A 42-year-old woman with BMI 49 kg/m² is posted for laparoscopic sleeve gastrectomy. Preoperative arterial blood gas shows PaO2 68 mmHg, PaCO2 52 mmHg, pH 7.36 and bicarbonate 30 mEq/L. Polysomnography confirms severe obstructive sleep apnoea. Which diagnosis BEST explains her gas exchange abnormality, and what is its principal perioperative implication?

• A Simple obesity with normal gas exchange, requiring routine care
• B Chronic compensated metabolic alkalosis, requiring no specific action
• C Chronic obstructive pulmonary disease, requiring bronchodilator optimisation
• D Obesity hypoventilation syndrome, carrying increased risk of postoperative respiratory failure and warranting planned non-invasive ventilation ✓

Explanation

Awake hypercapnia with PaCO2 above 45 mmHg and an appropriately raised bicarbonate above 27 mEq/L in an obese patient defines obesity hypoventilation syndrome, which is distinct from isolated obstructive sleep apnoea. These patients have blunted ventilatory drive and markedly higher rates of perioperative respiratory failure, reintubation and ICU admission, so plans for postoperative non-invasive ventilation and careful opioid sparing are essential. The normal pH reflects renal compensation over time, not a primary alkalosis.

Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.

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