What Is an Outer Cannula?

A cannula is a hollow piece of tubing used for medical purposes. It usually has an inner tube, and an outer tube, called the outer cannula. The outer part protects the inner part from damage and also holds the inner tube open. Cannulas are most often used to hold holes in the body open for access purposes, such as tracheostomy holes in the windpipe which let in air, but they can also be useful as biopsy equipment or as ways to extract or add fluids from an area of the body.

Cannulas are hollow tubes. In medicine, they are inserted into the body in some way. Often, the hollow cannula lets fluid out of a cavity, such as an abscess, or lets fluid in, such as medicine. A common use of a cannula is in a tracheostomy, where a surgeon cuts a hole in the windpipe to allow a patient with a blockage above the hole to breathe properly.

Whatever the cannula is used for, it normally comes in two parts. These are the inner cannula and the outer cannula. The outer cannula is typically the more robust of the two, and the inner cannula fits inside the outer cannula. The outer part is strong enough to prevent the hole inside the body from falling in and squashing the inner cannula. It therefore keeps the hole open and allows air or fluid to move in or out of the hole.

As well as tracheostomies, an outer cannula can find a use as a biopsy needle. Typically the inner cannula has a pointed end, called a trocar, which can cut through tissue. In this case the outer cannula is wide enough to prevent the entire cannula from making too big of a hole in the body.

The portion of the cannula which remains outside the body can also have a locking mechanism on it, which helps to keep the cannula in place. In the case of a tracheostomy, even pressure from a simple cough could otherwise dislodge it. The outer cannula can remain in place but the inner portion of the cannula is often removable, which helps keep it clean and prevent infection. Markings on the outer surface of the cannula can also tell doctors how deep to place the outer portion of the cannula, for optimum security. Although some cannulas are made of metal, many are made of plastic.