The Complete Handbook of Pulse Oximeters

Chapter 1: Understanding Pulse Oximeters

A pulse oximeter is a medical device that measures the oxygen saturation of hemoglobin in peripheral blood, expressed as SpO₂. It also reports pulse rate. The device works by passing light through a translucent part of the body—typically a fingertip, earlobe, or toe—and analyzing the changes in light absorption that occur as blood pulses through the tissue. The oxygen saturation value, which is represented as a percentage, indicates how much of the hemoglobin in the arterial blood is carrying oxygen. A healthy adult at sea level typically shows a SpO₂ reading of 95% to 100%. Readings below 90% are considered low and typically trigger clinical concern, though the exact threshold for intervention depends on the patient's baseline and medical context.

To understand how a pulse oximeter works, you need to know a little about the behavior of light and blood. Hemoglobin has different optical properties depending on whether it is bound to oxygen. Oxygenated hemoglobin (HbO₂) absorbs less red light at the 660 nanometer wavelength and more infrared light at the 940 nanometer wavelength. Deoxygenated hemoglobin (Hb) behaves in the opposite way. A pulse oximeter contains at least two light-emitting diodes (LEDs)—one red, one infrared—which alternately illuminate the tissue, and a photodetector on the opposite side that measures how much of each wavelength passes through. By calculating the ratio of absorption between the two wavelengths, the device derives the proportion of oxygenated hemoglobin.

The "pulse" in pulse oximetry comes from the device's ability to distinguish