Insulin Pump Therapy: What You Need to Know

The pump's continuous delivery allows precise glycemic control after meals and during exercise and eliminates the need for long-acting forms of insulin, which has several advantages, the authors say. They explain how pumps work, how to instruct the patient, and what complications to watch for.

By Jeff Unger, MD, and Alan O. Marcus, MD

Insulin pump therapy, or continuous subcutaneous insulin infusion, is designed to simulate normal pancreatic beta-cell function and deliver both basal and bolus insulin doses in patients with type 1 diabetes. Currently, there are 17 million Americans who have been diagnosed as having diabetes. On average, diabetes-related end-stage complications ultimately kill one American every three minutes. The cost of managing diabetes in this country exceeds $105 billion, with over 60% of that total allotted to caring for diabetes complications such as heart disease, stroke, blindness, amputations, and renal failure. Annual health care costs incurred by patients with diabetes average $10,000, compared to $2500 for people who do not have diabetes.

The number of Americans with diabetes is growing at an astronomical rate, increasing 33% in the past eight years, from a prevalence rate of 4.9% of the population to 6.5%. The fastest-growing diabetes population is the 33- to 39-year-old age group. The debate over the importance of achieving good glycemic control has been decided in favor of near-normal blood glucose levels, according to the Diabetes Control and Complications Trial (DCCT) published in 1993 and the United Kingdom Prospective Diabetes Study (UKPDS) published in 1998. The DCCT demonstrated that improved glycemic control can reduce the incidence of microvascular complications, such as retinopathy, neuropathy, and nephropathy, in patients with type 1 diabetes by 60%. At the conclusion of the DCCT, 42% of the intensively managed patients were using insulin pump therapy and 56% were on multiple daily injections. Patients in the DCCT who were on insulin pump therapy had a 0.3% lower hemoglobin A1c compared to the intensively managed individuals using multiple daily injections.

The UKPDS found that in patients with type 2 diabetes, glycemic control will eventually deteriorate due to pancreatic beta-cell failure. In time, these patients will find that using multiple oral agents will not allow them to maintain the targeted hemoglobin A1c level of less than 7%. Currently, only 40% of patients with type 2 diabetes use insulin therapy. A greater number could certainly benefit from intensive insulin management to ameliorate symptoms and decrease the frequency and severity of long-term diabetes complications.

Almost any patient on insulin therapy is an insulin pump candidate. More than 120,000 patients in the United States are now on insulin pump therapy. Approximately 90% of all patients with diabetes are managed by primary care physicians, many of whom are becoming adept at intensive insulin management using pump therapy.

In this article, we will review the protocols involved in insulin therapy, the advantages of pump therapy, precautions that need to be taken with certain patients, and the key mechanisms in the pumps themselves.

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Basal and Bolus Insulin

Any intensive insulin therapy program consists of two different forms of drug delivery: basal insulin and bolus insulin. Basal insulin is given in the form of NPH, ultralente, or glargine insulin. Basal insulin is provided to hold down fasting plasma glucose levels. In the fasting state, the liver breaks down glycogen, which is converted to glucose and released into the portal circulation, thus increasing blood glucose levels. The purpose of hepatic glucose production is to maintain adequate blood glucose levels for proper brain function.

Bolus insulin (in the form of regular insulin, insulin lispro, or insulin aspart) blunts the rise of postprandial glucose levels. Several studies have demonstrated that consistently elevated postprandial glucose levels can have a deleterious effect on endothelial function while at the same time increasing the risk of developing coronary artery disease.

Patients on multiple daily injections of insulin may need to mix basal and bolus insulins. Caution is required with NPH insulin because it is a cloudy solution that must be shaken thoroughly before being injected. Failure to do so can result in inconsistent insulin doses and significant variations in the insulin's effect on glucose levels. This form of insulin has a day-to-day absorption variance of 53%, which accounts for its unpredictability with regard to onset of action, duration of action, and peak activity level. For this reason, patients often experience difficulty in planning exercise and strenuous activity. Hypoglycemia may occur if exercise is performed as the insulin activity level peaks. Also, skipping a midday meal may result in hypoglycemia if NPH is injected before breakfast because its peak activity level occurs three to eight hours after injection.

