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Nov 21 2020

Blood Oxygen Measurement on Smartwatches: Is it Reliable?


In September of this year, Apple's new generation of watch Apple watch series 6 launched the blood oxygen measurement function. In the following month or two, smart watches with blood oxygen measurement function were released together, vivo WATCH, Huawei Watch GT2, Honor Watch GS Pro, Hua Mi Amazfit GTR 2 and GTS 2 flagship smart watches almost all include this feature.

In October, Huami Technology released the Amazfit Pop, which is priced at only 299RMB, and is also equipped with blood oxygen measurement. The blood oxygen sensor has begun to penetrate into low-end and mid-range computers, which may set off a greater wave.

So, what exactly is the blood oxygen measurement function on the smart watch? Is it reliable? After you finish reading this article, you will get the answer.




I Blood Oxygen & Blood Oxygen Saturation

 1.1 What is Normal Blood Oxygen Level?

 1.2 How Important is Blood Oxygen Saturation?

II How Does a Smart Watch Measure Blood Oxygen?

III What is the use of Measuring Blood Oxygen Saturation?

IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?

V Comparison with Pulse Oximeter

 5.1 What is a Pulse Oximeter?

 5.2 How Does a Pulse Oximeter Work?

 5.3 Main Components

 5.4 Can a Smart Watch be Used as a Pulse Oximeter?

VI Is it Necessary to Have an Oximeter at Home During Covid-19?

 6.1 Can a Pulse Oximeter Diagnose Covid-19?

 6.2 Is it Necessary to Buy a Pulse Oximeter?

I Blood Oxygen & Blood Oxygen Saturation

1.1 What is Normal Blood Oxygen Level?

Blood oxygen refers to the oxygen in the blood, and the human body's normal blood oxygen saturation is above 95%. The higher the oxygen content in the blood, the better the human metabolism. Of course, high blood oxygen content is not a good phenomenon. The blood oxygen in the human body has a certain degree of saturation. Too low will cause insufficient oxygen supply in the body, and too high will cause cell aging in the body.

Blood Oxygen

Figure1. Blood Oxygen

1.2 How Important is Blood Oxygen Saturation?

The cells in the human body rely on oxygen to survive. The oxygen that enters the human body through the respiratory system will be combined with hemoglobin and transported to various organs. Blood oxygen saturation is a measure of the percentage of hemoglobin combined with oxygen, so blood oxygen saturation is a key indicator to measure the health of the body. This parameter can be used to understand the oxygen content in the human blood.

Under normal circumstances, the normal blood oxygen saturation is between 95% and 100%. If the blood oxygen saturation is less than 90%, it can be considered as hypoxemia. When blood oxygen saturation is too low, it means that there will be hypoxia, which will affect the central nervous system, liver, kidney and other important organs. Therefore, blood oxygen saturation is an important indicator, and the measured data can assist in judging the current physical health.

How Important is Blood Oxygen Saturation?

Figure2. How Important is Blood Oxygen Saturation?

II How Does a Smart Watch Measure Blood Oxygen?

The blood oxygen measurement function of the smart watch actually judges health by measuring the human arterial blood oxygen saturation. The blood oxygen saturation specifically refers to the percentage of hemoglobin combined with oxygen in the blood, that is, the concentration of blood oxygen in the blood.

Generally speaking, if the blood oxygen saturation is below 94%, it will be regarded as insufficient oxygen supply. Many clinical diseases will cause insufficient oxygen supply, which directly affects the normal metabolism of cells, and in severe cases, it can be life-threatening. Therefore, blood oxygen testing is very important for clinical medicine.

The most primitive method of tracing blood oxygen measurement requires blood sampling first, and then electrochemical analysis by a blood gas analyzer to obtain the blood oxygen saturation. This method has complicated steps and cannot achieve continuous detection.

However, with the development of clinical medicine, non-invasive blood oxygen measurement is now widely used. As long as a finger pressure photoelectric sensor is worn for the patient, continuous blood oxygen detection can be realized. The essence is to use red light with a wavelength of 660nm and near-infrared light with a wavelength of 940nm as the intake light source, measure the light transmission intensity through the tissue bed, calculate the blood oxygen concentration and blood oxygen saturation, and display the results by the instrument.

The principle of measuring blood oxygen with a smart watch is similar to acupressure measurement, but the difference is that the part illuminated by the light source of the watch is the wrist, which is not as "transparent" as the finger. Visible light and infrared light cannot penetrate, so it is more challenging.

However, as a wearable device with a very high frequency of use, the stimulation brought by the development space of smart watches is far greater than the challenges they need to face.

How Does a Smart Watch Measure Blood Oxygen?

Figure3. How Does a Smart Watch Measure Blood Oxygen?

III What is the use of Measuring Blood Oxygen Saturation?

