Pulse Oximetry and other Non-Invasive Monitoring

Question 1

Where should the artery line on a blood pressure cuff be placed?

Answer 1

On the anterior arm- ideally roughly where the artery is

Check the arm diameter is within the specified range

Question 2

What does the cuff measure?

Answer 2

Oscillations- same principles as Korotkoff sounds

Absent when passed diastole
Start when flow returns after it is obstructed (systolic pressure)

The cuff use oscillometry to estimate the blood pressures.

Question 3

When are oscillations greatest?

Answer 3

Mean arterial pressure-

Oscillations start at systolic pressure
Greatest at mean arterial pressure
Reduce towards diastolic pressure

Question 4

What can cause errors in blood pressure monitoring?

Answer 4

Cuff size error- it too small over-reads
Poorly calibrated machine
Poor estimation in diastole
Atrial fibrillation- poor estimates of BP

Question 5

What arrhythmia can cause inaccuracies in blood pressure monitoring?

Answer 5

Atrial fibrillation

Question 6

What is meant by oxygen saturation?

Answer 6

This is the proportion of haemoglobin that is saturated with oxygen compared to the total haemoglobin (unsaturated and saturated)

Question 7

What colour is oxyhaemoglobin? How does this affect how pulse oximetry works?

Answer 7

Red (reduced absorption of red light)

Reduced haemoglobin is bluer in colour- reduced absorption of blue light but greater at red end of light spectrum

This principle underpins how oxygen saturation works

The more saturated blood is the redder it is- therefore reduced absorption of red light and greater absorption of blue light

Question 8

What are some limits to pulse oximetry?

Answer 8

False low readings are given for:
Nail varnish
Shivering
Methaemoglobin- reads 85%
Venous congestion/pulsatations- e.g. tricuspid regurge, valsalva

Question 9

When can pulse oximetry give a falsely high reading?

Answer 9

The oximeter cannot differentiate between oxyhaemoglobin and carboxyhemoglobin (occurs with carbon monoxide poisoning)

This does not carry oxygen so true oxygen saturation is low

Question 10

What causes falsely high readings for pulse oximetry?

Answer 10

Carbon monoxide poisoning
Poor perfusion
Anemia- reduced total oxygen content
No measure of CO2 but it has large effects on oxygen dissociation

Question 11

What measures expired CO2?

Answer 11

Capnography- plots CO2 against time