CGMS uses a sensor inserted under the skin for several days, and can measure the interstitial glucose value almost continuously. This is transmitted to a device for displaying data and trends. While not real time, this is a leap ahead of finger prick testing typically done 4 to 10 times per day.
This stream of data supports the mission of the artificial pancreas by:
- automatically providing a blood glucose reading every few minutes (the sampling rate dependent upon the particular design),
- recording and time-stamping blood glucose data for future download and analysis,
- monitoring trends pertaining to rising and falling blood sugars, which is helpful in the prediction of blood glucose levels in the immediate future,
- comparing blood sugar levels and predictions against a high blood sugar threshold, and then prompting the user that a correction bolus from an insulin pump is needed immediately,
- comparing blood sugar levels and predictions against a low blood sugar threshold, and then prompting the user to reduce the basal insulin from the pump or to eat something.
These capabilities suggest that a stream of real-time data of blood glucose levels can be used to "close the loop" and control the insulin pump directly. Closed loop control of the insulin pump in it's simplest form would be to correct immediate or pending hyperglycemia or hypoglycemia by adjusting the continuous basal rate to add or remove insulin as needed, automatically.
Some issues with the present performance of continuous sensing technology will require some study:
- continuous sensors require calibration a few times a day, by performing a manual blood glucose test with a finger stick, and then entering the blood glucose data into the continuous system for a sensor correction,
- continuous sensors are measuring interstitial glucose, so there is a time delay between the sensor data and the true blood glucose,
- automatic control removes the intellect of the user, which can be an additional safeguard when the data is subject to error and must be verified before taking action.