Intraoperative hypoxaemia: a stepwise approach

What it is

Hypoxaemia is a low partial pressure of oxygen in arterial blood; under anaesthesia it is usually first seen as a falling pulse oximeter reading. Because the oxyhaemoglobin dissociation curve is steep below a saturation of about 90%, a small further fall in oxygen tension then causes a large fall in content, so a sustained drop is an emergency. The causes follow the path of oxygen: a low inspired oxygen fraction (failed supply, wrong gas, a disconnected or leaking circuit), hypoventilation, ventilation-perfusion mismatch and shunt (atelectasis, endobronchial intubation, bronchospasm, secretions, aspiration, pneumothorax, pulmonary oedema), rarely a diffusion problem, and low venous oxygen from low cardiac output, embolism or high oxygen consumption. A falsely low reading from a cold, poorly perfused finger or a displaced probe is common, but it is a diagnosis made only after the patient has been checked. The safest approach is a fixed sequence: raise the inspired oxygen at once, hand-ventilate to feel the lungs, then work from the machine to the patient while calling for help, and treat what is found.

How to recognise it

• Oxygen saturation falling or below the target, often with a lower pitch of the oximeter tone.
• Cyanosis and darker blood in the surgical field are late signs.
• Changes in capnography (falling, absent or sloping trace) or in airway pressure point to the cause.
• Tachycardia and hypertension early; bradycardia, arrhythmia and hypotension when hypoxaemia is severe.
• An arterial blood gas confirms a low oxygen tension and separates a real fall from probe artefact.

Why it happens

• Low inspired oxygen: failed pipeline or cylinder, wrong gas, a leak or disconnection, or a low oxygen setting with a high fraction of another gas.
• Hypoventilation: an obstructed or kinked tube, a ventilator that is off, or a spontaneously breathing patient depressed by drugs.
• Mismatch and shunt: blood passes collapsed or flooded alveoli (atelectasis, one-lung intubation, secretions, aspiration, oedema, pneumothorax).
• Low mixed venous oxygen: low cardiac output, pulmonary or gas embolism, or very high oxygen consumption as in malignant hyperthermia or sepsis.
• Measurement error: poor perfusion, cold, movement, a misplaced probe or abnormal haemoglobins.

What to do at once

1. Give the highest oxygen concentration and say the problem aloud — more oxygen buys time and alerts the team.
2. Call for help and tell the surgeon — another pair of hands and a pause in surgery may be needed.
3. Look at the capnography trace and the airway pressure — they show disconnection, obstruction or collapse in seconds.
4. Hand-ventilate with the bag — resistance or a leak points to the airway; if the machine is suspect, switch to a self-inflating bag and separate oxygen.
5. Check the tube: depth, kinks, cuff, and pass a suction catheter — this finds a displaced, blocked or endobronchial tube.
6. Listen to both sides of the chest — absent sounds suggest one-lung intubation, pneumothorax or collapse; wheeze suggests bronchospasm.
7. Treat the cause: withdraw the tube, suction, bronchodilators, drain a tension pneumothorax — each is a quick reversible fix.
8. Recruit collapsed lung and add positive end-expiratory pressure if blood pressure allows — this reopens atelectatic alveoli.
9. Check the circulation: blood pressure, rhythm, blood loss, embolism — low cardiac output lowers venous and then arterial oxygen.
10. Send an arterial blood gas and consider a chest radiograph or bedside ultrasound — they confirm the problem and its cause.

Working along the oxygen path: what to check and the usual culprits

Costly mistakes

• Blaming the probe first and losing minutes while the patient is truly hypoxaemic.
• Missing endobronchial intubation after the head or table was moved, which pushes the tube deeper.
• Overlooking a tension pneumothorax, especially after central line insertion, trauma or high airway pressures.
• Ignoring a sudden fall in end-tidal carbon dioxide, which can mean embolism, low cardiac output or disconnection.
• Recruitment manoeuvres in a hypotensive or hypovolaemic patient, which can drop the blood pressure further.

Next step

• Once saturation recovers, find and document the cause; decide with the surgeon whether to continue or stop.
• Call a senior early if saturation does not recover with the first steps or the cause is unclear.
• Plan for recovery: extubate only when oxygenation is adequate, and give supplemental oxygen and close monitoring afterwards.
• Arrange high-dependency or intensive care for persistent hypoxaemia, suspected embolism, aspiration or pulmonary oedema.
• Check the machine and supply after the event, report equipment faults and debrief the team.

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Elsewhere in DurgiPedia

• The causes of hypoxemia
• Ventilation–perfusion matching and West zones
• Oxygen transport and the dissociation curve
• Reading an arterial blood gas step by step
• Pulse oximetry

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Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Hypoxia and Hypoxemia (StatPearls (NCBI Bookshelf), 2024).

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