Racial biases are built into the design of pulse oximeters


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One of the most indispensable devices during the coronavirus pandemic is the pulse oximeter, which clips onto a person’s finger, turns on a light, and reports a percentage of oxygen in the blood. Patients use pulse oximeters at home to monitor their condition, while hospitals use them to identify and prioritize the sickest covid patients. More generally, blood oxygen is considered the fifth vital sign, alongside body temperature, heart rate, breathing rate and blood pressure.

Pulse oximeters, however, don’t work as well in people with darker skin tones. There is a risk of “occult hypoxemia”, where the device indicates that oxygen levels are good but the patients’ actual saturations are dangerously low. Recent medical studies have quantified this bias and the consequences of an overestimated oxygen level. For example, Hispanic and black patients with the coronavirus were about a quarter less likely to be recognized as eligible for treatment. Getting an accurate oxygen reading can literally be a matter of life and death.

Despite the recent increase in attention to this issue, racial bias in pulse oximeters is nothing new; in fact, it has been integrated into the very development of this technology. A closer look at the history of oximeters reveals how prioritizing market expediency over fairness allows bias to seep into medicine.

The first oximeters were developed for military, not medical, use. During World War II, fighter pilots passed out at high altitudes, so American and German scientists developed oximeters for their respective air forces. These early devices clipped onto pilots’ ears and alerted them when they needed supplemental oxygen.

Hewlett-Packard (HP) later developed the atrial oximeter for healthcare in the 1960s and 1970s, with a remarkably liberal and transparent focus on fairness. In the October 1976 issue of their journal, for example, HP acknowledged how oxygen readings were affected by “skin and blood pigments, and surface characteristics of the skin”, before describing how they designed their own device so that oxygen readings are accurate, regardless of skin color. HP’s device was also tested on 248 black patients and was able to be personally calibrated with one patient’s blood. Meir Kryger, a Yale professor of medicine – who tested some of the early HP oximeter models as a pulmonology fellow at the University of Colorado – said the company “actually takes the business of pigments at a time when no one was”.

But HP’s oximeter was huge and cumbersome to use, not to mention the hefty price tag. It cost $13,000 in 1970, so the device was relegated to a few select research labs and considered clinically impractical. HP eventually discontinued its atrial oximeter and stopped manufacturing medical devices altogether.

In 1974, however, two Japanese companies took the next step in oximetry when Nihon Kohden and Minolta independently invented devices that measured oxygen levels via the beating of a patient’s arteries. The first “pulse” oximeters had arrived, with the two companies filing patents a month apart.

Although electrical engineer Takuo Aoyagi won this patent race for Nihon Kohden, the company did not pursue the device as it was only a side project for them. “Aoyagi made a prototype,” explained Katsuyuki Miyasaka – an anesthesiologist at St. Luke’s International University and Aoyagi’s close colleague – in an interview, “but there wasn’t much interest in developing it further. .”

Minolta continued and in 1977 released the OXIMET-Met-1471, possibly the first fingertip oximeter ever developed. With fiber optic cables sending light to and from the clip, the device was technologically advanced but, like HP’s, not clinically practical. It was extremely sensitive to motion, too heavy to be used on patients, and often overestimated oxygen levels in very sick patients, although it was otherwise quite accurate. The device has not been tested on any people of color because, in a country as ethnically homogeneous as Japan, “skin color may not be an issue,” Miyasaka said.

After becoming frustrated with a lack of success at home (only 200 devices were sold), Minolta attempted to market their pulse oximeter in the United States, distributing the device to various American hospitals for evaluation. William New, a former HP electrical engineer and anesthesiologist at Stanford University, quickly heard about the Minolta device and saw its shortcomings, but also its great potential.

New and two other colleagues founded the company Nellcor and, in 1981, launched their own pulse oximeter: the N-100. The device was designed to be clinically practical. With LED lighting and a flexible, paper-like sensor, Nellcor’s oximeter was disposable and largely insensitive to motion. One of the device’s most popular features was the way its tone changed based on a patient’s oxygen saturation, allowing for easier recognition of low oxygen levels.

Nellcor’s timing was extremely fortuitous. LEDs were becoming increasingly available in the early 1980s, just as a series of malpractice lawsuits were filed against anesthesiologists who monitored oxygen levels during surgery. As Nihon Kohden’s CEO recently wrote, Nellcor “seized the wave of technological innovation and market change at this time.” The N-100 dominated the market, “selling like hot cakes,” according to Miyasaka. In fact, a Canadian anesthetist described how “Nellcor” and “pulse oximeter” have become synonymous.

But Nellcor’s device was not fair. The company was so focused on developing an easy-to-use, clinically practical pulse oximeter that it overlooked the racial biases built into its devices. In 1987, Kryger compared the N-100 to HP’s oximeter and found that Nellcor’s device was not as accurate or responsive as HP’s.

Racial bias, of course, is not unique to Nellcor: most pulse oximeters have been calibrated only on fair-skinned people. And it’s not enough to say that the medical community didn’t know any better: they long ago understood how yellow skin color from jaundice, blue from sepsis, and naturally non-white skin color could lead to ” the skin pigmentation effect,” Miyasaka says, “but they thought, statistically or practically, you can neglect it.” The biased design’s supposed harmlessness continues to justify its existence.

HP’s device in the early 1970s was the exception that proves the rule. Although their oximeter was clunky and clinically impractical, it was a paragon of inclusivity as HP engineers made eliminating bias a priority. Racial bias in pulse oximeters, or any medical device, is never inevitable. Fairness requires intentionality.

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