During the period of the coronavirus epidemic, pulse oximeters became as common as thermometers, and are located on the shelves of many doctors' offices. But is there a place for them in a wristwatch?
In the early days of the coronavirus epidemic, it wasn't just face masks and hand sanitizers that were flying off store shelves. Pulse oximeters were also in short supply after it became known that a drop in the oxygen content in the blood could be a sign of a developing disease.
These inexpensive and non-invasive electronic devices use LEDs and photodiodes to measure the absorption of light by red blood cells. Oxygenated cells absorb more infrared color than red, while cells without oxygen absorb more. Armed with this information, the algorithms can calculate blood oxygen saturation levels. For most people, this figure should be well above 90%. In the case of coronavirus disease, it falls to around 80-something. So it seemed like a good idea to have such a device on hand - if, of course, you could find it.
The pandemic has been going on for more than half a year now, so it's no surprise that consumer electronics manufacturers are touting the benefits of adding pulse oximeters to wearable electronics. The sensors are low cost, do not consume much battery, and may attract some customers looking for a sense of security in our uncertain world.
Recently, Apple decided to add a pulse oximeter to its watch (Fitbit and Garmin had similar products even before the pandemic, they had to recognize sleep apnea). in mid-September it was announced that the Apple Watch Series 6four groups of green, red and infrared LEDs are used, as well as four photodiodes. The company promises a specialized advanced algorithm that determines the level of oxygen saturation in the blood. Red and infrared LEDs are busy measuring saturation; the green ones measure the pulse. The sensors are mounted on the back of the watch and touch the top of the wrist. They can be used to retrieve data on demand during the day and automatically at night.
Apple advertises its gadget as a "fitness and health" device. In other words, the company does not have permissionfrom the United States Food and Drug Administration (FDA) to advertise the device as medical. This is not surprising - it takes time to obtain such permission - but without it it is difficult to understand how accurately the device works.
Apple Watch Series 6 Sensors
Accuracy is a major issue for many pulse oximeters in the consumer market. Steve Xue, Physiological Engineer and Medical Director of the Center for Biointegrated Electronics at Northwestern University, says: "It's pretty easy to make a pulse oximeter, it's the level of an engineering student, but it's very difficult to make a device that is reliable for clinical use."
Did Apple make a good device? It's hard to say. In addition to the challenges of adjusting to different skin colors, working on the move, and other design challenges faced by all pulse oximeters, the difficulty level increases when placing sensors on the top of the wrist. Devices used in hospitals or sold in pharmacies usually cling to the tip of the finger, or sometimes to the earlobe.
"These body parts have advantages over the wrist," says Xu, "because they have more capillaries and a better signal-to-noise ratio."
Another disadvantage of wrist-based oxygen sensors is that while the fingertips are thin enough to allow light to pass through, wrist-based pulse oximeters have to rely on reflected light, which is inherently less accurate.
"It's not necessarily a bad thing," says Xu, "but all things being equal, the measurement won't be as accurate as a pulse oximeter designed for a more suitable place on the body."
“I would never put a pulse oximeter on my wrist,” says William McMillan, co-founder, president and director of science at Profusa , which develops implantable biosensors. “The wrist moves a lot, which is bad for continuous measurements” (Apple watches have motion sensors that can help track resting moments).
Xue says Apple can prove its watch can deliver accurate blood oxygen saturation measurements if it goes through the FDA approval process - a procedure for validating pulse oximeters has long been established.
But even without permission, Apple is launching several medical studies using pulse oximeters from the Apple Watch - one to prevent heart attacks, one to help with asthma, and one to look at changes in blood oxygen saturation levels as a sign of coronavirus and flu infection. Xue and Macmillan speak of these attempts with cautious optimism.
“The consumer device makers Apple and Fitbit have a much larger scale than most healthcare companies,” says Xu. - Few companies other than Apple, Fitbit and Samsung can throw a million devices into the world and process the incoming information. We need to conduct these studies and see how good predictions they can give, but we must understand that there will be many false positives. It may prove to be a useful research tool, but nothing is clear yet. "
“Since these devices are not particularly accurate and will not work with a control group, but with an uncontrolled environment, they will need a huge sample size to detect any phenomenon,” said Macmillan. "But in the end they will have millions of records in which to look for new ideas, and that's enough."
All this begs the question - does the average smart watch user need to wear a pulse oximeter around the clock?
Although such a wearable device may provide us with more data on fluctuations in blood oxygen saturation levels in the general population, says Xu, “in most healthy people, this level ranges from 97 to 99%. If we all wear them, and worry every time it drops to 92, then the consequences of the excitement of healthy people will overwhelm the clinical benefit, the magnitude of which is still unclear.