When Medtronic first announced they were launching a hybrid closed loop pump system, I was ecstatic. After all, not only was this a step toward a true, artificial pancreas, but it was a system meant to decrease the workload for diabetics everywhere.

At the time, I still had a year left on my current pump warranty and knew it would be a while until I could get my hands on the new system. Still, I was excited enough to bring it up to my endocrinologist at my next appointment.

What I learned about the 670g system that day surprised me.

How Auto Mode Works

I was acutely aware of how the hybrid closed loop system functioned even before I brought it up to my doctor. Instead of working off a list of preprogrammed basal rates for different times of day, the auto mode on the new MiniMed constantly adjusts how much insulin you are receiving based on your current blood sugar number.

If you are higher than you should be, it will give you a larger “micro-bolus” every five minutes to bring it down. If your blood sugar is lower than the target, it will give smaller doses or cut off insulin altogether until your blood sugar comes back up.

Over time, the system recognizes patterns in your numbers throughout the day and will start to predict these changes before they happen.

The reason this system is called a “hybrid” closed-loop is that it still requires you to do most of the work for meal boluses. You still need to carb count and enter your meal into the pump before you eat. Like most modern pumps, the 670g will calculate your bolus dose for you, but it will not respond to a missed bolus with enough insulin to bring it down on its own.

This system also has some limits when it comes to correcting for highs. If your blood sugar goes over 150 mg/dL, the pump will typically recommend a correction bolus. You will need to take a manual BG measurement and approve the recommended correction dose. If your sugars are hovering higher than you want, but still lower than 150 mg/dL there is no option to correct. Instead, you have to wait for the “micro-boluses” to bring it down slowly or trick the system with a fake meal bolus.

While there is still a lot of work that goes into managing your diabetes in auto mode, once the system has gotten to know your patterns you won’t have to do nearly as much to keep your blood sugar in range between meal times and overnight.

The Limits of the Hybrid Closed Loop

What surprised me the day I talked to my endo about the 670g had nothing to do with the finer details of how the system worked. Instead, it was the rigid nature of the system that I didn’t expect.

The hybrid closed loop was set up with a target blood sugar of 120 mg/dL and to stay within a range of 70-180 mg/dL. While these would be great numbers for the average diabetic, they by no means equate to healthy, “normal” blood sugars.

For me, the fixed target of 120 mg/dL was the deal-breaker.

An average blood sugar of 120 mg/dL is equal to an A1c of about 5.6, a number any diabetic should be proud of. But that doesn’t mean you should expect an A1c that low if you use the 670g in auto mode. In fact, most users report a number closer to 6.5 or 7.

That makes sense when you consider, that during user trials, the 670g auto mode system reduced lows by 44% but only reduced highs by 11%. The system seems to be calibrated to stay on the high side of 120 mg/dL. In fact, most users report their blood sugars tend to float between 130 mg/dL and 140 mg/dL between meals and even overnight.

Unfortunately, there is no way to adjust the 120 mg/dL target built into the 670g. And since auto mode won’t let you put in a correction dose for moderately high blood sugar, it’s impossible to force your average below 120 mg/dL.

Who Should Use Auto Mode and Who Shouldn’t

When I first talked to my doctor about the hybrid closed loop, my A1c was in the low 5’s. There was no doubt that my A1c would actually increase if I switched to the new system. It wasn’t an option for me.

But for anyone who is struggling to maintain an A1c below 7, you may want to consider giving the 670g hybrid closed loop a try. It can help regulate difficult basal rates that you don’t have the time or energy to solve yourself. And it is especially helpful for reducing overnight alarms and keeping your sugars balanced while you sleep.

You will still need to count carbs accurately and remember to bolus. In fact, since the algorithm depends on accurate bolusing and makes it harder to apply corrections, it can be more difficult to control your sugars on the hybrid closed loop if you do struggle in these areas. 

Of course, there are always opportunities to improve your skills. Medtronic provides free training to anyone switching to their pumps and your doctor should be able to guide you to free resources if they can’t help you themselves. Plus, there are a plethora of phone apps available to help you count carbs and track meals.

Looking Forward to Tighter Control In the Future

When the warranty on my old pump finally did expire, I contacted Medtronic to replace it with a 630g pump, a model with some of the features of the 670g but without the auto mode option. When I explained to the rep that I wasn’t interested in the hybrid loop because I was better at controlling my diabetes than it would be, she suggested I get the 670g anyway and just use it in manual mode.

As you might expect, the 670g has a more accurate CGM sensor than previous models. And, even though I wouldn’t be utilizing it for auto mode, it was still a better option.

I have been using the 670g exclusively in manual mode for over a year now. It is a huge upgrade from my previous MiniMed pump and my A1c remains in the low 5’s. Still, there are days when I long for the convenience of not having to watch and react to my blood sugar every single time it rises or falls. After a decade with this disease, I would love to know how that feels.

For now, I (and anyone else who considers 180 mg/dL way out of range) will have to keep waiting for the day technology allows for an auto mode smart enough to keep blood sugars closer to those of a normal, functioning pancreas.