A firefighter rescued from a burning building is confused with cherry-red skin. Pulse oximetry shows SpO2 of 97% and arterial blood gas shows a normal PaO2. The discrepancy between the pulse oximeter reading and his clinical condition occurs because:

• A Carbon monoxide increases dissolved oxygen content, keeping PaO2 artificially elevated
• B Carboxyhemoglobin absorbs light at the same wavelengths as oxyhemoglobin, so standard pulse oximetry cannot distinguish them ✓
• C Carbon monoxide converts hemoglobin to methemoglobin, which the oximeter reads as saturated hemoglobin
• D Carbon monoxide shifts the oxygen dissociation curve to the right, improving peripheral saturation readings

Explanation

CO binds heme iron with roughly 200 times the affinity of oxygen, forming carboxyhemoglobin. Standard two-wavelength pulse oximeters register carboxyhemoglobin largely as if it were oxyhemoglobin, giving a falsely reassuring saturation despite severe functional anemia. PaO2 measures dissolved oxygen and remains normal because CO does not affect gas exchange. CO shifts the curve left, not right, and does not produce methemoglobin.

Reference: Harper's Illustrated Biochemistry, 31st ed.

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Written and medically reviewed by the StethoPrep medical team.