Part Three: Missed Deco Stops - - -

[Dr Deco]

This is Part Three and a continuation of BillP’s question on MISSED DECO STOPS:
(See question from 10-31-2000.)

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?

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 NDLs 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 NDLs 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)?

Should it be OK for the diver to ignore the procedures for a missed deco stop and just continue diving?

For example, 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.

- - - - - -
The problem with “recovery” from missed decompression is that you do not know if gas phase separation (=bubble formation) has or has not occurred to any meaningful extent. This would be true with the Mplan software, also. :yelling: As I have stated repeatedly in the Ask Dr. Deco columns, decompression sickness is NOT a disorder whose etiology lies solely in supersaturation but rather in association with tissue nuclei concentration. If there where not any seed nuclei, the gas would remain dissolved up to the theoretical tensile strength of water; a diver could surface directly from six miles beneath the ocean. Nuclei concentration in the natural world varies, and it is not as great, for example, in water as in the atmosphere. Thus we are able to sustain some degree of supersaturation. (In the atmosphere, nuclei are present in much greater numbers, supersaturations break down with humidities of 100.02%, and cloud formation occurs abruptly. You can often observe that the bottoms of cumulus clouds are smooth because of the sharp limit. If we had the same nuclei concentration in our bodies as in the atmosphere, we could not ascent above 40 feet before sustaining DCS. )

Missed decompression stops are where these decompression algorithms begin to break down. They are not able to handle situations where bubble formation might occur or where it might be excessive. Decompression stops are given as mandatory to prevent this excessive bubble growth and "breakdown" of the algorithm. Most likely, bubble formation would not happen if it was just a few minutes from a deco stop and probably the dissolved nitrogen would be eliminated as per the algorithm. One does not known this for certain however. Even with a Doppler bubble detector, it would only indicate that a gas phase did or did not form in the capillaries of muscle and adipose tissue. It would not give an indication of local gas phase growth in the tendons and ligaments (and nerves). Thus this deco device (or software in the case of Mplan) is gambling on the fact that a free gas phase is not being carried over into the next dive. It is probably too liberal. If lodged in the tissues, this gas would act as a source for new gas in the volume and as a nidus for bubble growth on the next decompression.

It is for this reason that deco tables are in general very conservative. They allow diving by the vast majority of the population under almost all circumstances, although at times they do fail. Generally speaking, on any given day, any one individual might make a dive to quite a bit more than the NDL, not perform any deco stops, and walk away untouched. But this is sure not guaranteed. {See below for the Taiwanese divers.}

Once the decompression algorithm is exceeded (= missed stop or rapid ascent), the system (= diver’s tissues) must be “reset” before it is certain that the algorithm can be employed again. This means that the bubbles must be removed. This is what is done by the US Navy when they recompress their divers for missed stops. Now, I would wager in most cases, seeing the conservative nature of rec diving algorithms, that one could continue their dives, essentially with minimal impact if the error was trivial. However, one does not know this as a fact, since it is possible to have bubble formation and the "bends" on any given dive profile.

A report in Stockholm this year (Chao and Chang. Diving patterns and decompression sickness in Taiwanese diving fishermen) indicated what happens when you “wing it.” They report the following.

• Knowledge of decompression procedures: 29%
• No knowledge of procedures: 71%
  
• Never follow deco procedures: 55%
• Rare: 22%
• Occasionally follow: 15%
• Frequently follow: 4%
• Always follow: 4%
  
• Decompression with recompression treatment: 73% :upset:

These are the results of do-it-yourself diving tables.

It would be possible to implement a “recovery” profile that would be given by the computer. That may be the case in Uwatec devices, since it appears that they will send you down with a recommendation for additional time at depth (10 fsw). To my knowledge, this is completely theoretical although there may be empirical, laboratory evidence for it, since Professor Buhlmann performed many experiments over his lifetime. I do not know the basis of the “recovery” algorithm.

We can list some additional comments:

• [1.] As long as the deviations from the normal decompression procedure are small, and corrective action is instituted within a short interval of time, the device should/will continue to function. I believe that this is a theoretically reasonable concept. The reason being that gas bubble growth is not rapid but rather requires the diffusion of dissolved nitrogen across tissue; since considerable diffusion resistances are involve, this time will be on the order of several minutes. This was one of my initial observations and contributions to barophysiology {MR Powell. Leg pain and gas bubbles in the rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972)}.
  
