Blow a Safety Stop? Redescend ?

[Blackwood]

I know there are some really "in-the-know" people on here, whose opinion I respect. But is there anything "definite" on the theory of "bubble pumping"? Wouldn't the same issues of "bubble pumping" occur in a chamber? If not, why? Specific input would be most appreciated.

Chamber rides are much, much longer than safety stops. Yes, they shrink the bubbles, but then they give them adequate time to be offgassed and exhaled. That's the point of it.

Bubble pumping in a re-descent is a concern because of the short duration under pressure.

 

[Scott L]

Okay, this is really interesting stuff. I am learning something usefull, which I will noodle through. But, when I am told I am not understanding something correctly, I go find out.

Sooooo..... I sent an e-mail to a respected pulmonary cardiologist, who is a pulmonary specialist/professor of medicine, and wrote a really great book some years back on the subject. I asked him the basic question orginally posed; i.e. "would you redescend if you blew a safety stop?". I further clarified my question to him in that you knew nothing about the diver, their repetitive dive history, depth, bottom time, nothing.

The responses were as follows (please note this is NOT a recommendation from a doctor, and I am answering this as a point of discussion, NOTHING MORE, and and offering this only as a point of discussion):


It is really two questions, involving in-water recompression:
1. When you miss a recommended safety stop,
2. When you have (or about to develope) the bends

1. A safety stop in recreational diving is recommended because it builds a margin of safety; by definition, in recreational diving (i.e. diving by the tables) you will not get bent without a safety stop. It is 'no-decompression' diving. So if you missed a safety stop, and are worried about it, no harm at all in going back down and recompressing. Your theory is correct; it is the same (physics-wise) as the chamber in terms of increasing the pressure.

2. If you miss a decompression stop that is built in because it is necessary to avoid the bends (i.e., by definition not
recreational diving), then you have a problem. The problem is that if you get acutely ill (pain or mental confusion or whatever) and go back down, you could drown. This is probably why the docs recommend not to go back down - if the diver is unstable in any way, it could be a disaster. Controlled recompression is theoretically much safer.

So to answer your question, going back down is a good idea if there is no threat of illness (bends) that would lead to possible drowning AND (I should add) you have enough air to stay down awhile. In the real world I imagine there might be a lot of confusion over these issues if a diver surfaces without a stop and he/she asks 'what should I do?' Best to know and be prepared ahead of time.



So, I think there is lots of discussion left on this issue, and no clear consensus in the medical field. But my original point was that the physics and gas law properties do not change, but as pointed out, the controlled enviroment of a hyperbaric chamber does.

Good post,
Perhaps we should also bounce this question off of Doug Ebersole (debersole).

 

[Gombessa]

I have no idea if bubble pumping is a major concern for a re-descent to a recreational-depth, 3min safety stop, but if it's unnecessary to re-descend, and the risk of bubble-pumping is non-zero, why bother doing it? The disadvantages really seem to outweigh the benefits.

 

[Thalassamania]

I suggest that you e-mail him or her back and inquire about bubble pumping, I suspect that he or she overlooked that concern.

 

[TSandM]

Well, the issue with "bubble pumping" is allowing the bubbles to pass through the pulmonary filter, where they can reexpand on the arterial side. If you stay under pressure long enough, those bubbles will slowly offgas and resolve, and will not reexpand. How long you have to stay, and what gas you need to breathe to do that safely is probably not known. But going back down for three minutes on backgas and resurfacing is probably as close to a recipe for arterial bubbles as you can come up with.

 

[Thalassamania]

I'm sorry to get so insistent on this one, but this is one of the few times that I've seen "advice" being passed along that I really think might injure or even kill some one. Thanks Lynne, perhaps now we can put it to rest.

 

[dpaustex]

Okay, opinions are great. But "why" is redescending "is a close a recipe for arterial bubbles as you come come up with"?

 

[Thalassamania]

Well, the issue with "bubble pumping" is allowing the bubbles to pass through the pulmonary filter, where they can reexpand on the arterial side. If you stay under pressure long enough, those bubbles will slowly offgas and resolve, and will not reexpand. How long you have to stay, and what gas you need to breathe to do that safely is probably not known. But going back down for three minutes on backgas and resurfacing is probably as close to a recipe for arterial bubbles as you can come up with.

Because three minutes is not enough time, how much time is we do not know. It is, in my view, the height of foolishness to reenter the water when you are safely on the deck and put yourself at this risk in order to gild the lilly with a safety stop that you really did not have to make in the first place. Think about it for a minute.

 

[Blackwood]

For the sake of discussion, let's compare an average safety stop (a few minutes followed by at best a 3 minute ascent) with an average Type I chamber ride (60 for 20 on oxygen, 60 for 5 on air, 60 for 20 on oxygen, 30 minute ascent from 60 to 30 on oxygen, 30 for 5 on air, 30 for 20 on oxygen, 30 for 5 on air, half hour ascent from 30 to 0 on oxygen).

How are those things different?

 

[Scott L]

Because three minutes is not enough time, how much time is we do not know. It is, in my view, the height of foolishness to reenter the water when you are safely on the deck and put yourself at this risk in order to gild the lilly with a safety stop that you really did not have to make in the first place. Think about it for a minute.

I totally defer to your vast experience and credentials but if it can be argued that safety stops have prevented many DSC hits could we not logically assume that by not reentering the water with missed stops many unfortunate divers will suffer an occurence? Or, do you contend that the danger of becoming symptomatic underwater with the posiability of dire consequences outways DSC prevention in some cases with the practice?