cannula

A cannula with a magnified end.

A cannula (from Latin "little reed"; plural cannulae) is a flexible tube which when inserted into the body is used either to withdraw fluid or insert medication.

Decannulation is the permanent removal of a cannula (extubation)^[1], especially of a tracheostomy cannula.^[2]

Medicine

Several different kinds of cannulae.

Cannulae normally come with a trocar attached which allows puncturing of the body to get into the intended space. Intravenous cannulae are the most common in hospital use. A variety of cannulae are used to establish cardiopulmonary bypass in cardiac surgery. Nasal cannula is a piece of plastic tubing which runs under the nose and is used to administer oxygen.

IV cannulation

A venous cannula is inserted into a vein, primarily for the administration of intravenous fluids and medicines. An arterial cannula is inserted into an artery, commonly the radial artery, and is used during major operations and in critical care areas to measure beat-to-beat blood pressure and to draw repeated blood samples.

Complications may arise in the vein as a result of the cannulation procedure, the 4 main groups of complication are:

• hematoma: a collection of blood. Can result from failure to puncture the vein when the cannula is inserted or when the cannula is removed. Selection of an appropriate vein and gently applying pressure slightly above the insertion point as you remove the cannula may prevent this.
• infiltration: when infusate enters the subcutaneous tissue instead of the vein. To prevent this a cannula with accurate trim distances may be used. It is essential to fix the cannula in place firmly.
• embolism: this can be caused by air, a thrombus, or fragment of a catheter breaking off and entering the venous system. Such things can go on to lodge in an artery, blocking circulation to the corresponding area. To avoid air embolus, make sure that there is no air in the system. To avoid a thromboembolism use a smaller cannula. Avoid the catheter breaking by never reinserting the needle.
• phlebitis: an inflammation of the vein resulting from mechanical or chemical irritation or from an infection. Phlebitis can be avoided by carefully choosing the site for cannulation and by checking the type of infusate you use.

Veterinary use

A cannula is also used in an emergency procedure to relieve pressure and bloating in cattle due most commonly to their accidentally grazing wilted legume or legume-dominant pastures, particularly alfalfa, ladino, and red and white clover^[3].

They are also a component used in the insertion of the Verichip.

Body piercing

Cannulae are used in body piercing when using a standard IV needle (usually between 18GA and 12GA, although may be as large as 0GA, in which case the procedure is known as dermal punching and uses a biopsy punch without a cannula), and for inserting hooks for suspensions.

During piercing, the fistula is created by inserting the needle. The needle is then removed, leaving the cannula in place, which is sometimes trimmed down. The cannula is then removed and sterile jewellery is inserted into the fistula simultaneously, in order to minimise trauma to the fresh fistula caused by insertion of blunt-ended jewellery.

Non-medical use

Cannulae are used in laboratory chemistry to transfer a liquid between flasks without exposing it to the atmosphere. In this case, the cannula can be thought of as a double-ended needle, made of stainless steel or plastic. Larger bores (16-22 gauge) are usually used^[4] to avoid clogging. The sharp ends allow them to penetrate septa easily.

In biological research, a push-pull cannula, which both withdraws and injects fluid, can be used to determine the effect of a certain chemical on a specific cell. The push part of the cannula is fiilled with a physiological solution plus the chemical of interest and is then injected slowly into the local cellular environment of a cell. The pull cannula then draws liquid from the extracellular medium, thus measuring the cellular response to the chemical of interest. This technique is especially used for neuroscience.

In general aviation, a cannula refers to a piece of plastic tubing which runs under the nose and is used to administer oxygen in non-pressurized aircraft flying above 10,000 feet above sea level in Canada and above 12,500 feet above sea level in the United States.

References

1. ^ Children with Tracheostomies Resource Guide, by Marilyn K. Kertoy, page 15 (Google book search)
2. ^ Dorland's Illustrated Medical Dictionary
3. ^ Diseases of the Ruminant Forestomach : Bloat, Merck Veterinary Manual
4. ^ Rob Toreki (1 Dec 2004). Cannulas. The Glassware Gallery. Interactive Learning Paradigms Incorporated.

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