The problem with pulse ox

Disparities | June 29, 2022 | FREE

Most ACP Hospitalist content is available exclusively to ACP Members. This article is free to the public.


Research has long indicated that pulse oximeters can overestimate oxygen saturation
in patients with darker skin, but experts are now trying raise awareness of the problem.






Pulse oximeters are everywhere these days—not just in the ICU, on the floor,
and at outpatient practices, but even on the wrists of people walking down the street.

“These devices have become really mainstream. In addition to the pulse oximeters
that patients could buy off the internet or from a pharmacy or any store, really,
these devices are being integrated into wearable devices,” said Ashraf Fawzy,
MD, MPH, a pulmonology and critical care specialist and assistant professor of medicine
at Johns Hopkins University in Baltimore.

That prevalence makes researchers such as Dr. Fawzy particularly concerned about the
growing evidence that pulse oximeters don’t work as well in patients with darker skin
tones.

“For a long time now we have taken whatever the pulse oximeter says at face
value and made decisions day in and day out based on it,” said Valeria Valbuena,
MD, MSc, a general surgery resident at Michigan Medicine in Ann Arbor who has also studied pulse oximeters. “We really need to rely less on single pulse oximeter measurements as a profession.”

The problem

Evidence that pulse oximeters may overestimate oxygen saturation in those with darker
skin dates back to at least 1990, when a small study published in CHEST compared pulse oximetry in White and Black patients on ventilators, finding that in the latter group, a pulse oximetry reading of 92% was commonly associated
with significant hypoxemia.

The issue got little attention, however, until the pandemic brought both oxygen saturation
and racial equity to the forefront of health care. “It’s telling that there
was literature that existed describing the inaccuracy of the pulse oximeter by skin
pigment, but we weren’t even aware of it,” said Michael Sjoding, MD, MSc, an
assistant professor of pulmonary and critical care and hospital medicine at Michigan
Medicine in Ann Arbor. “It wasn’t in any of my educational curriculums.”

He first got an inkling of the problem early in the pandemic, when Ann Arbor starting
receiving COVID-19 patients transferred from Detroit.

“I and other colleagues were caring for a high number of racially diverse patients,
especially in the first wave. And anecdotally, several of us noticed that we were
seeing patients with pulse oximetry on monitors that were normal and then when, just
by chance, an arterial blood gas was performed, it showed the actual saturation was
lower,” he said. “None of us could figure out what was going on at the
time.”

Dr. Sjoding and his colleagues investigated and published their results as a letter
to the editor of the New England Journal of Medicine in December 2020. The study compared pulse oximetry and arterial blood gases from thousands of White and Black patients
at multiple hospitals before and during the pandemic and found that pulse oximetry failed to detect hypoxemia
almost three times as frequently in Black patients as in White patients.

Soon, a number of other researchers had built on their findings. A database analysis,
published by JAMA Network Open in November 2021, compared pulse oximetry and arterial oxygen saturation in more than 80,000 patients. It found that 6.9% of Black patients had oxygen saturation below 88% on blood gas
but not pulse oximeter, as did 6% of Hispanic patients, compared to 4.9% of Asian
and White patients.

The findings highlighted key questions about pulse oximeters, according to lead author
A. Ian Wong, MD, PhD, a critical care specialist and pulmonologist at Duke University
in Durham, N.C. “Myself, I’m neither White nor Black. How does this work in
people who don’t fit either of those categories?”

It was also important to determine if the inaccuracies had any effect on patient outcomes.
“We were trying to figure out, ‘OK, does this affect mortality?’”
he said. It appeared so in the study, with patients in whom the discrepancy, or “hidden
hypoxemia,” was found having higher mean sequential organ failure assessment
(SOFA) scores and higher in-hospital mortality.

“The third thing that was really interesting and that we did not expect to
find was that there appeared to be different rates at which different races got blood
gases,” said Dr. Wong.

Potential connections between the shortcomings of pulse oximeters and already well-established
racial disparities in outcomes, including from COVID-19, were further elucidated by a study Dr. Fawzy and colleagues
published in JAMA Internal Medicine on May 31.

“The renewed attention to the inaccuracies of pulse oximetry led us to ask
whether or not this discrepancy in pulse oximetry may translate into changes in how
we manage patients with COVID,” said co-lead author Tianshi David Wu, MD, MHS,
an assistant professor at Baylor College of Medicine in Houston.

The study found that Black patients were again more likely to have their oxygen saturation
overestimated by pulse oximetry and that this led to them being less likely to be
treated with oxygen when needed. Among the COVID-19 patients who eventually got oxygen,
there was a median treatment delay of an hour for Black patients compared to White
patients.

These recent studies may answer the question of whether pulse oximetry inaccuracies
are clinically important, according to Dr. Wong. “I’ve actually heard some
physicians say, ‘I know that this is an interesting finding, but how much does
it really make a difference?’” he said.

“On an absolute scale, the accuracy difference is small,” Dr. Sjoding
noted. “We’re talking one or two percentage points, on average, but the problem
is that we, in clinical medicine, make decisions—to treat or not to treat,
to admit to the hospital or not to admit to the hospital—often based on cutoffs.”

The search for fixes

In February 2021, the FDA issued a safety communication about pulse oximeters. It warned clinicians to “be aware that multiple factors can affect the accuracy of a pulse oximeter
reading, such as poor circulation, skin pigmentation, skin thickness, skin temperature,
current tobacco use, and use of fingernail polish.”

