For the shocked patient, insert two large-bore cannulas (16-18G), ideally in the antecubital fossae. Flow depends on the fourth power of the radius — one 16G cannula outperforms several 22G lines. Practical sequence for difficult access:
- Two attempts at visible peripheral veins (tourniquet on, arm dependent, gentle tapping).
- Try the external jugular (visible on the neck when head-down) or long saphenous at the ankle.
- In children — and adults where equipment exists — after two failed attempts move to the intraosseous route: proximal tibia, 2 cm below and medial to the tibial tuberosity. Any drug or fluid that goes IV goes IO.
- Take blood at cannulation for grouping and haemoglobin if your lab can run them.