20. Arterial Pressure Waveform

Question 1

What are the indications for direct arterial blood pressure

measurement?

Answer 1

1. > When non-invasive BP measurements are inaccurate –
   in obese patients,
   arrhythmias and
   during ambulance transfers.

2. 
> When extreme changes in blood pressure are expected – 
massive haemorrhage, 
cardiovascular instability and 
induced hypotension.

3.
> When frequent arterial blood samples are required.

4.
> When using pulse-contour cardiac output monitoring (e.g. LiDCO).

Question 2

What are the components of an arterial line used to measure blood pressure?

Answer 2

1. > Arterial cannula:
   short, stiff and 20 G size in adults.

2.
> Tubing: 
usually less than 120 cm, 
filled with 0.9% saline, 
connects cannula to transducer,
 must be free of kinks, 
clots 
and air bubbles.

3. > Three-way tap and flushing device.

4
> Pressurised fluid bag:
 usually 0.9% saline, 
pressurised to 300 mmHg,
with a drip rate of 4 mL/h 
to prevent clot formation within the cannula.

5. 
> Diaphragm: 
very thin membrane acts as an 
interface between the transducer 
and the fluid column.

6. 
> Piezoresistive strain gauge transducer 
connected to a Wheatstone bridge circuit 
(a null-deflection system 
consisting of a
galvanometer,
 two constant resistors, 
one variable resistor 
and a strain gauge): 

must be zeroed to atmospheric pressure and kept at the level of the right atrium

7. Microprocessor, amplifier and display unit

Question 3

What factors determine the shape of the arterial waveform?

Answer 3

1.
> Volume of blood ejected

2. > Speed at which this blood is ejected during each beat
3. > Ability of the vascular tree to
   distend and accommodate
   this ejected blood (i.e. compliance of arterial tree)

3. 
> Rate at which this 
ejected blood is able to 
flow from central arterial component 
into the peripheral tissues (i.e. systemic vascular resistance)

Question 4

What information can be derived from a direct arterial pressure wave

List 10

Answer 4

1. Systolic
2. Diastolic
3. Pulse Pressure
4. MAP
5. HR
6. Rhythm
7. Dicrotic Notch
8. Contractility
9. Compliance
10. Stroke volume
11. Pulse pressure variation

Question 5

More detail on 10 things can glean from Art line waveform

Answer 5

> Systolic blood pressure:
this is primarily influenced by stroke volume
and compliance.

This explains why elderly patients have a higher
systolic pressure due to a reduced vascular compliance secondary to atherosclerosis while neonates have a lower systolic pressure because of
a very compliant arterial tree.

> Diastolic blood pressure:
this is primarily influenced by arterial recoil. This
explains why elderly patients have a lower diastolic pressure as their stiff
arteries are not able to recoil effectively while neonates have a higher
diastolic pressure due to their good arterial elastic recoil.

> Pulse pressure
Mean arterial pressure
Heart rate
Rhythm

> Dicrotic notch:
this represents the nadir point that occurs immediately
after the aortic valve closes and is usually seen one-third of the way
down the descending limb of the pressure wave (i.e. when pressure in aorta is greater than the pressure in left ventricle). The position of this notch reflects peripheral vascular resistance. In presence of
vasodilatation (e.g. sepsis or epidural), there is a downward shift of the
dicrotic notch. Characteristic features of vasodilatation of the arterial wave form include a low systolic pressure, low diastolic pressure, wide
pulse pressure and delayed dicrotic notch.

> Left ventricular contractility: this can be estimated from the gradient of
the upstroke of the arterial waveform.

> Compliance of the arterial tree: this can be estimated from the gradient
of the downstroke of the arterial waveform.

> Stroke volume: this can be estimated from the area under the systolic
portion of the arterial waveform (i.e. from the start of the upstroke to the
dicrotic notch).

> Heart–lung interactions and fluid responsiveness: the changes in arterial pressure waveforms in response to changes in intra-thoracic pressures during mechanical ventilation (i.e. ‘swing’) can be used to determine fluid
responsiveness.

> Pulse contour analysis can be used to determine stroke volume and cardiac output (for more details, see Chapter 70, ‘Cardiac output
monitoring’).

Question 6

In what ways can a direct arterial
pressure transducer system give
you false information?

Answer 6

1. Calibration error
2. Transducer Height
3. Natural frequency + Resonance
4. Frequency response:
5. Damping:

Question 7

1.

> Calibration error:

Answer 7

1. > Calibration error:
   One-point calibration is suitable for
   highly accurate devices to remove the offset error.

Two-point calibration is required for
less accurate devices but with
assumed linear response,
in order to remove offset and gain errors.

Three-point calibration is used for devices
that are not very accurate or
that have a very non-linear response.

Arterial lines undergo
a one-point calibration by
zeroing the transducer to atmospheric pressure.

However, despite calibration,
drift of zero
and gain can occur over time.

Question 8

2.

> Transducer height:

Answer 8

2.
> Transducer height:
this must be at the level of the patient’s right atrium
(an error reading equivalent to 7.5 mmHg occurs for each 10 cm discrepancy in height).

Question 9

3.
> Natural frequency and resonance:

Nat freq

dir relate to

How should it relate to fundamental

in terms of the sytem relations

Answer 9

every system has a tendency to oscillate.

When a system is given a small oscillation (an external
push),
it will start to swing and the frequency at which it swings is the natural frequency
(also called resonant frequency)
of that system.

Natural frequency is
directly related to resonance.

It is important for the -
*natural frequency of the arterial transducer to be significantly different from the frequency of the arterial pressure wave or else it could amplify the signal

(natural frequency should be at least 10 times fundamental frequency).

Natural frequency is:
• Directly related to catheter diameter

• Inversely related to square root of the system compliance
• Inversely related to the square root of the length of tubing
• Inversely related to square root of the density of the fluid in the tubing.

Question 10

4. > Frequency response:

Answer 10

> Frequency response:
arterial pressure waveform is a
complex sine wave.

Fourier analysis allows this 
complex waveform to be broken
down into a series of 
simple sine waves 
of different amplitudes and frequencies. 

The fundamental frequency (or first harmonic)

is equal to the heart rate
(HR of 60 bpm = 1 Hz,
HR of 120 bpm = 2 Hz
and so on).

The first 10 harmonics of the
fundamental frequency contribute to
the waveform, and therefore,
in order to display the arterial waveform correctly,
the transducer should have a frequency response range
(i.e. bandwidth) of 0.5–40 Hz.

Question 11

> Damping:

Answer 11

> Damping:

this is the tendency of an object to resist oscillating

Overdamping underestimates SBP,
overestimates DBP
but MAP remains
the same

(e.g. blood clot, air
bubble or
excessive tubing compliance).

Under-damping 
overestimates SBP, 
underestimates DBP 
but MAP
remains the same

(e.g. tubing too long and non-compliant).

In order to minimise these errors, the monitoring system should apply an optimal
damping value of 0.64.

Question 12

What are some of the
complications associated
with arterial lines?

Answer 12

1. > Cannula disconnection leading to blood loss

2.
> Arterial thrombosis

3.
> Ischaemia distal to the cannula
(this is rare but can occur so collateral circulation should be checked, e.g. Allen’s test)

4.
> Infection

5.
> Inadvertent drug administration:
this can cause distal vascular occlusion and ischaemia. A-lines should be clearly labelled and colour-coded.