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Re: [IP] weird bubble movement


I have noticed this too with my MiniMed pump, but I don't think it
happened with my Disetronic (i.e. I think the bubbles moved smoothly
with the Disetronic, at least during bolus/primings).  I don't think
that this is something to worry about... as long as there is no
occlusion, you are still getting your insulin (maybe just with a slight
delay).  One thing to keep in mind is that the MiniMed delivers insulin
in 0.1 unit "pulses" (it has a stepper motor that pushes in discreet
amounts), where as the Disetronic pump delivers it in a steady stream
(it has a DC motor that turns continuously while delivering insulin) for
boluses.  For basal delivery, the Disetronic delivers in short bursts
every three minutes, which are amounts generally much smaller than the
MiniMed's 0.1 unit.  So the Disetronic would act more like the MiniMed
if you are just looking at Basal delivery.

The first thing to rule out is if the bubble is smaller than the
diameter of the tubing.  If it is smaller, then the insulin can get
around it and one would expect to see eratic movement whenever it
happens to break free of the side of the tubing.  If it blocks the
entire diameter of the tubing, however, then we have to look at other

I can't say for certain, but there is one probable explaination for why
this happens.  Since the MiniMed delivers in pulses instead of
continuously, you would naturally expect a "stop-and-go" type of
movement of the insulin in the tubing.  When the motor "clicks", it
pushes momentarily on the plunger, which in turn pushes on the insulin
in the plunger and increases the pressure there.  An important thing to
realize is that there is friction between the insulin and the tubing,
and between the plunger and the inside of the reservoir.  In order to
get insulin to move, this friction has to be overcome by the force the
motor is exerting.

Ok... this is a bit of a stretch but the physical principals are roughly
the same.  As a (rather dramatic) example, think about how an earthquake
happens.  At the fault line, two techtonic plates are trying to move in
different directions.  They are locked together by friction, so normally
no movement takes place.  When the forces/pressure builds up high
enough, the plates break free momentarily and the two plates move - this
is where the earthquake comes from.

Another, more eggheaded, example is to think about how you would push a
block of wood on pavemet.  Assuming the block is not initially moving,
to get it moving you would have to push increasingly harder on it until
you overcame the friction between it and the pavement.  This is "static
friction".  Once it is moving, you would find that you would not have to
push as hard to keep the block moving at a steady rate.  This is called
"dynamic friction".  Generally speaking, dynamic friction is less than
static friction - i.e. it is eaiser to keep something going (or flowing)
than it is to get it started.

So what might be happening here is with each click, the motor pushes a
little harder but isn't actually pushing hard enough to break the
friction of the plunger.  We are talking about VERY small movements
here, so I believe this is a realistic assumption.  It might take two or
three clicks, and then the motor has pushed hard enough to break the
plunger free and get the insulin moving.  So you would see the bubble
sit still for a short time and then jump ahead quickly.  And it might
take a different number of clicks each time, so the distance the bubble
travels might vary.  Longer tubing might amplify this effect, as there
is more "inertia" in the tubing.  A reservoir that doesn't have adequate
lubrication could also be to blame.

You also might want to take into account that the air bubble would
probably have different adhesion qualities to the tubing... i.e. it
might "stick" more to the tubing, thus introducing a sporadic effect.

Another effect that might influence this behavior is the fact that air
is compressible, while insulin relatively non-compressible (at least
under the conditions we are talking about).  If you had a rather large
air bubble (i.e. an air space of 1/8 or 1/4 inch or so in the tubing),
the insulin coming from the pump side could squeeze the air and make it
slightly smaller before the insulin on the downstream side would start
moving.  Thus the air would act, in essence, kind of like a spring and
introduce more complex motion into the system.  This would be like the
springs in a car's suspension system... if you hit a bump, the car
doesn't move up right away... there is a slight delay as the spring
compresses and then un-compresses.

Anyway, these are some possible explainations as to why you see the
sporadic movement with the pulse bolus delivery of the MiniMed and not
the steady delivery of the Disetronic.  I never tried this, but I
imagine if you were bored and wanted to watch an air bubble in a
disetronic line during basal-only delivery, it would act similar to the
MiniMed pump.

You know, one would think I'd have something better to do than write all
this stuff out, but I guess not.  Anyway, I hope someone out there finds
this useful or at least slightly interesting.  Like I said, these are
just some theories and I don't know for sure that they accurately
describe the bubble behaviour or not.

Happy pumping!


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