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Residual nocturnal hypoxaemia despite optimised CPAP for obesity hypoventilation syndrome — SCE Respiratory MC

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HardObesity Hypoventilation SyndromeResidual nocturnal hypoxaemia despite optimised CPAP for obesity hypoventilation syndromeSCE Respiratory

A 56-year-old woman with a BMI of 51 kg/m² was diagnosed with obesity hypoventilation syndrome after an awake arterial blood gas showed PaCO₂ 7.2 kPa and respiratory polygraphy showed severe obstructive sleep apnoea/hypopnoea syndrome (AHI 62 events/hour). She had no acute ventilatory failure and commenced fixed-level CPAP. Three months later, she uses CPAP for 6.8 hours nightly with minimal leak. Her morning headaches and daytime somnolence have resolved. Device download and repeat respiratory polygraphy show an AHI of 1.8 events/hour. Overnight transcutaneous CO₂ remains between 5.5 and 6.0 kPa without a sustained sleep-related rise. However, oxygen saturation remains below 90% for 38% of total sleep time, with a nadir of 84% and a sustained rather than cyclical pattern. An awake arterial blood gas on air shows pH 7.40, PaCO₂ 5.8 kPa and PaO₂ 8.5 kPa. Spirometry, transfer factor, chest radiography and echocardiography identify no additional cause of hypoxaemia. Which adjustment to her nocturnal respiratory support is most appropriate?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Add low-flow supplemental oxygen to CPAP

Explanation lettering: B = shown as A · D = shown as B · A = shown as C · E = shown as D · C = shown as E

CPAP has successfully treated both relevant ventilatory targets: the AHI is normal, adherence and mask leak are satisfactory, and nocturnal and awake CO₂ measurements show resolution of hypoventilation. The sustained residual desaturation is therefore not evidence of inadequate upper-airway splinting or persistent ventilatory failure. NICE recommends considering supplemental oxygen with CPAP or NIV when hypoxaemia persists despite optimal control of nocturnal hypoventilation and AHI, while addressing other causes of hypoxaemia. Those causes have been appropriately assessed here. A is inappropriate because clinically important nocturnal hypoxaemia remains untreated. B would be indicated for residual obstructive events but may increase leak or intolerance without correcting this non-obstructive desaturation. C is a plausible escalation in OHS, but NIV is indicated when hypercapnia, nocturnal hypoventilation, symptoms or sleep-disordered breathing persist despite CPAP; none does here. D preserves effective CPAP and adds low-flow oxygen for the residual hypoxaemia, with subsequent oximetric and blood-gas reassessment. E is incorrect because oxygen alone does not treat the underlying severe OSAHS or provide ventilatory support and should not replace effective positive airway pressure.

Reference: NG202, section 2: Obesity hypoventilation syndrome (20 August 2021) — https://www.nice.org.uk/guidance/ng202/chapter/2-Obesity-hypoventilation-syndrome NG202 Rationale and impact: Treatments for obesity hypoventilation syndrome—oxygen therapy (20 August 2021) — https://www.nice.org.uk/guidance/ng202/chapter/rationale-and-impact