Postoperative Hypoxaemia in a Patient with Obesity
A 58-year-old man with a BMI of 42 kg/m² undergoes elective laparoscopic cholecystectomy under general anaesthesia. He has obstructive sleep apnoea (OSA) diagnosed by polysomnography (apnoea-hypopnoea index 32 events/hour) and uses CPAP at home. He is extubated in the operating theatre and transferred to the post-anaesthesia care unit (PACU). Thirty minutes later, he is drowsy but rousable, with a respiratory rate of 8 breaths/min, SpO2 89% on 4 L/min oxygen via nasal prongs, and a PaCO2 of 58 mmHg on an arterial blood gas. His pain score is 3/10 at rest. He received intraoperative morphine 10 mg IV and fentanyl 100 mcg IV, and has a multimodal analgesic regimen including regular paracetamol and NSAIDs.
1. What are the likely contributors to this patient's hypoxaemia and hypercapnia? Prioritise them.
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The hypoxaemia and hypercapnia are multifactorial. The most immediate and reversible contributor is opioid-induced respiratory depression, evidenced by a respiratory rate of 8/min and a PaCO2 of 58 mmHg. The patient received intraoperative morphine and fentanyl, and residual opioid effect is likely. Second, his obesity and OSA predispose to upper airway obstruction and impaired ventilatory response to hypercapnia, especially in the postoperative period. Third, atelectasis from laparoscopy and anaesthesia reduces functional residual capacity and causes shunt. Fourth, residual anaesthetic agents (volatile or propofol) can depress ventilation. Pain may also inhibit deep breathing, but his pain score is low. The priority is to address opioid-induced respiratory depression and airway obstruction immediately, while considering atelectasis as a contributing factor.
2. Outline your immediate management and monitoring plan.
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Immediate management: call for help, ensure airway patency (head tilt/chin lift, consider oropharyngeal airway), provide assisted ventilation with bag-mask if inadequate, and administer oxygen to maintain SpO2 ≥94%. Consider naloxone 40–80 mcg IV titrated to respiratory rate and consciousness, avoiding full reversal to prevent pain and hypertension. If naloxone is used, monitor for re-sedation due to its shorter half-life than opioids. Place the patient in a semi-upright or lateral position to improve ventilation. Start CPAP if available and tolerated, as it splints the airway and recruits alveoli. Monitor continuously with pulse oximetry and capnography if available, and consider arterial blood gas recheck. Escalate to high-dependency unit if response inadequate. Avoid sedatives. Reassess pain and provide non-opioid analgesia. Document and consider informing the pain service.