Methemoglobinemia typically causes pulse oximetry to read around which value, despite a normal PaO2?

Methemoglobinemia typically causes pulse oximetry to read around which value, despite a normal PaO2?

• A. 90-92%
• B. 85%
• C. 98%
• D. 70%

Answer: (B)

Methemoglobinemia interferes with how a pulse oximeter estimates saturation. Pulse oximetry uses two wavelengths of light to distinguish oxyhemoglobin from deoxyhemoglobin, but when methemoglobin is present, it absorbs light at both wavelengths in a way that biases the reading toward a fixed value. As a result, the oximeter tends to show about 85% saturation, even if the true arterial saturation is higher or lower. Meanwhile, the arterial oxygen tension (PaO2) can remain normal because it reflects dissolved oxygen in the plasma, not the amount bound to hemoglobin. The iron in hemoglobin has been oxidized to the ferric state and can’t bind O2 effectively, so oxygen content is reduced despite a normal PaO2. Co-oximetry, which directly measures methemoglobin, provides an accurate assessment, and treatment with a reducing agent like methylene blue can be used if indicated.

Methemoglobinemia interferes with how a pulse oximeter estimates saturation. Pulse oximetry uses two wavelengths of light to distinguish oxyhemoglobin from deoxyhemoglobin, but when methemoglobin is present, it absorbs light at both wavelengths in a way that biases the reading toward a fixed value. As a result, the oximeter tends to show about 85% saturation, even if the true arterial saturation is higher or lower. Meanwhile, the arterial oxygen tension (PaO2) can remain normal because it reflects dissolved oxygen in the plasma, not the amount bound to hemoglobin. The iron in hemoglobin has been oxidized to the ferric state and can’t bind O2 effectively, so oxygen content is reduced despite a normal PaO2. Co-oximetry, which directly measures methemoglobin, provides an accurate assessment, and treatment with a reducing agent like methylene blue can be used if indicated.