Carbon monoxide poisoning
Carbon monoxide binds haemoglobin far more avidly than oxygen, so oxygen carriage collapses while the arterial oxygen tension and the pulse oximeter both stay deceptively normal.
Definition
Carbon monoxide poisoning is systemic toxicity from inhaling carbon monoxide, a colourless, odourless, tasteless product of incomplete combustion. The diagnosis rests on a compatible exposure history with compatible symptoms, supported by a raised carboxyhaemoglobin measured by co-oximetry. A normal carboxyhaemoglobin does not exclude it once the patient has left the source or received oxygen.
First principles
Haemoglobin prefers carbon monoxide 245 times over
The affinity of haemoglobin for carbon monoxide is 245 times that for oxygen, so it wins nearly every binding site it reaches. Around 80 to 90% of absorbed gas ends up on haemoglobin as carboxyhaemoglobin (COHb). Carriage falls, the remaining haem groups hold their oxygen more tightly, and unloading into tissue fails. The result is severe tissue hypoxia with a normal haemoglobin concentration and a normal arterial oxygen tension.
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