Slow tissue on gas from stops

[EFX]

...... your risk of DCS may be smaller if you did that extra time on your shallow stops, and omitted the deep stop(s). Simon M

Don't you mean you'll do less time at the shallow stops if you omit the deep stop(s)?

Diver-Drex:

The big difference between a deep stop and a multi-level dive is that the time spent at a multi-level depth is usually longer. You chose in your example to make the multi-level stop just 2 minutes which given the short time is really a deep stop. The longer you spend in the water the greater the risk. The risk is compounded if there's an emergency. Not only does adding deep stops add time but time is also added to shallower stops above that called by a plan without deep stops.

In your example profiles above, the inserted deep stop of 65 ft with a 50/80 gf and the 30/80 gf showed identical stop times (rounding the 4:40 to 5 minutes). The last profile (60 ft/5 min, 40/1, 20/10) is strange because of the long first stop time. My spreadsheet and Perdix show the least time spent at the first stop with increasing stop times at shallower stops. Your dive planner must be optimizing the ascent schedule by having you spend more time at the first stop on EAN50 that shortened the total deco time. Remove the deco gas and you'll probably find the 30/80 dive has more deco time than the 50/80. If it weren't for all the anecdotal evidence that deep stops make some divers feel better most divers would be running flat gf's instead of the shaped gf's everyone posts.

 

[Dr Simon Mitchell]

Don't you mean you'll do less time at the shallow stops if you omit the deep stop(s)?

No.

DD asked if it was OK to add in a deep stop and then let Buhlmann add in compensatory shallow stop time. The answer is that it is fine to do that. BUT if you are prepared to do the total amount of deco time prescribed by that exercise, then from a DCS risk point of view, based on the evidence we currently have about deep stops (and depending what he means by deep stops) he might be better off doing the same amount of decompression time but omitting the deep stop. The risk differences are likely to be small, and you could certainly do what he suggested doing (I suspect a lot of people are). But I also suspect he based the question on an assumption that he is increasing safety by doing deep stops and then adding extra shallow time to deal with the extra ongassing during the deep stops. The point is, that safety may be optimised further by just doing the extra shallow time. This is the concept of efficiency I have been trying to explain.... obtaining the lowest risk by optimally distributing your stops in the time you intend decompressing.

If it weren't for all the anecdotal evidence that deep stops make some divers feel better most divers would be running flat gf's instead of the shaped gf's everyone posts

I would not place too much faith in this sort of anecdotal evidence. People tend to feel what they have been told they are going to feel. Since people started backing away from deep stops there has also been a lot of anecdotal evidence that this also makes them feel better. I don't have a lot of confidence in these stories either way. The formal studies with objective data on dive outcome are what we should be focussing on and they all suggest that the deeper stops as prescribed by bubble models or RD in decompression diving are too deep.

Simon M

 

[KenGordon]

Simon, do you think people can ‘feel’ decompression stress before it gets to the level of symptomatic DCS?

 

[TrimixToo]

If we accept that ZHL-B is effective what is the potential harm in adding a deep stop and allowing the algorithm to make the adjustments? Aside from more overall time in the water what is the downside? How is it different than a multi level dive where the PDC tracks offgassing and ongassing during the shallower segment and calculates the ascent profile based on that data?

Deco effectiveness aside (as Simon has, once again, ably addressed that), consider the additional risks imposed by being in the water for a longer time with a deco ceiling. The shortest time that carries the deco risks you are willing to tolerate is always safest.

 

[Diver-Drex]

Thanks for the replies. I fully understand that more time at depth means more time to ascend. I used the multi level analogy to illustrate that divers are doing that all the time and their PDCs are handling without issue. Adding a ‘deep’ stop and following your PDC’s ascent profile won’t bend the computer.
But that doesn’t mean there isn’t added risk to the body.
- increased overall run time
- increased gas requirements
- further from the surface if a problem or emergency occurs
- increased thermal stress leading to less efficient offgassing
The last point is clearly potential added DCS risk. Currently no PDC factors in the above added risks into the ascent profile. Diving a VPM or GF 30/80 algorithm requires the deep stop and exposes the diver to the additional risks. Diving GF 50/80, or something similar, let’s the diver decide if any potential benefit outweighs the potential risks.

 

[Diver-Drex]

Deco effectiveness aside (as Simon has, once again, ably addressed that), consider the additional risks imposed by being in the water for a longer time with a deco ceiling. The shortest time that carries the deco risks you are willing to tolerate is always safest.

Agreed and understood. We were replying at the same time.

 

[Diver-Drex]

For Dr Mitchell - How much weight would you give to diver fitness in regards to ascent profiles? Defining fitness as VO2 max and percentage of muscle mass. Does the higher percentage of fast tissues (highly perfuse muscle) and increased cardio vascular efficiency of more fit diver increase deco efficiency enough to influence the distribution of deco time across depths?

