What Is an IV Cannula?

An intravenous cannula, or IV cannula, is a small length of tiny, flexible plastic tubing used to administer fluids and liquid medications to a patient through the venous system. The plastic cannula is inserted into a central or peripheral vein using an internal needle, or trocar, which pierces the skin and one side of the blood vessel. When the IV cannula is confirmed to be properly placed within the vein, the internal needle is withdrawn and discarded, and the exterior hub of the cannula is secured to the skin with tape. The IV cannula that remains within the vein is slightly flexible and less apt to re-pierce the vein wall and allow fluids to infiltrate the tissues. The external cannula hub is attached to the distal end of the primary intravenous tubing that, in turn, is connected to the IV bag.

Although they are manufactured by many different companies, IV cannulae gauge sizes are standardized and range from 14 G to 26 G, with the smaller numbers representing cannulae of larger diameters and length. Specific sizes are also standardized by color, so that they can be identified at first glance in an emergency. The size, or gauge, of an IV cannula is an important consideration. The gauge must be large enough to accommodate the amount and rate of fluid anticipated while small enough to be successfully placed within a vein's lumen or diameter. Emergency fluid replacement requires at least a 16 G, while nonsurgical adult patient IVs are started with an 18 G IV cannula and elderly patients with poor veins often can only be fitted with a 22 to 24 G.

There are different placement options for an IV cannula. The vast majority of adult IVs are placed in peripheral veins in the hands and arms and the site is changed every 72 hours to prevent complications such as clots, infection or inflammation. It is considered prudent to begin at the most distal area possible when starting a patient's IV as an unsuccessful start attempt or a discontinued site precludes an IVs later placement lower in the vein. When patients require long-term IV therapy or have very poor peripheral veins, larger cannulae are placed in a central vein — known as central lines, subclavian IVs, peripherally inserted central catheters (PICC) lines or Hickman catheters — where they can remain indefinitely if complications do not arise. Central line catheters are usually stitched into place to avoid disruption and their insertion points are evaluated frequently for infection.