Cannulation is a procedure regularly undertaken which involves threading a plastic tube into a vein, allowing access for fluid & drug administration. Blood samples can be taken during the initial cannulation process, but once in & flushed with saline, future samples are likely to be significantly contaminated.

Due to the physics of fluid dynamics, a larger cannula even if only slightly wider, will allow the delivery of fluids significantly more quickly. Therefore larger bore cannulas are preferred in resuscitation – however, it is better to have achieved access with a smaller cannula than not have access at all as with resuscitation the peripheral veins will dilate allowing larger cannulas to be inserted.

When considering whether to re-cannulate a patient, consider whether it is necessary or whether IV drugs can be given orally. Generally, you should try at most twice before escalating. The reason for this is multiple failed cannulas can make those veins significantly more difficult to cannulate so your seniors may have even less chance of success.

Equipment

• Gloves
• Hand sanitiser
• Tourniquet
• Appropriate size cannula 
• Alcohol swab
• Gauze
• Syringe 
• 0.9% sodium chloride vial
• Vacutainer/additional syringe if taking bloods too
• Cannula port/bung/octopus
• Cannula dressing
• Stickers to label cannula 
• Access to a sharps bin

The most common size of cannulas varies from 14-24 gauge. The larger the gauge number, the smaller the needle bore. Guage size and corresponding colour of cannula may vary between trusts, and it is advised to refer to local guides.

Procedure

1. Introductions, handwashing, consent, check patient identifiers
2. Prepare the saline flush and prime the octopus (if an octopus will be used). 
3. Apply tourniquet.
4. Identify a suitable vein (as straight as possible & bounces when palpated).
5. Clean with an alcohol swab and let dry.
6. Insert the needle of the cannula at a 30–45 degree angle to the skin, whilst applying gentle traction to the area. Shallow the angle of insertion once the skin is pierced and advance the needle of the cannula until a venous flashback is seen, carefully insert 1 mm further. This is because the plastic catheter is slightly shorter than the needle and you want to make sure both are in the vein.
7. Advance the cannula over the needle by simultaneously retracting the needle and advancing the plastic tubing. A second flashback is observed within the plastic catheter tubing. 
8. Once fully advanced, before removing the needle, ensure a gauze swab is placed between the cannula hub and the patient’s skin. This will absorb any blood spillages when the cannula needle is removed completely. 
9. Remove the tourniquet.
10. Apply pressure to the cannulated vein proximally and remove the needle. Dispose of the needle in the sharps bin.
11. If bloods or cultures are required attach a syringe or vacutainer and collect samples.
12. Attach the bung/primed octopus port.
13. Clean any blood from around the area.
14. Flush the cannula with 0.9% sodium chloride. This should be painless and cause no subcutaneous swelling, otherwise, it suggests the cannula has “tissued” (which means it is now in the tissue rather than the vein)
15. Fix the cannula using the dressings, whilst ensuring the cannula insertion site remains visible to allow timely detection of phlebitis.
16. Label the cannula with the date.
17. Using a local protocol for PVC bundle documentation, note the cannula size and area inserted i.e. Left ACF 

Tips

• Always question whether a cannula is necessary before placing one.
• Larger cannulas are better for giving a lot of fluids quickly. Radiographic contrast usually requires a cannula that’s pink or larger (i.e. smaller gauge).
• Try to go distally to allow more attempts to be had at the same vein proximally if unsuccessful. Areas such as the ACF’s may be restrictive for patients and, when the elbow is bent, may limit flow through the cannula.
• Tapping the vein, gravity, and submerging the hand in lukewarm water can all help improve your success rate.
• Ensure you anchor the vein (particularly in the elderly) as it will otherwise roll away from the needlepoint.
• If cannulating the hand, it may be useful to ask the patient to gently clench your fingers, creating a fist around your non-dominant hand. This will allow you to create a successful anchor with the patient’s fist and use your free non-dominant thumb to anchor the vein, whist cannulating with your dominant hand.
• Securing the cannula wings to the skin using the strips provided in the cannula-specific dressing pack helps ensure the cannula remains in place once inserted, before flushing. The position of the cannula may otherwise move whilst flushing if not secured.
• Prepare for a challenging cannulation by taking multiple cannulas or different gauge sizes of cannula with you. Larger cannulas will have longer lengths, making cannulation of patients with tortuous veins increasingly challenging. 
• Avoid cannulation of limbs with arterio-venous fistulas or with the presence of lymphoedema (e.g. history of cancer and ipsilateral lymph node biopsy)
• In patients with delirium or dementia, a light bandage may be wrapped around the cannula to minimise tampering of it once inserted. 

Alternatives

If unsuccessful despite multiple attempts at cannulation, alternatives include:

• Nursing staff or healthcare support workers (who often perform cannulation/venepuncture regularly)
• Senior support (including consideration of anaesthetics)
• Ultrasound-guided cannulation
• Definitive semi-permanent lines (e.g. midline, PICC)
• In emergencies, intraosseous access or arterial stab may be considered

Resources & References

• Demonstration video thanks to Geeky Medics

Written By Amina Aslam (FY2)
Edited by Pratyush Pradeep (FY2)
Video created by Dr Cesci Adams, Dr Michaela Gaston, Dr Siona Sabu (Clinical Teaching Fellows)