Saturday, February 20, 2016

More on Dexcom G4 (505 algorithm) trauma induced drops.

A month ago, it gave an example of what I considered to be a micro-trauma induced CGM sensitivity drop. You can see that post here. While, to be honest, I consider the issue to be almost settled for us thanks to previous observations, additional information never hurts. Let me start by giving a bit of background on the Saturday tennis training: training starts at 16:00 with a small warm up and about 10 minutes of controlled exchanges. After a 10 to 15 minutes, Max coach will start hitting a bit harder, make Max move forward and backwards and send a few lobs so Max can smash as he sees fit. The consequence of that heavy smashing and active play is that Max often finds himself in hyper extension, stretching himself as much as he can to catch those high balls and smash them down the court. That sequence happens around 16:10 to 16:15, after which we have our first BG check/recarbing pause.

We rotate Max's sensor location, but here is what happens when the sensor is on the same side of the abdomen.

The first chart is the one I previously posted. Max woke up with a perfect range that day and we more or less kept the levels until tennis with a bit of recarbing before the training began. After those first ten minutes and a bit of delay, we saw a non physiological (in terms of speed) sudden drop that almost exactly matched the later recalibration offset.



The second chart is a new one. This time, the unpredictable dawn phenomenon struck and Max woke up a bit higher than a month ago. This led to a somewhat more chaotic preparation, but identical values for the start of tennis. Then, after the exact same training chronology, we got the same non physiological drop at the same time. Likewise, we observed the same compensating offset when we recalibrated. (The recalibration came later this time because Max had another training scheduled from 18:00 to 19:30) . And of course, a delayed hypo hit us at about the same time, followed by an additional one, possibly because of the additional training...

This is so strikingly timed that I did not have to adjust the legend position in the code used to generate the chart.

On the Libre, one alternate explanation could be the use of a predictive algorithm. That explanation can't be excluded on the G4 505 since it can support an eventual limited predictive capability. However, had some predictive algorithm over estimated the drop rate, it would have corrected itself as additional data came in.

The most likely explanation as far as I am concerned is

- the sensor is stable and well embedded (on Saturdays, our sensors are typically 5 days old)
- the extreme extension of the abdomen during exercise breaks whatever scar tissue has built around the wire
- sensitivity drop suddenly because of the presence of blood or fluids on site (fairly typical in micro traumas of all kind).

This remains, of course, a working theory. But I like observations upon which theories are built to be reproducible and that one definitely is.

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