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Ari Eckman, M.D. and Christopher Saudek, M.D.
02-03-2011
- Real time monitoring of interstitial fluid glucose is available, with continuous display of glucose level for up to 5-7 days before changing sensor.
- Displays results on an external palm-held device or else on an insulin pump.
- Calibrated by self-monitored blood glucose, used to indicate immediate interstitial glucose as well as patterns of high and low glucose throughout the day.
- Consists of sensor, transmitter and receiver providing real time readings, graphs, trends, glucose and projected glucose alarms directly to patient.
- Glucose reported every 5-10 minutes, with capability of every minute for some CGM's.
- Small, flexible glucose oxidase sensor inserted under skin in abdomen or arm measuring interstitial glucose concentrations; water resistant transmitter sits on skin, sends glucose readings wirelessly to receiver; values downloaded to personal computer, glycemic profiles generated
- Portion of membrane polymer remains in skin after sensor removed. Long term effects of this not yet determined, although no health effects initially reported in clinical studies
- Devices currently available: Abbott Free Style Navigator Continuous Glucose Monitor, DexCom SEVEN Plus, Medtronic Guardian Real -Time Continuous Glucose Monitoring System, and MiniMed Paradigm Real-Time System
- Specific indications are yet to be established, but may be indicated for patients with unstable diabetes for purpose of improving diabetes management.
- May be useful for patients with type 1 diabetes who use intensive insulin therapy, with or without insulin pump, to help patient recognize fluctuations in glycemia and their causes.
- Also used to evaluate glucose control and in specific clinical situations such as gestational diabetes or intensive care units
- Useful in patients with hypoglycemia unawareness, repeated severe hypoglycemic episodes or undetected hypoglycemia
- JDRF study found that children and adolescents used it less regularly, and with limited use there was no benefit.
- JDRF study suggested more frequent CGM use associated with greater reduction in HbA1c after 6 months (Tamborlane).
- Adults (> 25 years old) with diabetes associated with greater CGM use compared to children and adolescents.
- With regular use, more time within target glucose range 71-180 mg/dL.
- Patients using CGM may spend less time in hypoglycemic and hyperglycemic range, and may have less nocturnal hypoglycemia (Garg).
- Valuable in guiding therapy adjustments: changing mealtime bolus dosage, adjusting basal insulin rate, changing insulin-to-carbohydrate ratio, etc.
- Used to diagnose and prevent postprandial hypoglycemia
- Results are not as accurate as with SMBG. Mean error about 15%.
- Physiological lag between capillary blood glucose data and interstitial fluid sensor data can be as much as 4-10 minutes, depending on rate of glucose change (Boyne).
- Not approved as replacement for SMBG; abnormally high or low reading should prompt SMBG before acting upon CGM result.
- Can have inflammation, slight bleeding or, rarely, infection at glucose sensor insertion site.
- Sensor may dislodge, new sensor must be inserted; sensors needs to be changed every 3-7 days, depending on CGM brand.
- Receiver must be within 5-10 feet of sensor for wireless range.
- No data collected during warm up period (can be between 2 and 10 hours depending on CGM device), required before 1st calibration each time new sensor inserted.
- Calibrations only permitted when blood glucose levels not changing rapidly, so calibrate after overnight fasting or at least 2-3 h postprandially.
- Not good choice for people who are technically challenged, and not adapted for visually impaired.
- Expensive; confirm insurance coverage prior to initiating CGM.
- CGM may enhance management of diabetes in highly motivated people, who are technically capable to incorporate it into personal daily diabetes management.
- Provides complete picture of glycemic control, by increasing number of glucose values available to make appropriate changes to insulin therapy, food intake, and activity in patients with diabetes
- CGM useful for detecting unrecognized hypoglycemia in type 1 and type 2 diabetic subjects
- Alarms may prevent severe, potentially dangerous hypoglycemic events.
- Useful in self-education of motivated patients, showing them what self-care events (insulin doses, diet, exercise) cause highs and lows.
- Valuable in controlling daily fluctuations in blood glucose, which may not be reflected in HbA1c levels.
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