A collaborative of critical care societies, including the Society of Critical Care
Medicine, American Thoracic Society, American College of Chest Physicians, and American
Association of Critical-Care Nurses, has pushed the agency to take more action. In a February 2022 letter, the group urged the FDA “to directly engage the developers and manufacturers
of all pulse oximeters to rectify this urgent situation in a timely manner.”

On June 21, the FDA updated its safety communication with plans to convene a public meeting of its medical device advisory committee later this year to discuss evidence about
the accuracy of pulse oximeters, recommendations for patients and clinicians, and
the amount and type of data that should be provided by manufacturers to assess the
devices’ accuracy.

The agency does already require some evidence that pulse oximeters are accurate in
patients of different skin colors, but some experts find the criteria insufficient.
“Right now, I would say that the standards aren’t rigorous enough to ensure
that these devices are performing the same in all people,” said Dr. Sjoding.
“It requires a sample size of 10 patients, two of whom are darkly pigmented.”

“There’s nothing in science that I could be like, ‘I tested it on three
people,’” said Dr. Valbuena. “We have had [pulse oximeters] around
for over four decades. When you think about the things that we do with other devices—ventilators,
machines to support your heart and lungs—we have constant innovation.”

At the University of California, San Francisco’s Hypoxia Lab, Michael Lipnick, MD,
has had a close eye on innovation in pulse oximetry. “We’re further behind
than we should be,” he said. “We don’t have solutions in the public
domain, so it’s not clear for all manufacturers how to address this. Some devices
perform better than others, but in almost all the devices that we’ve tested, we find
some bias, some inaccuracy, that we think is attributable to skin pigment.”

The lab tests pulse oximeters for manufacturers as well as large purchasers of the
devices, explained Dr. Lipnick, who is an anesthesiologist and intensivist at Zuckerberg
San Francisco General Hospital.

They’re also currently working on research that could lead to regulatory changes.
“We have received funding from the FDA to do a clinical trial to better understand
exactly how big of a problem this is and some of the reasons why this may be so,”
he said, noting that the inaccuracies likely involve not only skin color, but also
other factors such as perfusion.

For anyone outside of his lab, getting valid data about the accuracy of any one pulse
oximeter is difficult, according to Dr. Lipnick. “It’s not always so clear
what the performance of these devices is, and it may not always match up with what
some of the manufacturers are claiming,” he said.

Dr. Lipnick and his colleagues are hoping to remedy that by building a website to
share oximeter research. “We think that we can build what would be the largest
data set for this type of performance data, both for clinicians and users, and also
for researchers and manufacturers,” he said. Their site, openoximetry.org, is currently still in the pilot stage.

Clinical solutions

Of course, today’s desaturating patient can’t wait for data sharing, regulatory changes,
and technological innovation.

“The message that I think many researchers, including us, are trying to promote
is not that you can’t trust a pulse oximeter or that you shouldn’t use it, or that
these are not extremely valuable and essential tools for diagnosis, but rather just
to be aware that there are some devices out there that may be overestimating oxygen
saturation, particularly in those with darker skin pigment,” said Dr. Lipnick.

He noted that FDA-approved devices are likely more accurate than over-the-counter
alternatives. “I’ve observed a lot of clinicians who have their own fingertip
pulse oximeter or their own low-cost device that they’ve procured for a variety of
different reasons,” said Dr. Lipnick. “Be careful with any device that
hasn’t at least gone through FDA certification.”

Unfortunately, there’s no “easy bedside conversion” for overestimates
due to skin color, “like, oh, it seems like it’s five points lower for every
single measurement,” said Dr. Valbuena. Instead, she recommends considering
multiple readings from the device to get a more accurate picture. “We really
need to be looking at trends rather than single pulse oximeter measurements until
we have more equitable machines.”

Hospitalists should also trust their instincts, recommended Dr. Fawzy. “If
the pulse oximeter is giving you a different answer than what you’re seeing clinically,
it might be worthwhile to question it in certain populations,” he said.

Dr. Sjoding agreed. “If I have a patient, particularly someone who has darkly
pigmented skin, and I have a borderline pulse oximetry, I will be much less likely
now to weigh that information heavily in my decisions about how to care for that patient,”
he said.

There are other biomarkers available to help identify hypoxia, Dr. Valbuena noted.
“I look at things like lactate and like whether or not there are any derangements
in urine output or creatinine,” she said.

And when you really need to know the patient’s oxygen saturation, go to the gold standard
test, the experts agreed. “I hate to say it, but when you’re in doubt, especially
in hospitalized patients or in patients in the clinic that are not looking good, I
just think adding more blood gases,” said Dr. Valbuena. “There are consequences
to getting a painful procedure and it’s not done for free, but there are bigger consequences
to not having enough oxygen.”

Clinicians should be honest with patients about the problem, Dr. Wong suggested. “‘I’m
not choosing to subject you to this painful test just because I’m highly curious.
I’m trying to make sure you get the care you need,’” he said. “It’s
a delicate conversation.”

Conversing with colleagues about pulse oximeters can also be part of the solution.
“I am still surprised when I have conversations with trainees and physicians
that are not aware of it, because everybody’s using this device. Everybody,”
said Dr. Valbuena.

She would like to see clinicians join the push for better solutions. Remaking pulse
oximeters to be accurate across the range of skin colors and then replacing all those
in use in health care will be a large undertaking, she noted. “I really think
that every single provider needs to be aware and actively engaged,” Dr. Valbuena
concluded.

Leave a Comment