CO is a potent, multi-system toxin. Although its most famous and immdeiately lethal effect is its affinity for haemoglobin (around 240 times 'stronger' than Hb's affinity for oxygen) it has many other effects too, on many other systems.
It can damage neural tissue directly. It causes rhabdomyolysis. It causes platelet aggregation.
It does loads of other stuff too.
It's produced naturally in the body, at a rate of about 2mls per day, by the enzyme haem oxygenase. Some postulate that haemoglobin itself is actually designed to scavenge physiological CO - hence the otherwise evolutionary inexplicable affinity for Hb with CO in preference to O2.
For a very quick hint at all you never knew about CO, have a look here. More to follow...
Showing posts with label Myths. Show all posts
Showing posts with label Myths. Show all posts
Thursday, 9 November 2006
CO does NOT bind 'irreversibly' to Hb.
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.
Monday, 6 November 2006
Carbon monoxide does NOT turn you 'cherry pink.'
The red discolouration of mucous membranes caused by the presence of carboxyhaemoglobin is a post-mortem oddity, NOT a clinical sign. You need more than 40% or so COHb to make this colour visible to the human eye, by which time the victim will probably be unconscious.
The absorption spectrum of COHb does affect the way pulse oximeters behave, but it cannot, and must not, be used as a clinical “rule-out” sign for possible exposure.
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