Healthcare and Medicine Reference
Figure 20.1 Allen's test. (a) The patient's hand is elevated and pressure applied to both the radial and ulnar arteries. (b) The patient's hand will blanch white. (c)
On release of pressure over the ulnar artery the hand should reperfuse and lose its white colouration.
Figure 20.3 A FloSwitch™ type arterial cannula.
The fl uid is pressurised to 300 mmHg, and the giving set
incorporates a continuous fl ush system at 4 mL/h to help prevent
clot formation and resultant waveform dampening. There is also a
manual fl ush for clearing blood from the system after sampling.
A number of types of Tefl on-coated arterial cannulae are
available that differ in their mechanism of insertion. Most arterial
cannulae differ from venous in that the ends of the cannulae are
square rather than tapered. In younger patients particularly, it may
be helpful to make a small stab incision into the skin to avoid the
cannula tip catching.
The most commonly used types of cannulae in the UK are:
Figure 20.2 An arterial transducer.
(Becton-Dickinson UK) which looks simi-
lar to a venous cannula but with a switch for occluding fl ow,
and is inserted in a manner similar to venous cannulation
The arterial pulsations are transmitted along the column of
fl uid to the transducer, which converts the pressure changes
into an electrical signal displayed as the arterial waveform
The tubing containing the column of fl uid is of a specifi c
compliance in order to produce the optimum waveform. There
is a three-way tap included to allow for arterial blood sampling
without disconnection. Either heparinised or normal saline can
be used as the system fl uid. More centres are now discarding
heparinised solutions since there is little evidence of benefi t.
• (Vygon UK) which is a longer cannula, up to 10 cm,
that is inserted using the Seldinger cannula over wire technique
The type of cannula used and method of insertion is often down
to personal preference. Both cannulae have a maximum size
of 20G in order to minimise the risk of thrombosis and arterial