A cannula is a thin, flexible tube inserted into the body to deliver or remove fluids, gases, or medications. It’s one of the most common medical devices you’ll encounter in healthcare settings, from a simple oxygen tube looped over your ears to the plastic catheter taped to your hand during a hospital stay. The spelling varies (cannula or canula), but the device is the same.
How a Cannula Works
What makes a cannula different from a needle is its design. A needle is rigid and sharp, meant to puncture. A cannula is a hollow, flexible tube that stays in place after a needle creates the initial opening. In most cases, a sharp needle (sometimes called a trocar or stylet) sits inside the cannula during insertion, then gets removed once the tube is positioned correctly. The soft tube remains behind, letting fluids flow in or out without a sharp point sitting inside your body.
This “catheter-over-needle” approach is what happens when a nurse starts an IV line. You feel the initial needle stick, but what stays in your vein is the flexible plastic cannula. Some procedures use a different method called the Seldinger technique, where a thin guidewire is threaded through the needle first, the needle is removed, and the cannula is then slid over the wire into position. This is common for larger or more precisely placed lines.
IV Cannulas
The type most people encounter is a peripheral IV cannula, the small plastic tube placed into a vein on the back of your hand or inside your forearm. It’s used to deliver medications, fluids, or blood products directly into your bloodstream. These are short, typically a few centimeters long, and can stay in place for several days before needing replacement.
Peripheral IV cannulas are generally safe, but they can cause irritation. Vein inflammation (phlebitis) is the most common complication. Guidelines from the Intravenous Nurses Society put the acceptable rate at 5%, though real-world studies have reported rates ranging from 20% to 80% depending on the setting and how long the cannula stays in place. Signs of a problem include redness, swelling, warmth, or pain around the insertion site.
For patients who need long-term IV access or larger volumes of fluid, doctors use central venous cannulas. These are placed into bigger veins, usually in the neck, chest, or groin, and can remain for weeks or even months.
Nasal Cannulas for Oxygen
A nasal cannula is the lightweight tubing you’ve probably seen looped over someone’s ears with two small prongs resting just inside the nostrils. It delivers supplemental oxygen and is the simplest, most comfortable option for people who need a modest boost in oxygen levels.
A standard nasal cannula delivers oxygen at flow rates of 1 to 6 liters per minute. Each additional liter raises the oxygen concentration you breathe in by about 4%. At 1 liter per minute, you’re getting roughly 24% oxygen (compared to the 21% in normal room air). At 6 liters per minute, that rises to about 44%. For comparison, a face mask can deliver 35% to 55%, and a non-rebreather mask pushes that to 60% to 90%.
High-flow nasal cannulas are a more powerful version used in hospitals for patients with serious breathing difficulties. These deliver humidified oxygen at up to 40 to 60 liters per minute, with oxygen concentrations adjustable all the way up to 100%. Despite the high flow rates, the nasal cannula design lets patients eat, drink, and talk more easily than a face mask would.
Cannulas in Cosmetic Procedures
Blunt-tip microcannulas have become increasingly popular for injecting dermal fillers. Unlike the sharp needles traditionally used, these cannulas have a rounded tip and a small opening on the side rather than the end. A tiny puncture is made first with a regular needle, and the flexible cannula is then threaded under the skin through that single entry point.
The blunt tip pushes blood vessels aside rather than cutting through them. This translates to real differences for patients: less bruising, less swelling, and less pain compared to multiple needle punctures. One study in the Journal of Drugs in Dermatology found that bruising was noticeably more pronounced following injections with a traditional needle. A single entry point also means the practitioner can treat an entire area, like a full lip or cheek, without repeated punctures, which speeds up recovery.
Sharp needles still have advantages in precision for certain techniques, so many practitioners use a combination of both depending on the treatment area and the result they’re after.
Cannulas in Surgery and Critical Care
Larger cannulas play essential roles in major surgical and life-support procedures. During heart surgery or when a patient needs a heart-lung bypass machine, surgeons place cannulas directly into large blood vessels or even the heart itself to reroute blood flow.
In ECMO (extracorporeal membrane oxygenation), a life-support system for patients whose heart or lungs are failing, cannulas are the connection between the patient and the machine. The configuration depends on what organ needs support. Veno-venous ECMO supports lung function by drawing blood out of a large vein, oxygenating it externally, and returning it to the venous system. A specialized dual-channel cannula can handle both tasks through a single vein in the neck. Veno-arterial ECMO supports both the heart and lungs, requiring one cannula in a large vein and another in a large artery so that oxygenated blood returns under arterial pressure. In newborns and small children, surgeons sometimes place cannulas directly into the heart through an open chest.
What Different Sizes Mean
Cannulas are measured in gauge, and the numbering is counterintuitive: a higher gauge number means a smaller tube. A 26-gauge cannula is very fine, used for delicate work or as a “finder” needle to locate a vein. A 14- or 16-gauge cannula is much wider and used when large volumes of fluid or blood need to move quickly, such as in trauma or surgery.
The size matters because it determines flow rate. A large-bore cannula lets blood products or fluids pour in rapidly during an emergency, while a small-bore cannula is perfectly adequate for routine medications or slow IV drips. Your healthcare team chooses the size based on what needs to go through it and how quickly.