Reference design of commonly used electronic blood pressure monitor

In the medical system, there are a large number of non-invasive blood pressure monitors and multi-parameter monitors used in clinical practice. Their common feature is the dynamic non-invasive measurement of human blood pressure (systolic and diastolic pressure). The accuracy of blood pressure measurement is related to the life, health and safety of the general public. The state has listed sphygmomanometers as national mandatory measurement instruments. At present, most non-invasive electronic sphygmomanometers use the oscillometric method to measure blood pressure. The metrology department can only calibrate the static indicators of these blood pressure measuring instruments. There is no feasible method and means to calibrate the accuracy of their blood pressure (systolic and diastolic pressure). Therefore, it is necessary to discuss the calibration method of oscillometric electronic sphygmomanometers for measuring blood pressure.
1. Principle of oscillometric electronic sphygmomanometer
Blood pressure refers to the lateral pressure of blood in blood vessels on the unit area of ​​vascular wall, that is, pressure. That is to say, the static pressure of blood in blood vessels is blood pressure.
The blood pressure of the human body changes over time. Systolic pressure refers to the maximum value of blood pressure, and diastolic pressure refers to the minimum value of blood pressure, as shown in Figure 1.

The quantities measured by the sphygmomanometer are systolic and diastolic pressure. The oscillometric method for measuring blood pressure means that when the pressure of the cuff tied to the arm is about 60 mmHg higher than the systolic pressure of the blood vessel, the blood vessel is blocked. As the cuff pressure decreases, the blood vessel changes from blocked to conductive, and this process will generate a series of small pulses in the cuff, as shown in Figure 2. Pick up the small pulses, connect their peak values ​​into a curve, and obtain the envelope, as shown in Figure 3. According to the shape of the envelope, find the corresponding feature points to distinguish the systolic and diastolic pressures.

The shape of the envelope is mainly affected by parameters such as systolic pressure, diastolic pressure, blood vessels and arm mechanical properties. Among them, systolic pressure and diastolic pressure are the main influencing parameters. However, there is no perfect theoretical derivation for oscillometric blood pressure measurement, so the determination of characteristic points can only rely on the statistical induction of collected samples. Therefore, it can be said that oscillometric blood pressure measurement is based on a statistical method and has a certain degree of discreteness.

2. Verification method
The verification of oscillometric electronic sphygmomanometer is divided into two parts: static and dynamic.
Static verification refers to the verification of the accuracy of the sphygmomanometer sensor in measuring pressure. This is a routine verification and will not be discussed here.
Dynamic verification refers to the verification of the accuracy of the sphygmomanometer in measuring systolic and diastolic pressure. At present, there is no truly feasible dynamic verification method for sphygmomanometers at home and abroad. This article focuses on the discussion. 1. It can be seen from the measurement of the sphygmomanometer that the most effective method of dynamic verification is to generate a standard systolic and diastolic pressure for the sphygmomanometer to measure. The difference between the measurement result of the sphygmomanometer and the standard value is the basic error of the sphygmomanometer in measuring systolic and diastolic pressure.
△Pd=Pdj－Pdb；
△Ps=Psj－Psb；
where:
△Pd: basic error of diastolic pressure of sphygmomanometer;
△Ps: basic error of systolic pressure of sphygmomanometer;
Pdj: diastolic pressure measurement value of sphygmomanometer;
Pdb: standard value of diastolic pressure;
Psj: systolic pressure measurement value of sphygmomanometer;
Psb: standard value of systolic pressure;
This method has two main parts: generating standard systolic and diastolic pressures; and making the standard values ​​measurable by sphygmomanometer.
(l) Generating standard systolic and diastolic pressures
From the definitions of systolic and diastolic pressures, it can be seen that the generation of standard systolic and diastolic pressures is actually to generate a standard simulated blood pressure wave so that the accuracy of its peak and valley values ​​should meet the calibration requirements. From the definition of blood pressure, it can be determined that: since blood pressure refers to static pressure, whether the pressure medium of the simulated blood pressure wave is liquid or gas will not generate additional uncertainty in the calibration.
(2) Make the standard systolic and diastolic pressures measurable by sphygmomanometer.
The oscillometric electronic sphygmomanometer (hereinafter referred to as the sphygmomanometer) uses the process of the pressure in the cuff wrapped around the arm changing from high to low, and the brachial artery of the arm changes from blocked to open, so that the pressure in the cuff is superimposed with a series of small pressure pulses. The sphygmomanometer senses these signals and calculates the systolic and diastolic pressures of the human brachial artery after certain calculations. (The principle of the wrist-type oscillometric electronic sphygmomanometer is the same as that of the arm-type, which will not be discussed here.)
It can be seen that after the standard blood pressure wave signal is generated, there must be a mechanism responsible for transmitting the blood pressure wave signal to the sphygmomanometer. This transmission mechanism is equivalent to the human arm transmitting the blood pressure to the sphygmomanometer, which is called a simulated arm. The mechanical properties of the simulated arm are equivalent to those of the human arm.
The advantages of using this verification method are:
① It meets the requirements of value transfer.
The above verification method is based on the basic definition of blood pressure and the measurement of the sphygmomanometer to generate standard values. These values ​​can easily realize value traceability.
② The method is reliable.
The process of verifying the sphygmomanometer using this method is exactly the same as the working process of the sphygmomanometer measuring blood pressure. Therefore, it can be said that this method does not have additional errors in the method.
② It can completely calibrate the sphygmomanometer.
This method also regards the cuff of the sphygmomanometer as a sensor part of the sphygmomanometer for calibration. Therefore, it can completely calibrate the sphygmomanometer.
It is technically quite difficult to implement this calibration method. The generation of standard blood pressure waves and the production of simulated arms are very difficult. There is no means to implement this calibration method at home and abroad. The oscillometric electronic sphygmomanometer calibration device jointly developed by the Guangdong Institute of Metrology and the Guangdong Institute of Medical Devices implements this calibration method. The uncertainty of its standard systolic and diastolic pressure is 0.3kPa (2mmHg). This makes this calibration method from a concept to a reality.
2. Based on the working principle of the sphygmomanometer, another calibration method can be derived. (Strictly speaking, this method cannot complete the calibration of value transfer.)
The basic idea of ​​this method is to restore the envelope. It first collects a certain amount of population envelopes, and determines a typical envelope through statistical induction. This envelope implies the values ​​of systolic and diastolic pressures, that is, it nominally indicates the values ​​of systolic and diastolic pressures. During the calibration process, this typical envelope is restored to the sphygmomanometer through a device connected via a pipeline. The sphygmomanometer sensor senses the pressure change, picks up the envelope, and determines the blood pressure value. The difference between the value measured by the sphygmomanometer and the nominal blood pressure value of the typical envelope is the basic error of the sphygmomanometer in measuring blood pressure. The biggest problem with this method is that the systolic and diastolic pressure values ​​marked by the envelope cannot be calibrated by conventional experimental methods. In other words, it is impossible to trace the value.
At present, there are some devices based on this method abroad. These devices can be used for auxiliary calibration in the research and development and production process of sphygmomanometers, but it is inappropriate to use them in daily calibration.

3. Conclusion The
above mainly introduces two methods for calibrating oscillometric electronic sphygmomanometers, of which the first method should be more in line with the requirements of value transfer calibration. However, there are also some problems. The main problem is that the basic theoretical support of the oscillometric method is still lacking, which makes it very difficult to establish a simulated arm mechanical model, and it can only be processed by statistical induction. With the continuous improvement of the oscillometric method, we believe that its calibration methods and means will continue to improve.