Pulse oximeters measure how much of the hemoglobin in blood is carrying oxygen (oxygen saturation).
If you work in healthcare (or have been patient !) you are very likely to have come across pulse oximeters. You can find them in areas such as operating rooms, recovery, critical care, wards, and ambulances.
Pulse oximeters are in common use because they are:
• non invasive
• cheap to buy and use
• can be very compact
• detects hypoxaemia earlier than you using your eyes to see cyanosis.
Pulse oximeters measure oxygen saturation. Before we learn the principles of how pulse oximeters work, we need to have an understanding of what oxygen saturation is.
We all know that we need oxygen for life. ( Some of you may find this section a little too basic. In that case, to skip this short section, click this link to go to next section Physical properties used in pulse oximetry)
Oxygen enters the lungs and then is passed on into blood. The blood carries the oxygen to the various organs in our body. The main way oxygen is carried in our blood is by means of hemoglobin. You can imagine hemoglobin molecules( Hb) as “cars” and the “roads” being our blood vessels. The oxygen molecules get into these cars and travel around the body till they reach their destination.
Physical properties used in pulse oximetry
Pulse oximetry uses light to work out oxygen saturation. Light is emitted from light sources which goes across the pulse oximeter probe and reaches the light detector.
If a finger is placed in between the light source and the light detector, the light will now have to pass through the finger to reach the detector. Part of the light will be absorbed by the finger and the part not absorbed reaches the light detector.
The amount of light that is absorbed by the finger depends on many physical properties and these properties are used by the pulse oximeter to calculate the oxygen saturation.
The amount of light absorbed depends on the following:
1. concentration of the light absorbing substance.
2. length of the light path in the absorbing substance
3. oxyhemoglobin and deoxyhemoglobin absorbs red and infrared light differently
Don’t worry ! All the above will be explained in the next sections:
The physical properties that a pulse oximeter employs will be explained using the probe shown below. A finger is shown inserted into the probe. Above the finger are the light sources that emit light. In the finger is an artery which carries the blood the pulse oximeter is interested in and a vein through which the blood leaves the finger. Below the finger is the light detector.