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Pulse oximetry in adults

C Crawford Mechem, MD, FACEP
Section Editor
Polly E Parsons, MD
Deputy Editor
Geraldine Finlay, MD


Pulse oximetry measures peripheral arterial oxygen saturation (SpO2) as a surrogate marker for tissue oxygenation. It has become the standard for continuous, noninvasive assessment of oxygenation, such that is now known as the "fifth vital sign" [1-3]. Theoretical and clinical aspects of pulse oximetry will be reviewed here. Other measures of oxygenation and mechanisms of hypoxemia are discussed separately. (See "Oxygenation and mechanisms of hypoxemia".)


Pulse oximetry uses spectrophotometry to determine the proportion of hemoglobin that is saturated with oxygen (ie, oxygenated hemoglobin; oxyhemoglobin) in peripheral arterial blood. Light, at two separate wavelengths, illuminates oxygenated and deoxygenated hemoglobin in blood. The ratio of light absorbance between oxyhemoglobin and the sum of oxyhemoglobin plus deoxyhemoglobin is calculated and compared with previously calibrated direct measurements of arterial oxygen saturation (SaO2) to establish an estimated measure of peripheral arterial oxygen saturation (SpO2) [4].

The Beer-Lambert law — Pulse oximetry estimates peripheral SpO2 using a variation of the Beer-Lambert law. This law states that the absorption of light of a given wavelength passing through a non-absorbing solvent, which contains an absorbing solute, is proportional to the product of the solute concentration, the light path length, and an extinction coefficient. The Beer-Lambert law can readily be applied to co-oximeters used for a static column of arterial for blood gas analysis but needs to be modified to overcome the obstacles associated with interference from tissue and pulsatile flow when measuring SpO2 [5]. This modification involves measuring absorbance at using two different wavelengths, one to detect oxyhemoglobin and the other to detect deoxyhemoglobin [1].

Probes — Pulse oximeter probes consist of two light-emitting diodes and a photodetector.

Emitters – Deoxyhemoglobin absorbs light maximally in the red band of the spectrum (600 to 750 nm), and oxyhemoglobin absorbs maximally in the infrared band (850 to 1000 nm) [1]. Thus, the emitters emit light at 660 nm and 940 nm for optimal detection of these two substances.

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Literature review current through: Nov 2017. | This topic last updated: Nov 02, 2017.
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