Glucose Monitors
Continuous Glucose Sensing – Are Your Patients Candidates?
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Let’s start with a basic quiz for all you readers who do NOT have diabetes.
A mother with type 1 diabetes, on an insulin pump, is preparing to take drive to a soccer field 20 minutes away 2 hours after eating lunch. She has 3 small kids in her car. As a safety precaution, Mom decides to perform a glucose test. The meter reading is 90 mg/dL. Is Mom safe to drive?
If Mom had been wearing one of the 3 currently marketed continuous glucose monitoring systems (CGMS), she would have noticed the presence of 2 downward facing arrows as shown in Figure 1 indicating that her blood glucose level is actually dropping at the rate of 3 mg/dL/min. Even though her meter is reading “90 mg/dL” her blood glucose level is actually in decline, because the insulin dose she bolused at lunch is still being absorbed. Thus, in 15 minutes her blood glucose level will be at 45 mg/dL/min and she would become an impaired driver.
Two down arrows accompanied by an auditory or vibration alarm produced by the CGMS device would warn the user that her blood glucose level is dropping quickly. Blood glucose levels which are stable over time would be demonstrated on the sensor with a horizontal arrow. If the glucose level is rising at greater than 3 mg/dl/min an alarm would sound alerting the patient to appropriately manage her hyperglycemic event.
Crude self-blood glucose monitoring was initially developed in 1965. The first commercially available home blood glucose monitoring devices were sold in 1970, weighed over 3 lbs and cost over $350 per month to operate. The United States is home to 30 self-blood glucose meter manufacturers selling $2 billion worth of strips and meters annually. However, these devices provide only a static reading of one’s blood glucose levels rather than an indication of real time continuous trending. Using only a meter to manage and fine-tune diabetes would be similar to taking your wife to a movie on your “date night.” Instead of going inside to watch the film, you simply walk around the outside of the theatre and admire the movie posters! You can then tell your wife, “Wow, Honey, those were great pictures, weren’t they?” (I doubt she would let you off that cheaply!)
CGM systems operate by measuring the glucose levels in interstitial fluid. The devices consist of 3 components: a disposable sensor that measures glucose levels, a transmitter that is attached to the sensor, and a receiver that displays and stores glucose information. The information stored in the receiver is then converted into estimated mean values of glucose standardized to capillary blood glucose levels measured during calibration.
Using an applicator or self-insertion device, a thin plastic sensor is inserted just under the skin of the abdomen or the upper arm. These devices can display real-time glucose values and glucose trends, and some can also sound an alarm or vibrate when they detect hyperglycemia or hypoglycemia. The receiver can store information for later use, and long-term data can be downloaded to a computer.
A summary of the 2 commercially available CGMS products are depicted in Table 1. (The other 2 are available by prescription.)
The FDA lists the following indications for CGMS: detecting trends and tracking patterns of glucose values; serving as an adjunct to, but not a replacement for, information obtained from standard home blood glucose monitoring; aiding in the detection of episodes of hyperglycemia and hypoglycemia and minimizing glucose excursions; and facilitating acute and long-term therapy adjustments. All devices are indicated for patients 18 years of age and older, but only the MiniMed Guardian and Paradigm REAL-Time systems are approved for use in children age 7 through 17 years. The FDA advises that the use of CGMS is not intended to replace home blood glucose monitoring and that it should only be used in conjunction with patient self-monitoring. Further, treatment decisions based on CGMS results should be confirmed by a traditional blood glucose meter.
Table 2 lists provides a list of patients with type 1 diabetes who would most benefit from use of CGMS.
