25.5 Arterial Cannulation

Question 1

a) What are the indications for
arterial cannulation? (35%)

Answer 1

1. Measurement:

> > Blood pressure monitoring
(in ICU; on transfers;
for patients who have arrhythmias,
are on inotropes,
or who are critically ill).

> > Arterial blood gas analysis.

> > Cardiac output monitoring.

2. Diagnostic:
   » Angiography.
3. Therapeutic:
   » Thrombolysis.

> > Vasodilator administration.

> > Chemotherapy administration.

> > EVAR.

> > ECMO.

> > Stenting.

> > Embolisation.

> > Renal replacement therapy.

Question 2

b) How may an invasive arterial pressure measuring system be calibrated? (20%)

Answer 2

Calibration: to set or check the graduations by comparison with a standard.

I don’t believe we truly calibrate the system clinically,
but we do zero it,
level it
and then check that it gives roughly the
same reading as noninvasive blood
pressure monitoring.
The transducer will have been calibrated at the time of manufacture.

Before clinical use:
1.&raquo_space; Zero:
aseptic technique,
turn stopcock ‘off’ to patient,
open cap to air,
press ‘zero’ on IABP module,
check the trace is at zero
and the monitor states zero,
replace cap,

open three-way tap between
patient and transducer.
Atmospheric pressure is therefore
set as zero and blood
pressure is measured against that pressure.

2. > > Level:
   once zeroed,
   the transducer must be placed level
   with the heart in order to
   ensure that the hydrostatic pressure
   of blood is not included in
   the blood pressure recording
   (4th intercostal space midaxillary line).
3. > > Calibrate clinically:
   compare invasive with noninvasive blood pressure.

Invasive systolic blood pressure is usually
5–10 mm Hg higher than NIBP,
diastolic BP usually 5–10 mm Hg lower,
mean should be the same.

Question 3

c) Outline the sources of error when measuring invasive arterial pressure. (45%)

Answer 3

> >

1. Failure to zero.

2. > > Failure to keep level with heart
   (a 10 cm error in positioning height will
   lead to a 7.4 mm Hg error in
   blood pressure recording).
3. > > Transducer drift

(repeated exposure of the transducer to pressure causes distortion of the materials
with which it is made,
causing sensed value to
gradually drift away from actual value).

4. > > Resonance.

All objects have a natural frequency,
a frequency at which the object will
readily oscillate if force is applied to it at
a frequency close to the natural frequency.

This is resonance.
If the natural frequency
of the invasive blood pressure
measuring system was similar
to the frequencies of the sine waves
that make up the arterial pressure waveform,

then the system would resonate,
causing the output of the system to be greater
than it should be.

So, the natural frequency of the measuring system
is intentionally made higher than
the frequencies of the waveforms
that make up the arterial pulse.

It is important that a short,
rigid-walled cannula is used and
that the tubing does not exceed 120 cm in length in
order to maintain the
high natural frequency
of the measuring system.

5&raquo_space; Over- or underdamping.

Damping is a decrease in the amplitude of an
oscillation as a result of energy losses within a system.

Overly compliant tubing,
air bubbles in the column of fluid
between patient and transducer,

clots in the cannula and
an excess of three-way-taps will all result in
overdamping.

An underdamped system records an erroneously
low systolic blood pressure
and high diastolic pressure, although mean
arterial pressure remains less affected.

An underdamped system is
unlikely to occur if the correct
equipment is utilised in assembling an
invasive blood pressure monitoring system.