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Re: [IP] Pumping Questions

On 23 Nov 98 at 20:13, Richard Aleksander wrote:

> Every user swears by his pump.  Minimed users report what I consider a
> disturbing frequency of no-delivery alarms, a problem I feel stems from its
> lower pumping pressures and use of plastic cartridges (see below).  Even so,
> Minimed pumpers report being satisfied with the remediation to their problems
> and its users swear by their pumps also.

The "disturbing frequency" is probably due to the large number of Minimed users 
compared to the number of Disetronic users.  There's also a number of reports 
of Disetronic pumps going into runaway mode  in high RF or magnetic fields and 
emptying the cartridge in a few minutes.  You experience these kinds of fields 
when you are near an arc welder or radio or TV transmitter...   You look close 
enough and you'll find all kinds of horror stories for both pumps - usually 
problems that have been fixed in later models as the technology has developed.

> Here's some (objective I think) considerations that persuaded me to choose
> that unit.

Almost as objective as a pump rep... how much did D pay for the review? (VBG)

> Disetronic sends two pumps, as the pump is programmed to turn off for
> factory service after two years of pumping.  After service at the factory each
> pump is returned with an additional two years of warranted service. The extra
> pump can be used anytime as a back up.  Minimed sends one.
> NOTE:  Some Minimed owners say the two pump approach turns them off; that the
> unit must be bad if users need a spare.  Not so.  The reality:  I've never
> even powered up my spare.  But if I need to send in a unit for service, I'm
> assured I won't have to go back to multiple injections, or worry how long they
> have to keep the pump.  I haven't taken a single injection since I began
> pumping.

After twenty years in electronics and computers the idea of needing a "hot 
spare" bothers me - in my experience it does imply that the equipment will 
fail.  I've not had an injection since I began pumping either...

> Minimed uses a solenoid to push insulin a tenth of a unit at a time.  If a
> user has a basal dose of 8-tenths of a unit hourly, the Minimed will click
> eight times an hour, or once about every seven and half minutes.  In delivery
> of boluses of, say, 6-1/2 units, the Minimed will click 65 times and it will
> take several minutes to deliver.
> Disetronics pump an infinitely variable dose of insulin through a screw
> drive.  Five percent of hourly basal dosages are delivered each three
> minutes.   In the above case, 8/10 unit per hour, 4/100 unit is delivered each
> three minutes.  The above cited bolus of 6-1/2 units is silently pumped in a
> matter of seconds.
> Minimed would not match the delivery frequency capabilities of the
> Disetronic (that
> is, a click every three minutes) until the basal dosages exceed 2 units an
> hour.  That's much higher than the basal rate of virtually every user.
> With this in mind, I concluded that the Disetronic's delivery is smoother,
> faster and closer to natural than Minimed's.

The "infinitely variable dose" is really done via some more complex mechanical 
and electronic gyrations.  Both pumps use a screw drive and a solenoid to drive 
the pumping mechanism.  MiniMed verifies each click and since all clicks are 
the same size the circuitry is simpler.  Disetronic must be using a form of 
pulse-width modulation to drive their solenoid with variable step sizes.  
Neither company wants to discuss design issues in detail.  For small basal 
rates there is some benefit to the Disetronic's method.  

> I looked at the pumping pressures of the two pumps, in the belief that would
> have some bearing on pumping through any obstructions in the tubing.
> Disetronic pumps at 2-3x higher pressures.  It turns out that more Minimed
> users are complaining about non-delivery alarms.  I worried that the higher
> pressures would induce pain during boluses.  It doesn't.
> The Disetronic can be operated without damage underwater without
> modification or addition of waterproof casings.  There's a removable tappet
> designed to keep moisture out of the cartridge chamber, but even a flooded
> cartridge chamber has no detriment to the pump or its operation.  Just remove
> the cartridge and towel off the chamber.  All of the mechanical and electrical
> components are sealed.  Minimed's are exposed and the unit can be operated
> underwater briefly only after the user adds an optional, additional sports
> guard.  This turned out to be pretty important for me since I swim regularly
> for extended periods of time.  Hanging the pump around one's neck, the
> Disetronic can be taken into the shower as well.  Get your Minimed really wet
> and you'll have to return it for service.
> Humalog is the ideal insulin for use with the pump because it acts rapidly
> (<10 minute) and because such small amounts are used and in the body when
> delivered by pump.  This also has implications in making hypos more
> manageable.
> There's another fairly important difference between the two pumps related to
> Humalog, with the Disetronic holding an advantage.  The cartridges for the
> Disetronic can be plastic or glass.  Glass costs more and is the preferred
> material.  All Minimed cartridges are plastic.

Glass as the "preferred material" sounds odd...  it's also more dangerous 
because it can shatter if you drop your pump...

> Plastic cartridges tend to degrade Humalog molecularly.  The Humalog is
> attracted ionically to the plastic wall of the cartridge.  The plastic ones
> also present more opportunity or propensity for contamination with air.

Documentation for this?  Or is it a research proposal awaiting funding?
> Humalog has some degradability properties that are problematic for pump
> users.  Many of the Minimed users find they have potency/delivery problems
> before their cartridges empty, and they have to change them sooner.  Many also
> report they have to dilute their Humalog for various reasons.

Many Disetronic users are also mixing insulins (Humalog/Velosin) to get a 
smoother action...

> Plastic cartridges cannot be prefilled with Humalog that far in advance.
> And even if they fill them just before placing the cartridge in the pump, the
> Humalog remaining in the used factory-filled vial may be degraded as well,
> perhaps by as much as 30 percent.

Just taking it out of the vial using a plastic syringe degrades it?  The vial 
doesn't care about what the syringe looked like!  I'd like to see some real 
studies to back this up.  Why pre-fill anyway?  It only takes about two minutes 
to fill a syringe, about five to attache the tubing, prime and load it into the 
pump... then another five to prep the site and shoot the softset in...  Once 
you've opened a cartridge you've broken the sterile seal so you are increasing 
your risk of infection.

> Again, the pumpers on this listserv are using all the different hardware and
> each swears by the one he is using.

Each user swears by the one they are using.  If you ever get one to switch they 
swear by the one they use and at their previous one...  Definitely a case of 
your milage will vary.  People do defend these choices...

Randall P. Winchester
* The views expressed here are mine and do not necessarily *
* reflect the official position of anyone in particular.            *
* There's no guarantee on anything said here...
* If I say I understand something completely the only thing
* we can both be assured of is that I must have completely
* misunderstood something. 
Insulin-Pumpers website http://www.insulin-pumpers.org/