Last month, Apple launched the Apple Watch Series 6 at a virtual event that streamed from its Silicon Valley campus.
“The future of health is on your wrist,” flashed Apple’s slogan during the launch event, accelerating the current global conversation about health and technology. Vignettes showcased how the Apple Watch has saved lives, while Apple CEO Tim Cook asserted that healthcare providers have recognized the benefits of the smartwatch when working with patients.
At $400, Apple’s latest smartwatch boasts pulse oximetry—the ability to measure blood oxygen saturation levels—a feature that is especially relevant during the ongoing coronavirus pandemic. While companies like Oura and Fitbit have already been introducing technologies for flagging potential COVID-19 cases, Apple’s attempts to infiltrate the healthcare market are particularly significant due to the tech giant’s massive base of loyal customers.
What Is Pulse Oximetry and Why Is It Important?
“Blood oxygen and pulse oximetry are terms that we’ve heard a lot about during the COVID pandemic,” stated Sumbul Desai, Apple Vice President for Health, at the launch event. “As you breathe, your heart and lungs work together to deliver oxygen throughout your body. Blood oxygen saturation is an indication of how well this system is functioning and of your overall respiratory and cardiac health. And pulse oximetry is how you measure it.”
According to the World Health Organization, a healthy individual’s oxygen saturation of hemoglobin in arterial blood—or SpO2—should always be between 95–100 percent. If the oxygen saturation drops to 94 percent or lower, the patient is hypoxic and requires urgent medical treatment. Saturation levels of less than 90 percent constitute a clinical emergency, where the patient’s lips turn blue in a process called cyanosis. Low blood oxygen levels can ultimately lead to the failure of vital organs such as the brain and heart, which are particularly sensitive to the body’s lack of oxygen.
Pulse oximeters can detect signs of hypoxia before cyanosis sets in, and therefore serve as reliable early-warning devices. The readings are additionally important because they’re less subjective than pulse or blood pressure stats.
An oximeter probe consists of light-emitting diodes (LEDs) and a photodetector. The probe is placed where a pulse can be detected—medical-grade oximeters usually detect blood oxygen saturation through the finger. Beams of light are passed from the probe through blood and tissue, which absorb light based on the oxygen saturation of hemoglobin. The photodetector detects the amount of light transmitted, and the oximeter’s microprocessor calculates the SpO2 value.
During pulse oximetry, oxygen-rich blood shows up as bright red while desaturated blood appears darker. Pulse oximeter readings can vary depending on an individual’s age, medical history and elevation, with lower readings typically observed for people living significantly above sea level.
Pulse oximeters have proven to be especially useful for the early detection of COVID pneumonia due to their ability to flag low oxygen levels before an individual has even started to experience symptoms such as shortness of breath—a phenomenon termed ‘silent hypoxemia’.
“Pulse oximeters helped save the lives of two emergency physicians I know, alerting them early on to the need for treatment,” describes Dr. Richard Levitan, an emergency physician with 30 years’ experience, in a New York Times article. “When they noticed their oxygen levels declining, both went to the hospital and recovered.”
According to Dr. Levitan, the SARS-CoV-2 virus attacks lung cells that produce surfactant—a substance that helps the air sacs in the lungs stay open between breaths. As inflammation from COVID pneumonia begins, the lung’s air sacs collapse and oxygen levels start to fall. The lungs, however, initially remain “compliant” by continuing to expel carbon dioxide. Without a CO2 buildup, patients don’t experience shortness of breath, and continue to injure their lungs by breathing heavily to supplement their lack of oxygen. By the time patients notice any breathing trouble, their oxygen saturation levels could be as low as 50 percent, and their cases could turn critical in a matter of hours.
Other medical experts agree that in the midst of the current pandemic, a pulse oximeter could be the difference between life and death.
“In COVID-19 patients, we often see that they look comfortable, but their oxygen saturation is significantly worse than normal,” says Dr. Elissa Perkins, associate professor of emergency medicine at Boston Medical Center.
How Effective Is Apple’s Pulse Oximetry Technology?
The back crystal of the Apple Watch Series 6 employs four clusters of green, red and infrared LEDs, along with four photodiodes for measuring light reflected back from blood. A custom algorithm built into Apple’s Blood Oxygen app calculates oxygen saturation based on the color of the blood, with the smartwatch measuring blood oxygen levels between 70–100 percent. The user can perform a spot check—which takes 15 seconds—as well as periodic background measurements for tracking blood oxygen saturation trends over time.
Apple has been conspicuously silent about the error rate of its pulse oximetry feature, with spokeswoman Amy Bessette saying only that it “has been rigorously tested across a wide spectrum of users and across all skin tones.”
“For a small percentage of users, various factors may make it difficult to get a blood oxygen measurement including motion, watch placement on the wrist, skin temperature and skin perfusion,” concedes Bessette. “The blood oxygen app provides dynamic feedback to help users get the best reading possible.”
Unlike traditional pulse oximeters that clamp onto the finger, the Apple Watch takes measurements off the wrist. As it turns out, the supply of blood vessels is much richer on a finger’s surface than on a wrist—and this is likely the source of wildly inaccurate readings that have been experienced by multiple Apple Watch users.
“Sometimes the new Apple Watch Series 6 reports my lungs and heart are the picture of health, pumping blood that’s 100 percent saturated with oxygen,” writes Geoffrey Fowler for the Washington Post. “At other times, it reports my blood oxygen is so low I might be suffering from emphysema. (I am not.)”
