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Perioperative OSA/OHS — RACP Adult Medicine MCQ

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ModerateRespiratoryPerioperative OSA/OHSRACP Adult Medicine

A 65-year-old man with a body mass index of 44 is scheduled for elective knee replacement. He takes no regular medications. During pre-operative assessment, his ABG shows pH 7.36, PaCO₂ 48 mmHg, PaO₂ 65 mmHg, and HCO₃ 28 mmol/L. He reports excessive daytime somnolence. What pre-operative investigation is essential?

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Correct answer: B — Chest CT

This obese patient has chronic compensated respiratory acidosis (elevated PaCO₂ with compensatory elevated HCO₃) with hypoxaemia and excessive daytime somnolence – consistent with obesity hypoventilation syndrome (OHS) ± obstructive sleep apnoea. Pre-operative diagnosis is critical as undiagnosed OHS/OSA significantly increases perioperative risk (difficult intubation, post-operative respiratory failure, prolonged ventilation). Overnight oximetry or polysomnography should be performed and CPAP/NIV initiated pre-operatively if indicated.

Reference: eTG – 2025 – Respiratory; ANZCA – 2024 – Perioperative Management of OSA