Postoperative agitation and hypoxaemia after shoulder surgery in a patient with obstructive sleep apnoea
A 62-year-old man with a BMI of 38 kg/m2 and diagnosed obstructive sleep apnoea (apnoea-hypopnoea index 34, non-compliant with CPAP) undergoes arthroscopic shoulder repair under general anaesthesia with an interscalene brachial plexus block. He received 20 mL of 0.5% bupivacaine for the block, plus intraoperative fentanyl 250 micrograms and morphine 8 mg. In the recovery room 25 minutes after extubation, he is agitated, SpO2 is 88% on 4 L/min nasal cannula, respiratory rate is 8, and he is difficult to rouse. Blood pressure is 158/92 mmHg and heart rate is 104. The phrenic nerve is likely paralysed from the interscalene block.
1. What is your differential diagnosis for the agitation and hypoxaemia, and how do you prioritise management?
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Differential diagnosis:
- Opioid-induced respiratory depression: Fentanyl 250 micrograms plus morphine 8 mg in an OSA patient with a high BMI. Respiratory rate of 8 and difficult rousability fit.
- Residual anaesthesia or neuromuscular blockade: Confirm with a train-of-four and check for residual volatile agent.
- Hypoxaemia from phrenic nerve palsy: Interscalene block can cause hemidiaphragmatic paresis, reducing functional residual capacity, which is critical in obesity.
- Airway obstruction: OSA, obesity, and sedation predispose to obstruction. Agitation may be the first sign of hypoxaemia.
- Local anaesthetic systemic toxicity: 20 mL of 0.5% bupivacaine is 100 mg, which is within limits, but consider it if there is altered mental status, seizures, or arrhythmia.
- Other: Pneumothorax (rare with interscalene), aspiration, hypoglycaemia, and pain.
Prioritised management:
- Airway and oxygen: Sit the patient up, perform a jaw thrust, insert an oropharyngeal airway if tolerated, and apply continuous positive airway pressure or bilevel positive airway pressure if available. Increase oxygen delivery.
- Monitor: Apply full monitoring including capnography, and check arterial blood gases.
- Reverse opioids cautiously: Titrate naloxone in small aliquots (40-80 micrograms) to restore respiratory rate without causing severe pain or hypertension, given the blood pressure of 158/92 mmHg.
- Assess: Check train-of-four, temperature, and glucose. Exclude local anaesthetic toxicity with the lipid emulsion available.
- Escalate: If there is no improvement, call for senior help and prepare for reintubation with a difficult airway plan given the BMI and possible phrenic palsy.
2. He improves with naloxone and CPAP but remains drowsy. What postoperative disposition and analgesic plan would you choose, and why?
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Disposition:
- He is not suitable for a general ward or day-case discharge. He should be admitted to a high-dependency or monitored bed with continuous oximetry and capnography, at least overnight, because of the combination of OSA, opioid use, and phrenic nerve palsy.
- Consider a brief period of postoperative CPAP, and ensure the bed is 30 degrees head-up.
Analgesic plan:
- The interscalene block provides excellent somatic analgesia, so minimise systemic opioids. Use regular paracetamol and an NSAID if renal function and surgical bleeding allow.
- Avoid further morphine boluses; if opioids are needed, use a low-dose PCA with strict monitoring and no basal infusion, and consider opioid-sparing agents such as low-dose ketamine or dexmedetomidine with monitoring.
- Avoid sedative premedication and avoid benzodiazepines.
Why: The combination of obesity, OSA, residual anaesthesia, and hemidiaphragmatic paresis leaves little respiratory reserve. The priority is to prevent recurrent respiratory depression while maintaining adequate analgesia so that the patient can cooperate with physiotherapy and mobilisation. Document the plan and communicate with the surgical and nursing teams.