At present, the non-invasive detection method is mainly used for ordinary patients in clinical practice. It is roughly divided into artificial Korotkoff sound method and oscillometric method. Although the artificial Korotkoff sound method is relatively accurate, it is difficult to operate and is greatly affected by subjective factors; although the traditional oscillometric method is simple to operate, it has poor stability and individual adaptability, which is not conducive to the popularization and promotion of clinical application. Based on the oscillometric method, this paper improves the original measurement method from the aspects of hardware implementation and software design, and conducts comparison tests.
1 Hardware design
The main process of oscillometric blood pressure detection is to obtain the changing pressure signal in the cuff, analyze the pulse signal separated from it, find the position corresponding to the systolic and diastolic pressure, and get the data. The traditional oscillometric measurement is to amplify the signal from the sensor, low-pass filter the amplified signal to obtain the pressure signal, and then send it to the single-chip microcomputer by a set of A / D converters, and then carry out the pressure signal The band-pass filter obtains the pulse signal, which is sent to the microcontroller by another set of A / D converters. Its basic structure is shown in Figure 1.
After adopting ADμC848 of Σ-Δ single chip microcomputer, the circuit is simplified.
Due to the integration of a high-precision 16-bit Σ-Δ type A / D converter, and its A / D reference voltage can be programmed to adjust (the minimum can reach 10mV). Therefore, it can directly perform A / D conversion without the need to be amplified while ensuring accuracy and dynamic range requirements. In this way, a series of problems such as dynamic range change, noise, and voltage offset caused by the presence of the amplifier can be eliminated, and the use of devices is reduced, and the implementation cost is reduced.
Because the Σ-Δ type A / D converter provides a differential mode input method, the differential mode signal given by the sensor can be directly fed into the A / D converter, and in theory its common mode rejection ratio can reach infinity. Therefore, it can greatly reduce the common mode interference caused by the mismatch of the preamplifier circuit.
Because the conversion process of the Σ-Δ type A / D converter is filtered by a low-pass filter, it is not necessary to perform filter processing before A / D conversion. You can directly connect the sensor to the A / D and then perform digital Filtering.
Since a standard constant current source is integrated in the ADμC848, the constant current value can be adjusted by software programming. Therefore, a standard pressure output can be sampled according to different environments of product application, and then A / D conversion is performed, and then the constant current source is adjusted in time according to the conversion result until the desired conversion value is output to achieve automatic calibration of the product.
The hardware structure of the improved electronic sphygmomanometer is shown in Figure 2.
2 Software design
After the above hardware processing, the change curve of the pressure in the cuff is obtained. In the software processing, the pulse signal is first separated; then the interference point is removed, the envelope curve is fitted, and the corresponding average pressure is found; finally, the coefficient is calculated according to the coefficient Systolic and mean pressure.
Morphological filtering algorithm is introduced in the process of separating pulse signals. Because the pressure signal in the cuff is close to the frequency band of the pulse signal, the direct use of band-pass filtering will reduce the signal amplitude and signal-to-noise ratio, which will bring difficulties to the subsequent processing. The application of morphological filtering processing algorithm is to separate the signal from the morphological point of view, which can extract the pulse signal well. In order to be able to complete the signal separation in real time, the open operation will be used to process, smooth all the peaks in the original signal, and then use the original signal and the processed signal to make a difference to obtain the separated pulse signal. Figure 3 is the original signal diagram, and Figure 4 is the isolated pulse signal.
In order to effectively suppress interference and repair the defective pulse wave, the credibility of each pulse wave will be determined according to the angle relationship between the peak value of each pulse wave and the peak value of the pulse wave adjacent to it. Since the amplitude of the pulse wave does not change monotonously, such a judgment also needs to consider the amplitude factor. The specific method is shown in [1].
Use the weight information of each pulse wave obtained above to perform envelope fitting. Since the obtained envelope is obviously asymmetric (that is, the second-order fitting cannot meet the requirements), the third-order least-square fitting method with weights will be used. After the fitting is completed, the pressure value corresponding to the position of the maximum value on the curve is the value of the average pressure.
Finally, referring to the method in [2], the amplitude coefficient is determined based on the average pressure, and the corresponding position of systolic and diastolic blood pressure is calculated using the amplitude coefficient to obtain the size of systolic and diastolic blood pressure.
In order to verify the accuracy of the obtained sphygmomanometer, some typical samples were selected and their measurement results were compared with the Korotkoff sound method of manual auscultation.
First, the blood pressure value a1 is measured by the Korotkoff sound method of manual auscultation. After 15 minutes, the measured value b is obtained using the improved electronic sphygmomanometer; wait another 15 minutes, and then use the Korotkoff sound method of manual auscultation again Once measured, the blood pressure value a2 is measured, and the average value a of a1 and a2 is used as the measurement value obtained by manual auscultation of the Korotkoff sound method. The obtained measurement data are shown in Table 1 and Table 2.
As can be seen from the typical measurement results of the above groups, the application of the electronic sphygmomanometer described in this article to measure blood pressure can ensure the accuracy of blood pressure measurement within 5mmHg, which basically meets the accuracy requirements of blood pressure measurement.
in conclusion
The method has high hardware integration and easy design implementation; the software design integrates a variety of advanced algorithms such as morphological filtering, with high accuracy and strong anti-interference. Experiments show that this blood pressure monitor has good accuracy and can meet the general requirements of blood pressure measurement.
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