Because the blood oxygen saturation can be detected anytime and anywhere through smart wearable devices, the application scenarios of blood oxygen detection are very wide.

(1) Assist in judging the state of sleep breathing

Intermittent apnea may occur during sleep, which may cause insufficient oxygen supply. Through continuous blood oxygen detection, the blood oxygen saturation data during sleep can be recorded, and the data can be used to analyze whether there is hypoxia during sleep, so as to determine the sleep health status.

In addition, like some people who often work overtime, they can also use the blood oxygen saturation detection function to judge the current state. If the blood oxygen saturation is low, you may need to take a quick rest, and that is friendly to your health.

Work Overtime

Figure4. Judging the State of Sleep Breathing by Blood Oxygen Saturation

(2) Monitor the physical state during exercise

The blood oxygen saturation detection function not only allows you to know your physical condition at any time in your daily life, but also can play a better health support role in some special scenes. For example, in outdoor extreme mountaineering and other sports scenes, you can know your physical condition at any time through blood oxygen saturation, determine whether you need to rest or adjust the exercise intensity, so as to better cope with various sports scenes.

Monitor the physical state during exercise

Figure5. Monitor the Physical State During Exercise

(3) Monitor the health of parents

Generally, the APP on the mobile phone can synchronize the health data of the smart wearable device, so even if you are away from home, as long as you equip your parents with a smart wearable device, you can learn about your parents’ blood oxygen data and some other health data through remotely synchronized data. , adding more details for your concern.

IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?

At present, the blood oxygen measurement function on a smart watch has not been certified by NMPA or FDA, that is, it has reached the level of medical diagnosis.

The blood oxygen monitoring module of the smart watch consists of three parts: an optical sensor, a front-end signal acquisition system, and an algorithm. The principle is based on the different absorption rates of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the blood to red light and infrared light, the red light and infrared light are irradiated to the skin through an optical sensor, and then the blood vessels under the skin are obtained. After the reflection of the red light and infrared light, the blood oxygen is calculated by the algorithm.

The optical sensor is the core of the blood oxygen detection module, which is usually composed of several LED lights and diodes. Apple's official website shows that the blood oxygen sensor equipped with Apple Watch 6 consists of four groups of LED light clusters and four photodiodes, and is integrated in the crystal glass back.

If a smart watch is required to measure both heart rate and blood oxygen, it can be achieved through the same optical sensor.

Ams has launched an optical sensor that integrates blood oxygen measurement and heart rate measurement. Li Minghao, field application engineer manager at ams, said, “Heart rate measurement usually uses green light, and blood oxygen measurement uses red light and infrared light. Compared with optical sensors that can only measure heart rate, optical sensors that can simultaneously perform two functions The composition usually adds several red and infrared LEDs.

Smart watch

Figure6. Samsung Galaxy Series Smart Watch

As the core of blood oxygen measurement, the accuracy of optical sensors is very important to the accuracy of measurement results. For example, deviations in the wavelength of the red light produced by the diode may cause inaccurate measurement results.

"Smartwatch measuring blood oxygen is a signal obtained by measuring capillaries. The signal is very weak. Usually this signal is submerged on it, and the feedback signal may be only about 1% useful. Therefore, the design of optics and filtering is very important." Li Minghao said.

Currently, companies in the industry are taking various measures to improve accuracy.

The senior technical staff of Huami Technology said, "Any sensor has various non-ideal errors. In terms of control errors, Huami conducts strict testing and evaluation of all key parameters when selecting optoelectronic devices. Each product of Mi has undergone multiple measurements on the production line to ensure that the accuracy of the factory product parameters is within the design range. In addition, at the beginning of the design, select the relatively flat waveband of the human skin spectral absorption curve to limit the wavelength error to the blood oxygen The impact is within an acceptable range."

Increasing the number of sensors is also one of the ways to improve accuracy. The vivo watch is equipped with a self-developed 5-core optical heart rate blood oxygen sensor to achieve blood oxygen measurement. "5-core refers to a multi-sensor system composed of 5 sensors, in which the circuit and optical design are the results of vivo's independent research. Compared with the single sensor or multi-sensor design in the industry, it has a larger signal receiving area and receiving capacity. Stronger, can achieve more accurate monitoring effects." said a senior technical staff of vivo.

However, judging from the actual effect of the current smart watch for blood oxygen measurement, although there is an improvement in accuracy, there is still a certain gap between achieving medical-level blood oxygen monitoring. When the news that Apple Watch 6's blood oxygen measurement function was inaccurate, Apple came out to clarify that this function is only for health reference, not as a medical diagnosis standard.

The accuracy requirements of consumer and medical products are different. Taking the heart rate as an example, there is no problem with a few beats on the wrist, but in medical treatment, there can only be a difference of 1-2 beats.

