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Oxygen Therapy — SCE Respiratory MCQ

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HardOxygen TherapySCE Respiratory

A 42-year-old agricultural worker is admitted 6 hours after deliberately ingesting paraquat. He is alert and haemodynamically stable but has oral ulceration and increasing tachypnoea. His respiratory rate is 26 breaths/min, SpO₂ is 87% breathing air, and arterial blood gas analysis shows pH 7.39, PaCO₂ 4.5 kPa and lactate 1.6 mmol/L. Chest radiography is initially normal. He is transferred to a level 3 critical care unit for close monitoring and organ support. Which oxygen strategy is most appropriate at this stage?

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

Reveal the answer and explanation

Correct answer: D — Withhold oxygen at present; commence it only if SpO₂ falls below 85%, then reduce or stop it if SpO₂ rises above 88%

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

Paraquat is a specific exception to the usual treatment of acute hypoxaemia. Redox cycling of paraquat generates reactive oxygen species, and supplemental oxygen may aggravate its pulmonary toxicity. BTS guidance therefore recommends giving oxygen in paraquat poisoning only when SpO₂ falls below 85%, and reducing or stopping it when saturation rises above 88%. At 87% on air, this patient should be closely monitored in critical care without supplemental oxygen. B is tempting because 88–92% is the standard controlled-oxygen range for patients at risk of hypercapnic respiratory failure, but the normal PaCO₂ and absence of such a risk do not override the paraquat-specific threshold. C is the usual target for most hypoxaemic patients and most poisonings, but paraquat is explicitly excluded because oxygen may increase tissue injury. D is appropriate initial therapy for many seriously ill patients with marked hypoxaemia, but high-concentration oxygen is particularly undesirable here while SpO₂ remains above 85%. E reflects management of carbon monoxide poisoning, in which pulse oximetry is unreliable and maximal oxygen is indicated; it is the opposite of the required strategy in paraquat toxicity. Critical care admission remains appropriate because serious poisoning requires close monitoring even when oxygen is deliberately withheld.

Reference: BTS guideline for oxygen use in adults in healthcare and emergency settings (May 2017) — https://www.brit-thoracic.org.uk/document-library/guidelines/emergency-oxygen/bts-guideline-for-oxygen-use-in-adults-in-healthcare-and-emergency-settings/ BTS Guideline for oxygen use in healthcare and emergency settings (Checked 18 August 2026) — https://www.brit-thoracic.org.uk/clinical-resources/guidelines/emergency-oxygen/