• [2.] The deliberate violation of decompression is used in an occupational process known as “decanting” by caisson workers. They quickly leave the caisson, within five minutes enter the recompression chamber, and are brought down to a pressure just a little above the working depth. They complete the decompression on oxygen. A similar procedure is actually employed for hard-hat divers in a process known as “surface decompression.” In order to obviate the need for in-water decompression, the diver completes his decompression stop at 30 fsw, comes straight to the surface, and within five minutes enters a deck decompression chamber and is brought down to 40 fsw. The diver then breathes oxygen for a period of time (at 40 fsw) to complete the decompression. In both of these procedures, the men are at pressure, the pressure is reduced and then, within five minutes, the diver recompressed. These are variations from normal and special tables must be developed for this.
  
• [3.] NASA ascent procedures will produce numerous gas bubbles and cannot be used for staging unless there are no longer bubbles. All of our procedures are developed wherein bubbles are many but the residence time at supersaturation is limited and the subject is always breathing oxygen. You could use NASA protocols to go to 30,000 feet but could not use them to go, e.g., to 20,000 for one hour and them to 30,000 feet without a recalibration of the risk curves. This is because the subject started with no gas phase in the first but certainly had a gas phase in the second.
  
• [4.] To assure that the “system” is always at its initial point, no test subject is ever used who made a dive within 48 hours or a flight within 24 hours. It is obvious from the case of missed decompression and a lockout, that this caveat has not been followed. Thus the conditions under which the algorithm was developed have changed - - and that change might be vary significant.
  
• [5.] It is definitely known that certain dives will carry over a gas phase and produce later problems {Griffiths HB, Miller KW, et al. On the role of separated gas in decompression procedures. Proc R Soc Lond B Biol Sci. 1971 Sep 28; 178(53): 389-406; Gait D, Miller KW et al. The redistribution of vascular bubbles in multiple dives. Undersea Biomed Res. 1975 Mar; 2(1): 42-50}. This might well be the case in lockout situations.
  
  As far as the effect of missed stops, black coral divers of Hawaii perform some pretty amazing dives without any decompression. Sometimes they accumulate hours of missed deco time by the end of the afternoon. However, many of them are also paralyzed from this and remain so for the remainder of their lives. The ones remaining are natural selection’s “survival of the fittest” and possess, or lack, something [possibly surfactant] that is necessary for gas bubble formation.
  
  It is obvious that this question is very extensive. All of the "recovery" plans expressed here are for illustration only and are not necessarily given, or suggested, for actual use.:nono:

 

[Dr Deco]

Probably more will be added in this FORUM as the months continue. My thanks to BillP for this question.
_____________________________
Michael Powell (aka "Dr. Deco")

 

[BillP]

Dr. Deco:

Thanks for the detailed response. You stated:

"Because tables are expressed in numbers with several significant digits, they give an impression of precision and accuracy that does not in actually exist."

and

"The US Navy tables started with a set of limits derived by testing on US Navy divers. These were young men and in very good physical condition. [We can discuss this aspect of physical fitness in another FORUM.] Many table designers today will back off with respect to time at depth with the idea that recreational divers are older and not in as good shape as US Navy men. Probably this is a correct assumption in most cases."

and

"As far as the effect of missed stops, black coral divers of Hawaii perform some pretty amazing dives without any decompression. Sometimes they accumulate hours of missed deco time by the end of the afternoon."

Hopefully we all understand that decompression procedures are not a switch where if you stay within the limits you're perfectly safe, but if you exceed the limits set by the tables or computer and "flip the switch" you're bent. But many recreational divers seem to take in the above information and decide, "The limits don't apply to me. I'm in good shape, the limits are too conservative, and besides it can't happen to me." Maybe they're right, but they tend to forget your comment about the coral divers, "However, many of them are also paralyzed from this and remain so for the remainder of their lives."

My dive computer gives a NDL of 28 minutes on an 80 ft dive. The PADI dive table lists a NDL of 30 minutes for the same dive. Since decompression algorithms are really just best-guesses, isn't 31 minutes OK? How about 35 minutes? Heck, the Navy tables allow 40 minutes! It wasn't a strenuous or cold dive, so maybe 45 minutes will be OK (extending my bottom time ~60% from the original limit). So isn't it just fine to ignore the 28 minute NDL of the computer and dive however I want? Shouldn't the algorithm adjust itself to give me whatever limits I want to dive?

