Bouncing!

[Zeamonkey]

Guiding in the Red Sea often involve tying ropes on to the wreck of the Thistlegorm. first you take the first rope down and tie on at 14-20m, go back up, go down to tie 2nd rope, back up, take divers for a 30m dive, comeup and have your SI, then a 2nd dive to 25m, back up with clients, go back down and untie 2 ropes. What is the safest way to perform this task? if any?

 

[beezwax]

Hi Zeamonkey, I am not a doctor and I'm sure the medical mods will interject their expert opinion here, but I thought I'd sneak a comment in since I know exactly what you're talking about and wondered the same thing. I did my OW and AOW in Dahab in a week, and then did my very first dives as a newly minted diver on the Thistlegorm-- unforgettable. But I couldn't believe how many times the DM had to pop up/down to 80-100 feet.

I recently finished a Med Diver course and of course we discussed profiles and DCS, etc. It would seem to me that there is nothing inherently dangerous about a bouncing profile but that the critical thing, as always, is your ascent rate. The first thing to detemine in a diving emergency is whether the diver is sick (for example the bends) or injured (for example lung overexpansion). If you bounce to 100 feet and then rocket to the surface there's really no way for you to get DCS because you haven't spent any time at depth and thus haven't loaded any nitrogen. But you could blow a lung. On the other hand, when it's your fifth or sixth trip to the surface, as when you're bringing up the anchor lines at the end of two deep dives, your body definitely has loaded nitrogen and you could get bent. And you could still blow a lung. So in other words the key is to ascend slowly every time, but this is even more important at the end of the day since you now face the added risk of DCS.

 

[DandyDon]

There was some interesting dicussion about this on this thread: http://www.scubaboard.com/showthread.php?t=136048

I heard an answer to this in some recent training...

If bubbles had been formed on your rapid ascent, they would be confined to the "pulmonary" side of your heart and will hopefully be released through the alveoli in the lungs, allowing the diver to remain asymptomatic.

If one descends again, the bubbles will reduce in size, and may be able to pass by the lungs and into the side of the heart that pumps to the brain. At this time, the diver would have micro bubbles in the total circulation system, including neural tissue.

If the diver now ascends before these micro bubbles are completely purged from the blood, one or more of them could expand in neural tissue upon final final ascent.

For this reason, I was advised to never re-descend in this kind of situation.

This is also apparently the mechanism of danger in bounce dives.

 

[beezwax]

Hi Don,

I read that thread too, that's a good one. I would certainly agree that a bounce profile is less than desirable because every time you go down/up there is an ever greater chance to get bent. However, it can be done without incident (DMs on the Thistlegorm do it every day) but again the key is to control your ascents (and don't miss your deco). If anything, extend your deco on repeat ascents.

Regarding jagfish's post on bubbles, I think that's basically right. There are different ways that bubbles can get back into circulation. They don't really go backwards from the lungs to the left-side heart due to recompression (at least not directly). Recompression can cause the bubbles to actually pass from pulmonary veins directly into arterial circulation (and then back through the heart) rather than being diffused and exhaled. Although it is possible for bubbles in the ride-side heart to backflow into the left-side and out into arterial circulation that way, like with septal defects. I think there's still a lot that's not known about bubble formation (and I don't mean by just me!).

 

[DandyDon]

Yeah, I think maybe the emphasized statement could have been "...one of the mechnisms..."

If one had to do a last bounce or two after 2 previous bounces and 2 real dives, I'd want that final ascent to be as slow as possible - with longer, deep stops.
<edit>
A one minute or longer Surface Stop might also be a good idea - floating on the drift line for a while before climbing the ladder.

 

[beezwax]

I see there's another thread starting up about bounce dives:

http://www.scubaboard.com/showthread.php?t=138063

But the use of the term is different from how I've got it in my head-- they describe a bounce as a short duration dive with little or no deco stop(s). I've used it to mean a dive with little or no time at depth (like when bringing up an anchor, or when someone just wants to hit a mark on their depth gauge) but of course you've got to have a controlled ascent and do your deco, or else you're just being dumb. Is this how everyone else understands a bounce to be-- straight shot to the surface?

I just read about this on the WKPP page:

http://www.wkpp.org/articles/Decompression/why_we_do_not_bounce_dive_after_diving.htm

They say the same things. It's against their policy to do repeat bounce dives due to the bubble risks, but they also say that if you do have to do it then do the deco and ascend accordingly.

 

[vertaqua]

Guiding in the Red Sea often involve tying ropes on to the wreck of the Thistlegorm. first you take the first rope down and tie on at 14-20m, go back up, go down to tie 2nd rope, back up, take divers for a 30m dive, comeup and have your SI, then a 2nd dive to 25m, back up with clients, go back down and untie 2 ropes. What is the safest way to perform this task? if any?

Are you the only DM on this boat if so, maybe there should be one more DM. So that one of you can tie off to the wreck and then untie from it when your done diving, while the other leads the divers on the dive. This way the diver doing a bounce dive will have very little N2 buildup, on each bounce dive and a rather long SI in between bounces. The DM that does the bounces can then check out/ in divers since he's topside anyway.

 

[Dr Deco]

Hello readers:


Bounce dives are fine if they are short. When following dives that are long in duration, where real gas loads can accumulate in the tissues, bounce dives appear to become more dangerous.

I would avoid those following dives with substantial gas loads unless there is a generous surface interval.

Dr Deco :doctor: