Probably one of the hardest things about closed looping is when you find yourself not understanding the direction that your loop is taking. “WHY is Loop not giving more insulin right now?” or “Why is Loop doing nothing?”
To answer these questions, you have to Think Like A Loop. In other words, we need to know about Loop’s algorithm and understand what its decision points are. Tonight in Looped group, I posted this figure of a simulated BG curve and asked:
It’s 8pm and your predicted BG curve looks like the following. What do you expect Loop to recommend/enact at 8pm?
A. Zero (suspend) temp basal
B. Scheduled basal from your settings
C. High temp basal
D. Lower temp basal (between zero and scheduled)
Let me just say…LOVED the engagement and thought people put into it. The responses also help me focus on the next project I’m working on…super excited to release it in a week or two. But, back to question at hand.
What would your answer be? Before you give your final answer…consider this next twist. Would you give the same answer to this graph as you gave the graph above? If not, what would the answer be for this graph and why? (ignore the timestamp mismatch)
This second case has a dramatic drop happening. Suspend threshold is still at 60 mg/dL, correction range is still 90-110, the lowest value on the predicted BG chart is 75 mg/dL, and the eventual BG is 171 mg/dL for this example. So, pretty similar to the first example except this precipitous drop going on right now.
Let me tell you some of the common pitfalls we all can easily slip into when trying to answer these “Why is Loop giving me this basal?” questions.
- Wondering about IOB, COB, or DIA in order to answer…those aren’t a factor to answering the question as they have already been used to make the predicted BG curve. In other words, they are accounted for already in the information presented. All you need to answer this question is provided by the predicted BG curve, your suspend threshold, and your correction range.
- Thinking about this as a human…humans tend to say “well, I’m on a rise/fall right now so…[insert Loop action based on that]” Loop isn’t looking at the past BG movement, instead it’s looking at the prediction curve ahead and applying its rules based on that. Any drop or rise going on will have been added to the predicted curve through the BG momentum and retrospective correction components of the algorithm…so again they’re already incorporated into the predicted curve.
Restating for emphasis: All you need to answer this question is provided by the predicted BG curve, your suspend threshold, and your correction range.
The answer is B: Loop will give your scheduled basal in both situations shown above. When your predicted BG curve (A) drops for a time below correction range but (B) all of the curve still above suspend threshold, and Eventual BG is (C) above range or (D) within range…Loop will give your scheduled basal.
The logic is a bit of a wait and see. Scheduled basal will maintain the delivery of insulin. Your settings haven’t told it this is an “oh my gosh…stop the insulin!” moment (you’re predicted to still stay above suspend threshold), but we also don’t want to give high temps yet (to correct the eventual BG) because we’d like to safely make it through the part that is below correction range coming up.
- If BGs were to drop (enough) or keep dropping (enough), your predicted BG curve would likely slip to your suspend threshold and then Loop would suspend. (One important take away is to not to set your suspend threshold so low that it no longer acts as a safety in these situations.)
- If BGs were to rise enough such that the whole predicted curve comes back into or above the correction range completely, you’d then get high temp basals to correct for that eventual BG that is above correction range.
Want to practice a little more? Here’s a sequence of actual screenshots from a running Loop. Think for each of these…
A. Zero (suspend) temp basal
B. Scheduled basal from your settings
C. High temp basal
D. Lower temp basal (between zero and scheduled)
Time A
Time B
Time C
Ok, got your answers? You can see what Loop has done by looking at the middle basal rate change in the head’s up display.
Time A
Loop has provided no adjustments to the scheduled basal rate. The predicted BG curve has a temporary excursion below correction range, doesn’t dip as low as the suspend threshold and eventual BG is above correction range.
Time B
A few more BG readings have come in and affected the predicted BG curve. Now the predicted curve is entirely within or above the correction range…and since the eventual BG is above correction range…let the high temps begin. Loop is safe to correct for that eventual BG. (see Loop has increased temp basal by 1.5 u/hour)
Time C
And a few more BGs later, that BG has dropped some and the predicted BG curve has now gone below the correction range again. Hasn’t dipped low enough yet to hit the suspend threshold at the lowest predicted point…so Loop is going to continue to just provide scheduled basals.
Hopefully that little walk through helps illustrate what you can be looking at to understand why your Loop is recommending/enacting basals at a particular time. It’s all about the predicted BG curve, your suspend threshold, and your correction range.