By looking at the monitor’s long-term trend graphs starting approximately four hours after eating a meal and taking rapid-acting insulin, you can evaluate the effectiveness of your basal insulin. If your blood glucose level is gradually rising or falling after the food and mealtime insulin have worn off, your basal insulin probably needs adjustment.
In the example here, taken from an insulin pump user, a meal is eaten (and bolus of insulin given) at 11:30 AM. At around 4 PM, the glucose level begins to take a downturn. This indicates that the basal insulin level is too high in the late afternoon.
In this example, taken from a person using injections of glargine (Lantus) as his basal insulin, the blood glucose level is rising through the night from 2 AM until 8 AM. This indicates the need for a higher dose of glargine.
In addition to helping fine-tune basal insulin doses, long-term trend graphs can be used to determine the action curve of your rapid-acting insulin. That is, they can help you find out how long it takes for your insulin to finish working. (They are also valuable for determining how much “insulin on board,” or active, unused insulin, you have in your system.) Action curves can vary from person to person, with times ranging from 2½ to 5 hours. To determine your personal action curve, simply check to see how long it takes for your glucose level to stop dropping after you take insulin for a meal or to correct a high blood glucose level. Once the line graph flattens out, the insulin has pretty much run its course. For instance, in the example here, the insulin action curve is approximately three hours.
Long-term trend graphs are also useful to see the effects of high-fat foods and of intense or prolonged exercise on glucose levels, as well as to see blood glucose patterns during illness. The computer programs that can be used with CGM systems (Medtronic’s Carelink Web site, DexCom’s DM2 software) can be used to examine long-term trends over multiple days. The Sensor Daily Overlay at www.carelink. minimed.com lets users see up to seven days of 24-hour trend graphs superimposed and color-coded. The Hourly Statistics report on DexCom’s software provides statistical averages and graphic displays for virtually unlimited blocks of time. Both systems can reveal overall peaks and valleys in glucose levels over the course of a day, as well as patterns linked to weekends, schedule changes, menstrual cycles, and other variables.
Those lovely alarms
Perhaps the most valuable aspect of CGM systems is their ability to alert the user when blood glucose may be approaching dangerously high or low levels. You might say they act sort of like the highway rumble strips that keep you from veering off the road and into a ditch.
Although they may not detect every low, CGM systems will provide an early warning for the vast majority of them — and much earlier than most people would detect them on their own. In 2004, Dr. Bruce Bode, a renowned diabetes clinician and researcher who practices in Atlanta, Georgia, published one of the first studies on the effectiveness of CGM. He showed that using a CGM system with the low blood glucose alarm turned on reduced the amount of time spent in a hypoglycemic state by nearly 50% compared to using a system without the alarm option turned on. The low alarm makes it considerably safer for someone at risk for hypoglycemia to work, drive, exercise, and generally aim for tighter blood glucose control. Likewise, the high glucose alarm allows a person to be more aggressive at managing after-meal glucose spikes and preventing ketoacidosis, a life-threatening condition associated with very high blood glucose.