Blue Dye Pulse Oximetry Interference — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyBlue Dye Pulse Oximetry InterferenceFRCA Final

A 55-year-old woman undergoes mastectomy and sentinel lymph node biopsy under general anaesthesia. Patent Blue V is injected for lymphatic mapping. Which artefact may occur as a direct consequence of the dye?

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Correct answer: D — Falsely reduced peripheral pulse-oximetry readings

Patent Blue V can produce a falsely low SpO2 because circulating dye absorbs light used by the pulse oximeter, particularly near the red wavelength associated with deoxyhaemoglobin. The device therefore interprets the altered red-to-infrared absorption ratio as reduced arterial saturation, although arterial oxygenation may be unchanged. It does not directly cause falsely high blood pressure, ECG ST-segment artefact or increased end-tidal carbon dioxide. The original wording incorrectly treated methylene blue and Patent Blue V as the same compound; they are distinct dyes, although both can interfere with oximetry. Importantly, Patent Blue V may also cause delayed, severe anaphylaxis. A saturation fall must therefore initially be treated as genuine hypoxaemia, with clinical assessment and arterial blood-gas analysis when appropriate, rather than being dismissed as dye interference.

Reference: Royal College of Anaesthetists. NAP6: Perioperative Anaphylaxis, Chapter 18 — Patent Blue dye. 2018. https://www.rcoa.ac.uk/media/36806