Behaviour of the newly inserted sensor

Every time the transmitter is connected to the sensor, two electrical impulses are generated to help the sensor settle in. This happens every time except cases when the transmitter battery is low. It seems that to get operational, the sensor needs either its active substance being moisturized or more probably kind of interstitial fluid lake being formed around it. The transmitter indicates beginning of impulses generation by flashing green for about 10 seconds. Electrical current (ISIG) of these impulses is usually 4 – 10 times higher than sensor current measured under the normal operating conditions. The first impulse appears immediately after transmitter connection. The second impulse follows in less than an hour. Both of these impulses are followed by short periods when sensor current is low or equal to zero. Then the sensor current grows, reaches the maximum, then descends and stabilizes.

I have noticed great variance in times needed for sensor stabilization. It was up to 16 hours during first 2 years of my continuous glucose monitoring. Based on that, I thought that interstitial glucose values displayed by pump are not reliable within first 12 – 24 hours after new sensor insertion. Later, I realized that the sensor stabilization time reflects amount of interstitial fluid and intensity of its circulation at the insertion site. In other words, long sensor stabilization times may indicate problems with interstitial fluid circulation (usually caused by acidosis that lead to late difficulties of diabetes in the end). After suppression of my body acidity, sensor stabilization times decreased.

Typical ISIG profiles within first hours after new sensor insertion are shown on Fig. Newsensor 1. for sensor connected immediately after insertion and on Fig. Newsensor 2. for sensor inserted in advance.

Fig. Newsensor 1.    ISIG of the newly inserted sensor, sensor connected after insertion

The sensor connection time was set as 00:00. The first ISIG peak reached value of 107.96 nA at 00:05 after transmitter connection. Then sensor current dropped to zero values at 00:25, 00:30 and 00:35. The second peak reached value of 49.51 nA at 00:40 after transmitter connection and sensor current dropped to value of 1.14 nA at 00:45.
I undertook 2 days fast (a day prior and a day during the experiment) to keep my glucose profile as flat as possible or at least without rapid changes. The blood glucose profile on the figure indicates that my basal rate during the experiment was bit higher than needed. I had to get some fruit juice at 04:00, 09:00 and 18:00 to prevent hypoglycemia.

Fig. Newsensor 2.    ISIG of the newly inserted sensor, sensor inserted in advance

The sensor connection time was set as 00:00. The first ISIG peak reached value of 83.29 nA at 00:05 after transmitter connection. Then sensor current dropped to value of 13.77 nA at 00:10. The second peak reached value of 86.72 nA at 00:15 after transmitter connection and sensor current dropped to value of 15.55 nA at 00:20.
This experiment was done during a normal day. The glucose value peak at 01:55 is a post-meal height after brunch, the peak at 03:55 is a post-meal height after lunch and pit at 06:55 is low before afternoon snack.

Because sensor current is a linear function of interstitial glucose value according to formula {Principles 1} in section "Continuous glucose monitoring system principles", it is better to use the calibration factor instead of ISIG to demonstrate the sensor behavior. The calibration factor is described by formula {Principles 2}. Fig. Newsensor 3. shows the development of the calibration factor for sensor connected immediately after insertion and Fig. Newsensor 4. shows the development of the calibration factor for sensor inserted in advance.

Fig. Newsensor 3.    Development of calibration factor, sensor connected after insertion

The figure shows that sensor needed approximately 9 hours to stabilize its calibration factor and to provide reliable values of interstitial glucose measurements. The small peak of calibration factor at 10:00 is result of high blood reading used for calibration and the small pit at 18:00 is result of low blood reading used for calibration. This is caused by omitting of ADD constant in formula {Profiles 1} in section "Blood and interstitial fluid glucose profiles" and discussed in detail in section "Calibration process".

My experience with low intensity of interstitial fluid circulation shows that:

Fig. Newsensor 4.    Development of calibration factor, sensor inserted in advance

The figure shows that sensor needed approximately 2 - 3 hours to stabilize its calibration factor and to provide reliable values of interstitial glucose measurements. The peak of calibration factor at 03:55 is result of high blood reading used for calibration and the pit at 06:55 is result of low blood reading used for calibration. This is again caused by omitting of ADD constant in formula {Profiles 1}.

My experience shows that problems related to developing the calibration factor in cases of low intensity of interstitial fluid circulation do not appear either if the new sensor is inserted in advance or intensity of interstitial fluid circulation is high.