Glargine insulin has a more predictable absorption profile than NPH. However, glargine is clear and cannot be mixed with any bolus insulin. Therefore, patients using this type of insulin will have to give themselves a separate injection of short-acting basal insulin if both insulins are used before dinner. Most patients inject glargine at bedtime and use an insulin bolus before meals. In addition, once glargine is injected, its rate of absorption cannot be varied. Thus, patients who awaken with elevated fasting blood glucose levels due to the so-called dawn phenomenon may not be able to normalize their morning glucose levels on glargine. Increasing the glargine dose may result in diurnal hypoglycemia because insulin resistance decreases in the afternoon hours.

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Advantages of Pump Therapy

Insulin pumps use only a single type of insulin—either regular, velosulin, aspart, or lispro. These short-acting insulins have a much more efficient 3% day-to-day absorption variance than the basal insulin types. Unlike with multiple daily injections, pump therapy uses a single infusion site, usually in the abdomen. Multiple injection sites can result in unpredictable absorption during exercise and increase the risk of hypoglycemia.

Because the absorption of insulin from a pump site is so efficient and predictable, patients can decrease their daily insulin dose by 10% to 20%. Also, insulin is a growth hormone that can lead to weight gain. Improved insulin absorption and lower dosing can minimize weight gain and may even result in weight loss in pump users.

Patients who are intensively managed with multiple daily insulin injections are at increased risk for becoming hypoglycemic. In the DCCT, severe hypoglycemia, defined as a hypoglycemic episode requiring the assistance of another person, increased threefold in the intensive therapy group compared to the conventional group, which consisted of patients administering only one or two injections daily. Of the severe hypoglycemic episodes, 65% occurred during sleep and 35% occurred while patients were awake. Approximately 14% of all diabetes-related deaths in Europe are due to the "dead in bed" syndrome, where patients become severely hypoglycemic while asleep.

Patients who have had type 1 diabetes for more than five years often lose their counterregulatory mechanism for identifying and metabolically reversing hypoglycemia. These patients commonly develop hypoglycemic unawareness; they no longer recognize the common symptoms of low blood glucose levels, such as fatigue, sweating, blurred vision, dizziness, palpitations, and impaired cognition. Activities such as driving, exercising, and caring for children may be difficult and dangerous for these patients. (Common predictable causes of hypoglycemia in patients with type 1 diabetes are listed in the box below.)

One strategy for preventing hypoglycemia is to set a higher target blood glucose level. This is most easily accomplished with an insulin pump, using a lower basal insulin delivery rate and eliminating the wide glycemic swings that can occur with multiple daily injections. Blood glucose levels will be higher, and the patient will experience fewer episodes of hypoglycemia. For example, blood glucose levels can be targeted in the range of 120 to 180 mg/dl instead of 70 to 130 mg/dl. After six to eight weeks, the basal rate of the insulin pump can be increased, allowing the physician to lower the target blood glucose range so that it is closer to normal. However, hypoglycemic unawareness can recur in many patients using this technique.

Diabetic patients with gastroparesis, or delayed gastric emptying, present a unique challenge. Due to recurrent mismatches between the absorption of injectable insulin and carbohydrates from the gastrointestinal tract, patients often experience immediate postprandial hypoglycemia and delayed postprandial hyperglycemia. Attempting to manage gastroparesis with multiple daily insulin injections can be frustrating and risky. Some insulin pumps, however, allow the patient to give a mealtime bolus dose over several hours, rather than all at once. This reduces the incidence of wide postprandial glycemic swings by eliminating the mismatch between the insulin needed for carbohydrate metabolism and the insulin actually delivered by injection. Improving postprandial glucose excursions can result in clinical improvement of gastroparesis in six to eight weeks.

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Improving Postprandial Glycemic Control

The importance of improving postprandial glycemic control cannot be overstated. The risk of macrovascular diabetes-related complications, such as stroke, angina, and acute myocardial infarction, appears to be more dependent on postprandial glucose levels than on fasting glycemic control. Even among patients with type 1 diabetes who are able to maintain hemoglobin A1c below 7%, as recommended by the American Diabetes Association, about 39% will have significant postprandial hyperglycemia.