When Fowler first tried Apple’s pulse oximetry, the smartwatch displayed a shockingly low SpO2 reading of 88 percent. When retesting five minutes later, the oxygen level showed up as 95 percent.
To continue testing the Apple Watch, Fowler purchased an FDA-approved finger oximeter for $60 from Medline Industries, with a published error rate of two percentage points. While readings against the Apple Watch occasionally matched, they differed more often by two to three percentage points, sometimes going as much as seven percentage points lower.
Wall Street Journal’s Joanna Stern and The Verge’s Dieter Bohn reported similarly unreliable results.
Apple’s Additional Health Features
It’s a shame about the pulse oximetry, but at least the Apple Watch Series 6 inherits Series 5’s health features—such as the ability to measure an electrocardiogram and detect hard falls. Apple’s 2020 software update, watchOS 7, also supports sleep tracking.
Apple’s ECG app works similarly to a single-lead ECG, as opposed to a doctor’s office where a standard 12-lead ECG measures the heart’s electrical activity in three directions (right to left, up and down, and front to back). A titanium electrode resides on the smartwatch’s digital crown, while a layer of chromium silicon carbon nitride rests on the bottom of the watch. Holding a finger down on the crown with the watch securely fastened on the wrist creates a closed circuit that allows for an ECG to be performed. While Apple’s ECG app has been FDA-cleared to provide information about heart rate and rhythm—as well as enabling classification of atrial fibrillation (AFib)—it cannot detect heart attacks, blood clots, strokes, high blood pressure, congestive heart failure, high cholesterol or other forms of arrhythmia.
The Apple Watch can reliably detect falls using its accelerometer and gyroscope. The feature was robustly tested by a professional stuntwoman for a Wall Street Journal review.
During testing, the Apple Watch was not found to issue false positives from actions including a hand slam onto a desk, or a soft fall of the wearer onto a couch. It was only the stuntwoman’s slips, trips and falls that triggered an alert reading—”It looks like you’ve taken a hard fall”—with an option to call emergency services. If the stuntwoman remained immobile for a minute, the smartwatch automatically started a 15-second countdown for calling 911.
When it comes to sleep tracking, the Apple Watch offers only a basic report of the amount of time an individual has spent asleep along with a summary of heart rate data throughout the night. Competitors such as Fitbit offer far more comprehensive information including the quality of sleep and how much time was spent in light, deep and REM sleep.
Last (and probably least), with watchOS 7, the Apple Watch incorporates a hand-washing feature that caters to the COVID-19 pandemic by performing a 20-second countdown when you wash your hands. The smartwatch detects specific movements that are distinctive to hand-washing, as well as the sound of a running faucet. Once your hands are washed, Apple Watch displays a “Well Done” message constructed of soapy bubbles. While it’s a quaint feature for users who don’t wish to count to 20 themselves, the app has occasionally been found to pause the countdown timer even in the midst of proper hand-washing, simply because the hands weren’t close enough to running water. (What if I wanted to conserve water by scrubbing my soapy hands without the tap running?)
Apple Watch: Not a Medical Device
There is currently a troubling trend where tech companies are advertising health capabilities that may result in consumers treating gadgets as medical devices. If users ignore the fine print that says the Apple Watch’s blood oxygen app is “not intended for medical use”—and buy into Apple’s marketing about the smartwatch providing valuable pulse oximetry readings—the consequences could potentially be dangerous. While the most common scenario would likely be one where people are contacting their doctors too often because of false low readings, it could also go the other way.
“The devices [could] provide false reassurance [where] people don’t seek healthcare when they really need it,” warns Dr. Brian Clark, a pulmonologist and professor at the Yale University School of Medicine.
The bottom line is: Apple Watch Series 6 is not FDA-approved for pulse oximetry. In fact, Apple shouldn’t even be making claims of its ECG app’s life-saving abilities through flashy vignettes at launch events.
“Anecdotes are not data,” says Dr. Kevin Campbell, a cardiologist and expert in heart rhythm disorders. “You can’t make that type of sweeping claim with any accuracy or legitimacy.”
Additionally, the Apple Watch’s ECG app is FDA-cleared—not FDA-approved. It’s an important distinction because FDA approval requires far more rigorous scientific evidence than FDA clearance. Numerous medical experts agree that a company can only tout the life-saving capabilities of a product after it has achieved official FDA approval.
“We have toys, and we have things that are used for clinical purposes. And it really needs to be a clear distinction,” says Jessilyn Dunn, an assistant professor of biomedical engineering at Duke University.
What about using the Apple Watch’s pulse oximetry feature for “general fitness and wellness purposes”? Realistically, the sensors cannot provide measurements while working out. The slightest bit of movement—even breathing too heavily—has been found to send the Apple Watch into error mode.
That leaves us with “wellness”. What does that even mean if there’s no way to interpret the accuracy of the Apple Watch’s pulse oximetry readings?
Analysts predict that it may take more than a decade before smartwatches have the types of sensors and research necessary to be a comprehensive health solution for users. In the meantime, Apple fans may not necessarily need to splurge on the tech behemoth’s most expensive smartwatch in order to enjoy its best features. At $280, the Apple Watch SE offers a cheaper alternative where it lacks pulse oximetry and ECG readings, but still provides fall detection capabilities as well as other features found on the Apple Watch’s newer models.
Or you could simply invest in a $60 fingertip pulse oximeter.