Carbon Monoxide Displaces Oxygen from Hemoglobin to form Carboxyhemoglobin.
CO has a higher affinity for hemoglobin than oxygen does. As such, CO displaces oxygen from hemoglobin resulting in the formation of a complex called carboxyhemoglobin, which doesn’t carry oxygen. CO is very effective and occupies all oxygen carrying sites. Oxygen cannot displace CO.
What that means is that CO essentially takes out of service a certain percentage of the hemoglobin. As far as oxygen transport and tissue oxygenation are concerned, the percentage of hemoglobin that is bound by CO might as well be lost through an acute bleed.
For example, if a patient with a preexposure (i.e. to CO) hemoglobin of 12 mg/dL has a carboxyhemoglobin level of 25%, that means only 75% of their hemoglobin is left to carry oxygen. Their maximum oxygen saturation has dropped by 25% and is now only 75%. One can say that their effective hemoglobin level is only 9, a 25% drop from 12.
Pulse Oximeter Reading in CO Poisoning
The pulse oximeter will show a saturation of 100%. Why? Because pulse oximeters detect color and the color of carboxyhemoglobin is bright red
As such, pulse oximetry is not reliable in patients with CO poisoning.
Similarly, on an ABG, the PaO2 will be normal. Why? Because it is based on oxygen gas tension (pO2) and not oxygen content or true oxygen saturation.
The only ABG abnormality in CO poisoning may be metabolic acidosis, which is a late finding.
Inadequate oxygen delivery to the peripheral tissues → anaerobic cellular respiration → lactic acid production → Metabolic Acidosis.