Breaking it Down - How Pulse Oximeters Work

Pulse oximetry testing devices make use of light to determine the level of oxygen saturation in the blood. This light is emitted from a source that goes through the probe of the pulse oximeter and reaches the light detector on the other end. It’s really simple: If you place a finger in between the light detector and the light source, the light will indefinitely have to penetrate and pass through one’s finger to be detected. Part of this light is bound to be absorbed by the blood in the finger and part of it will go across to the light detector on the other end. The extent to which the light is absorbed by the pulse oximeter is there to calculate oxygen saturation. This amount depends a lot on the pulse oximeter which is used to calculate the oxygen saturation level. There are certain factors which determine the amount of light which is absorbed.

  • How the deoxyhemoglobin and oxyhemoglobin absorb infrared and red light differently. 
  • The length of the light path in the absorbing substance
  • The amount of light that is absorbed

In overnight oximetry testing, it is the hemoglobin in your body that absorbs this light. It only does so depending on the concentration of hemoglobin in every unit of blood. Therefore, the more hemoglobin present in an area, the more light will be absorbed. This quality is often described in physics as Beers Law.

The Wider Arteries

The light that has been emitted from the top of the pulse oximetry device has to pass through the arteries. Therefore, light passes through a comparatively longer path as wider arteries have greater cross-sectional length as compared to those which are narrow. Technically speaking, the concentration of hemoglobin could be the same in both, a wide and a narrow artery. However, in a wider one, this light absorbs more hemoglobin.

The Calibration Adjustment

Since blood is not a clean and clear liquid and is filled with irregular objects, such as different cells, this results in the light being scattered instead of going through in a straight line. Therefore, Beer’s Law and Lamberts Law both suggest that absorption of light varies according to the path it takes and the length it travels. As a result, concentration does not strictly apply to this device.

Since these laws cannot be applied, there can be errors in calculating accurate oxygen saturation. However, there is a solution to this: The calibration graph technique which corrects and minimizes these errors. A test pulse oximeter is first calibrated through human volunteers and is then attached to the person. After this, the volunteer is asked to breathe slowly for lower oxygen levels to prevail in the blood. Meanwhile, arterial blood samples are taken at intervals. As the oxygen saturation in the blood decreases, direct measurements made with respect to the arterial blood samples are simultaneously compared with the readings on the oximeter. It is recommended that for surefire results, you opt for overnight oximetry testing.