Detailed technical principle of abdominal pressure dynamic measuring instrument

Now for the measurement of abdominal pressure, people usually use the method of measuring bladder pressure. This method is inserted into the bladder through a catheter, and slowly injects about 50ml of normal saline into the bladder, and then measures the pressure of the liquid flowing out of the bladder. This method is simple and practical, and the measured value can truly reflect the abdominal pressure value of the human body. However, this method has a very fatal disadvantage: every time a bladder pressure is measured, it needs to be perfused once. It is impossible for medical personnel to continuously perfuse physiological saline. The general intensive care is perfused every 4 to 12 hours. Intermittent measurement may affect the medical staff to keep abreast of changes in the patient's abdominal pressure, and even delay the best time to implement the rescue.

This paper considers the shortcomings of the previous measurement methods, and designs an economical and practical abdominal pressure dynamic measuring instrument with the single-chip microcomputer as the core. It can realize the dynamic collection, storage and display of abdominal pressure data, so that the medical staff can grasp the patient's condition in time.

Measuring principle

The principle block diagram of the system is shown in Figure 1. The water injection pump injects physiological saline into the bladder of the person to be measured through the catheter. The catheter used here uses a double-channel catheter. The water injection pump fills the bladder from one channel, and the water injection speed is maintained. At 4 ml/h, the pressure sensor measures the pressure change of the effluent water from another channel.

The sensor converts the pressure change into a weak voltage amplitude change (0~75mv). After amplification by the analog amplifier, the voltage change value is changed from 0 to 4.5v, and then the single-chip microcomputer is used to control the a/d converter to the analog voltage. The sampling is performed, and the data is processed by the single chip microcomputer and sent to the display. This process uses a water injection pump to continuously inject physiological saline, the sensor continuously measures the pressure value, and the display dynamically refreshes the measurement result, so that long-term dynamic monitoring can be realized.

Figure 1 Block diagram of the abdominal pressure dynamic measurement system

From the measurement principle, there are two improvements compared with the traditional measurement method.

On the one hand, a two-channel catheter is used here instead of the conventional catheter in the traditional measurement method. With a two-channel catheter, one channel can be used for continuous fluid perfusion while the other channel is used to measure the pressure of the fluid as it flows out of the bladder. The catheter used in the traditional method has only one channel, so it cannot be measured during perfusion. Only after a certain amount of perfusion (usually 50 ml), the perfusion device is removed, and then the measuring device is connected to start measuring the flow of liquid from the bladder. pressure.

On the other hand, in order to achieve dynamic measurement, we use a single-chip microcomputer (at89s52) to control the dynamic operation of the entire system. At89s52 is mainly used to control the a/d converter for data dynamic acquisition in the system, and the collected pressure data is processed in real time to control the display to dynamically refresh the display result. In the conventional method, since only the pressure value is measured once, the microprocessor is generally not used for dynamic management, and the hardware and software of the measuring device are simpler.

System hardware design

The system uses at89s52 as the main controller to control data acquisition, calculation and display. The pressure sensor uses the npc1210 high-precision pressure sensor, and the pressure sensor outputs a weak voltage signal of 0 to 75mv. The output signal of the sensor is transmitted through the twisted-pair shielded wire and sent to the amplifier for amplification. The amplifier selects the high-precision operational amplifier of icl7650, which has the advantages of small input bias current, small offset, high gain, strong common mode rejection, and low price. The amplifier circuit is shown in Figure 2.

Figure 2 amplification circuit

The weak voltage signal output from the sensor is amplified by about 60 times and sent to the a/d converter. The 8-bit a/d converter ad0809 is used to convert the analog signal to the digital signal, and the at89s52 is used to control the a/d converter to collect the analog voltage signal of the amplifier output range of 0 to 4.5V. The interface circuit of at89s52 and ad0809 is shown in Figure 3. The obtained digital signal is sent to at89s52 for processing. The back end uses a 3-digit common anode digital tube to display the result, and the segment code is output by the p2 port of the single chip microcomputer, and the bit code is output by the p1 port.

Figure 3 interface circuit of at89s52 and ad0809 system software design

The main subroutine modules include: system initialization, a/d conversion program, average filtering subroutine, scale conversion subroutine, and binary to bcd code subroutine. The main program flow chart is shown in Figure 4. In order to prevent the patient from abrupt changes in abdominal pressure caused by accidental causes such as cough, in the data processing, an average filtering method is adopted to reduce the accidental error. Figure 5 is a flow chart of the average filtering subroutine. The subroutine adopts the method of average filtering to perform data processing, that is, the data which is continuously sampled 10 times is removed and the maximum value and the minimum value are removed, and the summation is performed, and the average value is obtained according to the 8 sample values, that is, the effective sample value is obtained and stored in the transmission buffer. Area.

Figure 4 main program flow chart

Figure 5 average filtering subroutine flow chart Conclusion

In this paper, a dynamic measurement system for abdominal pressure is designed. The dynamic measurement of abdominal pressure is realized by continuous perfusion and continuous measurement. The measuring instrument selects the low-power 8-bit single-chip microcomputer at89s52 for real-time analysis and processing of the abdominal cavity pressure signal collected by the pressure sensor. This abdominal pressure measuring instrument abandons the traditional manual periodic monitoring method and adopts the advanced continuous measuring method, which opens up a new way for the monitoring of abdominal pressure in critically ill patients.

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