Why engineers are working to build better pulse oximeters

This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.

Visit any health-care facility, and one of the first things they’ll do is clip a pulse oximeter to your finger. These devices, which track heart rate and blood oxygen, offer vital information about a person’s health. But they’re also flawed. For people with dark skin, pulse oximeters can overestimate just how much oxygen their blood is carrying. That means that a person with dangerously low oxygen levels might seem, according to the pulse oximeter, fine.

The US Food and Drug Administration is still trying to figure out what to do about this problem. Last week, an FDA advisory committee met to mull over better ways to evaluate the performance of these devices in people with a variety of skin tones. But engineers have been thinking about this problem too. In today’s Checkup, let’s look at the problem with pulse oximeters—why they are biased and what technological fixes might be possible.

To understand the problem, you first have to understand how pulse oximeters work. Most of these devices clamp onto some part of the body—usually a fingertip, but sometimes they need to be placed on earlobes or toes. One side of the clamp contains LEDs that emit light in two different wavelengths—red and infrared. A sensor on the other side of the clamp measures how much of that light passes through the tissue. The hemoglobin in oxygenated blood and deoxygenated blood absorbs these wavelengths differently, and by calculating the ratio of the red-light measurements to the infrared-light measurements—the R value—the device can tabulate blood oxygen saturation.

Here’s the problem: other factors can affect how much light is absorbed. Dark nail polish, for example, can throw off the reading. Or tattoos. Or melanin. “If a person has a darker skin tone, they’re going to be absorbing more light,” says Maggie Delano, an engineer at Swarthmore College who is interested in inclusive engineering design. Imagine there are 100 photons of light going through a finger. Some get absorbed by blood, some by bone, and some by melanin in the skin. “So if someone has a darker skin tone, maybe five photons get through instead of 20,” Delano says. “If your electronics don’t compensate for that in some way, there can be errors in that result.”

Those errors can have real clinical consequences. Blood oxygen is one of the key vital signs doctors use to determine whether someone needs to receive oxygen or be admitted to the hospital.   

Engineers are working to fix this problem in a variety of ways. At Tufts, Valencia Koomson and her colleagues have developed a device that can detect when the signal quality is poor or when the user has a darker skin tone and compensate by sending more light through. “We’re dealing with very weak optical signals that have to transverse through tissues with lots of [other] elements that absorb and scatter light,” she told Inverse. “It’s very similar to when you’re riding a car and you go through a tunnel. You lose signal because of the absorption of the materials in the tunnel, such that the signal being transmitted from the cell-phone tower is too weak to be processed by your phone.”

Koomson and her colleagues are collaborating with a medical-device manufacturing company to develop a prototype for clinical trials. Because their team was named a finalist in a recent challenge by Open Oximetry, they’ll be able to validate the device for free in the Hypoxia Lab at the University of California, San Francisco.

Meanwhile, engineers at Brown University are trying to find a workaround using special LEDs that can emit polarized light beams. Jesse Jokerst, an engineer at the University of California, San Diego, is working on an oximeter that uses light and sound, and also corrects for skin tone. Another team at the University of Texas at Arlington is hoping to swap the standard red light in pulse oximeters for green light, which bounces back instead of being absorbed. At Johns Hopkins, engineers have developed a prototype pulse oximeter that factors in skin tone when calculating blood oxygen saturation.

Neal Patwari, a mechanical engineer at Washington University in St. Louis, wants to keep the pulse oximeter’s hardware the same, but swap out the algorithm. A pulse oximeter takes four different measurements, two in each wavelength. One measurement takes place as the heart pushes blood through the arteries, when blood flow is at a maximum, and the other happens between pulses, when blood flow is at a minimum. Those four numbers get fed into an algorithm that calculates ratios—actually, one ratio divided by another. That gives you the R value. But, “when you take two numbers and divide them, you can get some strange effects when the denominator is noisy,” Patwari says. And one of the factors that can increase noisiness is darkly pigmented skin. He hopes to find an algorithm that doesn’t rely on ratios, which could offer up a less biased R value. 

Whether any of these strategies will fix the bias in pulse oximeters remains to be seen. But it’s likely that by the time improved devices are up for regulatory approval, the bar for performance will be higher. At the meeting last week, committee members reviewed a proposal that would require companies to test the device in at least 24 people whose skin tones span the entirety of a 10-shade scale. The current requirement is that the trial must include 10 people, two of whom have “darkly pigmented” skin.

In the meantime, health-care workers are grappling with how to use the existing tools and whether to trust them. In the advisory committee meeting on Friday, one committee member asked a representative from Medtronic, one of the largest providers of pulse oximeters, if the company had considered a voluntary recall of its devices. “We believe with 100% certainty that our devices conform to current FDA standards,” said Sam Ajizian, Medtronic’s chief medical officer of patient monitoring. A recall “would undermine public safety because this is a foundational device in operating rooms and ICUs, ERs, and ambulances and everywhere.”

