IV cannulation
Pick the right size for the job, find a straight vein you can feel, anchor it, go in shallow and advance the plastic, not the needle.
A dehydrated 70-year-old woman with gastroenteritis needs IV fluids. Her veins are hard to see, and two attempts on the back of her hand have failed. Her BP is 92/56.
A trauma patient arrives bleeding heavily and needs blood fast.
Which cannula and which vein for each, and what tricks help with difficult veins?
- Choose the size for the job18G for most adults, 14–16G for resuscitation and blood, 20–22G for small veins and medicines.
- Choose the veinStraight, bouncy, forearm. Avoid joints, injured limbs, fistula arms, the side of a mastectomy with node clearance.
- Warm, gravity, tourniquetHang the arm down, warm compress, tourniquet for up to a minute.
- Clean and let dryChlorhexidine 2% in alcohol, 30 seconds, then don't touch.
- Insert at 10–30°Flashback, advance 2 mm more, then slide the cannula off the needle.
- Release tourniquet, flush, secureClear dressing, label with date and time.
- Two failed attemptsAsk for help. Ultrasound if available. Intraosseous in emergencies.
- Two failed attempts in an unstable patient (intraosseous or senior)
- Arterial puncture (bright red, pulsatile)
- Swelling, pain or blanching at the site during infusion (extravasation)
- Red, painful, hard vein (phlebitis)
- Extravasation of irritant drugs (calcium, potassium, chemotherapy, dextrose 50%)
- Phlebitis and line infection
- Arterial cannulation
- Cannula left in without need
| 14G (orange) | About 270 mL/min |
| 16G (grey) | About 200 mL/min |
| 18G (green) | About 90 mL/min |
| 20G (pink) | About 60 mL/min |
| 22G (blue) | About 35 mL/min |
| Flow | Short, wide cannulas run fastest (radius matters most) |
| Review | Check daily, remove when not needed or if phlebitis |
Check every dose against a current drug reference and your hospital protocol.
On this page
Choosing the cannula
| Size | Colour | Flow (approx) | Use |
|---|---|---|---|
| 14G | Orange | 270 mL/min | Major haemorrhage, trauma |
| 16G | Grey | 200 mL/min | Resuscitation, surgery |
| 18G | Green | 90 mL/min | Most adults, blood, CT contrast |
| 20G | Pink | 60 mL/min | Small veins, IV medicines |
| 22G | Blue | 35 mL/min | Elderly, fragile veins, children |
| 24G | Yellow | 20 mL/min | Infants, very small veins |
Flow depends mostly on the radius (doubling the radius increases flow about 16 times) and length. Two short wide peripheral cannulas beat a long central line for resuscitation.
Choosing the vein
Forearm veins: straight, bouncy, not over a joint
Antecubital fossa, large cannulas, both arms
Fistula arm, lymphoedema or post-mastectomy node clearance side, injured limb, infected skin, a limb with a DVT
Ultrasound-guided, external jugular (trained), intraosseous in emergencies
Technique
- PrepareGloves, cannula, chlorhexidine wipe, dressing, flush (5–10 mL saline), sharps bin.
- Tourniquet5–10 cm above the site, not too tight (you should still feel an arterial pulse).
- Find the veinFeel for a bouncy vein; mark it mentally.
- CleanChlorhexidine 2% in alcohol, 30 seconds, let it dry. Don't touch again.
- InsertStretch the skin below, bevel up, 10–30°. Watch for flashback.
- AdvanceLower the angle, advance 2 mm more, then slide the cannula off the needle into the vein.
- FinishRelease tourniquet, press over the vein above the tip, remove needle into sharps, cap, flush, dress, label.
Tips for difficult veins
- Warm the arm with a warm towel or water for a few minutes.
- Gravity and time: arm hanging down, tourniquet on for up to a minute.
- Feel, don't just look. A bouncy vein you can feel is better than a visible one you can't.