 

[UWSojourner]

The last point is clearly potential added DCS risk. Currently no PDC factors in the above added risks into the ascent profile. Diving a VPM or GF 30/80 algorithm requires the deep stop and exposes the diver to the additional risks. Diving GF 50/80, or something similar, let’s the diver decide if any potential benefit outweighs the potential risks.

Grouping VPM with GF30/X or GF20/X under the banner "deep stop" algorithms is not accurate. GF in almost any configuration shows better supersaturation patterns than deep stop algorithms like VPM-B, RD, or RGBM.

See this link for a discussion.

 

[Diver-Drex]

Grouping VPM with GF30/X or GF20/X under the banner "deep stop" algorithms is not accurate. GF in almost any configuration shows better supersaturation patterns than deep stop algorithms like VPM-B, RD, or RGBM.

See this link for a discussion.

Thanks for the link and clarification. Didn’t mean to assign labels. For the dive I was talking VPM-B and ZHL-B GF 30/80 produce a first stop deeper than ZHL-B 50/80. Although not Dr Mitchell’s intent 50/80 has become a bit of a measuring stick.
For the dives I am concerned with, and I think are applicable to a lot of divers, within the limits of Tec50 and AN/DP, VPM-B and ZHL-B 30/80 produce very similar profiles. So in that context VPM-B and GF 30/80 are similar enough to each and both dissimilar enough to GF 50/80 to be categorized together. I’m sure at greater exposures the differences are more pronounced.
Again, didn’t mean to label anything.

 

[Dr Simon Mitchell]

Simon, do you think people can ‘feel’ decompression stress before it gets to the level of symptomatic DCS?

Hi Ken,

I don't have a definitive answer, but made a related comment to EFX. We often hear people talk about how they feel better (often related to degrees of tiredness) after diving after doing something different with their decompression (eg more deep stops or less deep stops!) but it is difficult to figure out how much of this perception is related to expectation and how much is truly reality. A good illustration of this is the nitrox thing. Many divers have claimed they feel less fatigued after nitrox diving where nitrox is used as though it is air for decompression purposes, and this strategy obviously reduces decompression stress. But in a study conducted in Adelaide 15 years ago, divers who were blinded to which gas they were using (air or nitrox 36) for a 40 min chamber dive (with exercise) at 18m (60') reported the same levels of post dive fatigue even though the nitrox dives would have provoked substantially less decompression stress.[1]

Having said all that, given that subclinical DCS (which we might define as high decompression stress with barely perceptible symptoms) must occur, then it is certainly plausible that divers may develop a perception of "not being right" without actually developing obvious symptoms. I guess my comment would be that because of the potential for psychological overlay in this matter (especially when doing things that you might expect to make things better or worse), such perceptions could not be relied upon as an outcome measure if trying something like a new approach to decompression.

Sorry, long answer to simple question.

For Dr Mitchell - How much weight would you give to diver fitness in regards to ascent profiles? Defining fitness as VO2 max and percentage of muscle mass. Does the higher percentage of fast tissues (highly perfuse muscle) and increased cardio vascular efficiency of more fit diver increase deco efficiency enough to influence the distribution of deco time across depths?

Hello DD,

Another difficult question to answer. There has long been a perception that having a higher percentage of body fat is a risk factor for DCS (or at least for greater bubble formation). Based on watching the literature over many years, I would say that the weight of evidence favours this idea but not all studies that have considered the issue agree. I won't list the references because there are too many.

To the more sophisticated question of whether your percentage body fat vs lean body mass might influence the ideal distribution of stop depth across a fixed amount of decompression time, I would say there is no evidence. One could hypothesise that a very fat person (with a higher percentage of slow tissue that has a high capacity for absorbing inert gas) might, for example, want to avoid deep stops more than a very lean diver. But other than musing about it, I would not take the notion any further. Most people fall somewhere in the middle anyway.

It is uncertain whether aerobic fitness per se affects DCS risk much, with two caveats. First fit people are likely to be less fat (see discussion above). Second, fit people are more likely to have indulged in aerobic exercise within 24 hours prior to diving, and there are several studies that suggest this reduces post-dive venous bubbling.[2-4]

Finally, in relation to your decompression question, there is some evidence that gentle exercise during decompression improves outgassing by improving tissue perfusion.[5]

Simon M

References:

1. Harris RJ et al. Measurement of fatigue following 18 msw dry chamber dives breathing air or enriched air nitrox. Undersea Hyperb Med. 2003;30:285-91.

2. Castagna O et al. Endurance exercise immediately before sea diving reduces bubble formation in scuba divers. Eur J Appl Physiol. 2011;111:1047-1054

3. Dujic Z et al. Aerobic exercise before diving reduces venous gas bubble formation in humans. J Physiol 2004;555.3:637-642.

4. Blatteau et al. Haemodynamic changes induced by submaximal exercise before a dive and its consequences on bubble formation. Br. J. Sports Med. 2007;41:375-379

5. Jankowski LW et al. Exercise during decompression reduces the amount of venous gas emboli. Undersea Hyperb Med. 1997;24:59-66.