Clinical assessment

The patient presents with….

Methaemoglobinaemia can be asymptomatic or cause a range of hypoxic symptoms.

Table 3: Symptoms related to percentage MetHb

MetHb as % of total Hb* Typical Symptoms+
<10 None
10-20 Cyanosis
20-30 Anxiety, headache, light headedness, tachycardia
30-50 Fatigue, confusion, dizziness, tachycardia, tachypnoea
50-70 Coma, seizures, arrhythmias, respiratory depression
> 70 Death

* for a Hb of 15 g/dL
+ symptom severity does not always reliably correlate to MetHb level but is affected by individual factors including pre-existing co-morbidities

Cyanosis is usually the first sign and patients can initially appear very well for the level of cyanosis. This is because cyanosis secondary to methaemoglobinaemia appears at 1.5 g/dL of MetHb, whereas cyanosis secondary to hypoxaemia appears at 5 g/dL of deoxygenated haemoglobin, which represents a much greater reduction in oxygen carrying capacity.

Patients often have a low SpO2 on pulse oximetry but again appear well for the degree of hypoxia. A notable feature of methaemoglobinaemia is a low SpO2 on pulse oximetry which does not improve with supplemental oxygen. This can be the first pointer to the diagnosis of methaemoglobinaemia [4].

Learning bite

In any patient who presents with cyanosis or low SpO2 out of keeping with their clinical state, and which does not improve with supplemental oxygen, think methaemoglobinaemia.

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