CSII stands for continuous subcutaneous insulin infusion, the medical term for insulin pump therapy. Instead of multiple daily injections, a small programmable device delivers rapid-acting insulin through a tiny tube inserted just under your skin, providing a steady flow throughout the day and extra doses at mealtimes. An estimated 400,000 people in the United States use CSII, and the technology has evolved significantly from simple pumps into systems that can automatically adjust insulin delivery based on real-time glucose readings.
How an Insulin Pump System Works
A CSII system has three core parts: the pump itself, a reservoir that holds the insulin, and an infusion set that connects the two to your body. The pump is typically a small, battery-powered device about the size of a pager. It holds a cartridge or reservoir filled with rapid-acting insulin and uses a tiny motor to push precise amounts through thin tubing to a soft cannula sitting just beneath your skin.
The infusion set is the disposable piece you replace regularly. Most sets use a soft Teflon cannula, available in different lengths (typically 6 mm or 9 mm) to accommodate different body types. The cannula inserts at a 90-degree angle into the fatty tissue of your abdomen, thigh, or upper arm. A spring-loaded insertion device makes the process quick, and you only feel a brief pinch. Once in place, the set stays put with an adhesive patch while the pump clips to your waistband, fits in a pocket, or tucks into a belt pouch.
The pump delivers insulin in two ways. A basal rate provides a slow, continuous trickle of insulin around the clock, mimicking what a healthy pancreas does between meals. When you eat, you tell the pump to deliver a bolus, a larger burst calculated to cover the carbohydrates in your food. You program both through the pump’s interface or, in newer systems, a smartphone app.
Tubed Pumps vs. Patch Pumps
Traditional “durable” pumps connect to the body through an infusion line that can be 30 to 100 cm long. The tubing gives you flexibility in where you wear the device, but it can snag on clothing or doorknobs and is visible under lighter fabrics.
Patch pumps eliminate the tubing entirely. The cannula and delivery mechanism are built into a small pod that sticks directly to your skin. You control it wirelessly from a handheld device or phone, making it more discreet. Patch pumps tend to be smaller and lighter, and many users find them easier to wear during sports or under fitted clothing. The tradeoff is that the entire pod is disposable, which can affect cost, and placement options are somewhat more limited since the device itself sits on your body.
How CSII Compares to Daily Injections
The main alternative to pump therapy is multiple daily injections (MDI), which typically involves one or two shots of long-acting insulin plus rapid-acting insulin before each meal. Both approaches can achieve good blood sugar control, but pumps offer finer adjustments that injections can’t easily replicate.
In clinical studies, people switching from MDI to CSII saw their HbA1c (a measure of average blood sugar over roughly three months) drop from 8.4% to 7.3% after six months. Just as notable, the number of low blood sugar episodes per month fell from about 16 to under 9. That reduction matters because frequent lows are one of the most disruptive parts of living with insulin-dependent diabetes, affecting sleep, driving, and daily confidence.
The practical advantage comes from the pump’s ability to deliver insulin in increments as small as fractions of a unit per hour. If your insulin needs shift overnight, during illness, or around exercise, you can program different basal rates for different times of day rather than relying on a single long-acting injection that can’t be adjusted once it’s given.
Adjusting Insulin for Exercise and Daily Life
One of the most useful pump features is the temporary basal rate. Before a workout, you can reduce your basal delivery to a percentage of normal, say 50%, for a set duration. This helps prevent blood sugar from dropping too low during and after activity. You can also increase it during illness or stress, when blood sugar tends to run higher. On an older-style pump, figuring out the right adjustment takes some trial and error. In newer closed-loop systems, the pump handles much of this automatically.
The ability to disconnect briefly is another everyday convenience. Most tubed pumps let you detach the infusion set at the skin site for showers, swimming, or intimate moments without removing the cannula. You simply reconnect when you’re done. Patch pumps are often waterproof enough to wear in the shower, though policies vary by manufacturer for full submersion.
Integration With Glucose Monitors
The biggest leap in CSII technology has been pairing pumps with continuous glucose monitors (CGMs). A CGM is a small sensor, usually worn on the arm or abdomen, that reads glucose levels in the tissue fluid every few minutes and sends the data wirelessly to the pump or a phone.
When a CGM communicates with an insulin pump through an algorithm, the result is called a hybrid closed-loop system, sometimes referred to as an “artificial pancreas.” The algorithm reviews glucose readings every 5 to 12 minutes (depending on the system), predicts where your blood sugar is heading, and automatically increases or decreases basal insulin delivery. You still enter carbohydrate counts for meals, which is why it’s called “hybrid” rather than fully automatic, but the system handles most of the moment-to-moment adjustments on its own.
Medtronic’s MiniMed 670G, released in 2016, was the first commercially available hybrid closed-loop system. Since then, several manufacturers have introduced competing systems using Bluetooth to connect the CGM sensor, the algorithm (which may run on the pump or on a smartphone), and the pump itself. Clinical trials and real-world data consistently show these systems improve time spent in a healthy glucose range while reducing the mental burden of constant decision-making.
Infusion Site Care and Rotation
The infusion set doesn’t last forever. Manufacturers recommend changing both the set and the insertion site every 48 to 72 hours. Research confirms that trouble starts around day three: skin irritation increases, insulin absorption becomes less reliable, and the risk of unexplained high blood sugar climbs. Infection at the infusion site is one of the most common complications of pump therapy, ranging from mild redness to, in rare cases, abscesses that require medical treatment.
To minimize problems, most diabetes educators recommend rotating among several areas of the body and spacing new sites at least an inch from the previous one. Using the same patch of skin repeatedly can cause the tissue underneath to harden or develop fatty lumps, both of which interfere with insulin absorption. A simple rotation pattern, moving clockwise around the abdomen, for instance, helps keep tissue healthy.
Risks to Be Aware Of
Because CSII uses only rapid-acting insulin with no long-acting backup, any interruption in delivery can cause blood sugar to rise quickly. A kinked cannula, an empty reservoir, a dislodged infusion set, or a dead battery can all stop insulin flow. Without that steady basal delivery, people with type 1 diabetes can develop dangerously high blood sugar within hours, and in some cases, this can progress to diabetic ketoacidosis (DKA), a serious condition requiring emergency care.
This is why pump users are taught to check their blood sugar if they feel “off,” carry backup insulin and syringes, and respond promptly to pump alarms. Modern pumps have multiple built-in alerts for occlusions, low reservoirs, and expired infusion sets, but no alarm system replaces awareness of how your body feels.
Who Uses CSII
CSII is most commonly prescribed for people with type 1 diabetes, but it’s also used by some people with type 2 diabetes who require intensive insulin therapy. Good candidates are generally those who are willing to check blood sugar frequently (or wear a CGM), count carbohydrates, and learn how to troubleshoot the device. Children, adolescents, and pregnant women with diabetes also use pump therapy, often because it allows more precise dosing than injections.
The learning curve is real. Starting on a pump involves training sessions with a diabetes educator, an adjustment period of several weeks to fine-tune basal rates and bolus ratios, and a willingness to wear a device on your body around the clock. For many people, that trade-off is worth it for steadier blood sugar, fewer lows, and the flexibility to eat, exercise, and sleep on a less rigid schedule.