Are Continuous Blood Glucose Monitors For You?
Now that you are well-educated about the latest in diabetes insulin therapy (for a review see "To Pump or Not to Pump"), I would like to discuss a relatively new form of technology: the continuous glucose monitoring system (CGMS). Since these devices are becoming more common in clinical practice and in response to several share posts, now is the time to clear up some common misconceptions about CGMS.
First, let's review some basics. There are currently three CGMS systems that are approved by the FDA: the Guardian, Dexcom, and the Navigator (just approved). These monitors require a sensor to be placed in a patient's subcutaneous tissue to measure glucose in the interstitial fluid. Interstitial fluid is within the layer of fatty tissue under the skin (not capillary blood). The sensing device must be calibrated appropriately before it can accurately start sensing blood glucose readings. This calibration period may take up to 12 hours, depending on the type of sensor. The key is to remember that you can't wear it for only a specific time period, such as overnight, and then take it off.
What does the sensor actually do? The sensor measures interstitial fluid glucose within the subcutaneous tissue in short time intervals. The most valuable information obtained with the sensing device is the observation and measurement of glucose trends over time. This is an extremely important tool and can indicate if your glucose levels are trending upward, the same, or downward. It is important to keep in mind that there is a 20-minute lag time from the actual blood glucose measurement to the same glucose level in the interstitial fluid. Therefore, one acts (or does not act) on information that is 20 minutes old. This is particularly helpful if the sensor indicates that glucose levels are dropping rapidly (two arrows in the down direction). This lets you know that you need to perform self-blood glucose monitoring with your meter to confirm a low blood sugar and then consume rapid acting carbohydrate. The same information helps you determine if your blood sugar is rapidly increasing after eating a particularly fun meal (two arrows in the up direction indicates a need to test and perhaps give a correction bolus). The sensor is equipped with alarms that are set by you and your diabetes educator to help remind you about calibration, when to test blood glucose, and if there is a malfunction.
What does the sensor not do? The sensor does not actually provide blood glucose readings. Instead (as mentioned above) it provides 20-minute delayed interstitial fluid readings. In some systems, such as Metronic Minimed, the Guardian communicates interstitial glucose information to the pump when worn together (trends on the screen), but it does NOT tell the pump how much insulin to bolus. You are still the brain behind the insulin pump. At this moment in time, the Guardian may be used in conjunction with the Medtronic Minimed pump or as a stand alone CGMS system. The Dexcom and Navigator are stand alone devices and can be used separately with or without any insulin pump, but it won't be able to communicate any information to the pump.
The CGMS along with the insulin pump is not an "artificial pancreas." At least not yet. This is the eventual goal of the pump and CGMS manufacturers.
How are CGMS systems best employed? The sensor should be used to observe trends in your blood sugars, not provide an actual blood sugar. As such, you still have to test blood sugars, perhaps even more so, if you want to be able to take advantage of the information available. It is important to note that you still need to check blood sugars when you are bolusing insulin and not base the amount of insulin on the information from the sensor, since it is not as accurate as the meter. Parents of small children may be somewhat comforted by the alarm system especially overnight, but as you know there are many false alarms. The CGMS is of particular help in the monitoring of the presence of evolving post-exercise induced hypoglycemia (low blood sugars resulting hours after strenuous exercise). The display arrows are very helpful in indicating the path you should take, such as if you need to use a lower temporary basal rate, need to give less insulin for a bolus, or need for carbohydrates. Keep in mind that one can become obsessed from observing those little arrows go up and down. Research has shown that many CGMS users tend to give too many corrective insulin boluses as a response to the arrows and go low! If you are the type of person who becomes obsessed with your blood sugars, you may even become over-anxious and look at the monitor every 15 minutes, which is not productive. You might even start going "high" because of anxiety.
To reiterate a theme that will be consistent in all of my blogs: there is no one right way to manage your diabetes. The CGMS system should be considered as a potential helpful tool to help you evaluate trends of blood sugars and as an adjunct to your overall diabetes care. Please discuss the positives and negatives with your diabetes team in order to determine if CGMS is right for you.