Healthcare and Medicine Reference
The pressurised fl uid system is then connected to the cannula,
not forgetting to open the FloSwitch™, allowing the cannula to be
The system must then be zeroed to give an accurate reading. The
pressure transducer is put at the same level as the patient's heart,
and the three-way tap is closed off towards the patient and opened
towards atmospheric air allowing the system to be zeroed.
The procedure including aseptic precautions and any complica-
tions should be documented.
The most common complication of arterial catheterisation is
thrombosis which occurs in up to 30% of cases. The risk of
thrombosis increases with the diameter of the cannula and the
duration it remains in place. Haematoma formation occurs both
after insertion and after removal and can be reduced through
minimising movement of the catheter and by applying adequate
pressure after removal. As the risk of infection at the site of punc-
ture increases with the duration of placement, cannulae should not
be left in place longer than absolutely necessary.
Information other than simply the systolic and diastolic blood
pressures and heart rate can be gained from inspecting the shape
of the arterial waveform (Figure 20.9). The slope of the upstroke of
the waveform refl ects the contractility of the myocardium, with a
poorly contracting heart having a less steep slope. Cardiac output
can be estimated by multiplying the area underneath the waveform
before the dichrotic notch (the stroke volume) by the heart rate.
In hypovolaemic patients the dichrotic notch is lowered; the slope
of the waveform after the dichrotic notch refl ects the degree of
vasoconstriction of the patient, with a gentle sloping waveform
seen in patients who are vasoconstricted. The mean arterial blood
pressure - the average pressure over the length of the cardiac
cycle - is calculated by integrating the pressure wave.
Vertical 10 mmHg/cm
Horizontal 0.2 s/cm
Figure 20.8 Seldinger technique. (a) A Seldinger wire in the artery is
used as a guide for the insertion of the cannula. (b) The cannula is
inserted over the wire. (c) The Seldinger type cannula in fi nal position
(with bung inserted).
Figure 20.9 Arterial waveform.