Why does carboxyhemoglobin cause high SpO2 reading?

[WashMe]

i've been chewing on this for a couple years now, and it intermittently comes up. I just can't figure it out. If oxyhemoglobin has high absorption at 940 nm and deoxyhemoglobin has high absorption at 660 nm, then why doesn't carboxyhemoglobin make it appear as if there is more deoxyhemoglobin (i.e. Low spo2)? I always read some bogus explanation about how the absorption spectra of oxyhemoglobin and carboxyhemoglobin roughly overlap at 660 nm so the machine "can't differentiate them", but this seems irrelevant/erroneous to me. The pulse ox just emits and measures two wavelengths of light, 660 and 940 nm; it has no way to be "confused by overlapping spectra". The ratio of absorption determines the calculated saturation. It seems that anything that absorbs light primarily at 660 would look more like deoxyhemoglobin, and anything that absorbs light primarily at 940 would look more like oxyhemoglobin. The device does not measure any sort of spectrum for hemoglobin species, it can only see light in and light out. Looking at the absorption spectra for these hemoglobins, it is clear that carboxyhemoglobin has no significant absorption at 940, and high absorption at 660. Thus, looking at overall light absorption, there will be increased relative absorption at 660, unchanged absorption at 940, so this results in an increased 660:940 ratio which would seem to lie on the deoxyhemoglobin side of things based on looking at the actual spectra. What am I missing here?

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[IlDestriero]

I suspect it is an artifact of how the pulse oximeter processes the absorption data at the 2 wavelengths to yield the (erroneous) result in the presence of a significant amount of carboxyhemoglobin.

 

[Planktonmd]

Correct! the problem is with with the algorithm used by the device to calculate the saturation from light absorption. It was not designed to handle carboxy hemoglobin.

 

[WashMe]

Thanks for the responses. I guess my question is why would it confuse it for oxyhb, not deoxyhb, when the absorption spectrum ratio of 660 and 940 more resembles deoxyhb? The machine only sees the ratio of light in and light out, it seems like the resultant calculation from the absorption ratio would point to a lower spo2.