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Re: [IP] Will the true canula volume please stand up?
email @ redacted wrote:
> Mary Jean, I was confused about this too, so I went to the lab to measure
> it. The silhouette cannula requires just a tad over 0.75 units (0.77) to
> completely fill it. So if there is no air at the end of the tubing
> (sometimes I find that banging the free end around causes some insulin to
> leave the end), then you should bolus 0.8 units just to fill the
> silhouette. I don't know where the 1 u on the package came from, but
> maybe when someone measured it after connecting it together, they had to
> give 1 U before they saw insuling coming out the teflon catheter. If so
> that extra 0.23 must have been required to fill the end of the insertion
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?
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