Dissociation of CO hemoprotein complexes

Dissociation of CO from hemoprotein complexes follows the laws of gaseous diffusion and mass action. The dissociation rate depends on the proportions of O2, CO, the respective heme moiety, and their relative affinities for O2 vs CO. So, to (1) remove a patient from the toxic atmosphere in order to remove pCO and (2) restore pO2 represent two fundamental therapeutic measures.

Carboxyhemoglobin dissociation begins as soon as the patient is removed from the CO-rich environment. It follows exponential elimination with an half-life of 230-320 min in room air. The dissociation rate is much increased when oxygen pressure is raised: in pure normobaric oxygen, the half-life of COHb dissociation is 90 min; it is 35 min in 2 ata pure oxygen, and 22 min in 3 ata pure oxygen26.

The dissociation rate of CO from other hemoprotein complexes is unknown but likely to be much slower as they depend on the quantity of oxygen delivered to the tissues, which is much lower than arterial values and subject to the effects of COHb levels as well. Miro et al. showed that the inhibition of mitochondrial cytochrome a-a3 peaked at 76 % during the acute phase of an even moderate CO poisoning (COHb between 0 and 25 %) and that it remained at a 48 % inhibition level until the 3rd day post exposure27. This persisting inhibition in the presence of normalization of COHb could account for the persistence of clinical manifestations.

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