Methaemoglobinaemia

Question 1

Methaemoglobinaemia - Causes

Answer 1

Congenital:

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

Acquire:
- DRUGS:
> Sulphonamides
> Nitrates
> Dapsone
> Sodium Nitroprusside
> Primaquine

• CHEMICALS: = Aniline dyes

Question 2

Methaemoglobinaemia - Features, Ix and Mx

Answer 2

• ‘Chocolate cyanosis’
• Dyspnoea
• Anxiety
• Headache
• Severe Sx: Acidosis, Arrhythmia, Seizures, Coma

ABG: Normal PO2 but DECREASED OXYGEN SATURATION (do you understand this?)

Mx:

• NADH-Methaemoglonaemia reductase deficiency = ASCORBIC ACID
• IV METHYLENE BLUE if acquired

Question 3

Methaemoglobinaemia - ABG

Answer 3

MIXED respiratory and metabolic acidosis with a relative hypoxia despite the inspired fraction of oxygen

Cyanosis and low SpO2 in the presence of a normal arterial PO2 by ABG

The diagnosis is confirmed by direct measurement of methemoglobin by a multiple wavelength co-oximeter. On a blood gas, normal PaO2 concentrations are usually found on analysis.

Clinical cyanosis in the presence of normal arterial oxygen tensions is highly suggestive of methemoglobinemia.

Rapid oxidisation of ferrous iron ions in haemoglobin to their ferric (Fe3+) states creates methaemoglobinaemia (MetHb).
> MetHb binds oxygen more avidly than haemoglobin and causes a shift in the dissociation curve to the left meaning that oxygen is not liberated in the tissues causing a cellular hypoxia (NB Shift to LEFT = Lower oxygen delivery, shift to left = more difficult to release O2)

> This manifests in a clinical cyanosis and often a deathly slate grey pallor to the patient. Oxygen saturations may not be as shocking as the patient appears since many pulse oximeters cannot distinguish between oxyhaemoglobin and methaemoglobin well.
However, supplemental oxygen will not improve recorded values (although it should still be given)