Postoperative Hypoxaemia

Postoperative hypoxaemia is arterial oxygen saturation under 90% (PaO2 under 8 kPa or 60 mmHg) while breathing room air after surgery. Mild hypoxaemia is 90% to 94%.

Epidemiology. It affects 20% to 50% of patients in the recovery room when breathing air. Continuous monitoring finds episodes in 40% to 70% of patients on the first nights after major surgery, mostly unrecognised and linked to opioids.

Causes by frequency. Hypoventilation (opioids, residual anaesthetic or neuromuscular block, sleep apnoea), atelectasis, airway obstruction, pneumonia, pulmonary oedema, bronchospasm, aspiration, pulmonary embolism, pneumothorax, diffusion hypoxia after nitrous oxide, low cardiac output, and measurement artefact (poor perfusion, nail varnish, motion, methaemoglobinaemia, carboxyhaemoglobin).

Pathophysiology.

Alveolar hypoventilation raises alveolar carbon dioxide and lowers alveolar oxygen. Ventilation perfusion mismatch and shunt widen the alveolar arterial gradient.

Functional residual capacity falls by 30% to 50% after upper abdominal surgery, which promotes closure of small airways.

Low cardiac output and shivering lower mixed venous oxygen, which worsens the effect of any shunt.

Restlessness, confusion, tachypnoea or bradypnoea, tachycardia progressing to bradycardia, accessory muscle use, paradoxical breathing, stridor, wheeze and drowsiness. Cyanosis is late (saturation under 85%). Arrhythmia and myocardial ischaemia may follow.

Red flags.

• Stridor or silent chest.
• Respiratory rate under 8 or over 30/min.
• Falling conscious level.
• Unilateral absent breath sounds or tracheal deviation.
• Sudden onset with chest pain or hypotension.
• Saturation under 85% on high flow oxygen.

Work stepwise through airway, breathing and circulation.

• Check the probe signal, then airway patency and respiratory rate, depth and chest movement.
• Auscultate and check tracheal position.
• Arterial blood gas for PaO2, PaCO2, pH, lactate and haemoglobin; co-oximetry if methaemoglobinaemia is suspected (saturation near 85% unresponsive to oxygen, chocolate brown blood).
• Portable chest radiograph, lung ultrasound, ECG, troponin and natriuretic peptide.
• Train-of-four count if neuromuscular block is possible.
• CT pulmonary angiography if embolism is likely.

Differential diagnosis.

• Airway obstruction.
• Opioid or sedative excess.
• Residual neuromuscular block.
• Atelectasis.
• Pneumonia or aspiration.
• Pulmonary oedema.
• Bronchospasm.
• Pulmonary embolism.
• Pneumothorax.
• Sensor error.

Immediate. High flow oxygen 15 L/min by non-rebreather mask, target saturation 94% to 98% (88% to 92% if at risk of hypercapnic failure). Open the airway with head tilt and jaw thrust or an oropharyngeal airway. Sit upright if haemodynamically stable. Call for senior help.

Treat the cause.

• Opioid depression, naloxone: 40 to 100 microgram IV, repeated every 2 to 3 minutes and titrated to respiratory rate over 10/min; reconsider the diagnosis after a total of 400 microgram; effect lasts 30 to 60 minutes so observe for at least 2 hours; monitor respiratory rate, sedation and pain; use small increments in opioid dependent patients and cardiovascular disease; no renal adjustment.
• Residual neuromuscular block, sugammadex: 2 mg/kg IV once when the train-of-four count is 2 or more; 4 mg/kg if the post-tetanic count is 1 to 2; 16 mg/kg for immediate reversal after rocuronium 1.2 mg/kg; maximum 16 mg/kg; confirm train-of-four ratio of 0.9 or more and watch for bradycardia and hypersensitivity; avoid if creatinine clearance is under 30 mL/min; counsel on reduced hormonal contraceptive efficacy for 7 days.
• Bronchospasm, salbutamol: 5 mg nebulised, repeated every 20 minutes up to 3 doses then every 4 hours; monitor heart rate and potassium; use cautiously in arrhythmia.
• Pulmonary oedema, furosemide: 40 mg IV slowly (no faster than 4 mg/min), repeat 80 mg after 1 hour if response is inadequate; usual daily maximum 200 mg; monitor urine output, potassium and creatinine; higher doses are needed in renal impairment.
• Tension pneumothorax: needle decompression in the fifth intercostal space at the anterior axillary line, then a chest drain.
• Pulmonary embolism: haemodynamic support and therapeutic anticoagulation once surgical bleeding risk has been weighed with the surgical team.
• Methaemoglobinaemia: methylene blue 1 to 2 mg/kg IV over 5 minutes, repeated at 1 mg/kg after 30 to 60 minutes; maximum 7 mg/kg; avoid in G6PD deficiency and with serotonergic drugs.

Respiratory support. CPAP 5 to 10 cmH2O, high flow nasal oxygen or non-invasive ventilation for persisting hypoxaemia. Intubate for exhaustion, falling conscious level or refractory hypoxaemia.

Monitoring. Continuous saturation, respiratory rate, end tidal carbon dioxide where available, and repeat blood gas at 30 to 60 minutes.

Referral and escalation. Critical care for hypoxaemia needing over 60% oxygen, ventilatory support, or unresolved cause.

Special groups. Obese and sleep apnoea patients need continuous monitoring and CPAP. In COPD, titrate oxygen to 88% to 92%. Children desaturate faster and need earlier airway support.