Dr Deco
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This is Part Three and a continuation of BillPs question on MISSED DECO STOPS:
(See question from 10-31-2000.)
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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 (= divers 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.
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:
(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 (= divers 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:
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 selections 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: