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Kit and numbers
🩹 Cannulae
Gauge, colour, flow rate — and which one a bleed actually needs
Colour coding is an ISO standard, so the colours are the same wherever you work — unlike most things on this screen. Flow rate is not standardised and varies between manufacturers; the figures below are typical gravity flow of water, which is the number quoted on the packet.
The reason a 16G moves five times the volume of a 22G is Poiseuille — flow rises with the fourth power of the radius and falls with length. One step up in gauge is not a small change, and a long line is slower than a short one of the same bore.
Gauge, colour and flow
| 14G | ~270 ml/minOrange. Major trauma and massive haemorrhage. Hurts going in. |
|---|---|
| 16G | ~180 ml/minGrey. The resuscitation cannula — GI bleed, sepsis, theatre. |
| 18G | ~90 ml/minGreen. Transfusion, contrast CT, most acute admissions. |
| 20G | ~60 ml/minPink. The default ward cannula. Fine for fluids and antibiotics. |
| 22G | ~36 ml/minBlue. Fragile or difficult veins, elderly, paediatric. |
| 24G | ~20 ml/minYellow. Neonates, and the vein that will take nothing else. |
What to reach for
| Actively bleeding | 2 × 16G or largerTwo short wide lines beat one long one. A central line is NOT faster — it is longer and often narrower. |
|---|---|
| Blood transfusion | 20G or larger22G will run blood in a small adult but slowly; check local transfusion policy. |
| CT with contrast | 18G or larger, antecubitalPump injectors need the bore and the position. Radiology will refuse a 22G in the hand. |
| Routine fluids or IV antibiotics | 20GA bigger cannula for no reason is a bigger hole and a shorter dwell time. |
Flow figures are approximate and manufacturer-specific — read the packet if the number matters. They are also measured with water by gravity: packed red cells are far more viscous and run considerably slower, and a pressure bag changes everything.
If you cannot get access in a shocked patient, stop escalating cannula size and escalate people — intraosseous access is faster than a third failed attempt.
Source: ISO 10555-5 colour coding; manufacturer flow data (BD, B.Braun, Vygon)