Science and Technology

How does a pulse oximeter work?

The pulse oximeter is a device used to measure oxygen saturation in the blood. Humans breathe oxygen in the air which enters the lungs. Then the oxygen is carried in our blood to the entire body by the hemoglobin in our red blood cells. Oxygen saturation refers to the percentage of the hemoglobin that carries oxygen in the blood.

There are two types of pulse oximetry. They are transmissive pulse oximetry and Reflectance pulse oximetry. Transmissive pulse oximetry is the most commonly used method. It uses a sensor device attached to a thin part of the patient’s body like fingertip, earlobe, or toes. Reflectance pulse oximetry is a less used method and does not require a thin section of the person’s body. They measure the reading on the feet, forehead, and chest, but it has some limitations.

In 1935, German physician Karl Matthes (1905–1962) developed the first two-wavelength ear O2 saturation meter with red and green filters which later used red and infrared filters. The original oximeter was made by Glenn Allan Millikan in the 1940s. The modern pulse oximetry was developed in 1972, by Japanese bioengineers, Takuo Aoyagi and Michio Kishi.

So, How does an oximeter works?

A typical oximeter uses light to measure oxygen saturation. It consists of two light-emitting diodes (LEDs) where one LED emits red light with wavelength 660 nm and another LED emits infrared light with a wavelength of 940 nm. The other side of the oximeter has a sensor that picks up the light that is transferred through the tissues we insert inside it.

Hemoglobin (Hb) absorbs light. More the Hb in the finger, more is the light absorbed. The hemoglobin with oxygen is called Oxyhemoglobin and the hemoglobin without oxygen is called Deoxyhemoglobin. Oxyhemoglobin and Deoxyhemoglobin absorb red and infrared light differently. Oxyhemoglobin absorbs more infrared light and lets more red light pass through it. Deoxyhemoglobin absorbs more red light and lets more infrared light pass through it. According to the Beer-Lambert Law in physics, the amount of light absorbed is proportional to the concentration of the light-absorbing substance.

Thus, the pulse oximeter calculates the oxygen saturation by comparing the amounts of red and infrared light received by the sensor after it is absorbed by the oxygen in the blood. So, By comparing the amounts of red and infrared light received by the sensor, the pulse oximeter knows how much light has been absorbed by the oxygen in the blood. Thus the instrument can calculate the oxygen saturation reading as a percentage of the maximum amount of oxygen in the blood.

Our blood is not the only thing that absorbs light. Our Skin and other tissues like fat, bone also absorb some light. Let us assume while taking the reading one person has a thin finger and the other has a fat finger. The tissues in the thin finger absorb only a little extra light, while the fatter finger absorbs much more light. The pulse oximeter has no way to measure if the finger is fat or thin. Then, how does the pulse oximeter know how much light is absorbed by the blood and how much light is absorbed by the tissues surrounding the blood? Luckily, arterial blood is the only thing pulsating in the finger and all the other things like the surrounding tissues are non-pulsating. This is because of the cardiac cycle where arterial blood volume increases during systole (heart contracts to pump blood out) and decreases during diastole (heart relaxes). Thus Pulse oximeters only consider the pulsatile blood ignoring the effects of the surrounding tissues that are non-pulsating.

Perfusion index ( PI ) in the oximeter is the ratio of the pulsing blood to non-pulsing blood flow in your finger that uses to indicate the strength of blood flow to your finger. PI is calculated by dividing the pulsatile signal (AC) by the non-pulsatile signal (DC) times 100, and is expressed as a percent ranging from 0.02% to 20%. Perfusion index ( PI ) is the ratio of the pulsing blood to non-pulsing blood flow in your finger that is used to indicate the strength of blood flow to your finger. The PI’s values range from 0.02% for very weak pulse to 20% for extremely strong pulse. A higher perfusion index means greater blood flow to the finger and a lower perfusion index means lower blood flow to the finger.

For a healthy person, the normal blood oxygen saturation level will be around 95–100%. If it falls below 90%, it may develop hypoxemia, an abnormal low level of oxygen in the blood. The right middle finger and right thumb have the most accurate value of the oxygen saturation in the right-hand Dominance.

Pulse oximeters are mainly used to monitor the health of a person with any condition that affects blood oxygen levels such as heart attack, heart failure or heart defects, Chronic obstructive pulmonary disease (COPD), asthma, pneumonia, lung cancer, anemia, or for diagnosis of some sleep disorders such as apnea and hypopnea. People with lung damaged may have a blood oxygen level lower than normal, so pulse oximetry can help to diagnose if there is a problem.

Pulse oximeter accuracy is highest at saturations of 90-100%, intermediate at 80-90%, and lowest below 80%. Factors that affect the accuracy of the measurement in the oximeter are poor circulation, strong movement, skin pigmentation, skin thickness, skin temperature (when hands are cold), current tobacco use, artificial nails, use of fingernail polish (especially black, blue, or green), Anemia, Dyes used in some surgical procedures and when oxygen saturation is very low (below 80%). For dark and light skin pigmentated people, the accuracy differences in pulse oximeters are typically small at saturations above 80% and greater when saturations are less than 80%.

When monitoring oxygen levels at home, consider other signs like Bluish coloring in the face, lips, or nails, Shortness of breath, difficulty breathing, or a cough that gets worse, restlessness, discomfort, Chest pain, or tightness, or fast pulse rate.

Be aware that some patients with low oxygen levels may not show any or all of these symptoms. If your symptoms are serious or getting worse, please contact a health care provider.

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Credits :
https://www.blf.org.uk/support-for-you/breathing-tests/tests-measure-oxygen-levels
https://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication
https://www.nejm.org/doi/10.1056/NEJMc2029240?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub++0pubmed%22+%5Cl+%22article_citing_articles – Journal
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4627972/ – Journal
https://www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_tutorial1_the_basics_en.pdf?ua=1
https://www.healthline.com/nutrition/10-benefits-of-exercise
https://www.thoracic.org/patients/patient-resources/resources/pulse-oximetry.pdf
https://www.sciencedirect.com/science/article/pii/S095461111300053X – Journal
https://en.wikipedia.org/wiki/Pulse_oximetry
http://incenter.medical.philips.com/doclib/enc/fetch/586262/586457/Understanding_Pulse_Oximetry.pdf%3Fnodeid%3D586458%26vernum%3D2
https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/pulse-oximetry
https://www.pulmolink.co.uk/pages/how-does-a-fingertip-pulse-oximeter-work
https://www.howequipmentworks.com/pulse_oximeter/
https://en.wikipedia.org/wiki/Hypoxemia
https://en.wikipedia.org/wiki/Hemoglobin
https://www.masimopersonalhealth.com/pages/how-to-use-it
https://www.thedailystar.net/toggle/news/all-you-need-know-about-pulse-oximeter-1938229

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