Last week I started a trial of Medtronic’s latest insulin pump and continuous glucose monitoring system, the Minimed 530G with Enlite. (See my previous post here.) I thought I’d devote today’s post to what’s arguably the most exciting part of the new system: threshold suspend.
As those of you who have been keeping up with the work toward developing a closed-loop insulin delivery system (the so-called Artificial Pancreas) know, threshold suspend is the first step in the long journey toward that goal. (You, like me, may also have been irritated by the marketing that surrounded the 530G and used the word “artificial pancreas,” since it was picked up and oversimplified by the news media to suggest that this system actually is an artificial pancreas, which it is most definitely not.)
Threshold suspend, which has been available on Medtronic’s Veo pump/CGM system in Europe since 2009, means that if your blood glucose reaches a certain predetermined level (anywhere from 60-90 mg/dL), your insulin pump will alarm, warning you of a potentially dangerous low. If you respond to the warning, nothing will happen. But if you do not respond to that warning, the pump will shut off basal insulin delivery for two hours. (Then it will go on for four hours, then off for another two, etc, till someone clears the alarm.) The idea is simple: if you’ve got a low blood sugar and are not responding, the last thing you need is more insulin.
When I set up my 530G system last week, I was hoping not to have to experience the benefits of Threshold Suspend firsthand. I’m lucky enough to never have passed out or been sent to the hospital as a result of a low blood sugar, and thanks to my CGM, usually am able to stave off bad nighttime lows. So I was surprised when I found myself roused from sleep my first night on the system by two-tone alarm of my pump going off, and the message on its screen that said Threshold Suspend. A quick scan of my body confirmed what it was suggesting: my heart was racing, and I felt shaky. My blood sugar was indeed low. And whereas I’m normally able to correct for a low with two glucose tablets (even that sometimes sends me too high), that night I took five glucose tablets and never had a spike above 130 mg/dL. In other words, I’m glad that it went off.
With that said, I’ve also found some things about the Threshold Suspend function to be unsatisfying/annoying. First, it isn’t predictive, meaning that it will only shut off once you’ve already reached your predetermined target (the next generation will hopefully have this capability). But second, there are some aspects of Threshold Suspend that I worry will limit its use. For example, you can only set your limit from 60 mg/dL to 90 mg/dL (kind of odd, since you can set other aspects of the pump as low as 40 mg/dL). I have mine set at 60 mg/dL, as low as it’ll go, but I don’t think it’s low enough: I often touch 60 mg/dL over the course of the day, and I don’t consider those lows to be emergency situations, since I’m alert and easily able to correct it. I would like to have the option to set it lower.
Second, I would like to be able to create two targets for Threshold Suspend: one for the nighttime, and one for the day (and a temporary snooze function). I say this because, as mentioned, a low of 60 mg/dL when I’m alert and awake is not an emergency situation. But the pump’s Threshold Suspend function INSISTS that it’s an emergency. The alarm does not start with a vibration — it goes straight into full on alarm mode. This means that if you are in a situation where you don’t want to risk having a siren emanate from your body — say a meeting or interview or yoga class — then your only option is to turn off Threshold Suspend. That might be fine for the moment, but it puts you at risk of forgetting to turn it back on for the times you really do need it, particularly before you go to sleep. And if the sensor is giving you an inaccurate reading — which is quite possible on any CGM system — then it’s even worse: you can have your pump start to go bat-sh*t crazy on you when you aren’t even low.
All this brings up a point that I predict I will be reiterating throughout this trial: I wish that there had been more patient input in the design of the 530G, particularly when it comes to alarms. I do not know what type of trials were done before the system was submitted to FDA for approval, but I cannot imagine following a real person with tightly controlled diabetes around for three days and not concluding that the current limits for Threshold Suspend are too high, and that you should be able to set different limits for different times of day (and that the nighttime alarm should be louder — by the time I woke up, insulin delivery had already been suspended). As it is now, I really do worry that the annoyance of having a siren go off every time my blood glucose hits 60 mg/dL is eventually going to make me either turn it off (and forget to turn it back on), or turn it off entirely. And that would defeat the main added benefit of this particular pump.
With that said, I think that the Threshold Suspend feature of the 530G system is a great advance in diabetes care. I also think that integrated pump and CGM systems — especially those that can ward off potential dangerous lows — are essential for people with diabetes, and I believe that insurance companies and payers like Medicare should cover them. That’s probably the most important message of all.