Practising venous cannulation

Nicholas B N Timmis and Geoffrey N Morris give an alternative method for developing this practical skill

All junior doctors need to be able to insert a peripheral cannula confidently, but the teaching of this technique to medical students is somewhat hit and miss. When we get an opportunity to practise in the wards, we often fail because we haven’t had the chance to practise in a relaxed environment, where we can iron out our early fumbling. The system that we describe here gives a good opportunity for students to practise cannulation on a simple but surprisingly realistic model so that cannulation in patients becomes more successful and, importantly, less painful.

Before beginning it is important to choose the correct size of cannula. The options, in order of decreasing bore size, are 16 gauge (grey) for surgical emergencies; 18 gauge (green) for blood transfusions; 20 gauge (pink) for maintenance of intravenous fluids; and 22 gauge (blue) for difficult veins, slow intravenous fluids, or intravenous drugs in a patient who can take oral fluids (blue).^{fig 1} ^{fig 2} ^{fig 3} ^{fig 4} ^{fig 5} ^{fig 6} ^{fig 7} ^{fig 8} ^{fig 9}

Step 1 Use a BD Venflon and a cooked piece of penne pasta. Using a BD Venflon is essential because the depth of its plastic casing means that the pasta sits nicely at an accessible height for cannulation (other brands often have deeper casings).

Step 2 Open the cannula, unfold its wings, and remove the plastic sheath that covers the needle. Insert the sheath through the pasta to stent it. The pasta simulates the skin, and the tapering end of the sheath creates a space to cannulate, simulating the vein

Step 3 Put the stented pasta into the cannula box ready for practice. In a real scenario remember to wear gloves, clean the overlying skin, and locate a sharps box before starting. Cannulation is easier if you first try to increase venous filling. It helps to use a tourniquet; to lower the arm below the level of the heart; to ask the patient to open and close their fist; and gently to tap above the vein

Step 4 Take a three point grip of the cannula, with your thumb on the white cap, index finger on the coloured cap, and middle finger on the wing. In a real scenario apply countertraction to the overlying skin with your other hand to help anchor the vein during insertion

Step 5 Approach at a 30° angle to go through the skin (the outer layer of pasta) then reduce to a 15° angle to advance the needle inside the vein (the space between sheath and pasta) until you see the first flashback (in a real scenario). The flashback provides visual indication of venous entry. The first flashback occurs as you enter the vein, and the second occurs as the needle is withdrawn and blood moves to fill this space. There are three main explanations for failed needle insertions—missing the vein; perforating the posterior wall of the vein; and hitting a valve within the vein

Step 6 Now change your grip, so the thumb and middle finger are on the white cap to withdraw the needle about 5 mm to produce the second flashback. Importantly the index finger provides countertraction on the wing

Step 7 With just the index finger remaining in place at the wing, advance the cannula along the vein. In a real scenario this is the time to release the tourniquet

Step 8 Fully withdraw the needle. Remove the white cap and use it to cap the cannula promptly. To prevent bleeding in a real scenario, occlude the vein with your other hand at the tip of the inserted cannula while you remove the needle until you cap the cannula.

Step 9 When finished practising, remove the cannula, return the needle to the cannula, and return this unit to its sheath for safe storage and further practice

What next?

In a real scenario, once the cannula is inserted you must apply a sterile dressing, dispose of your sharps, flush the cannula, and update the notes.

The cannula is flushed with 5 ml of normal saline from a 10 ml syringe through the coloured injection port. Flushing is vital to check that the cannula is correctly placed and is necessary before and after administration of a drug through the port. Any resistance, pain, or swelling on flushing indicates that the cannula is not in the vein.

The cannula should be removed at the first sign of infection and when no longer needed. If still indicated, the cannula should be re-sited after 72 hours. Always be wary of phlebitis, which can occur because of infection or chemical or mechanical irritation.

Competing interests: None declared.

Provenance and peer review: Not commissioned; externally peer reviewed.

Nicholas B N Timmis fifth year medical student University of Bristol
nicholastimmis@doctors.org.uk
Geoffrey N Morris consultant anaesthetist Southmead Hospital, Bristol
Student BMJ 2008;16:244-245 | 18