Oxygen Saturation
According to Philips Medical Systems, pulse oximeters are used to indirectly measure the amount of oxygen in the blood by measuring something called oxygen saturation. Oxygen saturation measures different forms of a protein called hemoglobin. Hemoglobin is a critical component of red blood cells because it can bind to oxygen, which allows these cells to transport oxygen throughout the body. Oxygen saturation measures the percentage of hemoglobin molecules that are bound to oxygen. In a normal healthy human, 94 to 100 percent of all molecules of hemoglobin in the arteries are bound to oxygen, which means that the arteries are at or near full oxygen carrying capacity.
Pulse Oximeters and Spectroscopy
Pulse oximeters are able to take advantage of the fact that oxygen-filled and oxygen-deficient blood are different colors. PulseOx.info explains that even small changes in oxygen saturation can be detected by pulse oximeters because they detect changes in light absorption. Pulse oximeters shine two different wavelengths of light: one that is red and another in the infra-red spectrum, which means it is invisible to the human eye. The light gets shined on a thin part of the body with arterial blood flow, such as the tip of a finger. Most of the light gets absorbed by the finger, but some passes all the way through. This light that makes it through the finger is received by a light-sensitive detector.
Determing Oxygen Saturation
The blood isn't the only thing in the finger tip that absorbs light, of course. Skin, muscle, and connective tissues also absorb light. Consequently, pulse oximeters measure the changes in the light absorption while blood is pulsing through the finger. By measuring the light absorption in the finger before and after the pulse, the pulse oximeter can isolate the effects of the blood from the effects of the rest of the tissue on light absorption. Once the light absorption of the blood has been measured a simple computer program uses that information to determine the oxygen saturation of the blood, which can then be displayed on the oximeter's monitor.
