Expansion
Carbon monoxide blocks oxygen carriage; cyanide blocks its use
Mnemonic
Both poison oxygen use while leaving the PaO2 and pulse oximeter normal:
Carbon monoxide
- Binds haemoglobin with 240 times the affinity of oxygen, and shifts the curve left, so it both reduces carriage and blocks release
- Pulse oximetry reads falsely normal, since it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin. Diagnosis needs co-oximetry
- Headache, nausea, confusion, and multiple household members affected. Cherry red skin is late and unreliable
- Treatment is high flow oxygen, cutting the half life from about 4 hours to under 1
Cyanide
- Poisons cytochrome c oxidase, so cells cannot use oxygen at all
- High venous oxygen saturation with a profound lactic acidosis, in a patient who is not cyanosed
- Smoke inhalation from burning plastics, and industrial exposure
- Treatment is hydroxocobalamin, which turns urine red, with sodium thiosulphate
“Normal saturations, normal PaO2, high lactate” should raise both, and in a house fire both may coexist.
Expansion
Carbon monoxide
- Binds haemoglobin 240 times more avidly than oxygen, and shifts the dissociation curve left, so the remaining oxygen is not released to tissue
- Also binds cytochrome oxidase and myoglobin
- PaO2 normal, pulse oximetry falsely high (it cannot distinguish carboxyhaemoglobin); diagnosis needs co-oximetry
- Treatment: high flow oxygen (reducing the half-life from about 4 hours to 1), hyperbaric oxygen in severe cases
Cyanide
- Blocks cytochrome oxidase (complex IV), so tissues cannot use oxygen
- Histotoxic hypoxia: severe lactic acidosis with a characteristically high venous oxygen saturation, because oxygen is delivered but not extracted
- Skin may be pink rather than cyanosed; bitter almond odour is unreliable
- Treatment: hydroxocobalamin (which turns urine red), or sodium thiosulphate
Both should be suspected in smoke inhalation from an enclosed space, and the combination is common. A patient rescued from a house fire with reduced consciousness and a raised lactate should be assumed to have both, and treated for both.