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Fig. 5.8 Intravenous catheter sizes. The smaller the gauge number, the larger the diameter of the catheter. The most common sizes are 18, 20 and 22 gauge catheters.

Size of Intravenous Catheter:

The intravenous catheter gauge refers to the internal diameter of the catheter. A small gauge (ga.) number indicates the catheter has a larger internal diameter. The larger the diameter the greater the potential flow rate, as flow increases to the fourth power of the catheter radius. Large-bore intravenous catheters (14 ga. and 16 ga.) are used for rapid infusion of large volumes of fluids and blood. A 20-gauge catheter is the most common general purpose catheter used for adults. When venous anatomy is challenging, a smaller 22-gauge intravenous catheter may be used to induce general anesthesia. Under general anesthesia, venous vasodilation occurs as a result of the effects of the volatile anesthetic vapours and the reduction of the patient’s emotional stress. With venous vasodilation, an additional larger gauge intravenous catheter can be secured if required.