A Rundown of Pulse Oximetry and Its Accuracy

Tanvir Minhas - 30 April 2021

The pulse oximeter: a medical device used to record the value of oxygen saturation in a patient’s blood. While it has been used in a variety of medical procedures for patient monitoring, fingertip pulse oximeters have become widely popular for household use in light of the coronavirus pandemic. Recent studies have shown, however that pulse oximeter readings may not be as accurate as we once thought, especially for patients with darker pigmented skin.

Oxygen Saturation Explained

When discussing oxygen saturation, we are ultimately looking at the saturation of hemoglobin in one’s body, particularly that of the arterial, or generally speaking, oxygenated blood.

Hemoglobin is a protein found in red blood cells. It exists as a quaternary protein structure, indicating that multiple fully folded and mature proteins come together to form a protein complex. In particular, hemoglobin is a tetramer: a combination of two alpha and two beta-protein subunits that join to form one hemoglobin protein.

Each of these subunits has their own iron atom-containing heme group that is used to bind oxygen and form oxyhemoglobin. The amount of heme groups allows each hemoglobin to hold four oxygen molecules, but as you already know, hemoglobin is just one type of protein found in your red blood cells.

This means that each of the trillion red blood cells found in a human body can contain millions of hemoglobin proteins that each have the capacity to bind four oxygen molecules.

As you can imagine, that is a lot of oxygen! Since oxygen has a low solubility in water, it cannot be transported efficiently in the blood’s primarily water-based component, plasma, and rather 98% of it is transported throughout the body as oxyhemoglobin.

Overall, this makes hemoglobin an incredibly important protein, and a main factor in oxygen saturation of blood.

The Science Behind Pulse Oximeters

In order to understand pulse oximetry, it is essential to know that hemoglobin is involved in the pigmentation of red blood cells. That being said, oxygenated blood presents as a brighter red colour than deoxygenated blood does.

Pulse oximeters utilize that difference in blood colour to detect the ratio of oxyhemoglobin to deoxyhemoglobin and provide an estimate of oxygen saturation based on that. To do so, beams of light are emitted from one face of the oximeter, and the absorbance of light is measured by the sensor on the other.

Oxygenated blood appears as a brighter red, which means that red wavelengths are reflected back towards the device. The opposite is true for deoxygenated blood, causing it to absorb more red light and instead, scatter infrared light.

To put it simply, by measuring the absorbance of two different wavelengths of light, red light and infrared light, the pulse oximeter can gauge how much of the blood is oxygenated.

Medical Applications

A pulse oximeter reading of anywhere from 95% and above is often considered an acceptable and healthy value. A percentage below that however, puts a patient at risk of hypoxemia: a deficiency of oxygen in their blood.

Hypoxia is a general term used to describe oxygen deficiency in tissues and organs following hypoxemia. It causes a multitude of health issues, most commonly shortness of breath, but can also be linked to headaches, dizziness, and other conditions. Hypoxemia’s effects on the respiratory system have been of particular interest recently because of its connection to COVID-19.

Pulse oximeters have become a popular device to detect coronavirus-related hypoxia because of the phenomena of silent hypoxemia: one that does not present any symptoms, including the telling sign of shortness of breath.

There are a multitude of theories as to why silent hypoxemia emerges in COVID-19 patients, with ongoing research to find the true cause. One such theory is that the virus causes the production of thrombi, or blood clots, in pulmonary vessels to cause inefficient oxygen exchange between the lungs and the blood.

Silent hypoxemia presents medical concern based on the fact that abnormally low oxygen levels that go undetected, and therefore untreated, may cause long-term negative effects on a patient’s body.

Patients that are deemed to be at risk of hypoxia, but are healthy enough to warrant against hospitalization, may be sent home with a pulse oximeter to monitor their health. Upon presenting oxygen saturation levels that indicate hypoxemia, they are advised to seek medical attention.

The Accuracy of Pulse Oximeters

While this use of technology allows for comfortable, at-home patient monitoring, there lies a concern in the accuracy of pulse oximeters in patients with darker pigmented skin.

It has been noted in some studies that for patients with darker skin, pulse oximeter readings can be inaccurate around 12% of the time, likely due to the absorbance of light by the patient’s skin rather than their blood.

Some physicians do still advise to utilize pulse oximeters for home-monitoring of COVID-19 patients, explaining that is it imperative to pay attention to any indication of a downward trend in values rather than the values themselves.

Beyond that though, the inaccuracy of such a widely used medical device does raise concern. It speaks to the fact that many aspects of the medical world are still lacking the level of inclusion that countless patients deserve.

This has been a recurring theme within medicine, evident especially by the lack of ethical studies performed to understand how certain medical conditions may present differently in BIPOC individuals. This results in our education systems failing to equip incoming generations of physicians with accurate information, continuing to perpetuate a cycle of exclusion and spread of potential misinformation.

The scientific community strives to provide our population with new information and findings that can be used to better inform the care of individuals. Holding studies, including those for new medical technology, that incorporate different genders, races, and sexualities is a start. By doing so, we move towards cultivating an extensive database that benefits patients, expands our knowledge, and ultimately promotes inclusivity.

Tanvir Minhas

Blogger

Tanvir is a second year Microbiology and Immunology student at the University of Saskatchewan. She has been involved with OMS since 2017 and is very excited to be a part of the launch of this blog! With a keen interest in medicine and a passion for inclusivity in the field, Tanvir is looking forward to being a part of a blog that aims to ignite important conversations within the scientific community.

Sources:

1 https://www.istockphoto.com/illustrations/pulse-oximeter

2 https://www.lung.org/lung-health-diseases/lung-procedures-and-tests/pulse-oximetry

3 https://www.sciencedirect.com/topics/materials-science/hemoglobin

4 http://themedicalbiochemistrypage.org/hemoglobin-and-myoglobin/

5 https://www.hsph.harvard.edu/nutritionsource/iron/

6 https://www.ncbi.nlm.nih.gov/books/NBK54103/#:~:text=The%20hemoglobin%20molecule%20has%20four,can%20bind%20four%20oxygen%20molecules.

7https://www.ncbi.nlm.nih.gov/books/NBK54103/#:~:text=Oxygen%20is%20carried%20in%20the%20blood%20in%20two%20forms%3A%20(1,has%20such%20a%20low%20solubility.

8 https://www.sciencedirect.com/topics/nursing-and-health-professions/hemoglobin#:~:text=Haemoglobin%20is%20the%20iron%2Dcontaining,of%20oxygen%20to%20the%20tissues.

9 https://medicine.uiowa.edu/iowaprotocols/pulse-oximetry-basic-principles-and-interpretation#:~:text=The%20oxygen%20saturation%20as%20determined,that%20tissues%20are%20sufficiently%20oxygenated.

10 https://www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_tutorial1_the_basics_en.pdf?ua=1#:~:text=All%20pulse%20oximeter%20probes%20(finger,is%20transferred%20through%20the%20tissues.&text=of%20arterial%20blood%20in%20the%20peripheral%20circulation.

11 https://acuclinic.com.au/pocit/O2Sat.htm