Methaemoglobinaemia — SAQ

Stem

A 23-year-old man is brought to the emergency department 45 minutes after an upper gastrointestinal endoscopy during which his throat was sprayed with benzocaine topical anaesthetic. He is centrally and peripherally dusky slate-grey, dyspnoeic and anxious. He has no cardiac or respiratory history and takes no regular medication.

On examination: GCS 14, respiratory rate 28/min, pulse 112/min, blood pressure 134/86 mmHg, SpO2 85 percent on room air which does not rise when 100 percent oxygen is applied by non-rebreather mask. There is no chest crepitation and no stridor. A venous blood sample drawn for gas is noted by the nurse to be chocolate-brown and to stay brown when the tube is shaken in air.

Investigations: arterial blood gas PaO2 96 mmHg, PaCO2 34 mmHg, pH 7.36, calculated SaO2 98 percent, lactate 3.1 mmol/L. Co-oximetry reports methaemoglobin 38 percent of total haemoglobin. Full blood count and renal function are normal. G6PD status is unknown.

Questions

a) What is the diagnosis, and explain why BOTH the pulse oximetry and the standard ABG gave misleading readings. (2 marks)

Diagnosis: acquired methaemoglobinaemia (benzocaine-induced), severe (MetHb 38 percent).

• Pulse oximetry reads ~85 percent and does not rise with oxygen because MetHb absorbs strongly at both pulse-oximeter wavelengths (660 nm red and 940 nm infrared); the device, calibrated only for oxy- and deoxyhaemoglobin, is driven toward a plateau at ~85 percent and becomes insensitive to changes in true arterial oxygenation.
• The standard ABG reports a falsely normal calculated SaO2 (98 percent) with a normal PaO2 (96 mmHg) because the machine does not measure SaO2 — it calculates it from PaO2/pH/temperature assuming all haemoglobin is normal. PaO2 reflects dissolved oxygen and is unaffected by dyshaemoglobins, so it stays normal.
• Only CO-OXIMETRY (multi-wavelength, measuring oxy-, deoxy-, carboxy- and methaemoglobin directly) reveals the true MetHb. The discrepancy between the calculated SaO2 and the true SaO2 is the saturation gap.

b) Describe the molecular mechanism by which benzocaine causes methaemoglobinaemia, and explain why the patient is hypoxic despite a normal PaO2. (3 marks)

• Benzocaine is an oxidant that overwhelms the red-cell reducing systems, forcing the haem iron from the ferrous (Fe2+) to the ferric (Fe3+) state, forming methaemoglobin.
• Ferric iron cannot bind oxygen, so each MetHb subunit is a lost carrying site (functional anaemia). Worse, ferric iron stabilises the R (relaxed) state of the neighbouring ferrous subunits in the tetramer, shifting the oxygen-haemoglobin dissociation curve to the LEFT — so the oxygen bound to remaining normal haemoglobin is held more tightly and released less readily to tissues.
• Under normal conditions spontaneous auto-oxidation forms ~0.5-3 percent MetHb daily, held below 1 percent by the major reducing pathway — NADH-cytochrome b5 reductase (~95 percent of reduction). Benzocaine overwhelms this. The minor NADPH-dependent methaemoglobin reductase pathway is normally dormant but is the target of the antidote methylene blue.
• The patient is hypoxic despite a normal PaO2 because PaO2 measures dissolved oxygen (unaffected by haemoglobin dysfunction); the problem is haemoglobin carriage and unloading, not lung gas exchange — hence tissue hypoxia (lactate 3.1) and the clinical picture with a normal PaO2.

c) Outline the stepwise definitive management, with drug doses and the mechanism of the antidote. (3 marks)

1. Resuscitate + stop the oxidant: ABCDE, high-flow 100 percent oxygen (maximises carriage by remaining functional Hb and dissolved O2), IV access, continuous ECG monitoring; withhold further benzocaine.
2. Methylene blue 1-2 mg/kg IV (= 0.1-0.2 mL/kg of the 1 percent solution) given slowly over 5 minutes — the first-line antidote. Mechanism: methylene blue is an artificial electron carrier for the NADPH-dependent methaemoglobin reductase pathway; it is reduced to leukomethylene blue (using NADPH from the hexose-monophosphate shunt — hence G6PD dependence), which non-enzymatically reduces MetHb-Fe3+ back to functional Hb-Fe2+. Response within 30-60 minutes (the patient visibly turns pink). Repeat 1-2 mg/kg after 30-60 min if symptoms persist; maximum 7 mg/kg in 24 h (larger doses are themselves oxidant — cause haemolysis and paradoxically raise MetHb).
3. Check G6PD as soon as possible — methylene blue is contraindicated in G6PD deficiency (fails — no NADPH — AND causes oxidative haemolysis). If G6PD-deficient, use ascorbic acid, N-acetylcysteine, and exchange transfusion for severe disease.
4. Supportive/ICU for severe disease (here MetHb 38 percent, confused — ICU admission); treat any arrhythmia or ischaemia; observe with serial MetHb every 4-6 h.

d) State THREE contraindications or cautions to methylene blue, and ONE alternative antidote for use when methylene blue cannot be given, with its limitation. (2 marks)

Contraindications/cautions (any three):

1. G6PD deficiency (absolute) — methylene blue fails (no NADPH) and causes haemolysis.
2. Serotonergic drugs (SSRIs, SNRIs, MAOIs, tramadol, linezolid) — methylene blue is a weak MAO inhibitor and can precipitate serotonin syndrome (FDA warning).
3. Large doses (over 7 mg/kg in 24 h) — themselves oxidant, causing haemolysis and a paradoxical rise in MetHb.
4. Haemoglobin M disease / cytochrome b5 reductase deficiency — response poor (structural defect persists).
5. Pregnancy and renal failure — relative caution (methylene blue crosses placenta; renally excreted).

Alternative (one): Ascorbic acid (vitamin C) 300-1000 mg/day orally (or IV for faster effect) — provides slow non-enzymatic direct reduction of MetHb over 24-48 hours; its limitation is its slow onset, so it is unsuitable as sole therapy in rapidly deteriorating severe disease (where exchange transfusion is the alternative). Other alternatives: N-acetylcysteine (glutathione precursor), exchange transfusion (physically removes MetHb), hyperbaric oxygen (provides dissolved O2, salvage only).