. The fact that no theory exists is why everything must be tested when outside of the limits of the model, and extrapolation is rarely possible. It is especially not currently possible to recover from a dive schedule when excessive tissue bubble formation has occurred. It is of course possible to end a dive series (with bubble formation) and possibly nothing untoward will occur as time passes and off gassing continues.

The idea in the minds of many divers is that the table is a license to dive with impunity. Imprudence can put a time bomb in their hands. A table shows you the "hills" in the “tissue pressure profile,” but that is all. You would not set out on a cross-country walk without checking the weather just because a trail is on the map . Neither does a table indicate the advisability of making a certain set of dives.:boom: