HYPOXEMIA

• Definition
SaO2 <90%, PaO2<60mmHg or fall >5% in SaO2
• Detection
Pulse oximetry
Cyanosis at SaO2 <85%, PaO245-50mmHg
Deoxygenated Hb> 5g/100mL - masked by anaemia
Associated: changes in BP, changes in HR, ischaemia, dysrhythmias, changes in EtCO2
Altered mental state

Mechanisms

• Low FiO2
Relative (inadequate for patient’s condition)
Absolute (problems delivering O2 to circuit)
• Inadequate VA
• V/Q mismatch
• Anatomic shunt
• Excess metabolic O2 demand
• Low cardiac output

Clinical Causes

• Inadequate ventilation
AIRWAY obstruction
HYPOVENTILATION during GA
• Endobronchial intubation
• Patients with increased A-a gradient
Pre-existing lung disease
>Pneumothorax
Pulmonary edema
Aspiration
Atelectasis
Pulmonary embolism
• Low cardiac output

Prevention

• Check anaesthetic machine
O2 analyser & alarms
• Adequate Ventilation (esp tidal volume)
• Monitor & adjust FiO2
• High normal range tidal volume
• Caution with spontaneous ventilation in lung disease

Manifestations

• Pulse Oximetry
Malfunction can occur: check waveform & probe position
• Hypothermia
• Poor peripheral circulation
• Artifacts: diathermy, motion, ambient lighting
• Dyes
• Cyanosis
• Dark blood in surgical field
• Late signs
bradycardia , myocardial ischaemia & dysrhythmias, hypotension and cardiac arrest

Management

• Assume low SpO2 = hypoxaemia
• Increase FiO2
Verify FiO2 increases
• Check pulse, BP
• Check EtCO2 & pulse oximeter (refer next slide)
• Hand ventilate - assess lung compliance, give large TV
• Check chest movements & auscultate chest
Exclude endobronchial intubation
• ABG’s
• Posture ­ sitting up

Verify Pulse Oximeter

• do not fixate
Assess signal amplitude
Check waveform
Check position
Correlate reading with diathermy
Shield probe
Change site

Persistent hypoxemia causes

• Pulmonary
Pneumothorax - consider CXR
Aspiration
Massive atelectasis
Pulmonary embolism
Aspiration of foreign body
Acute pulmonary oedema
• Extra-Pulmonary
Low cardiac output
Low Hb
Intracardiac shunting in CHD

Persistent hypoxemia management

• Use aggressive pulmonary toilet
Suction ETT
Consider bronchoscopy
• Consider addition of PEEP
Maintain large tidal volume 12-15ml/kg
• Restore circulating blood volume
Maintain CO and Hb levels (Hb>100g/L)
Consider inotropes

Unresolved hypoxemia

• Inform surgeons (earlier if appropriate)
Check retractors
Transfer to supine position
Terminate surgery ASAP
• Investigations in PACU
Incl. CXR, ABG’s
• Arrange transfer to ICU

Awake patient

• Detection ­ see previous slide
• Look for cause
Inadequate Ventilation ­ airway, depressed VA
Pulmonary and extra-pulmonary
Also diffusion hypoxaemia, laryngospasm, inadequate reversal, shivering
• Management
High flow O2 - CPAP - re-intubation
Drug reversal ­ relaxants, opioids