What was going through the minds of the computer manufacturer when they set the 28 minute limit? I can only imagine that their scientists (I hope) researched the available data from actual dives, animal studies, doppler bubble studies, incidence of decompression illness, etc. to come up with NDL's (starting with the Navy tables?). Then I see them thinking that the computer will be used by recreational vacation divers doing maybe 5 or more multilevel dives per day for a week (without a decompression chamber on the boat like the Navy divers often enjoy), so they backed off the NDL's some. Then they looked at me and saw a diver over 40 with something more than 2% total body fat and backed of a little more coming up with a NDL of 30 minutes for the 80 ft dive like the PADI tables. Then they ran it by the corporate lawyers who felt that the company would be ruined by the lawsuits from such a liberal limit and they insisted on a 10 minute NDL. Next the marketing department got wind of it, said they'd never sell a single computer with that limit, so they compromised at 28 minutes. That was the level of risk that they were comfortable with. I researched the limits of the computer and bought it.

There is a plethora of decompression algorithms out there. My own dive computer has no fewer than 90 different algorithms. Each table, computer, and deco program seems to have different limits, so obviously no one algorithm is correct for every dive and every diver. Some divers seem to think that every algorithm should adjust itself to fit them. Seems to me that it is the diver's responsibility to pick a decompression algorithm that best fits the level of risk that they are personally willing to assume. Don't blame the computer or table if it doesn't give you the limits you want. And don't ignore the limits that the algorithm that you choose gives you. Pick the decompression algorithm that fits you and your dive and live by it.

Just my 2¢,

Bill

(BTW, like many dive computers, the new Uwatec's will allow you to descend to the decompression ceiling within {I think} 5 minutes and complete your decompression obligation for a missed decompression stop. If you do not return to the ceiling within 5 minutes the computer will only display "SOS" and locks out for {again I think} 24 hours and gives no further decompression information on subsequent dives until you've been out of the water for a day. If the table designer "believes whole heartedly in their model" as you suggest, they've given you the limit for what they consider a safe dive, and you exceed that limit, how can the computer do anything but lock out?)



[Edited by BillP on 11-02-2000 at 06:50 AM]

 

[Dr Deco]

My dive computer gives a NDL of 28 minutes on an 80 ft dive. The PADI dive table lists a NDL of 30 minutes for the same dive. Since decompression algorithms are really just best guesses, isn't 31 minutes OK? How about 35 minutes? Heck, the Navy tables allow 40 minutes! It wasn't a strenuous or cold dive, so maybe 45 minutes will be OK (extending my bottom time ~60% from the original limit). So isn't it just fine to ignore the 28-minute NDL of the computer and dive however I want? Shouldn't the algorithm adjust itself to give me whatever limits I want to dive?

One can adjust tables to do this, but so far it can not be done with deco meters. We at NASA are working on methods to characterize individuals with respect to the decompression characteristics, since this could have an operational potential in the future. Such methods are not sufficiently advanced for the laboratory and are certainly not at all ready for divers in the field.

What was going through the minds of the computer manufacturer when they set the 28-minute limit? I can only imagine that their scientists (I hope) researched the available data from actual dives, animal studies, Doppler bubble studies, incidence of decompression illness, etc. to come up with NDL's (starting with the Navy tables?).

Since my entrance into this field in 1969, I have known many table and deco meter designers. I know that all of them have started with some type of empirical (= laboratory-derived NDLs) limits either from their own laboratory or from others lab studies. None of the NDLs in tables or meters are based on anything but data, although they may be reduced a bit if the designer personally believed that the test population differed from the individuals for who the table, or meter, is intended.

Then I see them thinking that the computer will be used by recreational vacation divers doing maybe 5 or more multilevel dives per day for a week (without a decompression chamber on the boat like the Navy divers often enjoy), so they backed off the NDL's some. Then they looked at me and saw a diver over 40 with something more than 2% total body fat and backed off a little more coming up with a NDL of 30 minutes for the 80 ft dive like the PADI tables.

It is correct to say that the tables are more conservative than the US Navy, but I do not know of any case where, after shortening the NDLs, the table designers then added time back. The tables were made more conservative from because of their initial NDLs; the deco meters were made more LIBERAL compared to tables, by actual calculating the inert gas loadings and not utilizing repetitive group designators. Liberalizing of recreational diving arises from the removal of this Repetitive Group system.

Then they ran it by the corporate lawyers who felt that the company would be ruined by the lawsuits from such a liberal limit and they insisted on a 10-minute NDL. Next the marketing department got wind of it, said they'd never sell a single computer with that limit, so they compromised at 28 minutes. That was the level of risk that they were comfortable with. I researched the limits of the computer and bought it.

