Using the Continuous Glucose Monitor (CGMS)

Abstracted from Dr. Bernstein’s book “Diabetes Solution”
© 2007 by Richard K. Bernstein, M.D.

Most of us have jobs that bring us into contact with other people during the day and family with whom we have contact after work. These contacts offer considerable protection from severe hypoglycemia, as colleagues and relatives will intervene if you start walking into walls or talking silly. A sleeping partner can frequently pick up on the labored breathing and cold, clammy skin or damp nightclothes that accompany hypoglycemia and then awaken you and ask you to check your blood sugar.

If you live or sleep alone, or if your sleeping partner is an extremely deep sleeper, however, you don’t have this protection at night.* A new backup is now available.

Several companies are now marketing continuous blood glucose monitors.† A continuous blood glucose monitor works via a tiny sensor implanted beneath the skin, using a technique similar to that used for insulin pump tubing. The sensor constantly measures glucose concentration in the tissue fluid present at its subcutaneous location. A combined power supply and radio transmitter attaches to your skin or clothing. The transmitter sends up to several hundred glucose readings daily to a small portable receiver that you can keep in a pocket.  The number displayed is approximately equal to the blood sugar about 20 minutes prior to the reading. So if you had taken a reading with your conventional method 20 minutes ago, this would be roughly the same as the reading from the sensor right now.

Also displayed is an up or down arrow to indicate whether blood sugar is increasing or decreasing. What’s most valuable is an audible alarm that can be set to sound at any selected blood sugar value and also to signal rapid drops in blood sugar.

There are some potential problems associated with these devices, so they’re not for everyone, and certainly not for me.

  • The sensor remains under the skin for 3 days. During this time, there is always a possibility for inflammation or infection (probably a low risk).
  • Fibrosis, or scar tissue, can potentially build up at the sensor site over time (although how long this would take is as yet unclear) and eventually make measurements less accurate.
  • Measurements are inherently less accurate than ordinary blood sugar monitoring, so the devices need to be calibrated against finger-stick blood sugars about twice daily. For now, at least, I would recommend that any blood sugar correction be made based on finger-stick measurements.
  • Both the equipment and the disposable sensors with associated supplies are quite costly and may not be covered by many insurance policies.
  • Advertising for these products may falsely imply (but does not state outright) that a sensor–insulin pump combination will automatically monitor and inject insulin to keep blood sugars on target around the clock.
  • The sensor is typically implanted in the abdomen, and has a bulky exterior.
  • The sensor will typically work for only 3 days, because the enzyme used is then depleted.
  • The setup requires training to use.

* Many insulin users will awaken automatically when blood sugar gets too low during sleep, so these people have built-in protection.
† Search the Web for “continuous glucose sensor” to compare different models.

While the sensors have at best limited usefulness at the moment, it is entirely feasible — just from an engineering perspective — that they will vastly improve over time. That said, I’m not holding my breath —this technology has been around for decades, and manufacturers have made plenty of money by employing it shoddily. One can still hope, however, that some brilliant entrepreneur will develop a highly accurate and timely sensor that can provide constant, accurate blood sugar readings.

But we’re not there yet. So to sum it up, if I were living alone, I’d use the sensor to protect from nighttime hypoglycemic episodes and forget about using an insulin pump.

Editor’s note:  Since this article was written the Dexcom Seven has become available. This stand alone CGMS can be worn for 7 days and does not need to be linked to a pump.