How does a pulse oximeter work?

Have you ever been curious about how a finger pulse oximeter works?

Hypoxemia is a condition in which oxygen levels in the blood fall below normal.  The condition is linked to a number of diseases affecting the respiratory and cardiovascular systems, which include lung diseases such as pneumonia, COPD (Chronic Obstructive Pulmonary Disorder), asthma, as well as anemia and heart diseases. A pulse oximeter works by detecting oxygen levels in the blood.

Pulse oximetry is a non-invasive test that measures the amount of oxygen in the red blood cells (oxygen saturation). It is the standard technique used in intensive care units (ICUs) to monitor critically ill patients as well as during general anaesthesia and the perioperative period (during and after surgery). Pulse oximetry is also used by family physicians during follow-ups on patients with COPD and asthma to check on their oxygen saturation levels.

Healthcare providers generally rely on pulse oximetry for accurate readings of blood oxygen saturation (SpO2) levels to assess their patients’ respiratory and cardiovascular health. Oxygen saturation levels between 95 and 100 percent, as estimated by pulse oximetry, are considered to be normal. Levels below 90 percent are too low and can be considered a clinical emergency.

What are pulse oximeters and who uses them?

Pulse oximeters monitor SpO₂ and heart rate. These medical devices are small, inexpensive, lightweight, and easy to use. They are considered painless as the device takes readings without inserting a needle. Pulse oximeters are generally placed on areas of the skin with higher density of blood vessels, such as the fingertip, earlobe, and toe.

There are a range of oximeters that can be used at home or the clinic. These include finger pulse oximeters, handheld pulse oximeters, or a combination of both. Finger pulse oximeters are currently one of the most commonly used pulse oximeters.

Home finger pulse oximeters are prescribed to patients with lung disease such as COPD. The oxygen saturation levels of these patients can then be measured by performing regular spot checks during the day. They are also used by athletes and mountain climbers who experience conditions associated with low blood oxygen saturation levels. These devices can help athletes and persons working out at a gym to assess their fitness levels and adjust their exercise regimen if necessary, by monitoring their blood oxygen levels.

Many hiking or mountain climbing expedition groups carry a low-cost handheld pulse oximeter that can fit inside a pocket to regularly check the blood oxygen saturation levels as they travel up to higher altitudes.  In a 2011 study, published in High Altitude Medicine & Biology, the researchers indicated that changes in oxygen saturation levels are quick in response to even minor changes in oxygen tensions at high altitude.  A 2014 study, published in High Altitude Medicine & Biology, suggests that a pulse oximeter can possibly predict acute mountain sickness, caused by sudden exposure to a drop in oxygen levels at high altitudes.

How do they work?

A home finger pulse oximeter clips over the fingertip. The pulse oximeter transmits tiny beams of light at the red and infrared wavelengths from a pair of diodes, located on one side of the clip arm. This technique is based on the principle that red and infrared light is absorbed by oxygenated and deoxygenated blood. The pulse oximeter measures the changes in the light absorbed by the haemoglobin to determine the oxygen saturation levels.

The readings only take a few seconds to display after the device is clipped on the finger and turned on. The two numbers displayed on the oximeters are the SpO₂ reading and the heart rate. The accuracy of the readings can be reduced by motion and low perfusion (decrease in blood circulation through the finger).

In most cases, the pulse oximeter is placed on the index finger. However, a 2015 study, published in Springerplus, reported that SpO₂ levels were significantly higher in the middle finger and the thumb of the dominant hand than the other fingers.

There are many home finger pulse oximeters widely available at pharmacies, retail, or online medical supply stores. However, these inexpensive pulse oximeters were not approved by regulatory agencies such as FDA. According to a 2016 study, published in Anesthesia and Analgesia, a majority of those low-cost finger pulse oximeters produced readings, found to be less accurate than the more expensive, FDA-approved devices. For this reason, lower-cost home pulse oximeters should not be used for diagnostic, prevention, or treatment purposes. These types of pulse oximeters should be used to monitor general health and wellness, fitness, and relaxation.

Written by Ranjani Sabarinathan, MSc        

References

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• Lipnick, Michael S. MD; Feiner, John R. MD; Au, Paul BS*; Bernstein, Michael BS†; Bickler, Philip E. MD, PhD* The Accuracy of 6 Inexpensive Pulse Oximeters Not Cleared by the Food and Drug Administration: The Possible Global Public Health Implications, Anesthesia & Analgesia: August 2016 – Volume 123 – Issue 2 – p 338-345 doi: 10.1213/ANE.0000000000001300

Image by Kelly Kirk from Pixabay