Apnoeic Oxygenation CO2 — FRCA Primary MCQ

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HardPhysiology - RespiratoryApnoeic Oxygenation CO2FRCA Primary

A fully preoxygenated, neuromuscularly blocked adult with a patent airway receives humidified high-flow 100% nasal oxygen during 10 minutes of complete apnoea. The SpO2 remains 100%. Compared with the onset of apnoea, which arterial blood-gas pattern is most likely?

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Correct answer: B — PaCO2 increased; pH decreased; PaO2 may remain high

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

A is correct. Oxygen uptake from the alveoli creates a pressure gradient that draws oxygen down a patent airway, allowing PaO2 and SpO2 to remain high during apnoea. This mass flow does not provide alveolar ventilation sufficient to eliminate metabolically produced carbon dioxide. PaCO2 therefore rises and produces acute respiratory acidaemia. High-flow nasal oxygen and cardiogenic oscillations may provide limited carbon dioxide clearance, but they do not match production. B is incorrect because PaCO2 cannot remain unchanged during complete apnoea. C and E give the wrong direction of PaCO2 change. D correctly predicts hypercapnia and acidaemia but incorrectly requires hypoxaemia despite the stated maintained saturation.

Reference: Patel A, Nouraei SAR. Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): a physiological method of increasing apnoea time in patients with difficult airways. Anaesthesia. 2015;70:323-329. https://pubmed.ncbi.nlm.nih.gov/32618626/