Postoperative respiratory depression after intrathecal morphine in an elderly patient
An 82-year-old woman weighing 55 kg has a hip hemiarthroplasty under spinal anaesthesia with 300 µg of intrathecal morphine. She has chronic kidney disease (eGFR 32 mL/min/1.73 m²) and takes regular oxycodone for osteoarthritis. In the recovery room, 90 minutes after the spinal, she is drowsy but rousable, with a respiratory rate of 7/min, oxygen saturation 89% on room air, and a sedation score of 3 on a 4-point scale. Her blood pressure is 112/64 mmHg and heart rate 68/min. The nurse calls you urgently.
1. What is your differential diagnosis and immediate management?
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Differential:
- Opioid-induced respiratory depression from intrathecal morphine (peak effect 6–12 hours, but early depression can occur, especially with renal impairment and pre-existing opioid use).
- Residual effects of sedatives given intraoperatively.
- Hypoventilation from a high spinal block (less likely at 90 minutes with a stable block).
- Hypoglycaemia, stroke, or other medical cause.
Immediate management:
- Call for help and start oxygen by face mask at 10–15 L/min, with a target saturation of 94–98%.
- Airway: position, suction if needed, and support ventilation with a bag-valve-mask if the respiratory rate remains below 8/min or the saturation does not improve.
- Naloxone: give 40–80 µg IV titrated every 2 minutes until the respiratory rate is above 10/min and the sedation score improves, while avoiding complete reversal of analgesia. If IV access is unavailable, give 0.4 mg intramuscularly.
- Monitor continuously with capnography and pulse oximetry, and check blood glucose.
- Reassess the cause and document the event.
Because of her renal impairment, the active metabolite morphine-6-glucuronide can accumulate, so she needs prolonged monitoring.
2. How would you adjust her analgesia and monitoring for the next 24 hours?
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Analgesia:
- Stop or reduce systemic opioids; use regular paracetamol and consider a low-dose NSAID only if renal function permits (it may not at eGFR 32).
- Consider a regional technique such as a fascia iliaca block or local infiltration for hip analgesia to reduce opioid requirements.
- If opioids are needed, use short-acting agents in small titrated doses with close observation.
Monitoring:
- Continuous pulse oximetry and respiratory rate monitoring for at least 24 hours, with a sedation score recorded hourly.
- Capnography if available, especially during sleep.
- Monitor urine output and renal function, and check for urinary retention (common with intrathecal morphine).
- Keep naloxone available at the bedside.
Communication:
- Hand over to the ward team and the pain service with a clear written plan.
- Inform the patient and family about the event and the plan.
- Document the intrathecal dose, the reaction, and the response to naloxone in the anaesthesia record.
In future anaesthetics, the intrathecal morphine dose should be reduced (e.g., 100 µg) or avoided, and the renal function and opioid tolerance should be considered.