Pulse Oximetry

The Basics of Pulse Oximetry

Pulse Oximetry is a method for determining the oxygen saturation of blood.  It functions by using 3 different LED's along with signal processing techniques.  The basic idea is that 2 LED's - One which is Red (~660nm) and one which is Infrared(~910nm) are shown through a site with oxygenated blood from arteries.  The blood will absorb differing amounts of light depending on it's oxygination at the time.

This demonstrates that more red light is absorbed by oxygenated blood and more IR by unoxygenated. 

Below is a basic layout for a pulse oximeter. 

The above picture demonstrates how pulse oximeters are normally configured for adults.  The light is shown through the translucent (relatively) finger, and collected by a photodiode on the other side.  The photodiode will create a current and voltage when light is collected.  By cycling the red and infrared lights on and off, one can determine the absorbtion of Red light when the IR light is off, the absorbtion of IR when Red is off, and Abient lighting when both are off.  By subtracting the Abient from the R and IR light, we get:

Red - Ambient Light         = R/IR ratio
Infrared - Ambient Light

This Ratio is determined by a microprocessor or other processing unit.  Then, a table lookup versus expermental calibration curves is performed to determine the current saturation.  In general, a ratio of .5 would mean 100% saturation, 1.0 would mean 85% saturation, and 2.0 would mean 0% saturation.  Due to the end part of this curve, values under 70% are considered inaccurate.

Potential Concerns

 The journal article linked below suggests that many false alarms in the hospital environment are due to Pulse Oximeters.  This suggests this may not be a good system for at home use.  Additionally, there is some concern over potenial for burns due to long term use.

Wikipedia Articles

LED's
Photodiodes
Pulse Oximetry
Pulse Oximeter