Pulse Oximetry – Carboxyhemoglobin
A pulse oximeter will measure red and infrared light transmitted through and reflected by the tissue bed. Any light is usable, since all light frequencies will transmit through and be reflected by the blood and tissue. The industry has chosen red and infrared light since it is technically possible to produce these wavelengths by an LED diode
There are actually four different hemoglobin species in normal blood: oxyhemoglobin, reduced (deoxy-) hemoglobin, methemoglobin, and carboxyhemoglobin. Each of these species has a different light absorption profile as can be seen on the graph below.
As the graph show the light profiles in the chosen areas (red = 660 nm and infrared = 940 nm) overlap considerably for carboxyhemoglobin and oxyhemoglobin (as it does for methgb and deoxyhgb). This means that the pulse oximeter will see the reflected light for carboxyhgb and oxyhgb as the same when measuring in the red light spectrum and thereby measure a combined reflection. Carboxyhgb is not reflected/absorbed in the infrared spectrum and therefore does not add to this part in the ratio. Remember, the ratio between these measurements is checked against the pulse oximeters database. This ratio will correspond with the displayed SpO2.
When the combined reflection includes the non-oxygen carrying carboxyhgb as equal to oxyhgb the result will be a false high reading. How much too high? This depends on the level of carboxyhgb. The higher the carboxyhgb level the bigger the discrepancy.
From Miller's Anesthesia 7th ed.p.1215