Pulse oximetry

Pulse oximeters rely on the differential absorption of oxyhemoglobin and reduced hemoglobin at two wavelengths of light, 660 mm and 940 mm.

Pulse oximeters estimate the functional saturation of oxygen, that is the oxygen saturation in arterial blood.

Pulse oximetry involves shining two wavelengths of light, red and infrared through peripheral tissue that has a pulsating vascular bed such as a fingertip or earlobe.
The two major forms of hemoglobin, deoxyhemoglobin and oxyhemoglobin, absorb light differently at these two wavelengths: deoxyhemoglobin absorbs more red light, and oxyhemoglobin absorbs more infrared light.

The functional saturation of oxygen is calculated as oxyhemoglobin divided by the sum of oxyhemoglobin and reduced hemoglobin

If a patient lacks a sufficient pulsation low the SPO2 may be inaccurate.
The administration of medical dyes  that absorb red or infrared light such as methylene blue, indigocyanine green,  indigo Carmine can interfere with SPO2 measurements and cause an oxygen saturation gap.

Pulse oximeters calculate an arterial absorption ratio by measuring the amount of light absorbed at each wavelength during arterial pulses and factors out static background absorption, and converts the information to oxygen saturation readings.

Low pulse oximetry measurements indicates hypoxemia.

Hemoglobin variants can alter light absorption spectra of hemoglobin and produce falsely low SpO2 levels.

Three abnormal hemoglobin variants: carboxyhemoglobin, sulfhemoglobin, methemoglbin absorb both red and infrared light and interfere with SPO2 measurements leading to an oxygen saturation gap.

Sickle cell anemia patients have unreliable pulse oximetry measurements.

When heme iron is in the ferric form it cannot bind oxygen and changes the absorption spectrum of hemoglobin resulting in inaccurate oxygen saturation measurements.

Methemoglobin has similar absorption at 660 mm and 940 mm wavelengths, it alters absorption ratio of the pulse oximeter and biases it toward an 85% reading (Tremper KK, Barker SI,)

Methemoglobinemia can cause low pulse oximetry levels, with under estimation of the SpO2 when the true values bar above 85%, and in over estimation of the oxygen saturation in patients below this level.

The presence of methemoglobulinemia underestimates the true oxygen saturation in most situations, but can still have significant clinical consequences.

Pulse oximetry measurements systematically overestimate arterial oxygen saturation in patients with darker skin colors, potentially leading to inappropriate medical decisions for Black patients.

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