V Comparison with Pulse Oximeter

5.1 What is a Pulse Oximeter?

Pulse oximeter has been a common medical device since the 1970s. It is most commonly used for people with respiratory diseases and sometimes for athletes and pilots who must monitor blood oxygen levels. They are mainly used for clinical testing and monitoring, but for certain groups of people, they may also be used at home.

Pulse Oximeter

Figure7. Pulse Oximeter

5.2 How Does a Pulse Oximeter Work?

Based on the change in light absorption during arterial pulsation. Two light sources located in the visible red spectrum (660 nanometers) and infrared spectrum (940 nanometers) alternately illuminate the tested area (usually fingertips or earlobes). The amount of light absorbed during these pulses is related to the oxygen content in the blood. The microprocessor calculates the ratio of the two spectra absorbed, and compares the result with the saturation value table stored in the memory to obtain the blood oxygen saturation.

A typical oximeter sensor has a pair of LEDs that face a photodiode through a translucent part of the patient's body (usually a fingertip or earlobe). One of the LEDs is red light with a wavelength of 660nm; the other is infrared light with a wavelength of 940nm. The percentage of blood oxygen is calculated by measuring the two wavelengths of light with different absorption rates after passing through the body.

5.3 Main Components

A microprocessor, memory (EPROM and RAM), two digital-to-analog converters that control LEDs, a device that filters and amplifies the signal received by the photodiode, and an analog-to-digital converter that digitizes the received signal to provide the microprocessor . The LED and photodiode are placed in a small probe that is in contact with the patient's fingertip or earlobe. Pulse oximeters generally also include small liquid crystal displays.

5.4 Can a Smart Watch Replace a Pulse Oximeter?

The answer is no. Fitness bracelets (including some Garmin devices) and smart watches with the function of detecting blood oxygen levels cannot be used as medical equipment.

Garmin said that blood oxygen saturation can help you understand your body's adaptation to high altitude (especially for mountain sports and adventure), and it can also be a reminder for symptoms of sleep apnea or overwork during exercise. But it also clearly states that these data cannot be used for medical purposes, nor can it diagnose, treat, cure or prevent any disease or condition.

The location where the device is worn may affect accuracy. The fitness tracker is worn on the wrist instead of the fingertips, it is easier to move when worn on the wrist, and the blood concentration on the skin surface is lower than that on the fingertips, so the obtained blood oxygen measurement may have a large error. However, although fitness trackers and smart watches are not medical devices, if they do detect abnormalities and alert you, then you should pay attention.

Apple Watch

Figure8. Apple Watch

VI Is it Necessary to Have an Oximeter at Home During Covid-19?

One of the powerful aspects of COVID-19 is that some patients with very low blood oxygen levels do not feel it themselves and are not aware of the severity of their illness. This has led some doctors (especially in the United States) to recommend pulse oximeters at home.

Sounds reasonable, doesn't it? However, you must know that although they are commonly used in hospitals, their value for healthy people at home is limited. Equipment problems or incorrect use may cause inaccurate readings, so it is not wise to rely solely on oximeters without comprehensive diagnostic support from medical staff.

6.1 Can a Pulse Oximeter Diagnose Covid-19?

The pulse oximeter may indicate a problem with the blood oxygen level, which may be related to the coronavirus, but it is only part of a comprehensive diagnosis. Blood oxygen saturation can help clinical decision-making, but it cannot replace clinical evaluation, nor can it be diagnosed alone.

Some doctors suggest that for patients with suspected symptoms of the new crown but not serious enough to be hospitalized, they can consider using a pulse oximeter at home for monitoring.

Covid Prevention

Figure9. Covid Prevention

6.2 Is it Necessary to Buy a Pulse Oximeter?

If you don’t have any breathing problems and have never used it before, then you don’t need to buy a pulse oximeter. It is usually only used if recommended by a doctor.

Professor Xu, a clinical assistant professor at the University of Hong Kong and honorary consultant of respiratory and intensive care medicine at the Royal Free Hospital in London, said that pulse oximeters are still useful as early home self-monitoring equipment. Those who are weak, prone to respiratory failure, suffer from chronic respiratory diseases or need oxygen therapy at home can prepare one.

Some people want to buy one for a self-test to find out their "normal" oxygen level, just in case. In fact, if you are not a suspected patient and have no symptoms, there is no need to buy one.

Without the help of medical personnel, the measured readings are not very useful, and if you have not used them before, misuse or misinterpretation of the numbers is easy to happen.

Dr. Andy Whittamore, the clinical director of the British Asthma Association and the British Lung Foundation, suggested: "Any surveillance at home should be part of the diagnosis, but not a substitute for clinical advice. All people who are concerned about their symptoms should see a doctor.

In addition, like many coronavirus-related products, such as masks, due to the promotion of pulse oximeters, demand has also begun to increase, so beware of unscrupulous sellers selling substandard products at high prices.

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