How does a pulse oximeter work?

A pulse oximeter is a noninvasive monitoring device that can indirectly measure a person’s functional oxygen saturation (SpO2) and help with early detection of hypoxemia. Pulse oximeters work by either transmitting light through tissue perfused by blood (ideally a site with a dense capillary bed like the fingertip). They are called pulse oximeters because they use the pulsations to discriminate between arterial blood and other tissue or venous blood. The probe on the pulse oximeter has light-emitting diodes (LEDs) which shine at least two types of light, red and infrared. The device is positioned so that the light shines through a translucent part of a patient’s body, such as a fingertip or earlobe, and measures the changing absorbance at each wavelength. Oxygenated hemoglobin absorbs more infrared light, whereas deoxygenated hemoglobin absorbs more red light. Since the absorption of light at these wavelengths differs between oxygenated and deoxygenated blood, the device can determine the absorbances due to just the arterial blood and thereby determine a patient’s peripheral oxygen saturation.

References: Lifebox Pulse Oximetry Learning Module; WHO Using Pulse Oximeters

Keywords: wavelengths, IR, infrared, red, LED