Insulin pumps allow several strategies for managing postprandial hyperglycemia. Before eating, patients can estimate the amount of insulin they need to cover their consumption of carbohydrates. ("Carb-counting," they call it.) The insulin is then given as a bolus via the pump. If a small amount of carbohydrates will be consumed, the insulin can be given as a normal or immediate bolus. A square-wave bolus (or extended bolus) allows a patient to give a single dose of insulin over a prolonged period of time. Square-wave boluses are helpful when foods that are high in both fat and carbohydrates are being consumed because the fat delays carbohydrate absorption. Foods such as pizza and ice cream, for example, can be eaten with a square-wave bolus given over two to four hours, thus avoiding erratic postprandial blood glucose levels.

A dual-wave bolus (or combination bolus) allows patients to use a normal and square-wave bolus at the same time. Normally, 50% of the insulin dose is given immediately at the start of a meal and the rest is given later as a square-wave bolus over two to four hours. The dual-wave bolus is the most effective way to deliver insulin postprandially because it simulates the predictable first- and second-phase insulin release that occurs in nondiabetic individuals.

Patients on insulin pumps should be encouraged to check their blood glucose levels two to four hours after eating. If their levels are above the target range (more than 160 mg/dl in most patients), an immediate correction bolus can be given. If the pump is in the process of delivering a postprandial square-wave bolus, the patient can deliver a correction bolus (an active square-wave bolus) at the same time. Most patients try to attain a blood glucose level of 150 mg/dl with their correction boluses to avoid overshooting the target and delivering too much insulin.

Patients should also check their glucose level before eating a meal. If it is high, the patient can add more insulin to the normal bolus, still keeping the square-wave bolus for later. For example, if the level before lunch is 210 mg/dl and the patient knows his meal will require 10 units of insulin, eight units can be given as a normal bolus and five units as a square-wave bolus given over two hours. (The different forms of pump boluses are illustrated in the box below.)

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Precautions with Pregnant Patients

Diabetic women who are contemplating pregnancy should attempt to improve their glycemic control as part of their pre-conception planning program. Pregnant patients should maintain fasting blood glucose levels of 70 to 110 mg/dl and two-hour postprandial levels below 130 mg/dl. Hyperglycemia in pregnancy can result in congenital abnormalities. Fetal mortality associated with diabetic ketoacidosis (DKA) approaches 50%. The patient's basal and bolus rates will need to be updated frequently because of the accelerated catabolism, increasing insulin demand, and intensified production of counterregulatory hormones with anti-insulin action during pregnancy.

For pregnant patients on insulin pump therapy, special precautions and protocols must be followed to prevent DKA. Due to the high rate of fetal mortality associated with DKA, a small bedtime dose of NPH insulin (0.2 U/kg) should be injected in case the insulin infusion via the pump is interrupted during sleep. This small dose of injected insulin will not result in nocturnal hypoglycemia.

A regular exercise program may help improve glycemic control and quality of life in nearly all patients with diabetes. The effect of exercise on glycemia depends on several factors: the intensity and duration of the exercise program; the patient's physical condition; pre-exercise carbohydrate consumption; the dosage, timing, and type of pre-meal insulin used; and the number of years the patient has had diabetes. To avoid exercise-induced hypoglycemia, the pre-exercise target blood glucose range should be 120 to 180 mg/dl. Exercising with a glucose level above 250 mg/dl can result in DKA because circulating insulin levels will be low while hepatic glucose production accelerates to meet increased energy requirements.

Patients on insulin pumps have several options when it comes to exercise. The basal insulin rate can be lowered during exercise by initiating a temporary basal rate. If a patient's normal basal rate is 1 U/hr, it can be lowered to 0.5 U/hr during the exercise period. If patients are contemplating a high-intensity form of exercise, the insulin infusion may be interrupted for up to two hours. Their blood glucose level should be monitored at 30-minute intervals while the insulin infusion is stopped.

Some pumps are waterproof and immersible to a depth of eight feet, allowing patients to swim while wearing the pump. The pump can also be disconnected during exercise. Because patients have so many options, exercising while on insulin pump therapy is much less complicated than attempting to deal with the unpredictability of NPH absorption.

One way to improve compliance with treatment for type 1 diabetes is to recognize the importance of issues related to quality of life. Patients who use multiple daily injections of insulin are required to play by the rules, so to speak, of such therapy. Three meals must be eaten daily, all at the same times. Sleep and wake-up times must be standardized because eight hours after NPH is injected, glucose levels will begin to rise. If a patient sleeps in on weekends, blood glucose levels will be very high, resulting in acute insulin resistance that may take six to eight hours to resolve using high-dose short-acting insulin.

