A cannula is a slender, hollow tube designed for insertion into a body cavity, duct, or vessel to manage the flow of substances. This common medical instrument allows healthcare providers to introduce fluids, medications, or gases into the body, or to remove fluids and collect samples. Cannulas are used across all areas of medical care, making them a fundamental tool in patient management.
Understanding the Components
A typical intravenous (IV) cannula is an integrated system consisting of several distinct parts that facilitate safe and effective placement. The main functional part that remains in the body is the flexible outer tube, often called the catheter or sheath, which is commonly made from biocompatible materials like polyurethane or Teflon. This sheath is responsible for the actual delivery or withdrawal of substances over a period of time.
To enable the initial puncture through the skin and into the target vessel, the sheath surrounds a sharp, inner component known as a stylet or needle. This needle provides the structural rigidity needed for insertion and is withdrawn immediately after the flexible sheath is properly positioned. At the external end of the device is the hub, a flared, color-coded connector that serves as the attachment point for intravenous tubing or syringes.
The size of a cannula is standardized using the French gauge system, which uses an inverse relationship between the gauge number and the diameter of the tube. For example, a lower gauge number, such as 14G, indicates a large-diameter cannula used for rapid fluid administration in trauma cases. Conversely, a higher gauge number, such as 24G, signifies a very fine-diameter cannula, generally used for delicate veins or in pediatric patients.
Categorizing Primary Uses
The most common application involves vascular access, where cannulas are inserted into veins to create an intravenous (IV) line. These peripheral IV cannulas are used to infuse medications, administer essential fluids, or provide blood products directly into the bloodstream. Arterial cannulas, a specialized form, are placed into arteries to allow for continuous, real-time monitoring of blood pressure and for repeated collection of arterial blood samples.
Another significant use is for respiratory support, most often through a nasal cannula. This device is a lightweight plastic tube with two small prongs that rest just inside the nostrils, delivering supplemental oxygen to patients with breathing difficulties. Nasal cannulas are used in various settings, including for patients with chronic lung conditions or those requiring oxygen during surgical recovery.
Cannulas are also indispensable in surgical and drainage procedures, where they operate as conduits for instruments or fluid removal. In minimally invasive surgery, such as laparoscopy, larger cannulas known as trocars establish a stable port through the abdominal wall, allowing surgeons to pass instruments and cameras into the body cavity. Specialized draining cannulas are also used to remove accumulated fluid, such as in the case of abscesses or in complex life-support systems like Extracorporeal Membrane Oxygenation (ECMO).
The Insertion Process and Maintenance
The insertion of a cannula, particularly for IV access, involves multiple carefully controlled steps to ensure patient safety and device longevity. The healthcare provider first cleanses the insertion site with an antiseptic solution, such as chlorhexidine, to minimize the risk of infection. A temporary tourniquet is often applied above the chosen site to engorge the vein, making it more accessible for puncture.
The sharp stylet, with the flexible catheter surrounding it, is then inserted through the skin and into the vessel at a shallow angle, typically between 10 and 30 degrees. Confirmation of successful entry is often provided by a “flashback” of blood appearing in a clear chamber at the hub of the device. Once placement is confirmed, the flexible catheter is advanced fully into the vessel, and the sharp inner needle is withdrawn.
The final placement is secured to the skin with a sterile, transparent dressing to prevent accidental dislodgement and to allow for continuous visual monitoring of the site. Ongoing patient care involves routinely checking the insertion point for any signs of complication, such as localized pain, swelling, redness, or heat, which may indicate phlebitis or infection. The cannula must also be regularly flushed with saline to ensure it remains patent and free from blockages that could impede medication or fluid delivery.