Which would NOT cause erroneous readings by dual-wavelength pulse oximeters?

Which would NOT cause erroneous readings by dual-wavelength pulse oximeters?

• A. Carboxyhemoglobin
• B. Methylene blue
• C. Fetal hemoglobin
• D. Methemoglobin

Answer: (C)

Dual-wavelength pulse oximetry estimates oxygen saturation by comparing how oxyhemoglobin and deoxyhemoglobin absorb red and infrared light. If a form of hemoglobin alters absorption at those two wavelengths in a way the device can’t distinguish from normal oxyhemoglobin, the reading becomes unreliable. Fetal hemoglobin behaves very similarly to adult hemoglobin at these wavelengths, so HbF doesn’t distort the ratio and SpO2 remains accurate. Carboxyhemoglobin, however, absorbs light at the red wavelength in a way that makes it look like oxyhemoglobin, so the pulse oximeter tends to read falsely high and may mask true hypoxemia. Methemoglobin absorbs light at both wavelengths more evenly, pulling the reading toward around 85% and making SpO2 unreliable. Methylene blue can also interfere with the optical signal and cause artifactual readings, especially when used to treat methemoglobinemia.

Dual-wavelength pulse oximetry estimates oxygen saturation by comparing how oxyhemoglobin and deoxyhemoglobin absorb red and infrared light. If a form of hemoglobin alters absorption at those two wavelengths in a way the device can’t distinguish from normal oxyhemoglobin, the reading becomes unreliable. Fetal hemoglobin behaves very similarly to adult hemoglobin at these wavelengths, so HbF doesn’t distort the ratio and SpO2 remains accurate.

Carboxyhemoglobin, however, absorbs light at the red wavelength in a way that makes it look like oxyhemoglobin, so the pulse oximeter tends to read falsely high and may mask true hypoxemia. Methemoglobin absorbs light at both wavelengths more evenly, pulling the reading toward around 85% and making SpO2 unreliable. Methylene blue can also interfere with the optical signal and cause artifactual readings, especially when used to treat methemoglobinemia.