>Years ago I went on some chamber runs (as part of a diver education
>program), and I still remember good old Jules Fortin (one of the pillars
>of the Canadian Navy underwater program) saying as our bottom time was up
>and he was about to start turning valves "Going up ...  and NO
>VALSALVA!".  The impression I got at the time was that this would be
>because it wouldn't be smart to do something to increase your internal
>pressure at a time when external pressure was decreasing, and embolisms
>were what was feared, but would this perhaps also help avoid PFO shunts? 
>
>I've always since then employed Valsalva strictly for descent only, FWIW.

Hard to second guess people I don't know, but it sounds like this might 
at least be part of the reason (if not the complete explaination). 

Truth be told, it usually is *NOT necessary* to clear on the way *UP* 
anyway, since the compressed gas trapped in the middle ear will expand on 
it's own as you ascend. The eustachian tubes at their openings at the 
roof of the mouth and throat have a little flap of tissue that acts like 
a flutter valve. It can be fairly large in some people ("patulant" 
eustachians in doc-speak) so as you descend (either while diving or 
flying) and gas is trying to get *INTO* the middle ear, this flap may 
slam shut and block the tube. It may take no more than 3-5 mm of pressure 
difference before you can't generate enough force with a Valsalva to 
overcome it (unless you do what the manuals say and start to ascend a few 
feet and try again). That's why it's important to get on your ears as 
soon as you hit the water so as not to create a pressure differential 
greater than your capacity to override.

But as you go *up*, the expanding gas pushes this flap out of the way 
(since it's hinged to "go with the flow"). Since a valsalva temporarily 
raises the right sided heart pressures, a patent foramen ovale *could 
theoretically* create a transient right to left shunt. (since such a 
manuever only lasts a second or less, it's hard to envision much of a 
shunt in practical terms, but I'll defer to the laboratory evidence).

Now if you inflame that flap of tissue (along with a bunch of other stuff 
in your ears and throat) with a head cold, it might not budge on the 
ascent either. But, no one here would dive with a cold now...*WOULD 
THEY*! 

Personally, it's very comforting to listen to my ears squeeking on hangs 
since it tells me the "system" is working! (damn little else is!)  

;-)

Robb Wolov  

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