Apple announced last week the release of Apple Watch Series 6 (AW6) which features a "revolutionary Blood Oxygen feature." That raised some questions:
- How accurate or useful (or revolutionary) is that blood oxygen feature?
- Should those who can afford the $399 rush out and buy one?
- Can the future of health really be on our wrist?
Although the Skeptical Cardiologist (especially the ) and thinks a home pulse oximeter can be useful during COVID-19, he hasn't yet ordered the AW6. Let's examine why.
What Is the Accuracy?
Apple's marketing release tells us that their blood oxygen sensor "employs four clusters of green, red, and infrared LEDs, along with the four photodiodes on the back crystal of Apple Watch, to measure light reflected back from blood. Apple Watch then uses an advanced custom algorithm built into the Blood Oxygen app, which is designed to measure blood oxygen between 70 percent and 100 percent."
Apple does not provide references to any studies or data that would support the accuracy of their new approach but does mention three health studies they are partnering in that will look at the AW6's utility in asthma, heart failure, and COVID-19.
Although we don't know anything about AW6's accuracy, we do have studies showing us that reflectance oximetry (used in AW6) is not as accurate or stable as transmittance oximetry (used in fingertip pulse oximeters.) It's important to understand this significant technological difference, so please indulge me for two paragraphs.
Oximeters determine oxygen saturation (SpO2) in tissues by quantifying the concentration of hemoglobin bound to oxygen (oxyhemoglobin or HbO2) and hemoglobin (Hb) without oxygen. Pulse oximeters, which are widely utilized in medical settings, utilize photoplethysmography (PPG) performed on pulsatile arterial blood at two different wavelengths to obtain a ratio of HbO2 and Hb. Optoelectronic sensors composed of light-emitting diodes and photodiodes that operate at red and near-infrared wavelengths capitalize on the differing absorptivity of Hb and HbO2.
Transmission-mode oximetry has been the gold standard but requires sensing at tissue locations that can be transilluminated with the light shining through the tissue such as the earlobes and fingers.
With reflective mode oximetry, the light sources and photodiodes are on the same side of the tissue and a thin measurement site is not required. Thus, it can be used at measurement sites such as the forehead, chest, and (most relevant for this discussion) the wrist, where a band or watch can be utilized.
Reflectance mode oximetry, the mode utilized by Apple Watch, has been to the transmission mode used by fingertip pulse oximeters, likely due to increased variability introduced by ambient light, the pressure applied, and positional variation inherent to the method.
Some smartphone apps have claimed an ability to measure SpO2 using reflectance, but when two were studied they were . Walter Schrading, MD, who co-authored the smartphone oximetry study, has told me that he does not recommend any smartphone app as a reliable pulse oximeter.
Pending the release of any data or objective study of the AW6's accuracy, it is difficult to recommend individuals purchase the device for the purpose of monitoring oxygen levels. Although Apple uses the adjective "revolutionary," its competitors (and ) have offered various bands and watches with SpO2 capability for several years.
Should You Purchase Apple Watch 6?
As soon as Apple announced ECG capabilities on its AW4 in the fall of 2018, I signed up to get one and I love it and cannot stand to be without it. I'm an unabashed Apple fan and tech junkie.
With a touch of the screen I can start a timer, check my heart rate or heart rate recovery, initiate a work-out, play music on my AirPods, check the weather, read and respond to text messages, or make a recording.
The ECG icon is not on there but readily accessible should anyone near me suddenly pass out or should I feel my heart palpitating.
So if you have no Apple Watch or an AW3 or less, have the money, and want all of the above nifty features seamlessly integrated with your iPhone plus handy ECG capabilities, go for the AW6.
Is It Worth Upgrading From AW4 or AW5?
Apple is really promoting the new Apple Watch for wellness and health enthusiasts:
"Apple Watch Series 6 delivers many notable hardware improvements, including a faster S6 System in Package (SiP) and next-generation always-on altimeter, along with its most colorful lineup yet, featuring a beautiful palette of new case finishes and bands. watchOS 7 brings Family Setup, sleep tracking, automatic handwashing detection, new workout types, and the ability to curate and share watch faces, encouraging customers to be more active, stay connected, and better manage their health in new ways."
Of those new features mentioned, the only one that I'm interested in is sleep tracking. I've assessed many different sleep tracking wearables, including the Oura ring, the Garmin VivoSmart (which offers nocturnal oxygen saturation, by the way), and multiple iPhone apps and have found them all to be inaccurate. They primarily rely on actigraphy to determine sleep versus wake.
We don't know any details about the AW6 sleep tracking. This shot from the Apple website suggests to me the information will be rudimentary.
Matt Walker, author of "," believes that all wearable sleep trackers have an accuracy of about 70% in determining sleep versus wake and the stage of sleep. (He serves as scientific advisor to Oura.) He makes an interesting comment in a recent Peter Attia podcast that "stickiness" is as important as accuracy when it comes to sleep tracking. Stickiness refers to how likely we are to continue using and wearing the device during sleep.
The Oura ring is very sticky. I continued to wear it for months after I had concluded it was not helpful in tracking or optimizing my sleep. The Apple Watch, due to its size and location, is not something I want to wear to bed frequently and therefore is not sticky.
My Apple Watch has long been reporting to me that I have an unbelievably good (54 mL/kg/min, whereas the average for men ages 20-29 years is 48).) Apple does not reveal how it calculates this key exercise parameter; like the AW SpO2 and heart rate variability, the number cannot be relied on. My AW VO2 max jumped up 10 mL/kg/min for no apparent reason 2 months ago and has remained there since.
Apparently, AW6 tweaks the VO2 measurement and reporting. The company needs to be transparent about how it is measured (really estimated, as there is no way they can measure it) before I would rely on it for anything.
Thus far, I haven't felt the urge to upgrade to the AW6. If you are primarily excited by the blood oxygen sensing capabilities of AW6, I would recommend instead purchasing a . At least we have on these devices, which use the more reliable transmission mode for measuring oxygen saturation.
If you are excited by the sleep tracking capabilities, consider whether you want to wear a bulky watch to bed that is likely inaccurately telling you how much you slept.
Anthony C. Pearson, MD, is a noninvasive cardiologist and professor of medicine at St. Louis University School of Medicine. He blogs on nutrition, cardiac testing, quackery, and other things worthy of skepticism at , where a version of this post first appeared.