Pulse Oximetry in Watches — Valuable Feature or Marketing Ploy?
One of the reasons I love autumn is that it’s a time of technical conferences and new products’ announcements. Anyone interested in gadgets knows that Apple recently introduced the Watch Series 6 with a blood oxygen sensor. Although, this feature is already available in wearable devices from other manufacturers, I believe that Apple will make it popular.
I like different technological devices, though, in my Amazfit Bip, I use only a pressure measurement feature while running. But maybe the blood oxygen level measuring is crucial, and it is worth thinking about buying new stuff? It was interesting to know more about this technology to see if it works, or it’s just another marketing ploy to encourage the purchase of new smart-watches.
The oxygen level in the blood of a healthy person is 96–100%. A rapid decline of this level indicates the development of the pulmonary or cardiovascular disease. A value below 90% demonstrates acute respiratory failure. Low levels of oxygen in the blood can cause hypoxemia, which can severely impact health.
Thus, the measurement of this indicator helps:
- to detect pathologies of the respiratory tract and lungs;
- to see variations in breathing while sleeping (can indicate sleep apnea — a disorder in which breathing repeatedly stops and starts);
- to increase the effectiveness of training without harm to health;
- in alpine mountaineering to prevent early signs of impending mountain sickness;
- in sports aviation for supervision and prevention of hypoxia.
In general, it seems that monitoring the oxygen level in the blood is quite important and can potentially detect the onset of health disorders. Providing that such measurements are accurate.
Medical devices estimate the level of oxygen in the blood as follows. The sensor with LEDs attaches to the finger. When light passes through the blood and soft tissues, it is partially absorbed, allowing it to determine the blood’s oxygen level. The device displays obtained data on the screen.
The approach to measuring oxygen level with modern smart-watches is somewhat different. The device emits light using LEDs on the vessels in the wrist area and gauges the reflected rays’ intensity using photodiodes. This is done to determine the blood color, which helps determine the amount of hemoglobin that carries oxygen.
As we see, the principles of measurement are quite different. Watch sensors detect the reflection of light from the body, not the passage of light through it. However, this affects the quality of the measurement. Scientists in this regard assert:
Monitoring at the wrist using the reflective mode presents challenges with regard to clinical use. Another limitation is that the reflected red and infrared pulses can only be used for specific areas, such as a radial artery; thus, most areas of the wrist are not available for monitoring. In addition, a slight position change at the measurement site significantly affects the performance of the oximeter.
Thus, another principle of measuring the level of oxygen has a strong effect on the degree of accuracy. But is this the only problem? I analyzed the manufacturers’ notes as to what else could affect the quality of the measurement.
Apple indicates that the accuracy of the measurement can be affected by:
- Skin perfusion (how much blood flows through the skin). Skin perfusion varies significantly from person to person and can also be impacted by the environment (for example, weather condition);
- Tattoos. The ink and pattern of some tattoos can block light from the sensor;
- Motion. Specific postures, such as arms hanging by your side or fingers in a fist position, will also result in unsuccessful measurements.
- High heart rate (above 150 bpm) while resting.
That’s why Apple, on its website, directly states that:
Blood Oxygen app measurements are not intended for medical use, including self-diagnosis or consultation with a doctor, and are only designed for general fitness and wellness purposes.
Garmin and Fitbit also set requirements for measuring oxygen level:
- There should be no space between the back cover of the watch and the wrist, and there is no visible light from the LEDs;
- The watch should not be located directly above the wrist;
- The hand must remain completely still during the measurement.
It should be noted that everyone wears smart-watches and bands differently. I like it to fit snugly around my wrist when I do sports. At another time, I loosen the strap to not press on my hand, in particular, at night. I’m sure I’m not the only one who does that. However, this dramatically affects the quality of the measurement.
So, what are the general conclusions that can be drawn?
It is good that manufacturers pay so much attention to sports and healthy functions in smart-watches and bands. Demand for such equipment will have a beneficial effect on its development and improvement of applicable technologies.
For all that, as of today, this technology is not perfect. Besides, many factors affect the accuracy of measurement, indicating the impossibility of using smart-watches as a medical measuring devices.
Therefore, I am convinced that this feature is more of a marketing ploy in its current form. At the same time, I hope that measurement technology will become more advanced in a few years and will be able to fulfill the goals set before it qualitatively.
