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Re: [IP] Will the true canula volume please stand up?

John, nice thinking, and you are right as far as you have taken it.   One
ml of air will be compressed about one microliter (0.1%) for each cm water
pressure change.  So if you have an air bubble of 1 ml and you drop your
pump from your waist to the floor (say 100 cm water pressure change) then,
you will compress it by 100 microliters.  This is equivalent to 10 U of
insulin.  So the issue is how sloppy you are when filling the tubing and
syringe.  Clearly 1 ml of air is a truly humongous bubble.  Normal small
bubbles are more likely on the order of a few microliters at best ( 1
microliter of air in the tubing occupies about 1/4 in (7 mm) or about 0.1
U.  So with a bubble of say 10 microliters, this same 100 cmH2O pressure
change could cause a compression of 1 microliter (or 0.1 U).  So the
solution here is to be careful to get all the bubbles out :-)
	But, there is another factor that will still play a role even if
there were no bubbles anywhere.  That is the distensibility of the tubing.
I think that one of the most  important technologic advances that Minimed
(disetronics , or whoever makes the catheters) has made is in their tubing.
To work efficiently in an insulin pump the tubing needs to be very
flexible, yet  indistensible, two seemingly opposite design criteria.  And
from my experience with all kinds of laboratory tubing, they have done an
outstanding job.   But as good as the new tubing is, there is still some
distensibility.  A 43 inch length has a measured distensibility of about
0.01 microliter for each cm water pressure change.  So for the same 100 cm
water pressure drop above, the tubing will expand by 1 microliter ( or 0.1
U).  For myself I don't worry very much about things plus or minus 0.1 U,
but I know there are some people and kids where this could be more
important.  There's nothing that can be done about the tubing, but the air
bubbles should be minimized as much as possible.

p.s.    In response to what I do when I'm not chatting on this listserver,
I'm a professor of physiology here at JHU.  My research involves lung
disease, and I've never worked on any diabetes research.  However, we did
have a grad student do a recent Ph.D. thesis looking at the very much lower
incidence of asthma among type I diabetics.   That maybe the only good
thing about being diabetic ;-)  We're not wheezers.
	Oh and by the way, with your seemingly keen quantitative mind, have
you ever thought about giving up your singing career and doing a Ph.D. in
Physiology?  We have NIH training grant support to pay tuition and living
stipend, and we'll even let you sing while at the lab bench *s*

<<<<<<<<<<<<<From: John Neale <email @ redacted>
Subject: Re: [IP] Will the true canula volume please stand up?

Mary Jean, Wayne, and anyone else...

This seemingly simple matter has also exercised my mind greatly. It
seems such an easy thing to test, and yet the results are so variable.
You'll waste a silhouette in the process, but just prime the tubing,
withdraw the insertion needle from the cannula, connect it to the
tubing, bolus 0.8, and watch for the insulin coming out of the end on
the last click. Easy. Except it doesn't work. The results can sometimes
vary. I took a Silhouette apart completely to see if there were any
cavities that might mysteriously fill up with insulin, but it all looked

Then I found another variable... Remove the cartridge from the pump, but
with the primed tubing connected, and draw the plunger back a tiny bit,
so there's a bit of air showing at the cannula end of the tubing. Raise
the pump high in the air, keeping the free tube end at waist level, and
hold you finger where the air bubble is. Then lower the pump right down
to the floor, again with the tubing at waist level, and see how far the
air bubble moves. It does move. But the amount varies. It depends on how
many of those little air bubbles are up in the cartridge. The head of
water (24", 48" or whatever) is enough to expand and compress the air
bubbles. This can sometimes amount to 0.5 units or even 1 unit if
there's a big bubble stuck in the cartridge.

So when you prime the pump and tubing, it may be important to keep
everything at the same level that you normally wear it, to prevent
insulin being pushed or sucked up and down the tubing.

There are other implications here that I've not seen mentioned anywhere.
If I have my pump deep in my trouser pocket in the morning, and then
move it high up into my shirt pocket, the result may be an unintentional
bolus of maybe 0.5 units. The extra insulin will slowly leak, under
gravity, into my body. Similarly, if you keep your pump high up in your
bra, then when you go to bed, the pump is lowered to roughly the same
level as the canula, the resulting compression of the air bubbles may
cause a back pressure, which will deny you your next hour's basal.

Sorry if this sounds a bit technical or confusing! I'm just trying to
logically describe what I've observed.

This might partly explain why some go high after set changing, while
others go low. It all depends on how much air is accidently trapped in
the system, and the relative levels of your pump and infusion site.

Any thoughts anyone?


Wayne Mitzner
Department of Environmental Health Sciences
The Johns Hopkins School of Hygiene and Public Health
615 N. Wolfe St.
Baltimore, MD 21205
Tel. 410 614 5446
Fax 410 955 0299

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