Carbon monoxide poisoning
USMLE Step 1Difficulty: mediumLast updated:
A 38-year-old man and his two children are brought to the emergency department in January after being found confused in their home, which has been heated with a gasoline-powered generator in the attached garage. The man reports a severe headache and nausea. Pulse oximetry on room air shows an oxygen saturation of 99%. Arterial blood gas analysis shows a PaO2 of 96 mm Hg. Co-oximetry shows a carboxyhemoglobin level of 28%.
Which of the following best explains why the pulse oximetry reading is misleading in this patient?
Answer and explanation
Correct answer: E. The pulse oximeter cannot distinguish carboxyhemoglobin from oxyhemoglobin
Carbon monoxide displaces oxygen from hemoglobin, reducing arterial oxygen content even though dissolved oxygen (PaO2) is unaffected. The source explains that ABGs and pulse oximetry are inadequate for diagnosis because the saturation reported in ABGs is derived from dissolved oxygen and is unaffected by carboxyhemoglobin, while the pulse oximeter cannot differentiate normal hemoglobin from carboxyhemoglobin and thus gives a falsely elevated oxyhemoglobin reading. The diagnosis therefore requires co-oximetry. The source also lists the mechanisms of toxicity: displacement of oxygen from hemoglobin (CO has greater affinity than oxygen), a leftward shift of the oxyhemoglobin dissociation curve that decreases oxygen release to tissues, and inhibition of mitochondrial respiration. Treatment starts with removal from the source and 100% oxygen by nonrebreather mask, which shortens the CO half-life from about 4.5 hours to about 1.5 hours.
Each option explained
- A. Carbon monoxide oxidizes heme iron to the ferric (Fe3+) stateIncorrect. Oxidation of heme iron to Fe3+ describes methemoglobinemia, not carboxyhemoglobin formation.
- B. Carbon monoxide lowers the arterial partial pressure of oxygenIncorrect. PaO2 reflects dissolved oxygen and is typically normal in CO poisoning, as in this patient (96 mm Hg).
- C. Carbon monoxide shifts the oxyhemoglobin dissociation curve to the rightIncorrect. CO shifts the curve to the left, which decreases oxygen unloading to tissues.
- D. Carbon monoxide binds hemoglobin with lower affinity than oxygenIncorrect. CO has a much greater affinity for hemoglobin than oxygen does.
- E. The pulse oximeter cannot distinguish carboxyhemoglobin from oxyhemoglobinCorrect. Standard two-wavelength pulse oximetry reads carboxyhemoglobin as oxyhemoglobin, giving a falsely high saturation.
Key point
In CO poisoning, PaO2 and pulse oximetry can be normal; carboxyhemoglobin must be measured by co-oximetry.
From the source
“the pulse oximeter cannot differentiate normal hemoglobin from carboxyhemoglobin and thus provides a falsely elevated oxyhemoglobin reading”