Anyway, here's such a situation, but also an illustration of many of the practical issues we face.
green segment: flattish around 100 mg/dl with a couple of mild compressions, no big deal.
By the way, a word about compressions: I often read very specific descriptions of compressions (transient sensor attenuations) in the T1D forums and groups. The compression should be abrupt, deep, and should end with a rebound. That is partly true: a major compression may indeed so unfold. But in practice, the compressions we detect and visually confirm can take almost any form. They can be partial, lead to fairly minor atenuations with no rebounds. They can be masked, as it is almost the case here, by a simultaneous increase. Be open: observe and learn: you may encounter compression lows, but also compression steady states or even compression highs (where the compression attenuates the ongoing rise)
third compression: that one is a major PITA. While it is detected, it masks - in a plausible way the rise that is happening at that moment.
compression exit: the trend starts to appear. But we need a few packets to make sure it is not one of those post-compression rebounds we see now and then. Unfortunately, another mild compression confuses the situation even more (and at that point, the compression detection algorithm, lacking a clear trend, has given up).
correction: the trend is now clear. Since we have seen such situation get out of hand quickly, the time has come for a quick Libre and blood check (see below): the Libre reports 230 mg/dl. The Roche Accu-check reports 225 mg/dl. The Dexcom still lingers at 160 mg/dl, one arrow up.
effect: as expected, around 6 packets later, the correction effect shows up.
Here's what the BG Meter and the Libre showed. Disregard time differences: both the BGM and the Libre are still running on winter time and both have drifting clocks. The actual time is 01:20 for everything.
- the dexcom is running the G4 share 505 algorithm. The sensor is 5 days old.
- the dexcom has been calibrated with the Roche Accu-Check BGM used here.
- the dexcom is on the right arm.
- the Libre is on day 12 of its life cycle.
- that particular Libre sensor has been eerily accurate through the session.
- the Libre is on the left arm.
I could be tempted to blame the Dexcom and praise the Libre and, to be honest, to some extent, I do.
- this is the ideal situation for the Libre "delay compensation" algorithm. None of the fancy factors where it goes a bit crazy are present.
- the Libre hasn't been compressed.
- this Libre sensor has been noticeably better than average (MARD of 5% vs Accu-Check over the whole period, but not enough data to be statistically significant).
- that Dexcom sensor has been underperforming a bit for reasons that I can't be certain of.
And what about the correction?
I hit hard. Very hard. Based on our experience, when the Levemir injection seems to fail, EGP can spiral out of control (we did get our first even 400 mg/dl on such an occasion). I used about 2.5 times more insulin that I would use to correct that trend in daytime.
There's always a bit of anxiety when using such a relatively high dose (8U) in the middle of the night. I do want to avoid the yo-yo situation where I have to correct a low later. And, at first, the huge drop after the plateau isn't reassuring. What is the fall accelerates? That is always a question that lingers.
As it turns out "insulin resistance", or EGP, or a mix of both is so high in those circumstances that the situation should evolve well. But that is an opinion based on our fuzzy experience and gut feeling, not a computable one, if only because the previous nights were OK and we have no definite idea about the current insulin sensitivity level.
As you can see, the trend settles quickly.
And even if I am usually very confident with my decisions, I will lose a few hours of sleep, keeping an eye on the situation just in case... and write this blog post to kill time.