But not everyone agrees that the benefits outweigh the harms. Last fall, a community health center in Oakland California, filed a lawsuit against some of the largest manufacturers and sellers of pulse oximeters, asking the court to prohibit sale of the devices in California until the readings are proved accurate for people with dark skin, or until the devices carry a warning label.

“The pulse oximeter is an example of the tragic harm that occurs when the nation’s health-care industry and the regulatory agencies that oversee it prioritize white health over the realities of non-white patients,” said Noha Aboelata, CEO of Roots Community Health Center, in a statement. “The story of the making, marketing and use of racially biased pulse oximeters is an indictment of our health-care system.”

Read more from MIT Technology Review’s archive

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No surprise that technology perpetuates racism, wrote Charlton McIlwain in 2020. That’s the way it was designed. “The question we have to confront is whether we will continue to design and deploy tools that serve the interests of racism and white supremacy.”

We’ve seen that deep-learning models can perform as well as medical professionals when it comes to imaging tasks, but they can also perpetuate biases. Some researchers say the way to fix the problem is to stop training algorithms to match the experts, reported Karen Hao in 2021. 

From around the web

The high lead levels found in applesauce pouches came from a single cinnamon processing plant in Ecuador. (NBC)

Alternating arms for your covid vaccines might offer an immunity boost over sticking to the same arm, according to a new study. (NYT)

Weight loss through either surgery or medication lowers blood pressure, according to new research. (CNN)

Pharma is increasingly building AI into its businesses, but don’t expect that to lead to instantaneous breakthroughs. (STAT)

Why engineers are working to build better pulse oximeters

This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.

Visit any health-care facility, and one of the first things they’ll do is clip a pulse oximeter to your finger. These devices, which track heart rate and blood oxygen, offer vital information about a person’s health. But they’re also flawed. For people with dark skin, pulse oximeters can overestimate just how much oxygen their blood is carrying. That means that a person with dangerously low oxygen levels might seem, according to the pulse oximeter, fine.

The US Food and Drug Administration is still trying to figure out what to do about this problem. Last week, an FDA advisory committee met to mull over better ways to evaluate the performance of these devices in people with a variety of skin tones. But engineers have been thinking about this problem too. In today’s Checkup, let’s look at the problem with pulse oximeters—why they are biased and what technological fixes might be possible.

To understand the problem, you first have to understand how pulse oximeters work. Most of these devices clamp onto some part of the body—usually a fingertip, but sometimes they need to be placed on earlobes or toes. One side of the clamp contains LEDs that emit light in two different wavelengths—red and infrared. A sensor on the other side of the clamp measures how much of that light passes through the tissue. The hemoglobin in oxygenated blood and deoxygenated blood absorbs these wavelengths differently, and by calculating the ratio of the red-light measurements to the infrared-light measurements—the R value—the device can tabulate blood oxygen saturation.

Here’s the problem: other factors can affect how much light is absorbed. Dark nail polish, for example, can throw off the reading. Or tattoos. Or melanin. “If a person has a darker skin tone, they’re going to be absorbing more light,” says Maggie Delano, an engineer at Swarthmore College who is interested in inclusive engineering design. Imagine there are 100 photons of light going through a finger. Some get absorbed by blood, some by bone, and some by melanin in the skin. “So if someone has a darker skin tone, maybe five photons get through instead of 20,” Delano says. “If your electronics don’t compensate for that in some way, there can be errors in that result.”

Those errors can have real clinical consequences. Blood oxygen is one of the key vital signs doctors use to determine whether someone needs to receive oxygen or be admitted to the hospital.   

Engineers are working to fix this problem in a variety of ways. At Tufts, Valencia Koomson and her colleagues have developed a device that can detect when the signal quality is poor or when the user has a darker skin tone and compensate by sending more light through. “We’re dealing with very weak optical signals that have to transverse through tissues with lots of [other] elements that absorb and scatter light,” she told Inverse. “It’s very similar to when you’re riding a car and you go through a tunnel. You lose signal because of the absorption of the materials in the tunnel, such that the signal being transmitted from the cell-phone tower is too weak to be processed by your phone.”

Koomson and her colleagues are collaborating with a medical-device manufacturing company to develop a prototype for clinical trials. Because their team was named a finalist in a recent challenge by Open Oximetry, they’ll be able to validate the device for free in the Hypoxia Lab at the University of California, San Francisco.

