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[IP] Re: Avoiding Bubbles


While in the grand scope of things the bubbles that we are speaking of will be
of little concern it is interesting to ponder.

>pressure in the *reservoir* will drop to ambient as soon as the insulin
is withdrawn into it. We don't really care what the pressure is inside the
*bottle*. IMHO, this effect can be neglected.<

If one assumes that there is an equal exchange of gas for insulin, I would
agree.  However, I have intentionally tried to equalize the pressure inside the
vial by inserting a open syringe and heard air escaping.  When working with a
new vial and filling a pump, the volumes are such that one could substantially
increase the remnant pressure.  Dissolved gas is directly proportional to gas
pressure.  I think there could be a 2X increase in pressure in an extreme case.
So, in general, I agree that it would be a negligible effect but it has the
potential to be more than negligible.  Most of the time users will leave the
vial equalized.

>With respect to changing altitude, an abrupt change, like taking an
airplane, will test the system far more than a gradual one, like hiking up
a mountain. <

Actually, rate of change will only effect the rate of bubble formation, bubbles
will form.  If the system is saturated the lower pressure will allow bubble
formation.  Assuming a rigid sealed container this expansion will force insulin
out and into the infusion site.  Any bubbles that have formed or were left in by
poor technique before the change in altitude will also increase in size forcing
even more insulin into the body.  6000 ft has an air pressure of about 11 psi
and airlines are generally pressurized to only 8000ft so there is substantial
volume changes in bubbles when flying.  Don't fly with bubbles.

>when the plane comes
back down, the pressure will simply equalize and the system will return to
the initial state. <

Not if insulin have been pushed out into the infusion site.  Either the piston
moves in or interstitial fluid is sucked back into the tubing as the bubbles
return to their initial state.  Both could create an insulin deficit after

I agree that it is very unlikely that the seals will leak.  The most likely is
the screw seal on the tubing.  It can be both to loose and too tight.  The
o-ring seals will probably withstand several atmospheres based on past design
experience.  No one at the pump companies would ever tell us the design pressure
of the seals.  I did just tender an offer letter to an engineer that
participated in the first Minimed pump design, he may know.  He claims to be the
one responsible for the solenoid pump design.

The fact remains that substantial bubbles are a problem for many pumpers.  I
would always suspect poor technique first.  The factors we have discussed are
part of the problem and when compounded could create an unfortunate situation.

So would you agree that we should.... Fill with warmed, agitated insulin that
has been stored with reduced pressure above the solution and we should check
often for those MAGIC bubbles?


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