Methaemoglobinaemia

Methaemoglobinaemia describes haemoglobin which has been oxidised from Fe2+ to Fe3+. This is normally regulated by NADH (nicotinamide adenine dinucleotide hydride) methaemoglobin reductase, which transfers electrons from NADH to methaemoglobin resulting in the reduction of methaemoglobin to haemoglobin. There is tissue hypoxia as Fe3+ cannot bind oxygen, and hence the oxidation dissociation curve is moved to the left

Congenital causes

• Haemoglobin chain variants: HbM, HbH
• NADH methaemoglobin reductase deficiency

Acquired causes

• Drugs: see box below
• Chemicals: aniline dyes

Clinical Features

• 'Slate grey' cyanosis refers to blue to grey skin colouration
• 'Chocolate blood' or 'chocolate cyanosis' refers to the brown colour of blood
• Dyspnoea
• Low SpO2 on pulse oximetry (often doesn't improve with supplemental oxygen)
• Patient will often tolerate hypoxia better than expected
• Normal PaO2 on ABG (but low SaO2)
• Anxiety
• Headache
• Severe: acidosis, arrhythmias, seizures, coma

Diagnosis & investigations
Diagnosis is made via direct measurement of MetHb level using co-oximeter (present on most modern blood gas analysers)

Other investigations

• FBC (anaemia may contribute to hypoxia)
• ECG (check for arrhythmia's)
• CXR (lung disease may contribute to hypoxia)
• bHCG (management thresholds different in pregnancy)

Management
Active treatment is required if:

• MetHb > 30%
• MetHb <30% but symptomatic
• Evidence of tissue hypoxia caused by MetHb e.g. myocardial ischaemia on ECG

Active treatment includes:

• Maintain airway and deliver high flow Oxygen
• Remove causative agents
• Treat toxidromes
• Consider IV dextrose 5%
• IV methylene blue (if acquired cause)
• If no response to methylene blue consider hyperbaric oxygen or exchange transfusion
• Ascorbic acid can be used if cause is NADH - methaemoglobinaemia reductase deficiency.