What is the role of pulse oximetry in the workup of methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Answer

Pulse oximetry is used extensively in the evaluation of patients with cyanosis and respiratory distress. Findings of bedside pulse oximetry in the presence of methemoglobinemia may be misleading. Pulse oximetry measurements with low-levels of methemoglobinemia often result in falsely low values for oxygen saturation and are often falsely high in those with high-level methemoglobinemia. The reason for these inaccuracies is as follows.

The pulse oximeter only measures the relative absorbance of 2 wavelengths of light (660 nm and 940 nm) to differentiate oxyhemoglobin from deoxyhemoglobin. The ratio of absorption of light at each of these wavelengths is converted into oxygen saturation by using calibration curves. Methemoglobin increases absorption of light at both wavelengths (more at 940 nm) and therefore offers optical interference to pulse oximetry by falsely absorbing light.

As a result, oxygen saturations by pulse oximetry in methemoglobinemia plateau at about 85%; therefore, a patient with a methemoglobin level of 5% and a patient with a level of 40% have approximately the same saturation values on pulse oximetry (~85%). The severity of the cyanosis does not correspond to the pulse oximetry reading: a patient may appear extremely cyanotic but still have a pulse oximetry reading in the high 80s.

However, newer multiwavelength pulse oximeters have been developed that can detect methemoglobinemia with an accuracy comparable to that achievable with co-oximeters.


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