Recently, during the COVID-19 pandemic, there was a high count of Blacks dying from this virus compared to Whites. Apparently, when Blacks visit the hospital to check their oxygen level using the pulse oximeter it gives data that says they are healthy, and in fact, they are not. The Food and Drug Administration(FDA) discovered this high death toll in the Black and investigated. They came down to the conclusion that 1 in 10 pulse oximeters gives wrong readings for people of color.
A research carried out by the University of Michigan Hospital did research on acute hypoxemia with a large cohort of 10,000 patients of both Black and White races. They tested for occult hypoxemia, arterial oxygen saturation of <88% despite an oxygen saturation of 92 to 96% on pulse oximetry. In this research a pulse oximeter and arterial oxygen saturation in arterial blood; this is basically the ultimate test where the blood sample is taken from the patient and tested in the lab for its oxygen level. Black patients had nearly three times the frequency of occult hypoxemia that was not detected by pulse oximetry as White patients.
HOW DOES THIS DEVICE WORK?
A commercial pulse oximeter is made up of a light sensor and two lights; the red light at 660 nanometers and the Infrared at 940 nanometers in the visible spectrum.
The above picture shows why only red and infrared lights are used because those lights can penetrate past the tissues and reach your arterial veins where blood flows from the heart.
Deoxyhemoglobin is a form of hemoglobin (blood) without oxygen and is represented as Hb. Oxyhemoglobin is a form of hemoglobin with oxygen and is represented as HbO2. The graph below shows the absorption level to the wavelength of the red and infrared light. From the below graph, you see that deoxyhemoglobin absorbs more red light while oxyhemoglobin absorbs more infrared light.
A group of doctors from Jerusalem College Technology(JCT) had a project for building a pulse oximeter and came up with the following data and formulas. This graph gives you the heartbeat of the patient. And it is measured from trough to peak. This uses light to measure blood flow.
AC is the trough to peak amplitude and DC is the mean value of the pulse basically the average and should not be mistaken for distance. The bigger the AC the more accurate your result tends to be.
The extinction coefficient is the absorbance divided by the concentration and path length basically how transmissive light can be. This formula has been labeled questionable because these values cannot be found from the data collected.
This explains the theory behind scattered and direct light when passed through something. Ideally when light passes through the finger of a person, the output or absorption level is dependent on how thick that finger is. That is if your finger is really thick the absorption level would be small due to the fact that lights do scatter and vice versa.
This basically is the combination of both the extinction coefficient and path length formula. This is assumed to be a more accurate formula for calculating the peripheral capillary oxygen saturation, an estimate of the amount of oxygen in the blood.
This is an empirical formula. K1 to K4 are not derived numbers they don’t look them up in the literature they literally took people’s heart ratio and SPO2 and took enough of those until they could fit those numbers. There is no fundamental science behind this formula this is just getting numbers that work.
Apparently, every formula that was derived before this empirical formula was thrown away and they ended up with this empirical formula. This raises a lot of questions. We do not know the extinction coefficient and path length which seems to be important in building a pulse oximeter.
The above graph is a piece of information I would like you to hold on to for further explanation and it shows us that red light is absorbed by melanin and infrared is barely absorbed by melanin. So basically red light is affected by how much melanin a person has and infrared is not.
DESIGN OF PULSE OXIMETER
My intention was to build a pulse oximeter similar to the ones used commercially using a 3D printer. This turned out to be a failure specifically because of the hinges that connect the led holder and sensor holder together. I spent a lot of time trying to perfect this design but I couldn’t. As Prof Eric Remy would say ” We are too focused sometimes on one particular goal we forget that there is more to do and achieve.”
My failures using a 3D printer to build the commonly used commercial pulse oximeter.
So I moved on from this design and made an All-in-One pulse oximeter that contains the light sensor and led lights. Below are my designs using the Tinkercad software and 3D prints.
Tinkercad design of All-in-One pulse oximeter
Tinkercad design of All-in-One pulse oximeter
Two types of data were collected the visible light(red) data and infrared light data. Like I stated earlier the bigger the distance from the trough to peak the more accurate it tends to be.
This data shows that the Black male’s graph for visible light(red) is difficult to read and you can barely get any information from it and this is due to the fact that melanin affects reds light just like we pointed out in the melanin absorption graph and that is why the graph seems unstable and the AC(trough to peak) is really low or sound to signal ratio is low. In the infrared light, we suddenly see that the Black male’s graph has a better sound to signal ratio and this proves the point from the melanin absorption graph that melanin barely affects infrared light.
The commercial pulse oximeters do not measure two major factors and this could result in the misleading of data analysis. This is especially for People of Color. These factors may include;
The thickness of one’s finger and
The percentage of melanin present in a person’s skin.
Therefore, I do not believe these devices can be trusted because of their inaccuracy. I would advise that people who try to check their oxygen level especially now that it is used at a high rate due to covid-19 should visit the hospital and opt to take the ultimate test that involves extracting the patient’s blood and taken to a lab for analysis and accurate results.
I thank God for the completion of this research project and appreciate my DTSF supervisors, fellow interns, and family. I wouldn’t have been able to accomplish this much without the love and care I got from everyone thank you! Thank you Gettysburg College. It has been an honor to be a part of this family.
Sjoding, Michael W., et al. “Racial Bias in Pulse Oximetry Measurement.” New England Journal of Medicine, vol. 383, no. 25, Dec. 2020, pp. 2477–78. Taylor and Francis+NEJM, doi:10.1056/NEJMc2029240.
Yossef Hay, Ohad, et al. “Pulse Oximetry with Two Infrared Wavelengths without Calibration in Extracted Arterial Blood.” Sensors (Basel, Switzerland), vol. 18, no. 10, Oct. 2018, p. 3457. PubMed Central, doi:10.3390/s18103457.
Zonios, George, et al. “Melanin Absorption Spectroscopy: New Method for Noninvasive Skin Investigation and Melanoma Detection.” Journal of Biomedical Optics, vol. 13, no. 1, International Society for Optics and Photonics, Jan. 2008, p. 014017. www.spiedigitallibrary.org, doi:10.1117/1.2844710.