Cannulae — or flexible tubes that can be inserted into a patient's body — are used for different purposes, resulting in cannula sizes that range from 14-22 gauge. Intravenous cannulae are used to administer medications and other fluids to patients and to remove blood for sampling. The purpose, the condition of the patient and the urgency at which intravenous fluid must be delivered determine which cannula sizes must be used. The smaller the gauge, the larger the diameter of the cannula and the faster the flow of fluid. Different cannula sizes are indicated by different colors, making them easier to differentiate, especially in emergency situations in which size might be critical.
A blue-colored cannula indicates a 22 gauge, which allows a fluid flow rate of 36 milliliters per minute. This cannula size usually is used for children because of their small size and for patients who have veins that are difficult to cannulate, including the elderly and oncology patients. These cannulae generally are used only to infuse normal saline, antibiotics and anti-coagulant medications such as heparin.
The standard cannula for routine use is pink-colored. A 20-gauge cannula, it has a fluid flow rate of 61 milliliters per minute. This cannula size is used mainly for routine blood sampling but also can be used for routine blood transfusions and intravenous fluid infusions. Green-colored cannulae also are considered to be a standard size. Measuring as an 18 gauge, with a fluid flow rate of 90 milliliters per minute, this cannula size is used for routine blood transfusions, transfusing large volumes of fluid, intravenous feeding of patients and the harvesting and separation of stem cells.
The white-colored cannula size is seldom used. A 17-gauge cannula, it has a fluid flow rate of 140 milliliters per minute. Instances in which this cannula size might be used include for patients who require significant amounts of intravenous fluids or for blood transfusions that, while not emergencies, need to be given as rapidly as possible.
Emergency situations and operating theaters tend to utilize gray and brown cannulae. Gray-colored cannulae have a 16-gauge size and a fluid flow rate of 200 milliliters per minute, and 14-gauge brown-colored cannulae have a flow rate of 300 milliliters per minute. These larger cannula sizes are necessary for rapid transfusion of blood, intravenous fluids and medications.
Choosing the correct cannula size is important so that treatments can be carried out properly. For example, smaller cannula sizes result in greater pressure in the vein. This can cause hemoloysis during the aspiration of a blood sample if the cannula is too small. Furthermore, if a patient requires a bolus or the rapid infusion of medication, a cannula that is too small will not allow the fluid to flow quickly enough and, combined with increased pressure, could result in the blowing out of the vein.