What is Pulse Oximetry?                              

Pulse oximetry is not new. A sensor attaches to the finger or the ear and shines alternating red and infrared light across the extremity to a receiving photosensor on the other side. Shine a penlight across your hand to see how red light penetrates--infrared is invisible. The amounts of light getting through tissues to the sensor changes according to the amount of oxygen bound by hemoglobin in the blood.  

The oxygen being carried is called  the oxygen saturation, SaO2, SpO2 or the "Sat", in units 0-100%. In operating rooms, the ER, hospital floors or your doctor's office low measurements guide treatments. Low levels can reveal that breathing is too slow or shallow, or that there is lung or heart disease preventing oxygen crossing from the lungs into the bloodstream. 

Pulse oximetry is used everywhere to check blood oxygen levels in patients. Sometimes all that is done is one quick check, like a snapshot at one point in time. Otherwise, pulse oximetry can be monitored continuously, like the speedometer in an automobile. The critical difference is that  humans do not behave as predictably as vehicles using cruise control, especially when they fall asleep while using respiratory depressants like opioids.