Pulse oximeter

A pulse oximeter is a device used to measure oxygen as a percentage of blood gases in a non-invasive manner. It uses a sensor that is placed over a thin part of a patient's body, usually their fingertip, but in the alternative an earlobe. Two different wavelengths of light are emitted on one side to be picked up by a sensor on the other side. These wavelengths are chosen because they will not be affected by other substances, including fat, bone, veins or even nail polish. They will only be affected by blood in the arteries. The sensor results are analyzed with a computer that displays the result on a monitor. 
Although a pulse oximeter is not a complete solution to determine whether a patient is getting proper respiration or bloodflow, it does have the advantage of being able to provide a quick and continuously updated result. 
Pulse oximetry at Wikipedia
Category:Medical devices