Cannulation is the process of inserting a thin, flexible tube called a cannula into a blood vessel, most commonly a vein in your arm or hand. It’s one of the most frequently performed procedures in hospitals, used to deliver fluids, medications, blood transfusions, or to draw blood samples without repeated needle sticks. You may also hear it called “getting an IV” or “having a line put in.”
Why Cannulation Is Done
The most common reason for cannulation is to create a reliable access point into your bloodstream. This allows healthcare staff to give you IV fluids for dehydration, administer medications that work faster when delivered directly into a vein, run blood transfusions, or provide anesthesia during surgery. Rather than a fresh needle stick each time you need treatment, a single cannula stays in place and serves as a port for multiple uses.
Arterial cannulation is a more specialized form where a cannula is placed into an artery rather than a vein. This is typically reserved for intensive care or operating rooms, where doctors need continuous, real-time blood pressure readings or frequent blood samples to monitor oxygen and carbon dioxide levels. The radial artery at the wrist is the most commonly used site for arterial lines.
Where the Cannula Goes
For standard IV cannulation, the nondominant arm is preferred because it’s more comfortable and less likely to get bumped or dislodged. The most common sites start at the back of the hand, where the metacarpal veins sit close to the surface, and move up the forearm to larger veins like the cephalic and basilic veins. The inner elbow (antecubital fossa) has easily visible veins and is often used when quick access is needed, though it can be awkward since bending your arm may block the flow.
Ideal veins are straight, feel soft and spongy when pressed, and sit away from branching points where valves can interfere. A vein that feels hard or rigid is likely clotted and won’t work. In some populations, finding a suitable vein takes more effort. This includes young children, people with obesity, pregnant women, and anyone whose veins have been affected by chemotherapy or prior IV drug use. For newborns and infants, scalp veins are sometimes used when limb access isn’t possible.
What Happens During the Procedure
The process typically takes just a few minutes. A tourniquet is tied above the chosen site to make the veins fill and become more visible. You may be asked to make a fist to help plump the vein. The clinician will feel the area with their fingertip, looking for a vein with good size and bounce. Sometimes tapping the skin or applying a warm compress helps veins stand out.
Once a vein is selected, the skin is cleaned with antiseptic in expanding circles outward from the insertion point. A topical numbing agent may be applied, though this isn’t routine for all patients. The cannula device consists of a needle inside a flexible plastic tube. The needle pierces the skin and enters the vein, and once blood appears in the device (confirming correct placement), the plastic tube is slid forward into the vein while the needle is withdrawn. The tube is then secured with a transparent adhesive dressing.
You’ll feel a brief sharp sting as the needle goes through the skin. Once the plastic cannula is in place and the needle removed, it should not hurt. If it continues to be painful, that can signal a problem with placement.
Cannula Sizes and What They Mean
Cannulas come in standardized sizes measured in gauge, and each gauge has a color-coded hub so clinicians can identify them at a glance. Counterintuitively, a higher gauge number means a smaller tube. The size chosen depends on what the cannula needs to deliver.
- 24 gauge (yellow): The smallest option at 0.7 mm, flowing about 20 mL per minute. Used for newborns and patients with very fragile veins.
- 22 gauge (blue): 0.9 mm, flowing around 35 mL per minute. Common for elderly patients, children, and slower infusions.
- 20 gauge (pink): 1.1 mm, delivering about 60 mL per minute. The go-to choice for routine IV fluids and medications.
- 18 gauge (green): 1.3 mm, flowing at roughly 90 mL per minute. Used for blood transfusions and situations needing faster fluid delivery.
- 16 gauge (gray): 1.7 mm at 180 mL per minute. Selected for surgery and rapid fluid replacement.
- 14 gauge (orange): The largest standard size at 2.1 mm, capable of 240 mL per minute. Reserved for trauma and massive fluid resuscitation.
How Long a Cannula Stays In
A peripheral IV cannula is generally replaced or removed within 72 hours of placement to reduce the risk of infection and vein irritation. If the cannula is no longer needed, it’s taken out sooner. Some hospitals follow a “clinically indicated” approach, meaning the cannula stays as long as it’s functioning well and shows no signs of complications, rather than being replaced on a fixed schedule.
While the cannula is in place, it needs regular flushing to prevent blockages. The standard practice is to flush with normal saline before and after any medication or fluid is given. For peripheral cannulas, a 5 mL flush is typical, while larger central lines require at least 10 mL. After blood transfusions or thicker substances, a 20 mL flush is recommended because blood components can deposit along the inside of the tube. The flushing technique matters too: short, pulsing bursts (rather than one steady push) are more effective at clearing the cannula’s interior.
Common Complications
Cannulation is generally safe, but problems do occur. The most frequent complications are local, meaning they happen at or near the insertion site.
Infiltration is one of the most common issues with IV therapy. It happens when the cannula slips out of the vein or punctures through it, causing fluid to leak into the surrounding tissue. Signs include swelling, tightness, and coolness around the insertion site, and you may notice the fluid dripping more slowly or the IV pump alarming frequently. The area can also start leaking fluid visibly. This isn’t dangerous with standard fluids, but it means the cannula needs to come out and be replaced elsewhere.
Phlebitis is inflammation of the vein wall. It shows up as redness, warmth, pain, and swelling along the path of the vein, sometimes with a firm cord you can feel under the skin. It can be caused by the physical presence of the cannula irritating the vein, by certain medications, or by infection. Phlebitis is more likely with larger cannulas, with certain drugs that are harsh on veins, and the longer a cannula stays in place.
Bleeding from the insertion site can occur if the cannula shifts or becomes partially dislodged. This is usually minor and controlled with pressure, but it can be more significant in patients on blood-thinning medications.
Infection is a less common but more serious risk. Bacteria can enter through the insertion site and potentially reach the bloodstream. This is why sterile technique during insertion, regular site inspection, and timely removal or replacement of cannulas are all critical.
What Makes Cannulation Difficult
Not everyone is an easy stick. Dehydration causes veins to flatten and retreat deeper under the skin. Chronic illness, repeated hospital stays, and long-term IV therapy can leave veins scarred and harder to access. Cold temperatures constrict blood vessels, which is why warm compresses or letting your arm hang down can help veins become more prominent before an attempt.
When veins can’t be found through standard methods, clinicians may use ultrasound guidance to visualize veins beneath the skin, or in rare emergency situations, a small surgical incision can be made over a known vein to allow direct access. Children and infants present unique challenges because of their smaller vessel size, which is why specialized training and smaller gauge cannulas exist for pediatric patients.