A cannula is a medical device defined as a thin tube inserted into the body to create access to a cavity, vessel, or duct. Its purpose is to allow for the delivery or removal of substances like fluids, medication, or blood samples. This device provides reliable, sustained access, allowing medical professionals to administer necessary treatments.
Anatomy and Mechanism
The standard intravenous (IV) cannula system is composed of two main parts: a sharp inner needle and a flexible outer tube, which is the catheter. The inner needle, often called a stylet or trocar, provides the rigidity necessary for the initial puncture of the skin and vessel wall. This stylet fits snugly inside the catheter, which is typically made of flexible plastic like polyurethane or Teflon.
The insertion mechanism is a deliberate two-step process designed to achieve safe placement. First, the entire assembly pierces the skin and enters the target vessel, such as a vein. Once the tip is positioned inside the vessel, often confirmed by a visible “flashback” of blood, the sharp stylet is carefully withdrawn. This leaves only the flexible catheter tube remaining inside the patient’s body. The catheter end connects to a hub, allowing connection to IV lines or syringes for continuous or intermittent access.
How a Cannula Differs from a Standard Needle
The fundamental difference between a cannula and a standard hypodermic needle lies in their structure and function. A standard needle is a singular, rigid, sharp-tipped instrument designed for immediate, one-time penetration to inject or draw a substance. In contrast, a cannula is a system designed to establish a semi-permanent port of entry.
The design of a cannula prioritizes flexibility and reduced trauma once access is secured. The sharp inner needle is immediately removed, leaving the flexible, blunt-tipped catheter in place to minimize the risk of accidental vessel wall damage. This flexibility allows the tube to remain in the patient for an extended period without causing significant tissue injury during movement. The flexible plastic catheter maintains the access point, reducing the need for repeated skin punctures.
Common Medical and Cosmetic Uses
Cannula applications vary widely across medical disciplines, depending on the required size and purpose, which is indicated by its gauge. The gauge refers to the internal diameter of the tube; a lower number signifies a larger diameter. For example, in emergency medicine, large-bore cannulas (14G or 16G) are used to allow the rapid flow of large volumes of fluids or blood products during trauma.
For routine hospital care, such as administering antibiotics or basic IV fluids, a mid-range size (20G or 22G) is commonly selected to balance flow rate with patient comfort. Cannulas are also used for arterial line placement, typically in the radial artery, allowing for continuous blood pressure monitoring and frequent blood sampling in intensive care units. Specialized nasal cannulas deliver supplemental oxygen directly into the nostrils for patients with respiratory issues.
In cosmetic procedures, a specific blunt-tip microcannula is frequently used for injecting dermal fillers. Because the tip is rounded, it tends to push aside nerves and blood vessels rather than piercing them. This significantly reduces bruising, swelling, and the risk of injecting filler into a blood vessel. This technique often allows the injector to treat a larger area from a single entry point, enhancing safety and improving patient recovery time. Cannula size selection (typically 14G to 26G) is based on the patient’s condition, the treatment goal, and the viscosity of the substance being administered.