Diabetic Life

What Is A Continuous Glucose Monitor And How Does It Connect To An Insulin Pump?

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Last night on my daily walk with Fiona I was talking with my mom and she asked me a question that I thought I would post.  I figure if my mom, who I talk to about everything, was asking then there is pretty good chance that some of my Sweet Basal readers likely had the same question.

“How does that sensor gather your blood sugar readings and why doesn’t it fall out during the week?”

As you have read in previous posts, (Why A Trip To The Wisconsin State Fair Convinced Me To Get An Insulin Pump, and “It’s an Insulin Pump. Not an iPod, Pager, or Phone.” “Oh…Where’s the Insulin?”)  I am currently on the Medtronic 670g insulin pump.  I have been wearing this particular pump for almost two years now and it is the one of the first “hybrid closed-loop” devices on the market.

But First, Definitions Are Needed

Before I address the definition of hybrid closed-loop I want to provide some understanding of two other important terms.

Basal Rate – the small, continuous amount of insulin generated by a healthy pancreas throughout the day based on the immediate needs of the current moment in order to maintain a healthy, steady blood sugar level.  Traditional insulin pump patients require a basal rate to be programmed into their pump which typically varies throughout the day based on body chemistry changing throughout the day.

Bolus – the dose of insulin given based on the amount of carbohydrates consumed.  A healthy pancreas automatically senses the amount of carbohydrates consumed and creates the appropriate amount of insulin to cover them.

An insulin pump patient must count the number of carbohydrates in everything consumed and enter this info into the pump.  The pump is programmed with a series of ratios. These ratios represent the number of units of insulin : number of carbohydrates entered.  These ratios typically vary throughout the day.

Basal and bolus rates need to be increased or decreased due to numerous factors such as activity level, time, hormones, illness, fat or protein in food, stress, weight gain or loss, heat, pregnancy, changes in the wind, an ant sneezing in Alabama, laughing too hard, or for absolutely no reason at all.

This is a bit of joke but also fairly true.  Somedays I am convinced there are insulin fairies and gnomes at work.

Hybrid Closed-Loop Pump – an insulin pump that does not operate on a programed basal rate of insulin.  Instead, this pump is wirelessly connected to a Continuous Glucose Monitor (CGM) that I wear on my thigh.  The CGM reads my blood sugar every five minutes and sends this data to my pump.  Based on these readings my pump doses a basal rate of insulin when needed.

This is a graph display on my pump.  Each pink dot on the top is a basal dose and each yellow icon on the bottom is a bolus dose from my pump.  The blue line is my blood sugar readings from my sensor.  The ideal range is within the green bar.

In addition, when my CGM detects that my blood sugar levels are rising or falling it will cause my pump to alarm.  Based on this alarm I either take a bolus of insulin to correct for the high if needed or eat something to treat the low.  If my CGM senses an oncoming low it will automatically suspend dosing any basal insulin to help prevent a more severe blood sugar drop.

At any time I can look at my pump and see the blood sugar value provided by my CGM.  This removes the extreme fear of the unknown high or even worse the unknown and potentially dangerous low.

The arrow pointing down means my blood sugars are trending low.  Additional arrows are displayed based on how rapidly my blood sugar levels are falling.

Even with a hybrid closed loop pump I still need to enter the number of carbohydrates consumed and the pump calculates the dose based on the ratio set for that time of the day.  I still need to consider all the factors listed above (ie. exercise, illness, stress, hormones, gnomes etc.) when dosing but the pump gives me a fairly firm starting point.

But HOW does the CGM read my blood sugars?

A new CGM is inserted once a week.  A video of the process can be seen here.  The needle pokes the very thin filament under my skin and this wire or filament takes the reading from the…get ready, big word coming…. interstitial fluid under my skin.  This reading is then sent to the white transmitter connected to the sensor which wirelessly communicates with my pump.


No, I cannot feel the wire under my skin.

Yes, the needle used to insert the wire comes out immediately.  Only the thin wire is under my skin and connected to the piece you see on top of my skin.

Yes, is hurts a little when the needle goes in but staying alive is pretty cool so a weekly poke isn’t that big of a deal.

No, you cannot poke it like a magic button if you see one.  (This has happened to me and other diabetics)

Yes, I can swim and shower with it on.

The white piece is the transmitter and the small grey pieces is the sensor connected to the wire under my skin

But HOW Does It Stay On When You Swim, Shower, Run, Play with Kids, etc.?

The underside of the sensor (the piece with the wire on it) is sticky.  This stickiness, in addition to a product called Skin Tac make the sensor and the covering tape stick very well.  Skin Tac makes my skin sticky like a sticker.  Imagine sticking two stickers together and trying to peel them apart.


Skin Tac was suggested by a family with two diabetic girls at a waterpark.  My sites kept falling off while theirs were still stuck every time I saw them.  I finally went to talk to the parents and the dad actually went out to the car to give me a pack to try (diabetic families are good people).  It was the first product that could withstand my sweat, showers and swimming.

The only true nemesis of the tape covering my sensor is sun lotion.  The oil in sunscreen combined with swimming and heat can loosen the tape and require an additional tape to put over the top.


So that is my very low level, non-technical explanation of how my pump works.  I am thankful every day, no matter how many beeps and alarms, that there are extremely smart people committed to developing new medical technology.  Insulin pumps have come so very far in a relatively short period of time and it gives me hope that one day these developments will take us to a cure.

Diabetic Mama of Twins fueled by my family, working out, eating, dog walking, getting lost in the woods and insulin. Writing to share the journey this Type 1 diagnosis has taken me on since 2007.

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