To determine blood oxygen (arterial blood oxygen saturation (SpO2)) and pulse rate, many biosensors use the reflectance of red and infrared (and sometimes green) light. Based on photoplethysmography (PPG), an optical technique that measures blood volume variations, dual-wavelength PPG pulse oximeters noninvasively measure/calculate the arterial oxygen saturation (SpO2).
An oximeter transmits radiation at a known wavelength(s) through blood to measure the blood oxygen saturation based on the amount of reflected or scattered radiation. In addition to finger clamps that have become quite common for professional and consumer usage, nose, ear lobe and forehead locations are also used. The reason is that the skin in these areas has a much higher vascular density than other locations, such as the skin of the chest wall.
Since red and near-infrared light have significant differences in absorption and penetrate tissues well, clamp-type pulse oximeters emit two wavelengths of light, red at 660 nm and near-IR at 940 nm, from a pair of small light-emitting diodes located in one arm of the finger probe.
More recently, digital watches or wrist-worn devices take advantage of this technique.
For example, in Apple Watch Series 6, the blood oxygen sensor uses four LED clusters and four photodiodes integrated into the back of the watch case. Green, red, and infrared LEDs shine light onto the blood vessels in the wearer’s wrist, and the photodiodes assess the amount of reflected light. The color of the detected blood provides an indication of how much oxygen is present. The light sources used by the reflectance sensor comprise red, IR, and green LEDs operating at wavelengths of approximately 660, 850, and 525 nm, respectively.
Normal blood oxygen saturation ranges from 95% to 100%. Medical devices are used to detect hypoxemia, a sign of a problem related to a pulmonary or cardiac (breathing or circulation) issue, where the user’s measurements are below normal blood oxygen levels. In addition to historical use in treating pulmonary vascular disease (PVD) and/or interstitial lung disease (ILD), oximetry became an integral part of COVID-19 condition monitoring and treatment.
The issue: darker skin pigment may absorb some of that light and make the readings higher than they should be.
Previously, in 2013, the FDA developed a guidance document to assist the industry in preparing premarket notifications (510(k)s) for pulse oximeters. The FDA documents specifically state, “Your study should have subjects with a range of skin pigmentations, including at least 2 darkly pigmented subjects or 15% of your subject pool, whichever is larger.”
On February 19, 2021, the FDA issued a warning about pulse oximeters to inform the public and healthcare providers of the limitations of these devices, stating, “While pulse oximeters may be useful for estimating blood oxygen levels, these devices have limitations that can result in inaccurate readings.”
The safety communication identified several factors that can affect the accuracy of a pulse oximeter reading, including poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use, and fingernail polish.
In October 2022, Apple published a white paper on the Blood Oxygen app on its smartwatch. In it, the company noted its awareness that melanin is a strong light absorber at the wavelengths that the Apple Watch uses, potentially making PPG measurements more difficult in users with darker skin tones. However, to account for this, the sensing platform senses the amount of detected light signals and automatically adjusts the LED current (and the light output), photodiode gain (its sensitivity to light), and sampling rate to ensure adequate signal resolution across the range of human skin tones.
References
Lawsuit alleges Apple Watch’s blood oxygen sensor ‘racially biased’; accuracy problems reported industry-wide – ABC News (go.com)
Pulse Oximeters – Premarket Notification Submissions [510(k)s]: Guidance for Industry and Food and Drug Administration Staff | FDA
ISO – ISO 80601-2-61:2017 – Medical electrical equipment — Part 2-61: Particular requirements for basic safety and essential performance of pulse oximeter equipment
Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations – ScienceDirect
Pulse oximeters are less accurate for Black patients, study finds : Shots – Health News : NPR
How Accurate Is the Apple Watch’s Blood Oxygen Sensor? (honehealth.com)
How to use the Blood Oxygen app on Apple Watch – Apple Support
Monitor your heart rate with Apple Watch – Apple Support
Comparison of SpO2 and heart rate values on Apple Watch and conventional commercial oximeters devices in patients with lung disease | Scientific Reports (nature.com)
Blood-Oxygen-app-on-Apple_Watch-October-2022
Pulse oximeters are less accurate for Black patients, study finds : Shots – Health News : NPR
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