How does pulse oximetery work?

Pulse oximetry measures the percentage of oxygen carried through the arteries, compared with the maximum level. A healthy level at resting state is between 95%-99%.

When a pulse oximeter is attached to a finger, earlobe or toe, two beams of light (one red, one infrared) are shone through.

Because blood absorbs light differently depending on the level of oxygen it contains (with oxygenated haemoglobin absorbing infrared light and allowing red light to pass through, and deoxygenated blood doing the opposite), the light beams allow the pulse oximeter to determine what colour the arterial blood is by detecting a slight change in the blood colour each time the heart beats.

This allows the oximeter to calculate the overall oxygen saturation (SpO2).