Today marks three years since I first started Loop, the do it yourself automated insulin dosing system. It has improved my glucose management by giving me better results with less effort.
My CGM time in range increased, often to a rarified level of 90%+ and I set my range limits to 65-140 mg/dL. Serious hypos, those < 55 mg/dL are limited to less than a few minutes, on average, each day. Standard Deviation as a proxy for glucose variability is often under 20 mg/dL and rarely over 30 mg/dL.
My case is not typical
Now, I know my situation does not represent the average person with T1D. I’ve been using a cab limited way of eating (<70 grams/day) to tame glucose extremes since 2012. Starting one year ago, I further restricted any foods containing grains or any added sugars. I did this in response to a coronary artery disease diagnosis. I don’t actually count my carbs each day but I’m easily under 30 grams/day.
I feel certain that Loop would not perform as well for someone who wanted to consume a diet of highly processed carbs on a regular basis.
Changing basal insulin more powerful than it appears
Loop performance is subject to how well the user selects appropriate basal profiles, a daily schedule of Insulin sensitivity factors, and insulin to carb ratios for meals. Honoring good pre-bolusing hygiene is essential to reasonable post-meal traces, even with Loop. Subcutaneous insulin injection will never compare well to a non-diabetic’s pancreas release of insulin directly into the portal vein of the liver. Loop can’t change that fact.
Back in the '90s, when I first read of the concept of a closed-loop insulin dosing system, I was highly skeptical that modulating basal insulin delivered by an infusion pump could ever catch up with nutrition metabolism. This is one of the things that really surprised me about Loop.
An amazing system that I like to watch in action
With Loop settings optimized, I never get tired watching Loop slow, stop, and then reverse a rising or falling glucose line to return it to the target I set. It can affect positive change within about 15 minutes, an amazing feat considering the sub-q insulin delivery route.
Unlike commercial systems, Loop permits setting your glucose target to whatever you want. I set mine at 83-84 mg/dL as I consider that a normal and optimal glucose level.
Loop is not perfect, neither am I
My metabolic life is not perfect, however, with Loop. A recent incident with an errant glucose reading from a newly inserted sensor was made worse by me making a rookie mistake. Seems Loop can’t fix my human shortcomings!
Here’s what happened. I put a new sensor into service and like many sensors in the first 24 hours, this one was not ready for prime time. In fact, it was much worse than the average first day sensor errors.
It first dove for the hypo range showing over a two-hour period persistent lows in the low '40s. I was fingersticking, holding back my desire to over-calibrate, and knew that my actual blood glucose was in the mid to low-60s.
Then this ill-behaved new sensor decided to move in the other direction. Now, when sensors over-react to the low side, Loop just shuts off basal delivery. Its response to a low-60s trend is the same as a low 40s trend – it can’t step on the brakes any harder than a zero basal rate.
On the high side, however, Loop can continue to add more basal insulin subject to user-adjustable delivery limits. I am fairly liberal setting these limits and this didn’t help the situation. The false sensor glucose topped off at 274 but a fingerstick only showed 136.
Humility still applies
My mistake was not paying close enough attention to what was actually happening. Loop was over-delivering basal insulin and I should have stopped it. This is a rookie mistake and a reminder that even with an automated system like Loop, you always have to keep your head in the game.
Before I knew it, my insulin on board number had ramped up to 5 units and my blood sugar was not at 274 but 136. I had to eat several glucose tabs to compensate. Here’s the graph of my mistake.
CGM accuracy can be Achilles’ heel
I’ve read criticisms here that automated insulin delivery systems are not reliable due to the underlying inaccuracy of glucose sensors. The case I show above illustrates well that weakness. But this situation for me is an outlier. This set of circumstances, including my inattention, doesn’t happen even once per year. And my safety can be improved with a lower delivery limit.
In summary, I love my Loop system. It has improved my glucose management and my quality of life. Almost every morning, I wake up in the 70-99 range. These are early days of automated insulin dosing and I think the algorithms will get better. I also think the user interfaces will do a better job of balancing the safety settings without inducing alarm fatigue.
Of course nothing will work as well as the glucose system that mother nature produces but without the mythic cure-in-five-years on the horizon, I will continue to happily live with my Loop.
Past Loop posts