On September 15, Apple introduced the Watch Series 6 with blood oxygen measurement, improved battery and display. And an always-on altimeter. Sales of the device in Russia started on September 25. Editor TJ has been testing the new Apple smartwatch for a week and discusses what’s interesting about them.
Blood oxygen measurement is the main innovation and disappointment of the Series 6
Why measure oxygen? The Apple Watch Series 6 is Apple’s first smartwatch to feature a SpO2 sensor that measures how oxygenated the blood is. In fact, since iOS 13, Apple Watch owners have been able to track at least one oxygen-related metric. Maximum oxygen uptake, or VO2Max.
The watch allowed you to measure the approximate value, but this did not require a separate sensor. Only a heart rate monitor, which is in all models, starting from the first. However, do not confuse maximum intake (VO2Max) and oxygen saturation (SpO2) – these are completely different indicators. Both in terms of measurement and usefulness for tracking health.
VO2Max is measured with a decent error. Only during walking or running workouts and only after a few weeks of training. First, the watch needs to collect data so . That there is something to calculate the approximate value of oxygen consumption from. This indicator does not reflect the state of health. Only preparedness or endurance for physical exertion, specifically for walking or running. It is possible to be healthy and have a reasonably low VO2Max.
How is VO2Max Useful?
But despite the narrow range of applications (athletes and amateurs), VO2Max can be useful to many more people than SpO2. The blood oxygen saturation level reflects the work of the lungs and the supply of oxygen to the internal organs, and in the vast majority of people, it always stays at the level of 95-98%.
The SpO2 sensor (or pulse oximeter as it is more commonly known) is commonly used in hospitals during surgery, intensive care, and rehabilitation. This is the same device on the patient’s finger, often seen in foreign films and TV shows.
While VO2Max can show training and endurance progress, oxygen saturation is more applicable to people with specific diseases or conditions. Among them are respiratory failure, asthma, COPD and some sleep disorders.
The level of oxygen saturation is not a very reliable indicator of a person’s condition, unlike the pulse, because there is not always a direct relationship between well-being and oxygen in the blood. You can feel great at 95% (the lower limit of the conventional norm) or terrible at 100% blood oxygen saturation – in people without respiratory or respiratory diseases; the indicator can remain normal regardless of external conditions.
Testing of Apple Watch Series 6
We tested this out by taking the Series 6 into a dry sauna. Even after five minutes of being in oxygen-limited conditions (in a closed space with very hot air), the watch showed 95%. Immediately after entering the dressing room, the figure jumped to 97%, but on the street, it fell again to 95%.
There is no specific rate of oxygen saturation in the blood. This figure is above 95% for most healthy people, but the percentage can drop to 88% for some, and they will continue to feel normal – their body is adapted to this.
Before the release of the Series 6, it seemed that the watch’s new feature could be especially useful during a pandemic, as Covid-19 attacks the lungs. Many doctors even began to use pulsexometry as an early diagnosis of coronavirus: in Moscow, 93% oxygen saturation was enough for hospitalization. But in the end, the researchers did not find evidence that this is a reliable method. They recommended that doctors not convince patients that the oxygen level will allow them to know about Covid-19 infection.
How pulse oximeters work?
Pulse oximetry is not a new technology: in 2020, household pulse oximeters can be bought for a couple of thousand rubles, and Apple was not even the first to transfer them to wearable devices. For example, Xiaomi Mi Band 5 can measure oxygen. The first pulse oximeters were created in Germany in 1935, since which the basic principle of operation has not changed.
Pulse oximeters cannot measure the percentage of oxygen in the blood because they do not have direct access to it. Instead, the devices measure the amount of the protein haemoglobin, which also carries oxygen in the blood.
To measure oxygen levels in pulse oximeters, two LEDs are usually used – red with a wavelength of 660 nm and infrared with a wavelength of 940 nm, as well as a photodiode to capture light. Typically, LEDs shine through the patient’s finger, under which there is a photodiode and fixes the amount of light transmitted.
If there is a lot of oxygen in the blood, and therefore, most often, haemoglobin, it transmits infrared light worse but transmits red well. If there is little oxygen, more infrared radiation will pass. Based on this, the device calculates the percentage of oxygen saturation.
How does oxygen measurement work on the Apple Watch?
The main innovation that wearable electronics manufacturers have made to pulse oximeters is that they managed to get rid of the “clothespins” circuit and the need to stick fingers into the devices. Unlike household options, smartwatches have both LEDs and a photodiode on top. Therefore, the devices give a result based on the reflection of light from the blood and not due to transparency.
In the case of Watch Series 6, a sensor of four LED clusters and four photodiodes was used. The watch has three colours of LEDs – red, green and infrared. Which illuminates the blood while the sensor determines the colour. And, based on this, gives the percentage of oxygen saturation.
Pulse oximeter in Watch Series 6 during operation Picture of Apple
While it usually takes several minutes to measure oxygen, Apple Watch measures oxygen in a maximum of 15 seconds. But there are some nuances: the hand with the watch should lie on the table. Or your knees with the display up, and the watch itself should be slightly moved away from the wrist and not move. Even a small movement can throw off the sensor, giving the watch a “failed measurement”.
On the move, oxygen cannot be measured: for this, you need to calm down, sit down and wait 15 seconds.
As with measuring the pulse, the oxygen level in the blood can be checked through a separate application on the Apple Watch. The process itself is very pleasant: during the operation of the sensors, the watch shows a beautiful animation of blood flow, as if you are looking through the watch directly at the blood in the vessels, and at the end, they seem to “capture” oxygen and give the result. If you remove your Apple Watch in the process or start moving your hands, the animation will be interrupted.