Medtronic Minimed 670G Insulin Pump showing "Sensor Warm Up Started" message

Sunday is Sensor Start Day



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I have goals, dreams, family, and a career. I didn’t choose to have diabetes and managing Type 1 diabetes* is time consuming.

I use the Medtronic Minimed 670G Insulin Pump which connects to the Guardian CGM sensor and automatically adjusts my insulin dosage. Medtronic calls this a “Hybrid Closed Loop System.” Before this device existed, people set up their own systems and called it “looping.”

For this system to work, I have to always have a functioning CGM sensor. CGM stands for “Continuous Glucose Monitoring.” The sensor has to be replaced every 7 days. Replacing the sensor and getting the new one working takes about 3 hours. First, the transmitter has to be re-charged. That takes at least 30 minutes. The second part of this time is the 2 hour “warm up” period. During this time, the sensor is unusable. At the end of the 2 hour period, you are able to calibrate the sensor and start using it.

So, changing the sensor means going without a sensor for about 3 hours. This means that there is no automatic insulin adjustments made. The looping is out of service. To make this even trickier, accurate calibration requires the blood glucose to be stable and within normal range.

To do this whole process easily, you have to have a basal rate that will keep you in range during this time period. Additionally, if you eat anything, you have to do the correct bolus insulin dose.

I try to not eat during this warm up period. Once the sensor has warmed up, I calibrate and then eat! Back to auto mode is such a relief.

Sensor change is the biggest hassle in managing my diabetes

The “Hybrid Closed Loop System” works super well for me. I love auto mode. But every 7 days, you have to change the sensor, and deal with this 3 hour process.

To minimize the impact of diabetes in my life, I’ve decided to always do this on Sunday morning. It’s a way I can build this into my routine and not have the stress of sensor change during my work week. It also reduces the chance of having a sensor fail during the week. In general, sensors tend to last the full 7 days for me, so this works out.

I wish these sensors lasted longer than 7 days…

Some people do re-start the sensors and extend them to last 10, 14, or even 21 days! I’ve tried re-starting the sensor myself, and usually can get a few more days out of it, but after day 10 or 12, the sensor must start deteriorating, because it will require more frequent calibration. I don’t like the surprise of being woken up in the middle of the night to calibrate and I don’t want the sensor to fail on me when I’m counting on it.

But, even if you re-start the sensor, you still have to go through the warm-up period, so it doesn’t save you any time.

Re-starting the sensor increases risk and stress, as the sensor WILL fail one day, and it may be at a very inconvenient time. There is a chemical on the sensor that gets used up, so sensors really do have a limited lifespan.

Freestyle Libre makes a 14 day sensor but it doesn’t communicate to my pump, so I can’t use it for Medtronic’s auto mode.

I LOVE auto mode – this is a normal night for me!

Medtronic Minimed 670G Insulin Pump graph in auto mode

I always calibrate before bed. If my blood glucose is high, the pump will suggest a bolus and the adjustment it suggests usually works perfectly!! In the above graph, the spike was dinner. The blood drop symbol at 10pm was when I calibrated and did the recommended insulin bolus.

Improving Day 1 Sensor Accuracy

The sensor readings can be less accurate on the first day. I read a tip online about how the accuracy can be improved by “marinating” or “soaking” the sensor for at least a few hours beforehand. What this means is that you insert a new sensor ahead of time, but you don’t connect the transmitter to it until later.

I’ve started inserting the new sensor on Saturday night, and then when I start it on Sunday morning, the sensor has already been in my body 12+ hours. For me, this has dramatically improved the accuracy of the sensor on day 1. It does take more planning, as I can’t take a shower when I have the sensor soaking because it’s not waterproof when not connected to the transmitter.

My Routine

Shower on Saturday. After the shower, insert a new sensor. Don’t do anything with the new sensor until Sunday.

On Sunday morning, eat breakfast. Remove old sensor, re-charge transmitter, and then start new sensor. Wait 2 hours for it to warm up. Calibrate and then eat lunch.

* I actually have LADA Diabetes, treated as Type 1 Diabetes.



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