Maximum Flow Rate of Fluid with 18G (Green) IV Cannula
The maximum flow rate of fluid with an 18G (green) IV cannula is approximately 80-105 mL/min under gravity conditions, and can reach up to 180-200 mL/min with a pressure bag. 1, 2
Flow Rate Characteristics of 18G Cannulas
- 18G cannulas are commonly color-coded green and are considered medium-sized peripheral venous access devices 1
- Flow rates through 18G cannulas are significantly higher than smaller gauge cannulas (20G, 22G), making them suitable for moderate volume fluid resuscitation 2
- When using a pressure bag (300 mmHg), flow rates through an 18G cannula can be doubled compared to gravity flow alone 2
Factors Affecting Flow Rate Through 18G Cannulas
Cannula-Related Factors
- The internal diameter of the cannula is the primary determinant of maximum flow rate 3
- The length of the cannula affects flow rate - shorter cannulas have less resistance 3
- Catheter material and design can influence flow characteristics 4
Infusion Setup Factors
- Addition of extension tubing decreases flow rates 3
- Needle-free valves significantly impair flow (up to 36% reduction) 4
- Pressure bags significantly improve flow rates in all IV devices 4, 2
- The choice of infusion tubing can be rate-limiting for larger cannulas 3
Fluid-Related Factors
- Fluid viscosity significantly impacts flow rate - more viscous fluids flow more slowly 3
- Blood products flow approximately 4.5 times slower than crystalloids through the same cannula 3
- Temperature of fluids affects viscosity and therefore flow rate 3
Comparison with Other Cannula Sizes
- 14G cannulas have significantly higher flow rates (approximately 50% higher) compared to 18G cannulas 2
- An 18G peripheral IV catheter has equivalent flow rates to a 14G single-lumen central venous catheter with standard infusion sets 3
- Two 20G IVs can actually provide faster flow than a single 18G IV (41.3 mL/min vs 35.6 mL/min) despite theoretical calculations suggesting otherwise 5
Clinical Applications and Considerations
- For rapid fluid resuscitation, consider using pressure bags with 18G cannulas or larger (14G) 1, 2
- For administration of blood products through an 18G cannula, expect significantly slower flow rates than with crystalloids 3
- For CT angiography procedures 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 1
- The smallest practical size of cannula should be used for patient comfort while meeting clinical needs 1
Common Pitfalls
- Manufacturer-quoted flow rates are often significantly higher than actual clinical flow rates 5
- Flow through IV cannulas is not purely laminar at clinically relevant flow rates, making Poiseuille's law less applicable than commonly believed 6
- Adding multiple extension sets, stopcocks, or needle-free connectors can dramatically reduce flow rates 4, 3
- Cannula position (kinking, partial occlusion against vessel wall) can significantly reduce actual flow rates below theoretical maximums 1