The glycemic index is an important consideration. This 0-to-100–point scale is used to indicate how quickly specific foods raise blood sugar. Foods higher on the index (such as white potatoes and fruit juice) raise blood sugar more quickly than do foods that are lower on the index (whole grains, milk, nuts, and seeds).23 The caveat to using the glycemic index is that foods are often consumed as part of a mixed meal of proteins, fats, and carbohydrates—each of which will have a different effect on how rapidly blood sugars rise. Since fat slows digestion, meals that are high in both fats and carbohydrates can cause extended elevations in blood glucose.
To address this concern, smart pumps are equipped with an extended bolus feature that allows delivery of the bolus over a preset period of time.24 A percentage of the bolus may be delivered immediately and the remainder over time (dual-wave bolus). A typical split may be 50% now and 50% over the next two hours. This takes into account not only carbohydrate intake, but the components of a meal high in carbs and fat, such as pizza.
Several factors can impact the accuracy with which the patient calculates boluses to lower blood sugars for meals and other times of elevated blood glucose. Optimally, the total amount of correction insulin should represent only a small portion of the total daily bolus amount if the patient is being proactive. Ask patients about their carbohydrate-counting. Are they counting at a high skill level? Is their insulin-to-carbohydrate ratio correct? What about their inputs into their bolus calculator? Are their targets set too high or too low? Is their sensitivity correct? Is their active insulin time correct?
To assess postprandial control, patients should be instructed to check their blood glucose two hours after they started to ingest each meal.15 Consistent elevations mean that an adjustment to the insulin-to-carbohydrate ratio is indicated; alternatively, the patient may choose to consume fewer carbohydrates if weight is a concern.
If patients are not consistently measuring their foods (a discipline that is important but challenging to maintain), portion creep may be occurring. Encourage the patient to start measuring food again, at least for a while. A refresher visit to the nutritionist or diabetes educator may also be helpful. If patients are eating out often, they should be encouraged to limit themselves to meals with a known carbohydrate content.
Response to various foods is often idiosyncratic. Having patients keep a detailed log of foods eaten, exercise performed, and corresponding blood glucose levels will often clear up mysterious blood sugar elevations.
Correction Insulin
Depending on their personal approach and regimen, patients are taught to monitor their glucose either prior to eating, after eating, or both; and to take insulin to achieve a correction if the glucose is outside their target zone. A typical preprandial target range is 80 to 120 mg/dL, with 140 to 180 mg/dL two hours postprandial. (The American Association of Clinical Endocrinologists, while emphasizing the importance of individualizing glucose targets, suggests a fasting plasma glucose level below 110 mg/dL and a two-hour postprandial concentration below 140 mg/dL to achieve a target A1C level ≤ 6.5% in the nonpregnant adult.16 The American Diabetes Association’s comparable recommendations are 70 to 130 mg/dL preprandial and 180 mg/dL peak postprandial.15)
In today’s pumps, as part of the bolus calculation, a number can be programmed that represents the number of points in mg/dL by which one unit of insulin will reduce blood sugars. This may be known as a sensitivity or correction factor.
For example, a pumper who is making a postprandial correction and is not eating has a target blood sugar between 140 and 180 mg/dL and a sensitivity of 40; at present, his blood sugar is 230 mg/dL. Assuming no active insulin, the pump would calculate a dose of about 1.25 units of rapid-acting insulin.
How active insulin, or insulin on board, is calculated differs among pump manufacturers but is loosely based on original insulin pharmacodynamic studies.15,22 In smart pumps, the bolus calculator takes into consideration how much insulin is still active when it recommends a correction dose for hyperglycemia. This allows the user to address hyperglycemia quickly and effectively but prevents “stacking” of insulin doses and subsequent hypoglycemia.18
The active insulin time is preset in some pumps, but many diabetes clinicians use three or four hours (anecdotal evidence). The pumper can test the validity of active insulin by administering a correction bolus when blood glucose exceeds 250 mg/dL, then checking blood glucose levels every 30 to 60 minutes for six hours with no further food intake.9 The length of time it takes for blood sugars to return to target and remain steady is determined to be the correct duration of insulin action. In the example above, if the pump determined that there was active insulin remaining from the prior meal, it would subtract that amount from the calculation to prevent insulin stacking—which would most likely lead to overcorrection and hypoglycemia.