What is the role of pulse oximetry and carbon monoxide (CO)-oximetry in the workup of smoke inhalation injury?

Updated: Oct 15, 2021
  • Author: Keith A Lafferty, MD; Chief Editor: Joe Alcock, MD, MS  more...
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Answer

Pulse oximetry readings can be misleading in the setting of carbon monoxide (CO) exposure or methemoglobinemia because these devices use only 2 wavelengths of light (the red and the infrared spectrum), which detect oxygenated and deoxygenated hemoglobin only and not any other form of hemoglobin. Readings are falsely elevated by CO-bound hemoglobin (carboxyhemoglobin).

In methemoglobinemia, light reflection is similar to that in reduced hemoglobin. Pulse oximetry may show a depressed oxygen saturation, but the decrease does not accurately reflect the level of methemoglobinemia. In fact, as methemoglobin levels reach 30% or higher, the pulse oximetry reading converges on approximately 85%.

CO-oximeters use 4 wavelengths of light and are capable of detecting carboxyhemoglobin and methemoglobin as well as hemoglobin and oxyhemoglobin. Some newer co-oximeters use 5 wavelengths and are also able to measure sulfhemoglobin. The percent of oxyhemoglobin measured by CO-oximetry is an accurate measure of the arterial oxygen saturation. The difference between saturations obtained by CO-oximetry and calculated figures is known as the saturation gap and is an indicator of dyshemoglobinemia.


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