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Obesity-related restriction — SCE Respiratory MCQ

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HardRespiratory PhysiologyObesity-related restrictionSCE Respiratory

A woman with BMI 48 kg/m2 has severe OSA on sleep study. Awake ABG shows pH 7.37, PaCO2 7.1 kPa and bicarbonate 31 mmol/L; spirometry shows proportional FEV1/FVC reduction with preserved ratio. What diagnosis explains the daytime gas abnormality?

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Correct answer: D — Obesity hypoventilation syndrome causing chronic awake hypercapnic ventilatory failure

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

D is correct. Obesity plus sleep-disordered breathing and awake PaCO2 above 6.0 kPa, after excluding other causes, defines obesity hypoventilation syndrome. OSA alone need not cause daytime hypercapnia. The compensated bicarbonate indicates chronicity. IPF and PE more often cause hypocapnia until advanced disease, while emphysema lowers transfer factor rather than producing the characteristic obesity physiology with preserved ratio and reduced ERV.

Reference: NICE NG202 OSAHS and obesity hypoventilation recommendations: https://www.nice.org.uk/guidance/ng202/chapter/Recommendations