I am not aware of any changes made by a legal department. There was no interference from any outside groups with regard to DSAT’s Recreational Dive Planner. Nor was there any changes made by a marketing department. In part, there relative similarity of the NDLs is because they all can be traced to a common origin and their deviations are straightforward. Some of the books on deco meters actually give the pedigree. For example, Uwatec comes from the work of Professor Buhlmann and Dr. Max Hahn in Switzerland, DSAT tables and Oceanic meters are derived from the limits of Dr. Spencer’s Doppler research (and the testing work of Drs. Powell and Rogers), The DCIEM tables come from Canadian research of Kidd and Stubbs.

There is a plethora of decompression algorithms out there. My own dive computer has no fewer than 90 different algorithms. Each table, computer, and deco program seems to have different limits, so obviously no one algorithm is correct for every dive and every diver. Some divers seem to think that every algorithm should adjust itself to fit them. Seems to me that it is the diver's responsibility to pick a decompression algorithm that best fits the level of risk that they are personally willing to assume. Don't blame the computer or table if it doesn't give you the limits you want. And don't ignore the limits that the algorithm that you choose gives you. Pick the decompression algorithm that fits you and your dive and live by it.

The deco meters ALL assume (with the exception some of the SUUNTO meters that have been programmed with the Reduced Gradient Bubble Model) that there is essentially NO GAS BUBBLE FORMATION. As soon as this phase separation occurs (= gas bubbles forms), the who system changes. This is because the strong driving force for gas elimination only occurs when the nitrogen remains dissolved. When bubble formation occurs, there is a rapid equilibration and the nitrogen diffuses into the bubble. There it remains at ambient pressures with virtually no driving force for elimination. Naturally when you dive again, that gas reservoir dissolves once again adding that much more gas to the tissue. It is just as if one took a hypodermic needle and injected a hefty dose of nitrogen into you tissues! Obviously this is a bad idea :upset:

If the table designer "believes whole heartedly in their model" as you suggest, they've given you the limit for what they consider a safe dive, and you exceed that limit, how can the computer do anything but lock out?)

The LOCK OUT has more to due with the inability of decompression algorithms to mathematically treat a separated gas phase. Carrying this gas phase along is diving with a time bomb. :boom:

 

[Warhammer]

Originally posted by Dr Deco


The deco meters ALL assume (with the exception some of the SUUNTO meters that have been programmed with the Reduced Gradient Bubble Model) that there is essentially NO GAS BUBBLE FORMATION. As soon as this phase separation occurs (= gas bubbles forms), the who system changes. This is because the strong driving force for gas elimination only occurs when the nitrogen remains dissolved. When bubble formation occurs, there is a rapid equilibration and the nitrogen diffuses into the bubble. There it remains at ambient pressures with virtually no driving force for elimination. Naturally when you dive again, that gas reservoir dissolves once again adding that much more gas to the tissue. It is just as if one took a hypodermic needle and injected a hefty dose of nitrogen into you tissues! Obviously this is a bad idea :upset:

So do you feel that Suunto's RGBM is an advancement making the computer a better choice. Or is the residual nitrogen it computes a mere guesstimate?

 

[BillP]

My description of the decision process for a computer algorithm was definitely meant as tongue-in-cheek, but thanks for your explanation.

 

[Dr Deco]

Dear Warhammer:

The Reduced Gradient Bubble Model is a generalization for all diving situations, and it does not apply in specific instances where stops have been missed. While not exactly a guesstimate, it is not exactly the answer to missed decompression calculations. The algorithm would not know how much gas phase formed and how to handle the changes in diving from that point onward.

This variation in missed stops could only corrected from additional information such as number of bubbles as determined by Doppler ultrasound. Such coupling of bubble detection, gas phase retention, and deco meters does not yet exist. The reduction in off gassing by gas bubbles blocking the lung capillaries is a part of the algorithm in the Uwatec, but again, it is a generalization.

We were aware of “carry over” in repetitive dives, and these testing methods were employed during laboratory and open water trials of tables such as the DSAT Recreational Dive Planner. [Hamilton, RW, RE Rodgers, MR Powell, and RD Vann. Development And Validation Of No-stop Decompression Procedures For Recreational Diving. Diving Science and Technology. February 28, 1994, (pp. 78 + appendix)]. In the case of these tables, the tested recomendation was four dives per day.