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[IP] Variable pumping pressure (Re: Disetronic v. Minimed pump comparison)

I wanted to address a response to my post on Disetronic vs. MiniMed pump
comparison that dealt with "variable pumping pressure".  In that
posting, the author stated that the Disetronic pumps at a pressure 3
times higher than the MiniMed.  I can't confirm or deny that statement,
but I believe a more correct wording of the statement would be that the
Disetronic is CAPABLE of delivering at 3 times the pressure... not that
it does this at all times.

This would only be the case when a bolus was being delivered (a large
amount of insulin in a short amount of time).  Basal deliveries are
generally very small, and so the pressure would not be 3 times during a
basal delivery.  If an occlusion occured during a period of basal-only
deliveries (like at night), the pressure would have to build up over

Disetronic says that it's pump uses "variable pumping pressure" to
overcome occlusions in the infusion line, which is basically saying that
the pump increases the pressure in the tubing until the occlusion is
pushed clear.  MiniMed says that ALL pumps have "variable pumping
pressure", but that its occlusion detection is more sensitive.

I'm not going to get into the physics of the thing here, but basically
if there is something blocking the tubing and the pump is still actively
running (in an insulin pump, or actually in any other pump delivery
system such as a pump delivering water from a well), pressure will build
up in the tubing until one of the following happens: (a) the blockage is
pushed clear, (b) the pressure becomes too great and the pump stalls,
fails, or is turned off by a safety mechanism (occlusion alarm), or (c)
the tubing fails (springs a leak or blows up).  As far as I know, the
motors in insulin pumps are not strong enough to blow the tubing, so we
are left with (a) or (b).

If (a) happens, you will probably not know about it as no alarm will go
off (the occulsion is cleared).  If it takes a while for the occlusion
to clear, you might get some high BG readings warning you of the
problem.  Obviously, a higher pressure would be better able to clear an
occlusion.  So assuming that the Disetronic is capable of producing a
higher pressure behind an occlusion, it would be better able to clear
that occlusion.  BUT - you don't necessarily want to count on the
occlusion being cleared by higher pressure...  Read on...

On to (b)... the pressure did not clear the occlusion, so now we are
waiting for the alarm.  With an occlusion, the problem is that while the
pressure is building up YOU ARE NOT GETTING ANY INSULIN.  If this
happens during a large bolus, the pressure will build quickly and so the
occlusion alarm is more likely to sound in a timely manner.  But say
that the occlusion occurs during a period of small or no bolus (say at
bedtime).  With the small amounts of insulin that are being moved for
basal rates (especially if those rates are low), it can take a while for
the pressure to build up enough for the occlusion alarm to sound.  In
this case, a more sensitive occlusion alarm would warn you earlier of
the problem.

So, anyway, why did I bother posting all this stuff about variable
pumping pressure?  Not to scare anyone.  My opinion (and this is just my
opinion) is that you don't want to count on a higher pressure to "push
through" the occlusions.  If there is an occlusion, even if you do push
through it, you have a problem and should probably be looking at
changing your infusion set, reservoir, or both.  A more sensitive
occlusion alarm would give you an earlier warning of the occlusion,
regardless of how much pressure the pump can put behind it.

But I guess the main point I want to make is that as long as you are
doing regular blood glucose testing, you should be able to catch any
problems with your pump before they become serious.  I would not feel
comfortable counting on an occlusion alarm in lieu of BG testing to let
me know that something has gone wrong.

It is important to note that if you are using Lispro (Humalog) insulin,
this topic becomes more important than if you are using a Regular type
of insulin.  Lispro insulin has about a 4 hour lifespan once it is
injected.  If you have an occlusion that lasts for over 4 hours, after
that 4 hours you basically don't have ANY insulin on board and DKA and
set in relatively quickly.  Regular types of insulin (Velosulin, Humulin
R) stay in the system for a slightly longer period of time so the clock
doesn't tick quite as fast (I think this is one of the reasons that
Lispro is not yet officially approved for pump use).  With Lispro, it
becomes more important to have a sensitive occlusion alarm, especially
at night when most people don't test for 8 hours or so.

For the record, I have used both the Disetronic H-Tron and the MiniMed
507C with Lispro insulin, and have not had any serious problems
resulting from occlusions.  I HAVE had occlusions with both pumps, but I
was able to catch them with my regular BG testing before they turned
serious.  Also, I think that the new Disetronic D-Tron pump is supposed
to have a more sensitive occlusion alarm system than the H-Tron.

Hope someone finds this interesting/useful.

Best Regards,


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