It binds haemoglobin 240 times more avidly than oxygen and shifts the curve left
Expansion
Carbon monoxide causes harm in three ways:
- Competitive binding with about 240 times the affinity of oxygen, reducing oxygen carrying capacity
- Left shift of the dissociation curve for the remaining haemoglobin, so what oxygen is carried is released less readily to the tissues
- Direct cellular toxicity, by binding cytochrome oxidase and myoglobin
The second effect is the one often forgotten, and it is why carbon monoxide is far more toxic than an equivalent degree of anaemia.
Clinical features are non-specific: headache, nausea, confusion, then coma. The classic cherry red skin is a late and unreliable sign.
Key diagnostic trap: arterial oxygen tension is normal, because dissolved oxygen is unaffected, and pulse oximetry reads falsely high, since it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin. Diagnosis requires co-oximetry.
Treatment is high flow oxygen, which reduces the half-life from about 4 hours to about 1 hour, and hyperbaric oxygen in severe cases.