iNO Methaemoglobinaemia Monitoring — FRCA Final MCQ

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ModerateIntensive Care MedicineiNO Methaemoglobinaemia MonitoringFRCA Final

A 55-year-old woman on ICU has severe ARDS. She is commenced on inhaled nitric oxide at 20 ppm. After 4 hours her oxygenation has improved but the iNO is being weaned. What metabolic complication must be monitored during iNO therapy?

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Correct answer: C — Hypernatraemia

Inhaled nitric oxide is oxidised to methaemoglobin (MetHb) and nitrate. MetHb cannot carry oxygen. Continuous co-oximetry or intermittent MetHb levels should be monitored during iNO therapy. MetHb >5% is clinically significant; >20% causes tissue hypoxia, cyanosis, and cardiovascular compromise. Treatment of significant methaemoglobinaemia: methylene blue 1-2 mg/kg IV (contraindicated in G6PD deficiency). Other iNO monitoring: NO2 levels (toxic nitrogen dioxide formed from NO + O2).

Reference: ESC – 2022 – Pulmonary hypertension; ICS – 2024 – ARDS adjunctive therapies