Many of our actions will have an impact on the heart rate. For example, when smoking or drinking, because the oxygen carrying capacity of blood is reduced, in order to maintain the normal operation of the body, the heart needs to transport more blood to ensure oxygen supply, and the heart rate will accelerate; When the body needs more oxygen during exercise, the heart rate will also increase. Therefore, heart rate is an important indicator to reflect our physical condition.
At present, there are three mature heart rate monitoring methods, namely ECG signal method, arterial blood pressure method and photoelectric method.
Electrical pulse measurement uses dry electrode technology to obtain ECG data of human body, and detects heart rate through peak value. The principle is similar to electrocardiogram.
Before the mechanical contraction of the heart, there will be an electrical excitement, which can be transmitted to the body surface through human tissues. Electrical pulse measurement is to measure the heart rate by monitoring this electrical stimulation.
The accuracy of electrical pulse measurement is the highest, but because of the long wavelength of this electrical signal, the two electrodes used for monitoring must keep a certain distance, which is also difficult to achieve on small wearable devices.
Arterial blood pressure method monitors heart rate by sensing arterial blood pressure. When the heart contracts, arterial blood flow increases and blood pressure increases; When the heart relaxes, arterial blood flow decreases and blood pressure decreases. This change in blood pressure is so obvious that you can feel it with your hands, so there was a diagnostic method of "pulse-taking" in the past.
The shortcomings of arterial blood pressure method are obvious. Because it takes a certain amount of pressure to feel this change in blood pressure, wearing this device for a long time will definitely bring a sense of oppression and discomfort, so it is rarely used in other heart rate monitoring devices except sphygmomanometer.
Photoelectric method is based on photoelectric plethysmography (PPG) technology, which can be divided into transmission type and reflection type. Transmission type generally measures heart rate through blood oxygen content.
Hemoglobin in the blood transports oxygen to all parts of the body. After oxygen is consumed by the body, the heart pumps blood, sends oxygen-free hemoglobin to replenish oxygen, and brings new hemoglobin with oxygen. This cycle is consistent with the change of heart rate, but the absorption rate of light by oxygenated hemoglobin and anaerobic hemoglobin is different. The oxygen content of blood is measured by emitting light with a specific wavelength, so as to further obtain heart rate data.
The advantage of this method is that it can measure heart rate and blood oxygen saturation at the same time, but because light must penetrate human tissues, it can only be used in specific parts such as fingertips and earlobes. Bracelets need to be worn on the wrist, which does not meet the use conditions of transmissive PPG, so they are rarely used in wearable devices.
Reflective type senses the change of blood vessel volume when blood flows through light reflection, so as to measure heart rate. At present, most wearable devices on the market, such as Apple Watch and Xiaomi Bracelet 2, adopt this method. So you can clearly see the emitting end and receiving end of the heart rate sensor of these devices. The emitting end emits green light, and the receiving end receives the light reflected by the body tissue.
Green light is not used because blood is red, but because there are other tissues besides blood and blood vessels, which will bring some signal "noise" and affect the accuracy of monitoring, and green light has the strongest anti-drying ability. In transmission PPG, red light is used, because red light has longer wavelength and stronger penetration.
As for the accuracy, our usual exercise, posture and even skin color will have an impact on the heart rate measurement results of photoelectric method. Therefore, the heart rate measured by most bracelets and watches is not completely accurate, but it can basically reflect the trend of heart rate change, which is enough for the exercise heart rate monitoring of ordinary people.