How a Pulse Oximeter Works

Pulse oximetry is a very simple, non-invasive procedure of monitoring the oxygen saturation of the hemoglobin component of the red blood cell. Basically, it works by measuring the absorption of red and infrared by pulsatile blood.

Principle of Pulse Oximetry

The basic principle of the way it operates is centered on the red to infrared light absorption qualities of oxygenated and deoxygenated hemoglobin. The pulse oximeter consists of a probe that is sometimes connected to a digital handheld unit (although in some cases, the hand held unit I incorporated with the finger probe). Inside the probe or sensor is a set of two light-emitting diodes facing a photodetector. A segment of the body such as the earlobe or finger, should be clipped in between. When the device is turned on, the light coming from the LEDs (red and infrared wavelengths) is partially absorbed by hemoglobin. Specifically, oxygenated hemoglobin takes up more infrared light and allows far more red light to go through. On the other hand, deoxygenated blood absorbs more red light and permits infrared light to go through. Consequently, the actual calculated percentage of red to infrared absorption provides the quantity of saturated oxygen in your blood.

There are two known methods of transmitting light through the sample site, either by transmission or reflection. In transmission, the LEDs are facing or opposite the photodiode (photodetector) and the light passes through the subject’s finger or earlobe. In reflection, both LEDs (transmitter) and photodetector (receiver) are adjacent to each other so that when light is shone, it bounces from the transmitter across the site to be measured to the receiver.

The Pulse in Pulse Oximetry

Every time the heart beats, a surge of arterial blood goes through the site, thereby resulting to more light absorption during the surge. But the machine inside the pulse oximeter will differentiate pulsatile blood flow from other constant absorbers such as the skin, tissue, and venous blood and only display arterial blood.

And because the device is capable of identifying pulsatile blood, pulse oximeters can therefore measure the pulse rate of a person—and will only function in the presence of a pulse.