IV Cannula and fluid flow rates
- Maximum achievable flow rate is mainly limited by the size of the IV
cannula and its length
- Other important factors include pressure of infusion and viscosity of
fluid (e.g. saline faster than blood)
- Flow is inversely proportional to the 4th power of the radius
[Pouseuille's law] - i.e. small changes in cannula diameter = large changes
in flow
- IV cannula size uses old wire gauge system (a 20G cannula is equal to
1/20", 18G is 1/18" etc.)
- Fluid resuscitation requires at least 16 G cannula
| Cannula size |
Colour |
Time to infuse 1000ml Normal saline under ideal circumstances |
| 22 G |
Blue |
22 min |
| 20 G* |
Pink |
15 min |
| 18 G |
Green |
10 min |
| 16 G |
Grey |
6 min |
| 14 G* |
Red |
3.5 min |
* French system - 4F = 20G, 6F = 14G
French system
| French Size |
Diameter |
Examples of use |
| 4F |
1.3 mm |
Paediatric long lines |
| 6F |
1.9 mm |
Paediatric long lines |
| 8F - 20F |
3.8-6.4 mm |
Vascath |
| 10F-18F |
3.2-5.7mm |
NGT |
| 24F - 32F |
7.6-10.4 |
Adult chest drains |