Meanwhile, engineers at Brown University are trying to find a workaround using special LEDs that can emit polarized light beams. Jesse Jokerst, an engineer at the University of California, San Diego, is working on an oximeter that uses light and sound, and also corrects for skin tone. Another team at the University of Texas at Arlington is hoping to swap the standard red light in pulse oximeters for green light, which bounces back instead of being absorbed. At Johns Hopkins, engineers have developed a prototype pulse oximeter that factors in skin tone when calculating blood oxygen saturation.

Neal Patwari, a mechanical engineer at Washington University in St. Louis, wants to keep the pulse oximeter’s hardware the same, but swap out the algorithm. A pulse oximeter takes four different measurements, two in each wavelength. One measurement takes place as the heart pushes blood through the arteries, when blood flow is at a maximum, and the other happens between pulses, when blood flow is at a minimum. Those four numbers get fed into an algorithm that calculates ratios—actually, one ratio divided by another. That gives you the R value. But, “when you take two numbers and divide them, you can get some strange effects when the denominator is noisy,” Patwari says. And one of the factors that can increase noisiness is darkly pigmented skin. He hopes to find an algorithm that doesn’t rely on ratios, which could offer up a less biased R value. 

Whether any of these strategies will fix the bias in pulse oximeters remains to be seen. But it’s likely that by the time improved devices are up for regulatory approval, the bar for performance will be higher. At the meeting last week, committee members reviewed a proposal that would require companies to test the device in at least 24 people whose skin tones span the entirety of a 10-shade scale. The current requirement is that the trial must include 10 people, two of whom have “darkly pigmented” skin.

In the meantime, health-care workers are grappling with how to use the existing tools and whether to trust them. In the advisory committee meeting on Friday, one committee member asked a representative from Medtronic, one of the largest providers of pulse oximeters, if the company had considered a voluntary recall of its devices. “We believe with 100% certainty that our devices conform to current FDA standards,” said Sam Ajizian, Medtronic’s chief medical officer of patient monitoring. A recall “would undermine public safety because this is a foundational device in operating rooms and ICUs, ERs, and ambulances and everywhere.”

But not everyone agrees that the benefits outweigh the harms. Last fall, a community health center in Oakland California, filed a lawsuit against some of the largest manufacturers and sellers of pulse oximeters, asking the court to prohibit sale of the devices in California until the readings are proved accurate for people with dark skin, or until the devices carry a warning label.

“The pulse oximeter is an example of the tragic harm that occurs when the nation’s health-care industry and the regulatory agencies that oversee it prioritize white health over the realities of non-white patients,” said Noha Aboelata, CEO of Roots Community Health Center, in a statement. “The story of the making, marketing and use of racially biased pulse oximeters is an indictment of our health-care system.”

Read more from MIT Technology Review’s archive

Melissa Heikkilä’s reporting showed her just how “pale, male, and stale” the humans of AI are. Could we just ask it to do better? 

No surprise that technology perpetuates racism, wrote Charlton McIlwain in 2020. That’s the way it was designed. “The question we have to confront is whether we will continue to design and deploy tools that serve the interests of racism and white supremacy.”

We’ve seen that deep-learning models can perform as well as medical professionals when it comes to imaging tasks, but they can also perpetuate biases. Some researchers say the way to fix the problem is to stop training algorithms to match the experts, reported Karen Hao in 2021. 

From around the web

The high lead levels found in applesauce pouches came from a single cinnamon processing plant in Ecuador. (NBC)

Alternating arms for your covid vaccines might offer an immunity boost over sticking to the same arm, according to a new study. (NYT)

Weight loss through either surgery or medication lowers blood pressure, according to new research. (CNN)

Pharma is increasingly building AI into its businesses, but don’t expect that to lead to instantaneous breakthroughs. (STAT)

How wastewater could offer an early warning system for measles

Measles is back with a vengeance. In the UK, where only 85% of school-age children have received two doses of the MMR vaccine, as many as 300 people have contracted the disease since October. And in the US, an outbreak has infected nine people in Philadelphia since last month. One case has been reported in Atlanta, another in Delaware. An entire family of six is infected in Washington state. 

On January 23, the World Health Organization issued a warning. “It is vital that all countries are prepared to rapidly detect and timely respond to measles outbreaks, which could endanger progress towards measles elimination,” said Hans Kluge, WHO regional director for Europe. 

Catching measles outbreaks early is tricky, though. Like many other respiratory viruses, it starts off with a cough, runny nose, fever, and achy body. The telltale rash doesn’t appear for two to four more days. By then, a person is already infectious. Very infectious, in fact. Measles is one of the most contagious diseases around.

Maybe there’s a solution. The US developed a vast wastewater sampling network to detect covid during the pandemic. Could we leverage that network to provide an early warning system for measles?

“I actually think you could make the argument that measles is even more important to [detect] than covid or influenza or any of the other pathogens that we’re looking for,” says Samuel Scarpino, an epidemiologist at Northeastern University in Boston.