- Smaller cannula: a 20G that works beats an 18G that blows.
- Dehydrated patients: after a little oral or subcutaneous fluid, or one IO bolus, veins often appear.
- Ask for help after two attempts; ultrasound if available.
Intraosseous access
In emergencies (cardiac arrest, shock, burns) when IV access fails quickly:
- Sites: proximal tibia (2 cm below the tibial tuberosity, medial flat surface), proximal humerus (adults), distal tibia.
- Drill or manual needle; confirm by aspirating marrow and a smooth saline flush.
- Any resuscitation drug or fluid can be given (pressure bag for fluids).
- Remove within 24 hours once other access is available.
Complications
| Complication | Clues | Action |
|---|---|---|
| Phlebitis | Red, painful, hard vein | Remove; warm compress; antibiotics if infected |
| Extravasation | Swelling, pain, pale or cool skin at the site | Stop infusion, aspirate, elevate, specific antidote for some drugs |
| Infection | Pus, spreading redness, fever | Remove, culture, antibiotics |
| Arterial puncture | Bright red, pulsatile | Remove, press 5 minutes |
| Haematoma | Swelling at site | Remove, press |
Common mistakes
- Using a 22G cannula for a bleeding patient.
- Putting cannulas over joints (kinking, extravasation).
- Touching the site after cleaning.
- Leaving cannulas in "just in case".
- Running potassium, calcium or 50% dextrose through a tiny hand vein.
- Many attempts instead of asking for help.
Frequently asked questions
What size cannula should I use?
18G for most adults; 14–16G for resuscitation and blood; 20–22G for small or fragile veins.
What should I do after two failed cannulation attempts?
Stop and ask for help. Use ultrasound if available. In an emergency, place an intraosseous needle.
Which cannula gives the fastest flow?
A short, wide one: 14G (orange) gives about 270 mL a minute.
How do you recognise extravasation?
Swelling, pain, coolness or blanching at the cannula site during an infusion. Stop the infusion immediately.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. Which cannula allows the fastest fluid flow?
Answer: 14G. Flow increases with the radius; short wide cannulas are fastest.
- 20G: Slower.
- 22G: Slowest.
- 16G: Slower than 14G.
- 18G: Slower.
2. After two failed cannulation attempts in a shocked patient, what is next?
Answer: Intraosseous access or senior help. In emergencies, intraosseous access is fast and reliable.
- Subcutaneous fluids: Too slow.
- Central line by a junior: Not first.
- Keep trying for 30 minutes: Delay.
- Oral fluids: No.
3. A patient's cannula site is swollen and painful during an infusion. What is likely?
Answer: Extravasation. Stop the infusion and remove the cannula; some drugs need specific treatment.
- DVT: Unlikely.
- Arterial cannulation: Different signs.
- Normal: No.
- Allergy: Less likely.
4. Which vein is commonly used for coronary bypass grafts and lies in front of the medial malleolus?
Answer: Great saphenous vein. The great saphenous vein starts at the medial end of the dorsal venous arch, passes in front of the medial malleolus (good for venous cutdown), runs up the medial leg and thigh with the saphenous nerve below the knee, and joins the femoral vein at the saphenofemoral junction (about 4 cm below and lateral to the pubic tubercle). It is used for grafts.
- Small saphenous vein: Behind the lateral malleolus; joins the popliteal vein.
- Femoral vein: Deep vein.
- Popliteal vein: Deep vein.
- Anterior tibial vein: Deep vein.
5. According to Poiseuille's law, halving the radius of an arteriole changes its resistance by what factor?
Answer: Increases 16 times. Resistance is proportional to viscosity × length ÷ radius⁴. Halving the radius increases resistance by 2⁴ = 16 times (and reduces flow 16-fold). Arterioles are therefore the main site of control of peripheral resistance.
- Doubles: Underestimates.
- Increases 4 times: That would be radius squared.
- Halves: Opposite.
- No change: Radius has a large effect.