What are the different gauges of cannula and how can I remember them?

Intravenous Cannula Gauges and Mnemonic for Clinical Use

The optimal cannula gauge should be selected based on the clinical situation, with 14-22G being the most commonly used sizes, where smaller gauge numbers indicate larger diameter cannulas.

Standard Cannula Gauges and Their Applications

• 14G (Orange): Largest commonly used peripheral cannula, ideal for trauma, major surgery, and rapid fluid resuscitation 1

• 16G (Grey): Large bore cannula suitable for surgery, blood transfusions, and moderate resuscitation 2, 3

• 18G (Green): Medium-sized cannula with flow rates of 80-105 mL/min under gravity and up to 180-200 mL/min with pressure bags; commonly used for routine surgery, blood transfusions, and CT angiography 2, 3

• 20G (Pink): Smaller medium-sized cannula suitable for most routine IV therapies and short procedures; provides adequate flow for most non-emergency situations 4, 5

• 22G (Blue): Small cannula used for pediatrics, elderly patients with fragile veins, or when larger cannulas cannot be placed 6, 5

• 24G (Yellow): Very small cannula primarily used in neonates, pediatrics, and patients with extremely fragile veins 3

Mnemonic for Remembering Cannula Gauges: "OBGPBY"

Orange (14G) - Biggest bore for Blood and Bulk fluids
Grey (16G) - Great for General surgery and blood products
Green (18G) - Good for most hospital General needs
Pink (20G) - Perfect for Planned procedures
Blue (22G) - Better for Babies and elderly
Yellow (24G) - Young ones (neonates) and Yielding veins

Flow Rate Considerations

• Flow rate decreases as gauge number increases (smaller diameter) 2, 3
• An 18G cannula allows flow rates of approximately 80-105 mL/min under gravity and up to 180-200 mL/min with a pressure bag 2
• For rapid fluid resuscitation, use 18G or larger with pressure bags 2, 3
• For CT angiography requiring contrast media injection, an 18G peripheral venous catheter allows flow rates of 5.0-7.5 mL/s (300-450 mL/min) when used with pressure injectors 2

Clinical Selection Guidelines

• For initial cannulation of arteriovenous fistulae: Start with the smallest needle available (usually 17G) and increase size based on performance 1
• For trauma/resuscitation: Use 14-16G to allow rapid fluid administration 1, 2
• For routine IV therapy: 18-20G is typically sufficient 4
• For pediatrics/fragile veins: 22-24G is appropriate 3

Important Clinical Considerations

• The smallest practical size of cannula should be used for patient comfort while meeting clinical needs 1, 5
• Pain during cannulation is not significantly different between 18G and 20G cannulas, contrary to common belief 4
• Cannula position (kinking, partial occlusion against vessel wall) can significantly reduce actual flow rates below theoretical maximums 2
• For arterial cannulation, smaller gauge cannulas (20G) have significantly lower occlusion rates compared to larger ones (18G) (8% vs 34%) 7
• Ultrasound guidance can improve success rates for difficult cannulations 1

Pitfalls to Avoid

• Don't select cannula size based solely on ease of insertion; consider the clinical requirement 1
• Avoid using cannulas that are too large for the vessel as this increases risk of thrombosis and vessel damage 7
• Don't forget to consider local anesthetic for cannulation regardless of gauge size, as studies show that IV cannulation with 18-22G cannulas is more painful than subcutaneous lidocaine injection 5
• For arteriovenous fistula cannulation, avoid using a cannula larger than the vein diameter as this increases risk of infiltration 1