Vital technology with a fatal flaw

At the start of the COVID-19 pandemic, Ashraf Fawzy recalls a patient who arrived in intensive care at Johns Hopkins Hospital, a black woman with asthma. Despite her pulse oximeter readings indicating normal oxygen levels, the woman insisted she was out of breath.

“His symptoms were severe, but the numbers told a different story,” recalls Fawzy.

Perplexed, the doctors performed an arterial blood test – the gold standard for measuring oxygen, but a more invasive and time-consuming method than the clamp-on pulse oximeter. The second test validated the woman’s pleas. “It turned out that we needed to give her more oxygen and we needed to keep her in intensive care longer,” Fawzy says.

This experience was not unique. Fawzy, an assistant professor of medicine at Johns Hopkins University School of Medicine, was noticing a sporadic pattern of inaccurate numbers from pulse oximeters, apparently more common in patients of color. Comparing his notes, he discovered that a colleague in Texas, David Wu of Baylor College of Medicine, saw the same thing. In December 2020, a letter published in the New England Journal of Medicine confirmed that they were not alone.

“Because we light the skin to see how much light is absorbed by hemoglobin, different skin tones can interfere with how light is absorbed and how it is measured.”

Ashraf Fawzy

Johns Hopkins School of Medicine

Since its invention in 1974, the ‘pulse ox’ device has become a staple of routine medical practice, its readings being known by some as ‘the fifth vital sign’. Attached to a finger, toe or earlobe, the device shines wavelengths of light onto the skin to provide immediate estimates of blood oxygen saturation levels, helping to detect a range of problems common health conditions, including respiratory and heart problems.

During the pandemic, pulse oximeters have become more widespread than ever before, almost as available as thermometers in pharmacies and home medicine cabinets, and are popping up in wearable devices like smartwatches. With COVID-19 affecting lung function, a pulse-ox is commonly used to determine if and when a patient enters the hospital, and once there, how their treatment is progressing. “In COVID, the pulse-ox has become central to making clinical decisions,” Fawzy said.

But for years it has been known that pulse oximeters can harbor racial bias, with studies dating back to the late 1980s suggesting a flaw in the way the device measures blood oxygen in people of color. But according to Fawzy, the pandemic has finally brought the issue to the forefront of medicine. “In my name and in Dr. Wu’s name, we were never taught this in medical school,” he says. “It was never mentioned that beef pulses should be beware of in certain populations or that these could potentially be inaccurate.”

Although the issue needs more research, darker skin tones are thought to affect how well a pulse oximeter absorbs light. “Because we light the skin to see how much light is absorbed by hemoglobin, different skin tones can interfere with how light is absorbed and how it’s measured,” Fawzy says.

Through their own research published in JAMA internal medicine in May, Fawzy and Wu confirmed that there is a disparity between legumes and beef. Eliminating data from more than 7,000 patients from the Johns Hopkins Precision Medicine Center of Excellence for COVID-19, led by Brian Garibaldi, associate professor of medicine at Johns Hopkins University School of Medicine, the study found that black patients and Hispanics were 29% and 23% less likely than white patients, respectively, to have pulse oximeters recognizing their eligibility for more aggressive COVID-19 treatment.

These findings are concerning, says Fawzy. “Pulse counts are used to determine what drugs are given and when – important decisions.”

The U.S. Food and Drug Administration recently announced a plan to convene the Medical Devices Advisory Committee to review regulatory requirements for pulse oximeters, which Fawzy hopes will result in guidelines for testing and more inclusive validation.

Is it possible that some patients have died due to pulse oximeter defects? Fawzy says there isn’t enough data yet to answer that question, but follow-up studies will aim to determine exactly that. “What we do know is that our study showed a potential delay in care, and other studies showed that ethnic and racial minority patients had worse COVID outcomes,” Fawzy says. “Pulse oximeter inaccuracy may be one of many factors involved, but more research is needed.”

In the long run, updating the design of the pulse oximeter, which has remained largely unchanged for decades, is essential. A number of researchers are now taking up this challenge, including a group of biomedical engineering students at Johns Hopkins that Fawzy commissioned. The EquiOx team, led by recent graduate Jerry Zhang and undergraduate students Stanley Zhu and Vivek Chari, has developed a prototype for a new pulse oximeter that accounts for skin tone and other factors physiological when calculating blood oxygen saturation. Simulations have shown that the device could potentially be 2.8 times more accurate than traditional pulse oximeters at detecting low oxygen levels in dark-skinned patients. The team is currently testing the device in a clinical study at Johns Hopkins Hospital.

Beyond COVID-19, an improved pulse oximeter would have implications for a slew of diseases where oxygen level monitoring is critical, including pneumonia, asthma, chronic obstructive pulmonary disease, emphysema , cystic fibrosis, heart failure, lung cancer and sleep apnea.

Right now, Fawzy says, it’s not realistic to stop using pulse oximeters or switch to arterial blood testing entirely. “Pulse ox is still a very useful tool and right now it’s the best we have for non-invasively measuring oxygen levels,” he says. “It would not be practical or reasonable for every person of color to have their oxygen level monitored solely by blood tests, which are more invasive, can be painful, and do not provide ongoing or timely monitoring.”

So for now, he says, the best strategy is to raise awareness. “It is important to know that this device has limitations. If the pulse does not match what you are feeling, seek medical care as the numbers may not be accurate.”

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