Wearable devices are generally worn for long periods of time, so they have been given an important mission since their inception – monitoring the body’s biological indicators, such as sleep monitoring, heart rate monitoring and more. At present, heart rate monitoring has become the standard function of smart bracelets and smart watches, and more than one hundred products are also equipped with heart rate monitoring sensors. How do these wristbands and watches monitor heart rate? Is the result accurate? Today we will discuss this issue.
Start with the basic principles of heart rate monitoring. Heart rate refers to the number of heartbeats per minute in a normal person’s quiet state, also called a quiet heart rate. Adults have a normal heart rate of 60-100 beats per minute, and athletes have lower heart rate than normal people because of their stronger cardiopulmonary function. The heart is like a pump that delivers blood to various parts of the body through contraction and relaxation. Transportation is the basic function of blood. Both oxygen and nutrients need to be transported by blood to reach the tissues of the body.
Many of our behaviors have an effect on heart rate. For example, when smoking or drinking, due to the reduced oxygen carrying capacity of the blood, in order to maintain normal functioning of the body, the heart needs to deliver more blood to ensure oxygen supply, and the heart rate will increase; As the body needs more oxygen, the heart rate will increase. Therefore, heart rate is an important indicator of our physical condition.
At present, there are three main methods for monitoring heart rate, such as ECG, arterial blood pressure, and photoelectric method.
Electrical pulse measurement uses dry electrode technology to obtain human ECG data. The peak rate is measured by heart rate, and the principle is similar to ECG.
Before the mechanical contraction of the heart, an electrical activation is generated. This electrical activation can be transmitted to the body surface through human tissue. The electrical pulse measurement method measures the heart rate by monitoring the electrical activation.
Electrical pulse measurement has the highest accuracy, but since the wavelength of this electrical signal is long, the two electrodes used for monitoring must be kept at a certain distance, which is difficult to achieve in a small-sized wearable device.
The arterial blood pressure method monitors heart rate by sensing arterial blood pressure. When the heart contracts, the blood flow in the arteries increases, and the blood pressure rises; when the heart is dilated, the blood flow in the arteries decreases and the blood pressure decreases. This change in blood pressure is very obvious and can be felt by hand, so in the past there was a diagnosis method of “diagnosis”.
The shortcomings of the arterial blood pressure method are obvious. Because of the need to exert a certain pressure to feel this blood pressure change, long-term wearing of this device will definitely bring pressure and discomfort, so in addition to the sphygmomanometer, it is very good in other heart rate monitoring devices. Use less.
Photoelectric method is based on photoelectric volume map (PPG) technology, which is divided into transmissive and reflective, and transmissive generally measures heart rate by blood oxygen content.
The hemoglobin in the blood carries oxygen to various parts of the body. After the oxygen is consumed by the body, the heart pumps blood and sends hemoglobin without oxygen to the oxygen, and brings a new oxygenated hemoglobin. This cycle is consistent with changes in heart rate, while oxygenated hemoglobin and non-oxygenated hemoglobin have different rates of light absorption. Heart rate data is further obtained by measuring the oxygen content of the blood by emitting light of a specific wavelength.
The advantage of this method is that both heart rate and blood oxygen saturation can be measured at the same time, but since it is necessary to let light penetrate human tissue, it can only be used in specific parts such as fingertips and earlobes. The bracelet needs to be worn on the wrist and does not meet the conditions of use of the transmissive PPG, so it is rarely used on wearable devices.
The reflective type measures the heart rate by sensing the change in blood vessel volume when the blood flows. At present, the wearable devices on the market, such as Apple Watch, Xiaomi Bracelet 2, etc., mostly adopt this method. Therefore, you can clearly see the light emitting end and the receiving end of the heart rate sensor of these devices. The transmitting end emits green light, and the receiving end receives the light reflected by the body tissue.
The use of green light is not because the blood is red, but because the detected parts are in addition to blood and blood vessels, and other tissues, which will bring certain signal “noise”, affecting the accuracy of monitoring, while green The anti-drying ability is the strongest. In transmissive PPG, red light is used because the red light has a longer wavelength and a stronger penetrating power.
As for accuracy, our usual movements, postures, and even skin colors all have an effect on the heart rate measurement of photoelectric methods. Therefore, the heart rate measured by most wristbands and watches is basically not completely accurate, but it can basically reflect the trend of heart rate changes, which is enough for ordinary people’s exercise heart rate monitoring.