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COVID-19 OXYGENATION ASPECTS have familiarized many people with the pulse oximeter, a little clothespin gizmo that attaches to one’s fingertip to measure pulse and blood oxygen level. An informative summary exists at houstonmethodist.com.
As reported by Tara Parker-Pope in The New York Times, December 23, 2020, “Home pulse oximeters have been the personal tech gadget of 2020 and a reassuring way for patients to monitor their health at home during Covid-19. But a new study found that even in a hospital setting, pulse oximetry devices can sometimes be inaccurate, particularly in Black patients.”
Roni Caryn Rabin offers background information in The New York Times, December 22, 2020: “Pulse oximeters work by shining two wavelengths of light, a red light and an infrared light, that pass through the skin of a finger. The device detects the color of blood, which differs depending on the amount of oxygen. Oxygenated blood is bright cherry red, and deoxygenated blood has a more purplish hue.”
“Depending on the hue,” Rabin continues, “different amounts of light from the device are absorbed, and the oximeter analyzes the proportions of the absorption and calculates the amount of oxygen.”
Inaccurate readings may occur because of the way the light is absorbed by darker skin pigments. Rabin cites Dr. Philip Bickler, the director of the hypoxia research laboratory at the University of California, San Francisco, who said that pigment “scatters the light around, so the signal is reduced. It’s like adding static to your radio signal. You get more noise, less signal.”
Rabin notes that dark nail polish also reduces the accuracy, as do cold fingers. Direct measurement of oxygenation is more accurate, but it cannot be done without intervention and must be taken from arterial blood, not just any vein. The pulse oximeter will continue to be a useful medical tool, but with this caveat of its limitations.
Related Earlier Research. By the way, as noted in previous research, “It is obvious clinically that skin of the palms and soles (volar skin) of Blacks is lighter in color than Black glabrous [other hairless] skin. The same difference obtains in whites, but is much less obvious.”
Also, this oximeter shortcoming has been known for some time: In December 2007, the National Library of Medicine of the National Institutes of Health published a Comparative Study titled “Dark Skin Decreases the Accuracy of Pulse Oximeters at Low Oxygen Saturation: the Effects of Oximeter Probe Type and Gender.”
The paper’s Abstract notes, “Pulse oximetry may overestimate arterial oxyhemoglobin saturation (Sao2) at low Sa02 levels in individuals with darkly pigmented skin, but other factors, such as gender and oximeter probe type, remain less studied.”
I suspect the reason for the newsworthiness of additional oximeter research is Covid-19’s ubiquity of these fingertip devices. ds