13. Hypoxia

Question 1

What is hypoxia?

Answer 1

Definition:
Arterial O2 saturation < 90%
or PaO2 < 8 kPa

Question 2

> Detection:

Answer 2

Pulse oximetry

• Cyanosis occurs at SaO2 < 85% or PaO2 < 6.7 kPa
• Correlates with deoxygenated Hb > 5 g/dL

> Associated:

• Changes in BP or changes in HR

• Altered mental state
• Late signs: myocardial ischaemia, arrhythmias, bradycardia,
hypotension and cardiac arrest

Question 3

What are the causes of hypoxia?

Answer 3

> Low FiO2 (hypoxic hypoxia)

• Relative (inadequate for patient’s condition)
• Absolute (problems delivering O2 to circuit)

> Inadequate alveolar minute ventilation

> V./Q. mismatch

> Anatomical shunt

> Anaemia (anaemic hypoxia)

> Low cardiac output (stagnant hypoxia)

> Histotoxic hypoxia (e.g. cyanide poisoning or carbon monoxide)

> Excess metabolic O2 demand

Question 4

Clinical causes:

Answer 4

> Inadequate alveolar minute ventilation

> Obstructed airway

> Endobronchial/oesophageal intubation

> Increased alveolar-arterial gradient

> Pre-existing lung disease
(e.g. COPD and pulmonary fibrosis)

> Pneumothorax

> Pulmonary oedema

> Aspiration

> Atelectasis

> Pulmonary embolism

> Low cardiac output

Question 5

Prevention:

Answer 5

> Check anaesthetic machine

O2 analyser and alarms

Adequate ventilation (especially tidal volume)

Maintain tidal volumes in normal range

> Monitor and adjust FiO2

Caution with spontaneous ventilation in lung disease

Question 6

What are the causes of artificially
low pulse oximeter saturation
readings?

Answer 6

> Pulse oximeter malfunction –
check waveform and probe position

> Hypothermia

> Poor peripheral circulation

> Artefacts – diathermy, motion, ambient lighting

> Dyes (e.g. methylene blue)

Question 7

Describe your management of hypoxia.

Answer 7

State that this is an anaesthetic emergency
and that you would call for senior anaesthetic assistance.

> Low SpO2 on pulse oximetry
is due to hypoxaemia until proved otherwise

> Increase FiO2

> Verify FiO2 increases – check oxygen analyser

> Verify pulse oximeter –
check position,
assess signal waveform and amplitude,
and consider changing site

> Check other vital signs –
HR rate, blood pressure, ECG, end-tidal CO2

Hand ventilate to assess lung compliance
and confirm adequacy of ventilation

> Check chest movements and auscultate chest

> If an LMA is in situ,
consider intubation to secure the airway

> Confirm endotracheal tube position
and exclude endobronchial intubation

> Inform the surgeon, ask them to stop operating and to check retractors if applicable

> Check arterial blood gas to further define the degree of hypoxia If saturations remain low, establish the cause and treat as appropriate:

Question 8

Causes

Pulmonary:

Answer 8

• Pneumothorax
• Bronchospasm
• Lobar collapse
• Mucous plugging
• Aspiration
• Massive atelectasis
• Pulmonary embolism
• Aspiration of foreign body
• Acute pulmonary oedema

Question 9

> Extra-pulmonary:

Answer 9

• Low cardiac output
• Anaemia
• Intracardiac shunting (e.g. congenital heart disease)
• Histotoxic hypoxia

Question 10

> M anagement options in persistent hypoxaemia:

Answer 10

Consider the use of the following interventions;
the exact interventions will
obviously depend
upon the working clinical diagnosis.

• Addition of PEEP
• Ensure adequate tidal volume 6–10 mL/kg
• Pulmonary toilet – suction endobronchial tube
• Restore circulating blood volume
• Bronchoscopy
• Maintain cardiac output and Hb levels (Hb > 10 g/dL)
• Transfer patient to supine position if applicable
• Terminate surgery as soon as safely possible
• Optimise ventilation and decide if extubation can be achieved safely or whether a period of prolonged ventilation will be required, this will depend on diagnosis
• Arrange check CXR
• Arrange transfer to ICU for further management
• Document sequence of events in patient’s medical records and complete critical incident form