Healthcare and Medicine Reference
Table 10.1 Cannula sizes and their uses.
Colour Size Flow rate
Paediatric or elderly patients with
small, fragile veins
Digital dorsal veins
IV maintenance fl uids, drugs,
Rapid infusions of fl uids, drugs
and blood products.
Unstable patients, emergency
of a central line is not appropriate. Examples of such drugs include
50% glucose for the treatment of hypoglycaemia and amiodarone
An 18G or 20G (green or pink) cannula is appropriate for situa-
tions where maintenance fl uid or IV drugs are required. 22G (blue)
cannulae should be reserved for children or those with very diffi cult
IV access. Blood products should be run through a 18G (green) or
bigger cannula to minimise the risk of clotting.
Figure 10.1 Veins of the hand.
Choosing the site of cannulation
Choosing the ideal vein for cannulation should take into con-
sideration factors such as patient comfort and convenience, size
of cannula required, and the size, mobility and fragility of the
patient's veins. Where possible, the patient's non-dominant hand
should be chosen. The back of the hand or lower arm should be
chosen in most situations, as it is relatively comfortable, the can-
nula is unlikely to kink and it is easily inspected and accessed.
Cannulation of the hand is also associated with a lower incidence
of phlebitis compared with cannulation of veins of the wrist or
upper arm. The distal cephalic vein, known as the 'houseman`s
vein' because it is often chosen by junior doctors, is normally
large and well tethered, making it easy to cannulate. Veins in the
antecubital fossa are often large and easy to cannulate, but can be
awkward and obstruction of fl ow through the cannula tends to
occur if the elbow is fl exed.
Veins on the underside of the arm and wrist are often painful
when cannulated so should be avoided if possible. Veins in the foot
can be used as a last resort but tend to be painful and inconvenient
for the patient and are associated with a higher risk of phlebitis
and thromboembolism. Finally, experienced practitioners will
occasionally cannulate the external jugular vein, particularly in
emergency situations when IV access elsewhere is diffi cult.
Luer Lok TM
Catheter hub + wing
Figure 10.2 Cannula.
practitioner is competent to insert should be put into the patient's
largest peripheral vein. This will usually be a 14G or 16G (orange
or grey) cannula in the antecubital fossa. These cannulae have the
largest radius and therefore the highest fl ow rate, allowing a large
volume of fl uid to enter the circulating volume in a short period
of time. By doubling the radius of the cannula, the fl ow through it
is increased 16-fold.
A cannula inserted into a large vein is needed in situations where
potentially irritant drugs need to be administered and the insertion
Step-by-step guide: intravenous cannulation
Give a full explanation to the patient in simple terms and
ensure they consent to the procedure (if able).
Set up your trolley (Box 10.1 and Figure 10.3).
Prepare your trolley as a sterile fi eld. Wear a plastic
disposable apron and non-sterile gloves, and take alcohol
hand rub with you.