I’ve been using the Medtronic Minimed 670G insulin pump in auto mode for 20 days now.  I’m a new insulin pump user (I switched from doing Multiple Daily Injections) and my insulin pump trainer set me up with auto mode after using my pump for one week.

On auto mode, the 670G changes the basal rate automatically based on the Guardian 3 CGM sensor reading.  It tries to maintain a glucose level of 120 mg/dL.

The basal rate is represented as a series of purple dots above the graph.  As you can see in the following pictures, the pump decides when to reduce my basal rate (fewer purple dots) and sometimes will even turn it off completely.  When my blood glucose increases, it will increase the basal rate.

I’ve found that by optimizing the timing and amount of the bolus insulin, I can take advantage of this insulin pump’s “brain” and get flat lines for long parts of the day!!

I feel that this auto mode is a perfect fit for my relatively low carb diet (~150 carbs a day) and my low insulin needs (~19 units a day).

Sunday evening dinner graph

I ate meatloaf and potatoes.  My blood glucose was going low before dinner, so the pump automatically suspended the basal rate.  Since I’m in auto mode, it didn’t alert me, because it was able to keep my blood glucose around 120 mg/dl automatically.  I just can see that it wasn’t giving me basal insulin because there is a large gap in the purple dots that appear above the graph.

Around 5:30, I gave myself a pre-meal bolus.  You can see the yellow fork and knife symbol at the bottom of the graph.  Because my glucose was already in the normal range, and because I was going to eat a high fat meal which delays glucose release from the food, I actually didn’t do this bolus more than a couple of minutes before eating.

As you can see, the basal rate didn’t run while this bolus insulin was working and the basal rate really started up as my glucose began to rise about an hour after eating.

Sunday Afternoon Graph

See the blood drop on the graph?  That’s when I tested my blood glucose and calibrated the CGM sensor.  This Minimed 670G System using a Guardian 3 Sensor which requires calibrations between 2-4x a day.  I usually calibrate when I first wake up, before lunch, and at bedtime.  Often, at bedtime, my blood glucose will be high, and the pump will suggest a bolus correction dose.  It’s very good at getting that dosage correct, and it’s amazing to wake up in the morning with glucose around 120-130 mg/dl!   The correction dose, in my case, is sometimes very small, like 0.1 to 0.3 units of insulin.   A small amount of insulin has a large effect on me, and I could never make these corrections with doing insulin injections, since the smallest amount I could inject was 0.5 unit.

Before Breakfast Graph Showing Dawn Phenomenon

The above insulin pump graph shows that in auto mode, the pump’s basal rate was able to effectively control the normal rise in my blood glucose levels that happens after waking up.  I’ll admit, that some mornings I “cheat” by telling the pump that I’m having 5 grams of carbs and then I don’t really eat them until later (if at all).  Programming in “Phantom carbs” allows me to get it a bit lower than it would do on its own.

How I Interact with my Insulin Pump

• I calibrate the CGM by doing a blood glucose test when I wake up and before I go to bed.
• If my glucose is trending high for a period of time, I do a blood glucose test and enter that into the pump.  If the pump suggests a bolus correction, I accept that.
• I bolus before I eat for the number of carbs I’m going to eat.
• Sometimes I bolus as much as 30 minutes before I eat.  I do this if I’m trending high.
• In the morning, I have a higher insulin/carb ratio programmed in than for the rest of the day.  The pump settings allow for this to be different based on time of day.
• When I’m not exactly sure of carb count, I try to over estimate by a bit, rather than under estimate.  I’ve found that doing MORE bolus insulin creates a flatter line… This is the exact opposite rule than what I’d do for injections.  With injections, I had to bolus less because I’d run the risk of going low.    Insulin pump turns off the basal rate so I don’t go low as frequently.
• For exercise, there is a “temp basal target” that you can set, to tell the pump to aim for 150 mg/dl.  Honestly, this hasn’t been sufficient for me.  When I exercise, my blood glucose drops quickly.  Therefore, I’ve found that drinking some juice before exercising (15-20 carbs) and then suspending the pump has worked out better than leaving it in auto mode.

Not always perfect

As you can tell, I’m quite happy with the results in general.  But, sometimes it doesn’t work out perfectly.  There are still periods where my glucose is higher than I’d like, and the occasional low blood glucose event too.

Most amazing things about this pump

• The Guardian 3 CGM is often VERY CLOSE to my actual blood glucose reading!!
• In both regular and auto mode, it will turn off the basal insulin to prevent going low.
• The pump is able to correctly calculate a correction bolus before bedtime and usually there are no alerts through the night and I wake up with a normal glucose reading.

What could be better

• On the first day of the Guardian Sensor, it requires more frequent calibrations.  Sometimes it needs to be calibrated in the middle of the night.  (To hopefully avoid this, I’m going to try starting my sensor earlier in the day.)
• I wish it could be programmed to beep 15-30 minutes after doing a bolus, so that I can be reminded to eat.  (Instead, I use my cell phone to set a timer.)
• I wish it had a higher target that could be set for exercise.  As noted above, the 150 mg/dl target hasn’t been working for me.  I think if it allowed me to set it for like 180 mg/dl, that might work better and would be safer than suspending it.
• I got very frustrated with the Quick-set infusion sets that came with it because I had a hard time with the twisting the mechanism to release it.  Also, one time the inserter spring was too forceful and it hurt.  I’ve since bought some of the mio infusion sets which are easier to disconnect, although with those I also wasted 2… I guess insertion sets take some learning.  Maybe I lack dexterity, but I think about all of the people with arthritis and other conditions that would also make these things difficult…

Related Reading

• Protocol for Hybrid Closed Loop Technology with the MiniMed 670G System – PDF from Medtronic about how to use the auto mode.

Related Amazon Links

• Energizer AA Lithium Batteries – This pump’s manual recommends lithium batteries.  These Energizer AA ones are the ones I’m using.  The pump ships with alkaline batteries, so those do work, but probably lithium ones last longer.  And lithium batteries aren’t as expensive as they used to be.
• Pumping Insulin: Everything For Success – A very helpful book with detailed information about pumping insulin!  Lots of information about how insulin pumps work and how to solve common problems.