What is an insulin pump and how does it work?

What is an Insulin Pump and How Does It Work

An insulin pump is a small, battery-operated, programmable device that delivers rapid-acting insulin continuously throughout the day via subcutaneous infusion, mimicking the body's natural insulin secretion through both basal (continuous background) and bolus (mealtime) dosing. 1

Device Components and Physical Design

The insulin pump system consists of several key components that work together to deliver insulin:

• The pump body contains the insulin reservoir (typically holding 200-300 units), battery, and programmable computer that controls insulin delivery 2
• Traditional pumps use flexible plastic tubing connected to a subcutaneous cannula (metal or plastic) that is inserted under the skin and changed every 2-3 days 1
• Patch pumps (tubeless systems) attach directly to the skin with an integrated cannula and are controlled by a separate handheld device via Bluetooth 1
• Only rapid-acting insulin analogs (Novorapid, Humalog, Apidra) are used in pumps, never long-acting insulin 1, 3

How Insulin Delivery Works

Basal Insulin Infusion

Basal insulin is delivered as a continuous programmed infusion 24 hours per day to maintain blood glucose levels during fasting and prevent ketosis. 1, 3

• The basal rate typically accounts for approximately 50% of total daily insulin requirements, though this varies by individual 1, 4
• The hourly infusion rate is calculated by taking 50% of total daily insulin dose and dividing by 24 hours 4
• Basal rates can be programmed to vary throughout the day to account for phenomena like the dawn effect (early morning glucose rise from counter-regulatory hormones like cortisol and growth hormone) 1
• Different basal rate programs can be set for varying activity levels, such as weekdays versus weekends 1, 4

Bolus Insulin Dosing

Bolus doses are administered at mealtimes to cover carbohydrate intake and correct high blood glucose levels. 1, 3

• The user inputs their current blood glucose level (from fingerstick testing) and the amount of carbohydrates to be consumed 1

• The pump's onboard calculator determines the appropriate insulin dose based on:

  • Insulin-to-carbohydrate ratio (how many grams of carbohydrate are covered by one unit of insulin) 1
  • Insulin sensitivity factor/correction factor (how much one unit of insulin lowers blood glucose, typically 3 mmol/L or ~50 mg/dL) 1
  • "Insulin on board" memory that tracks active insulin from previous doses to prevent "stacking" and hypoglycemia 1

• The user can accept the calculated dose or override it based on anticipated variations in insulin sensitivity (e.g., during exercise or stress) 1

Advanced Pump Systems

Sensor-Augmented Pumps

These partial closed-loop systems integrate three components: an insulin pump, continuous glucose monitor (CGM), and an algorithm that automatically suspends insulin when glucose is low or predicted to drop within 30 minutes. 1

• The ASPIRE trial demonstrated that sensor-augmented pumps with low-glucose suspend significantly reduced nocturnal hypoglycemia without increasing A1C levels 1
• Predictive low-glucose suspend reduced time with glucose <70 mg/dL from 3.6% to 2.6% without rebound hyperglycemia 1

Automated Insulin Delivery (AID) Systems

AID systems represent the most advanced technology, automatically adjusting insulin delivery in real-time based on CGM readings to mimic physiologic insulin secretion. 1

• All current AID systems adjust basal delivery continuously, and some deliver automatic correction doses 1
• The algorithm can be hosted in the pump body, insulin pod, or phone app depending on the system 1
• Insulin delivery is modulated by increasing, decreasing, or pausing insulin based on CGM feedback and predicted glucose trends 1
• AID systems are preferred over non-automated pumps and multiple daily injections in people with type 1 diabetes 1, 2
• Users still must manually enter carbohydrates consumed or announce meals, and most systems require activity announcements 1

Clinical Benefits

Pump therapy provides modest but meaningful advantages over multiple daily injections, with A1C reductions of 0.30% and reduced severe hypoglycemia rates in both children and adults. 1

• Real-world data show benefits in A1C levels, hypoglycemia reduction, and total daily insulin dose reduction 1
• Pumps offer greater flexibility for lifestyle variations and more precise insulin dosing 3
• Treatment satisfaction and quality of life measures improve with pump therapy 1

Critical Safety Considerations

Because pumps use only rapid-acting insulin, disconnection or pump failure creates relative insulin deficiency within 1 hour and absolute deficiency within 4 hours, with severe risk of hyperglycemia and ketoacidosis. 1, 3

• Patients must always carry backup rapid-acting insulin syringes or pens 2
• Regular glucose monitoring is essential, especially when changing settings 3
• Potential risks include mechanical problems, infusion site infections, hypoglycemia, and lipodystrophy 5
• If glucose exceeds 14 mmol/L (252 mg/dL), check the pump and infusion set and perform ketone testing 3

Patient Selection Requirements

Before starting pump therapy, patients must demonstrate carbohydrate counting proficiency, frequent glucose monitoring capability, technical aptitude, and high motivation. 2

• Pumps are approved for type 1 diabetes and selected patients with type 2 diabetes 1, 6
• The American Diabetes Association recommends considering pump therapy for adults and youth with type 2 diabetes on multiple daily injections who can safely manage the device 2
• Close monitoring is necessary during the first 3 months with daily contact to adjust settings 2
• Socioeconomic barriers including race/ethnicity, insurance status, and income should be addressed upfront 2