Insulin pens were first introduced in the 1980s, in an attempt to make insulin delivery more convenient and possibly less fear-inducing. By definition they contain some form of insulin, although there are also pens prefilled with pramlintide to administer along with insulin. Pens tend to be preferred by patients when compared to vials and syringes.
Insulin pens may also be more accurate. However, pens are devices and must be used appropriately. Some require reusable cartridges, others are prefilled. Each time a new needle is attached; it must be primed with a flush of two to four units of insulin so that a full dose is delivered. Occasionally pens malfunction and do not deliver the desired amount of insulin. If a patient using an insulin pen has unexplained high blood sugars, the use of a new pen and reinforcement of the need for priming should be considered. The patient should see a stream of insulin flow from the tip of the pen needle before assuming the pen is ready for use. All patients should know how to use a vial and syringe, in case pens are not available. Most insulin preparations are now available in pen form.
Continuous Glucose Monitoring
Short of a cure for type 1 diabetes, technology that can continuously monitor blood glucose levels, particularly if coupled to a pump to create a closed loop system, has long been sought. Initial, less successful attempts at continuous glucose monitoring included a 3-day Minimed Continuous Glucose Monitoring System (CGMS) and the GlucoWatch G2 Biographer. The former did not provide real-time data to patients and was not widely used, although it did provide a 3-day retrospective report of blood glucose levels and trends. The GlucoWatch Biographer was a large wristwatch-like device that drew up interstitial fluid through the skin and measured glucose levels every 10 min for up to 13 h. Alarms alerted the user to high, low, and falling glucose levels. Neither device was reliable for detecting hypoglycemia and the GlucoWatch had a high rate of false alarms.
It did not improve control beyond what is possible with standard SMBG. Current devices have been shown to be more useful. The Medtronic Minimed Continuous Glucose Monitor (CGM), the Dexcom STS, and the Navigator (not FDA approved at the time of this writing) provide nearcontinuous monitoring of interstitial glucose levels every 1–5 min and a continuous readout of glucose values and trends. These devices can be set to alarm when reaching a high or low targets as well as when glucose levels are rising or falling rapidly. However, because they read interstitial fluid rather than blood glucose concentration, there is a physiologic lag between meter reading and the corresponding blood glucose levels. This lag is greatest during periods of rapid glucose change and can be up to 20 min. Due to this lag, and the lack of large clinical trials assessing the accuracy of these devices in clinical use, CGM systems are approved for adjunctive use, not as replacement for SMBG. Traditional models for assessing the accuracy of single point-in-time blood glucose testing, such as the traditional Clarke error grid, do not accurately assess the functionality of CGM because they do not factor in the benefits of knowing the direction and rate of glucose change.
Therefore, the continuous glucose-error grid analysis (CG-EGA) was developed to measure these dynamic properties. The CG-EGA works by analyzing pairs of reference and sensor readings as a process in time, with point and rate comparisons combined into a single accuracy analysis for each of three blood glucose ranges hypoglycemia (<70 mg/dl), euglycemia (70–180 mg/dl), and hyperglycemia (>180 mg/dl). Results are then plotted on a grid subdivided into five zones representing the clinical outcome that might occur if the patient acted on the CGM data. Values in zones A and B represent accurate or benign error readings; values in zone C may result in unnecessary corrections; values in zone D signify a dangerous failure to detect and treat; and values in zone E denote erroneous treatment. In a study to assess the accuracy of the FreeStyle Navigator CGM 58 subjects with type 1 diabetes wore two sensors simultaneously and had frequent venous measurements taken while residing for 5 days in a clinical research center.
