Effects of HBO

Elimination of CO depends on its release from hemoproteins. The latter will occur in the absence of ongoing exposure as a function of recovering pO2.

COHb dissociation begins as soon the exposure is terminated. However, given the higher affinity of CO to hemoglobin, the half-life for elimination is between 230 and 320 minutes breathing ambient air. COHb Dissociation is greatly increased by oxygen: its half-life is 90 minutes during NBO; 35 minutes during 2 ata HBO and 22 minutes in 3 ata HBO55.

Even though the affinity for CO is less than for hemoglobin, dissociation of CO from other hemoproteins is slower due to the low tissue pO2 levels compared to the arterial pO2. This is exacerbated by the COHb-related reduced oxygen delivery. Accordingly, detoxification cannot begin until peripheral oxygen delivery is sufficient, i.e., once COHb dissociation is well on the way. This may also explain why detoxification of the other hemoproteins is delayed relative to haemoglobin.

HBO, by increasing the quantity of dissolved oxygen, immediately restores to normal peripheral oxygen delivery. As a consequence, the hypoxemic hypoxia ceases prompting the recovery of the histotoxic hypoxia. Cytochrome a-a3 reverts to its functional form which enables the mitochondrial respiratory chains to recover their ability to reduce oxygen. The production of oxygen free radicals decreases, promoting an improved recuperation of cellular functions. Brown & Piantadosi have reported an experimental confirmation of these mechanisms56,57.

Finally, Thom reported HBO decreases the vascular lesions caused by the reoxygenation phase by decreasing lipid peroxidation58. The mechanism involved was a decrease in leukocyte adhesion to the vascular endothelium. The latter was due to a decrease in beta-2 integrin expressure by means of cyclic GMP reduction caused by the effects of HBO effect on membrane guanylate cyclase59. This favourable action of HBO has also been observed in other situations of ischemia-reperfusion60.

In summary, there is a large body of experimental data showing the greater effectiveness of HBO as compared with NBO.

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