surfacing during a dive

[Genesis]

that IF you have bubbles in your veinous system from your ascent, and you decend again, they can compress and get past your lungs.

That turns a non-event (small bubbles in your lungs are filtered by them and emitted into the atmosphere - gone from your body) into a type II DCS hit - the kind you DO NOT WANT!

People have gotten SERIOUSLY bent bounce diving - coming up from a 100' profile, then going back down to 20 or 30' to retrieve a deco bottle that has been left hanging, for example, and immediately re-surfacing.

The problem is that it is very common to get non-symptomatic bubbles in the venous side of your circulatory system during the final 15-30 feet of your ascent. This, by the way, is where the 15' 3 minute safety stop recommendation came from - doppler studies showing that this happens frequently to divers on what are "NDL" dives. The safety stop is an attempt to reduce the incidence of these micro-bubbles by giving you time to offgas some of that overpressure before you come to the surface.

There is no way for you to know if you have small, non-symptomatic and non-harmful bubbles in your lungs after a dive.

But, if you go back down and allow them to compress and get through the lungs, you now place them into the arterial side of your circulation - and that can nail you good.

BTW, do not freedive after a scuba dive either - for exactly the same reason.

Now, these risks are relative. If you're well within the NDL on a 60' dive its far less likely that you have microbubbles than if you are close to the NDL on a 100' one! Common sense has to be applied - but if you've been deep its a bad idea to do anything that could be considered a "bounce".

 

[budgy]

I guess most of us really don't have a clue why some people get a bend. A person can repeat the same kind of dive in the same conditions all within recommended tables and one day for no apparant reason they get bent.

 

[devilfish]

Genesis once bubbled...
[]

But, if you go back down and allow them to compress and get through the lungs, you now place them into the arterial side of your circulation - and that can nail you good.

[/B]

Silent bubbles on recompression from the surface to say 33fsw will not compress enough to "get through the lungs". In fact from surface to 33 fsw they will compress only 21% of it's original physical diameter size. It would take a descent to 8ata for the physical diameter size to bcome 50% of it's original size from the surface.

 

[MikeFerrara]

In general I don't think up and downs are good. With that said, big and or fast up and downs are worse than little ones. I prefer to avoid them. I also don't like to get lost in the first place. I parted company with a DM who couldn't learn to navigate and was always surfacing with student to see where they were. On these profiles I wasn't so much woried about DCS as I was the fact that you are more likely to run into somekind of problem during ascents and descents (seperation, equalization, rapid ascent) than you are just swimming along the bottom. I don't think DM when DMing should get lost. Anyone can screw up their navigation a little in an unfamiliar site but we don't teach or DM in site we don't know so we don't get lost and we don't drag students up and down. There are already enough up and downs in an OW class.

I know divers who will surface from a 80 or 90 ft dive to see where they are and then go back down. I wouldn't do that and if I am working the boat nobody else will.

 

[Genesis]

Devilfish, the WKPP people have a lot of experience with this, and they had a LOT of people get bent this way - coming up after a mandatory deco dive (using O2 to deco at that!) and then bouncing down to 30' or so to get a deco bottle.

They stopped doing it for this reason - they had a lot of "undeserved" DCS hits.

Nobody originally thought it was a big deal, due to the relatively small amount of bubble size change and the fact that at 30' it takes a LONG time to run out of NDL time.

You're right about the percentage of change in the bubble size. Where you're wrong is that the entire microbubble theory is that the bubbles that are being talked about here are EXTREMELY small - small enough that it doesn't take much of a decrease in their size for them to pass the lungs.

The problem with bubbles is that they get together and make fewer, but bigger bubbles. If this happens in the lungs as they are being gas-exchanged out its not a big deal, so long as they are small enough. But if it happens in the ARTERIAL side of your circulation, you have major trouble.

Your venous circulatory system has lower pressure than your arterial side. This is why, normally, these microbubbles don't bend you - the differential overpressure is low enough that they come out of solution ONLY on the venous side of your circulation. This doesn't get you unless there are a lot of them, because they collect in the lungs and diffuse out as you breathe - no harm, no foul.

This is why a PFO is so dangerous to divers, and if you have one there is a much greater risk of you getting bent than if you don't.

From my understanding of the science and theory behind this once you come up you should stay up until your SI expires or, if you must go back down, you must STAY DOWN and do a full ascent again, including the safety stop. The "bounce up and down", from what DAN and others have had to say, is a risky undertaking.

 

[devilfish]

Genesis,
I didn't say it's ok to go back down, it isn't for many of the reasons you mentioned. I only said that bubbles will not get reduced in size enough to go through the lung. Many believe that pressure will reduce the bubbles to "force them back into solution", that is absolutely wrong. That is not what is happening.
As for the PFO, if bubbles do pass through then it's not bends, it becomes AGE regardless of the what gas it is on the arterial side.
Up and down is definately not good and should be avoided.