Carboxyhaemoglobin (COHb) is NOT a totally stable compound. It is much less likely to release its CO than oxyhaemoglobin is to release oxygen, but it still does so - it is not an 'irreversible' reaction as some of the books would have you believe.
The half-life of COHb is around 4 hours when breathing 'clean' air (i.e. 21% O2.) This shortens as you breathe a higher concentration of oxygen.
This is especially important to bear in mind if you are faced with testing someone for COHb levels some time after their possible exposure.
If someone comes to see you, say, 12 hours after possible exposure, they will have gone through three half-lives. So they might have had near-fatal levels of 40% COHb at the time, but their measurable levels will now be down to 5% or less.
'Within normal' COHb levels cannot be used to rule out exposure if there is a delayed presentation. As ever, an accurate history is the best tool.
The half-life of COHb is around 4 hours when breathing 'clean' air (i.e. 21% O2.) This shortens as you breathe a higher concentration of oxygen.
This is especially important to bear in mind if you are faced with testing someone for COHb levels some time after their possible exposure.
If someone comes to see you, say, 12 hours after possible exposure, they will have gone through three half-lives. So they might have had near-fatal levels of 40% COHb at the time, but their measurable levels will now be down to 5% or less.
'Within normal' COHb levels cannot be used to rule out exposure if there is a delayed presentation. As ever, an accurate history is the best tool.
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