An alternative method of delivering an intensive insulin therapy basal-bolus regimen is by CSII via an external pump. The insulin is delivered through a fine catheter from the pump to a subcutaneous insertion site. The pump delivers insulin as a preprogrammed, variable rate basal infusion as well as patient-directed boluses given before meals or snacks or in response to elevations in BG concentration outside the prespecified target range.
The basal insulin infusion rate for the adult patient with DM1 commonly falls between 0.2 to 1 U/hr, although it can be higher. The basal rate can be programmed either to continue at a constant rate over the 24-hour period or more commonly to increase and decrease at predetermined times of the day to prevent anticipated excursions in the BG concentration, for example, morning rises in glucose associated with the dawn phenomenon. Typically the patient with DM1 will require anywhere from 3 to 4 or more alternative basal rates to enable tight glycemic control. Pumps provide the ability to use multiple alternative basal profiles to deal with recurrent patterns that require adjustment of insulin doses (e.g., menstruation, weekend lifestyle, and a variety of levels of exercise/activity). In addition they offer profiles, such as dual wave bolus or extended bolus to accommodate anticipated variations in eating patterns, e.g., for eating a large or extended multicourse meal.
Only rapid-acting or regular insulin is used in the insulin pump. Adjustments to the basal insulin infusion rate or changes in the size and timing of the insulin boluses generally allow more timely responses in BG concentration than are seen when adjustments are made to doses of intermediate-acting or long-acting insulin. All of these features confer a potentially greater flexibility for the patient in terms of lifestyle and insulin dosing. It has been suggested that use of lispro insulin may lead to a lower risk of hypoglycemia than the other rapid insulin analogs and/or regular insulin in pumps (98,99).
Considerations in the Decision to Start CSII Pump Therapy
There are some practical considerations that the patient and provider must consider together when weighing a decision to use an insulin pump:
• Patients treated with insulin pump therapy must always monitor glucose frequently (four or more times daily) and must always be alert to the possibility of failure of the infusion system, otherwise unexplained hyperglycemia develops.
• The pump insertion set must be changed every 24 to 72 hours to assure uninterrupted insulin delivery and to prevent insertion site infections as described below.
• There is a risk of infection at the subcutaneous insertion site. Infections may occur on average once annually per patient even when best of practices for insertion and site care are used. Such infections are usually minor and can be treated by changing the site of infusion and using a topical antibiotic; a short course of oral antibiotics may also sometimes be required. If an insertion site abscess develops, surgical drainage in conjunction with antibiotic treatment will be necessary.
• Because rapid-acting insulin is most commonly used, pump failure as a result of mechanical malfunction or catheter-related problems can quickly result in severe hyperglycemia with ketoacidosis that will develop in a matter of hours, as mentioned earlier, in the patient with DM1 (100,102).
• The initial cost of an insulin pump itself is high ($4500-6000 in 2007). One must then also purchase pump supplies on an ongoing basis. The relatively recently released patch, OmniPod pump system requires a lesser initial payment for the PDA device that controls Pod functions and programs insulin dosing (~ $600). Single 72-hour use Pod units are then purchased in prospective fashion on a monthly basis for about $35 each, spreading out cost. Most health-insurance payors will cover 80% to 100% of the cost of a CSII pump system and supplies.
• Some patients consider pump to be uncomfortable, embarrassing, or otherwise awkward.
Calculation of Initial Insulin Doses for CSII Pump Therapy
Basal will typically be delivered to meet between 40% and 50% of the patient's total daily insulin requirement. The balance of the daily requirement is given as premeal bolus doses, which will control postprandial glucose excursions. In a patient with reasonably controlled previous MDI injection regimen (e.g., A1c < 7.0%), the initial total daily dose of insulin (TDDI) administered by pump may be 10% to 20% less than the TDD of the previous regimen, as absorption of insulin from the subcutaneous delivery site is more efficient. Conversely, patients with a prior trend to hyperglycemia may start with the same TDD as they had been using with their SC injection regimen. In general, as in the MDI regimens described earlier, approximately one-half of the total daily insulin dose is administered as basal insulin apportioned equally at the time of start up into an hourly delivery rate by dividing the desired total daily basal insulin dose by 24 hours to determine the number of units of insulin to be delivered per hour. For most patients, basal rates are in the range of 0.01 to 0.015 U/kg/hr (i.e., for a 60-kg woman approximately 0.6 to 0.9 U/hr), but they can range from under 0.5 to more than 2.0 U/hour. Premeal boluses for pump initiation may be estimated as follows: 20% for breakfast, 10% for lunch, 15% for dinner, and 5% for bedtime snack, or may be determined by carbohydrate counting and an individualized insulin-to-carbohydrate ratio (103).
The total daily basal rate can alternatively be calculated by multiplying the patient's weight (in kg) by 0.3. Assuming that this basal rate represents 50% of the total daily insulin requirements for the person with DM1, an equivalent number of units of insulin used for basal delivery can be distributed between meal insulin boluses as described for MDI above or again will alternatively be calculated using carbohydrate counting ratios.
Controlled clinical trials have indicated that on an average, intensive insulin regimens that use multiple insulin injections lead to levels of glucose control similar to those achieved with the insulin pump. On the other hand, there are some patients who never achieve adequate control with multiple daily injections but experience dramatic improvements with pump therapy. According to the Clinical Practice Recommendations of the American Diabetes Association (104), the insulin pump should be used only by candidates strongly motivated to improve glucose control and willing to work with their health-care provider in assuming substantial responsibility for their day-to-day diabetes self-management.
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