Table 2. Type 1 Diabetes CGMS Candidates
- History of hypoglycemic unawareness, especially nocturnal hypoglycemia
- Pregnancy or pre-conception planning
- Patients with erratic glycemic control
- Patients who are initiated on insulin pump therapy should consider wearing a sensor as a “test drive” for the first 2 weeks of therapy so that pump parameters may be fine-tuned efficiently
- Patients who use pramlintide; a drug known to induce hypoglycemia
- Patients who travel frequently
- Athletes. The sensors will display interstitial glucose readings while the individual is exercising
- Patients who complain of “hypoglycemic symptoms” after having had gastric bypass procedures. These individuals may actually develop a condition known as islet cell hyperplasia (nesidioblastosis) which can result in frequent episodes of paroxysmal hypoglycemia detectable with CGMS.
- Patient requests to purchase the device for personal use
- Patients using corticosteroids which tend to favor post-prandial hyperglycemia
- Patients who wish to understand the effects of various foods on their glycemic control
CGMS does NOT cure diabetes and requires patients to work even harder to self-manage their disease. Therefore, CGMS should not be prescribed to patients who are unwilling to wear or utilize the devices properly. Using the sensor does NOT reduce one’s obligation to monitor blood glucose values on a timely basis. Sensors still need to be calibrated at least twice daily. Alarms must be monitored and appropriately managed. “Alarm fatigue” is likely to occur in new sensor patients if the warnings are set within the normal glycemic range. For example, if a “high alarm” alert was set at 150 mg/dL or a low alarm set at 90 mg/dL, alarms would be sounding continuously and soon they would be ignored. Therefore, prescribers of CGMS for patients with hypoglycemic unawareness may choose to set the low alert at 70 and eliminate the high all together to minimize the frequency of sensor alarms.
As mentioned above, one of my favorite uses of CGMS is for patients with hypoglycemic unawareness. Figure 2 A-B shows the CGMS graph of a 78-year-old retired physician who has a long history of nocturnal hypoglycemia and “insulin stacking.” Dr. X tends to bolus insulin from his pump whenever his blood glucose levels approach 180 mg/dL. One month prior to wearing the sensor, the patient’s wife administered glucagon to him on 4 separate occasions.
Figure 2-A
This continuous glucose sensor download from “Dr. X” shows 7 days of interstitial glucose readings. The target range for acceptable glucose levels has been set at 80-180 mg/dL. This was a “blinded sensor study” meaning that the patient was asked to simply wear the device that had no alarms set. He was told NOT to respond to the high or low readings on the real time sensor. The down arrows show significant declines in the patient’s glucose values due to “over correction” of his post-meal glucose values. The circles indicate the length of time that the patient was noted to be hypoglycemic (with a glucose value = 40 mg/dL), which, on one day, lasted 7 hours!
Figure 2-B
Follow-up of Dr. X’s CGMS download taken 8 weeks after using the sensors continually and being re-educated about the dangers of self-induced hypoglycemia. The patient’s A1C has actually improved from 7.8 to 7.4 % and he did not experience any severe hypoglycemic episodes. More importantly, he reported improvement in his mentation and quality of life.
CGMS is a new technology which may offer patients with diabetes a major advance in improving A1C values while reducing the occurrence of disruptive hypoglycemia. Although the long-term danger of hyperglycemia is an increase in diabetes complications, the short-term hazard of hypoglycemic unawareness can be devastating. An automobile accident, a fall resulting in fracture, or a death from severe hypoglycemia is reason enough to consider using CGMS. I encourage all of our diabetic patients who experience hypoglycemia to consider purchasing a CGM system. Insurance companies are now beginning to cover the costs of these devices more frequently for patients who meet certain criteria. CGMS can save and improve the quality of lives of our patients with type 1 diabetes.
Have you had an experience with any of the CGM systems? We would like to hear from you.
Jeff Unger, MD
Published on August 17, 2010
Dr. Unger is the Associate Director of Metabolic Studies at Catalina Research Institute in Chino, California. He is a long time insulin pumper and has used all of the currently available CGMS devices. Dr. Unger is also a Board Member of Primary Care Network and a consulting physician for the LA Angels of Anaheim baseball club.
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