Multiple daily injections pose other problems. Shift workers and business travelers can be difficult to manage on four injections of insulin daily. Carrying around syringes, needles, and insulin pen injectors or vials can complicate a daily insulin regimen and hinder compliance. Women have a difficult time controlling their blood glucose levels during menstruation. Insulin pump therapy simplifies treatment regimens, allowing patients the flexibility that promotes compliance and improves quality of life. Shift workers, for example, can maintain good glycemic control on days when they work a graveyard shift, and by changing the basal rate on the pump they can have equally good control on their days off. During menstruation, women can shift to a different basal rate and improve glycemic control.

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How Insulin Pumps Work

The first insulin pump, introduced in 1974, was the size of a large backpack and filled with metal receptacles and tubing. Today, three different types of insulin pumps are available in the United States: Medtronic MiniMed, Animus, and Disetronic. (Deltec pumps will be available later this year.) These pumps weigh less than four ounces and are the size of a beeper (see photo, below).

 
Insulin pumps use a flexible infusion set with catheter lengths of 24 or 42 inches. Several different types of infusion catheters are available. Most patients prefer 24-gauge Sof-sets (see photo, left) or Silhouettes to the 27-gauge bent needle. The infusion site—either the abdomen, hip, buttock, or leg—is prepared with an intravenous prep pad; the catheter is inserted using an autoinsertion device (see photo, below). The pump contains an insulin-filled syringe, which must be changed with each new infusion set every two or three days. Infusion catheters have a quick-release feature that allows patients to detach the catheter from the pump's insulin reservoir for bathing, exercise, or sexual activity.

The pump is powered by batteries that last three to six weeks, depending on the amount of insulin the patient is using. Each pump comes with several alarms to prevent inadvertent insulin delivery and to warn the patient when the batteries are running low or when the infusion set has become clogged or dysfunctional. Patients are trained to troubleshoot their pump's performance and make the necessary corrections. They are also advised to call an 800 help number inscribed on the back of the pump for further assistance.

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Establishing the Pump's Parameters

It is the clinician's responsibility to establish the pump's working parameters. The first step is to determine the approximate total daily dose (TDD) of insulin that the patient will need. This can be done by taking the patient's current weight in kilograms and multiplying by 0.7. Thus, a 70-kg man would require a TDD of 49 units over a 24-hour period. Pumps work on the basal/bolus system, in which 50% of the TDD is given as basal insulin and 50% by the patient via pre-meal bolusing. A patient with a TDD of 49 units would therefore be allotted 24.5 units (rounded off to 24) per 24 hours, or a 1 U/hr basal rate. If a patient experiences the dawn phenomenon, a second basal rate can be programmed. Although multiple basal rates could be programmed, most patients require only one or two.

Patients can choose to initiate a temporary basal rate, bypassing the rate set by the clinician, for between 30 minutes and 24 hours. This may be useful for treating mild hypoglycemia. Slowing the rate of insulin delivery will allow the blood glucose level to gradually rise. Patients can thus avoid "eating their way out of hypoglycemia," which may result in excessive weight gain as well as rebound hyperglycemia.

Any intensive diabetes treatment plan carries potential risks to patients. Proper education, frequent blood glucose monitoring, and adherence to recommended guidelines are the cornerstones to managing problems unique to patients on pump therapy. The potential complications that insulin pump users must be able to recognize, troubleshoot, and manage are listed in the box below.

Patients on insulin pump therapy should always have access to injectable insulin and syringes in case their pump malfunctions. Assessment of high blood glucose levels in insulin pump patients must always be addressed. Hyperglycemia may imply that insulin delivery is compromised or that an underlying infection may be present.

Some patients should not be placed on insulin pump therapy without extreme caution. This would include individuals with psychiatric illnesses, such as drug and alcohol abuse or eating disorders, as well as those who are unwilling to perform home blood glucose monitoring.

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Role of the Primary Care Physician

Primary care physicians should encourage their patients to intensify their diabetes regimens so that long- and short-term complications can be avoided or delayed. If these physicians can learn to manage such regimens, placing a patient on insulin pump therapy becomes a relatively simple matter. Insulin pump companies offer specialized training to physicians who want to advance their patients to this higher level of diabetes care.

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