Question: why is pao2 normal in methemoglobinemia?
Answer: Methemoglobin absorbs light at both of those wavelengths, thus the presence of these additional hemoglobin species makes SpO2 calculation inaccurate. Arterial blood gas measurement of PO2 is not affected by methemoglobin, resulting in a normal (and often elevated due to supplemental oxygen) calculated SaO2.