Comparison of the FreeStyle Navigator measurements with the laboratory reference method gave mean and median absolute relative differences (ARDs) of 12.8 and 9.3%, respectively. The percentage in the Clarke error grid zone A was 81.7% and in zone B was 16.7%. Readings were similar on days 1 and 5 of sensor use. Sensors last from 3 to 7 days, with the Dexcom system marketing the longest-lasting sensor.Each system is somewhat different and requires training on its features. In all there is a physiologic lag between the interstitial and the capillary glucose levels, which is most pronounced when the blood sugar levels are falling or increasing rapidly. Calibration of the CGM system is best done during periods of relative glucose stability. Small studies have been done to assess the utility of CGM. These studies produced largely positive results, although changes in HbA1c levels were small. In a small study (n = 24) of adults with type 1 diabetes over 3 months there was a significant decrease in HbA1c level of 0.4 ± 0.5% from baseline (starting HbA1c = 7.43 ± 1.0%). This difference was significant when compared to a control group who did not wear the CGM. Although there was a difference in the time spent within the target range, there was not a significant reduction in rates of hypoglycemia. In a larger home use study, 140 adults with diabetes were followed in a 12-week observational study using the Dexcom 3-day CGM device.
Overall, a reduction in HbA1c of 0.4 ± 0.05% (least squares mean ± SE, P < 0.0001) was found. The greatest HbA1c reductions were found in patients with the highest initial HbA1c values as well as in those who used the CGM device most frequently. A multicenter study evaluated the safety and effectiveness of the 7-day Dexcom CGM in 86 subjects. Subjects wore a sensor for 7 days during each of three consecutive periods. During the first period subjects were blinded to the data and were unblinded for the second two periods. Overall, 97.2% of the 6811 matched (CGM to SMBG) data points fell in the Clarke error grid zones A and B, with an 11% median absolute relative difference. After unblinding, subjects reduced the time spent in hypoglycemic range (<55 mg/dl) by 0.3 h/day, reduced the time spent in hyperglycemic range (>240 mg/dl) by 1.5 h/day, and increased time in the target zone (81–140 mg/dl) by 1.4 h/day. This study was not designed to assess the change in HbA1c levels. In another randomized controlled study of insulin-requiring patients using the DexCom STS sensor, those given access to continuous glucose readings and alerts/alarms managed episodes of hyperglycemia and hypoglycemia more effectively than control subjects blinded to this information. On average, the unblinded group spent 21% (P < 0.0001) less time in hypoglycemic (<55 mg/dl), 23% (P < 0.0001) less time in hyperglycemic (>240 mg/dl), and 26% (P < 0.0001) more time in the target glycemic range (81–140 mg/dl). The application of the CGM systems to help people with type 1 diabetes when performing exercise or doing other strenuous activity has been studied.
In one study, patients with type 1 diabetes performing vigorous physical activity were studied with a Guardian RT CGM. Five subjects with type 1 diabetes were monitored before, during, and after a 60-min vigorous spinning class using the Guardian RT CGM. Three of the subjects were found to have late-onset hypoglycemia which was identified with the CGM technique, suggesting that CGM could be a useful tool in monitoring the response to exercise. CGM has been found to be helpful in marathon runners. Another application for CGM has been to assess glycemic control following islet cell transplantation. In a study of eight subjects with type 1 diabetes following islet cell transplantation, CGM revealed that less glycemic variability and hypoglycemia were present in C-peptide-positive patients after transplantation. Many of the studies on CGM have been performed in children. DirecNet is a consortium of investigators who are examining the utility of CGM and other treatment modalities in children with type 1 diabetes. DirecNet studies should help define the benefits and limitations of CGM in this population. In summary, the benefits of CGM could be multiple, although large clinical trials have not been performed to prove these benefits.
Potential advantages of CGM include its ability to alarm when patients fall below a certain threshold blood sugar as well as when the blood sugar is trending toward hypoglycemia. This could allow for earlier treatment or even avoidance of hypoglycemia. Real-time data can provide the user with the knowledge of whether glucose levels are rising or falling before a snack or meal and insulin dose and carbohydrate load consumed can be adjusted. High alarms can alert users to significant hyperglycemia. Finally, the stored data can be used for retrospective analysis by the health-care provider, with a focus on adjusting basal rates, overnight blood sugars and correction boluses, and carbohydrate ratios. Drawbacks include the high cost, the inaccuracies of measurement, the risk for insulin stacking (giving insulin too often when the blood sugar level is high), and technical difficulties with the devices.
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