Wastewater surveillance relies on standard lab tests to find genetic evidence of pathogens in sewage—DNA or RNA. When people are infected with covid, they shed SARS-CoV-2 in their stools, so it’s easy to see why it would show up in wastewater. But even viruses that don’t get pooped out can show up in the sewers. 

Although measles is a respiratory virus, people shed it in their urine. They also brush their teeth and spit in the sink. They blow their noses and throw the tissue in the toilet. “We shed these viruses and we shed bacteria and fungi in so many ways that end up in the sewer,” says Marlene Wolfe, an environmental microbiologist and epidemiologist at Emory University and one of the directors of WastewaterSCAN, a program based at Stanford that monitors infectious diseases through municipal wastewater systems. 

The literature on wastewater detection of measles is scant, but encouraging. In one study, a team of researchers in the Netherlands tested wastewater samples collected in 2013 during a measles outbreak in an orthodox Protestant community for evidence of the virus. They found measles RNA, and the positive samples matched the locations where cases had been reported. They even managed to confirm that the virus in one sample was genetically identical to the outbreak strain. But not every measles case showed up in the sewers. Some samples taken where cases had occurred didn’t harbor any measles RNA. 

In another study, researchers from Nova Scotia developed a tool to screen wastewater for four pathogens simultaneously: RSV, influenza, covid, and measles. When they tested it in Nova Scotia, they didn’t get any positive hits for measles, which didn’t surprise them as no cases had been reported. But when they seeded the wastewater samples with a surrogate for measles, they were able to detect it at both high and low concentrations

The real question, Wolfe says, is whether detecting measles in wastewater would have any public health value. Because measles is rarely asymptomatic and the rash is so distinctive, cases tend to get noticed. “Some of our other systems can work pretty well at identifying measles cases as they come up,” she says.

Wolfe could see value in monitoring, she says, if people really shed high quantities of the virus before those signs are visible. “Then it really could provide an early warning,” she says. But that’s not known at the moment. 

What would a wastewater surveillance program for measles look like? “If we had the ability to target places where the vaccination coverage was lower, that would be a place to prioritize resources,” Scarpino says. “Airports and other ports of entry are going to be really important as well.” Earlier this month, someone infected with measles passed through both Dulles and Ronald Reagan airports just outside of Washington, DC. Finding measles RNA in airport sewage doesn’t necessarily mean a local outbreak might occur, but “it definitely means that the risk profile is there and we should be monitoring much more actively,” he says. 

While measles isn’t part of wastewater surveillance yet, plenty of other pathogens are. Health officials around the globe have been testing sewage for polio since the late 1980s. Because people who contract polio shed large amounts of the virus in their feces, and because so many people are asymptomatic, “it’s like a perfect use case in a lot of ways,” Wolfe says. But wastewater surveillance didn’t really become fashionable until 2020, when covid hit. 

The National Wastewater Surveillance System, which the Centers for Disease Control and Prevention (CDC) launched in 2020 to monitor covid, now also tests for mpox. WastewaterSCAN currently tests for 10 different pathogens, including covid, mpox, RSV, influenza, norovirus, and rotavirus. The team publishes that data on a dashboard on its website and shares it with the CDC. Wolfe and her colleagues also recently worked with Miami-Dade County in Florida to assess the feasibility of testing for dengue. Even though dengue is rare in Florida, the team picked up a signal in the wastewater. 

In fact, wastewater surveillance works for most of the pathogens they’ve tried, Wolfe says: “The potential for leveraging this tool to effectively support measles surveillance is absolutely possible.” 

Another thing

The complement system may be the most important immune defense you’ve never heard of. And now two teams of researchers say that this microbe-fighting protein cascade is abnormal in some people with long covid, pointing researchers toward new potential therapies. 

Read more from MIT Technology Review’s archive

Wastewater with its wealth of microbes could help researchers track the evolution of antibiotic resistance in bacteria, Jessica Hamzelou wrote last year. 

Health officials used wastewater surveillance to track the spread of mpox in 2022 and helped scientists estimate how many people in California’s Bay Area might be affected, Hana Kiros reported. 

Way back in 2021, Antonio Regalado covered some of the first efforts to track the spread of covid variants using wastewater.  

From around the web

The FDA slapped a black box warning on CAR-T cancer therapies, which rely on engineered T cells to fight the disease. The decision comes after the agency received 25 reports of new blood cancers in people who received these treatments. (NBC)

My latest for Nature is a deep dive into efforts to restore immune tolerance in people with autoimmune diseases. Researchers are finally having some success addressing the cause of these diseases and are even talking about (gasp!) the possibility of a cure. (Nature)   

An 11-year-old boy who was born deaf can now hear after receiving gene therapy as part of a clinical trial. “There’s no sound I don’t like,” he told the New York Times. “They’re all good.” (NYT)