Design of a dynamic abdominal pressure measurement system (Fig.)

The abdominal cavity is a closed space in which any increase in the volume of any organ can lead to intra-abdominal hypertension. The purpose of intra-abdominal pressure monitoring is to provide effective evidence for timely intervention and nursing by observing the changes of intra-abdominal pressure after abdominal surgery and in severe patients. Nowadays, abdominal pressure is usually measured by inserting a catheter into the bladder, slowly pouring about 50ml saline into the bladder, and then measuring the pressure of fluid flowing out of the bladder. This method is simple and practical, and the measured values can truly reflect the values of human abdominal pressure. However, this method has a very fatal disadvantage: every measurement of bladder pressure requires perfusion, it is impossible for medical staff to keep deperfusion of saline. In general, intensive care is perfusion measurement every 4-12 hours. Such intermittent measurement may affect medical staff to timely understand the changes of abdominal pressure of patients, or even delay the time of rescue. Considering the drawbacks of previous measurements, this paper designs an economical and practical peritoneal pressure dynamic measuring instrument based on single-chip computer. Its main features are: painless and non-invasive continuous peritoneal pressure measurement for patients, which can realize the dynamic collection, storage and display of peritoneal pressure data, so as to facilitate medical personnel to grasp patients'condition in time.

measuring principle

Continuous monitoring is necessary for abdominal pressure testing in critically ill patients, so we have adopted a new method to measure abdominal pressure. System schematic diagram Fig.1 shows that a water filling pump uses a catheter to inject saline into the bladder of the person being measured. The catheter used here uses a two-channel catheter. The water filling pump fills the bladder from one channel at a rate of 4 ml/h. The pressure sensor measures the pressure change of the water flowing out of another channel. The sensor converts the pressure change into a slight change of voltage amplitude (0-75mV). After amplifying the analog amplifier, the value of the voltage change with the range of 0-4.5V is obtained. Then the analog voltage is sampled by A/D converter controlled by single chip computer. After processing the data by single chip computer, the analog voltage is sent to the display. This process uses the water injection pump to continuously inject saline, the sensor also continuously measures the pressure values, and the monitor dynamically refreshes the measurement results, so that long-term dynamic monitoring can be achieved.

Fig.1 Diagram of the peritoneal pressure dynamic measurement system

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

On the one hand, the double-channel catheter is used instead of the conventional catheter. A dual-channel catheter allows continuous fluid infusion through one channel while another is used to measure pressure as fluid flows through 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 50ml), the perfusion device is removed and then connected to the measuring device to begin measuring the pressure of fluid flowing out of the bladder.

On the other hand, in order to achieve dynamic measurement, we use single-chip computer (AT89S52) to control the dynamic operation of the whole system. AT89S52 is mainly used in the system to control A/D converter for data dynamic collection, real-time processing of collected pressure data, and control display dynamic refresh display results. In traditional methods, because only pressure values are measured, microprocessors are generally not used for dynamic management, and the measurement device is simpler in hardware and software.

System Hardware Design

The system uses AT89S52 as the main controller to control data collection, operation and display. The pressure sensor uses NPC1210 high pressure sensor, and the output of the pressure sensor is a weak voltage signal of 0-75mV. The output signal of the sensor is sent out through the twisted-pair shielding cable and sent to the amplifier for amplification. ICL7650, a high operational amplifier, is selected as the amplifier. It has the advantages of small input bias current, small misalignment, high gain, strong common mode rejection and low cost. The amplifier circuit is shown in Figure 2.

Figure 2 Amplifying circuit

Amplify the weak voltage signal from the sensor by about 60 times and send it to the A/D converter. An 8-bit A/D converter AD0809 is used to convert analog signals to digital signals. AT89S52 is used to control the A/D converter to collect analog voltage signals with the output range of 0-4.5V from the amplifier. The interface circuit between AT89S52 and AD0809 is shown in Figure 3. The digital signal obtained is sent to AT89S52 for processing. The back-end uses a 3-bit common-anode digital tube to display the results, and the segment code is output from P2 port and P1 port of the single-chip computer.

Figure 3 Interface circuit between AT89S52 and AD0809

System Software Design

The main subprogram modules include: system initialization, A/D conversion program, average filter subprogram, scale transformation subprogram, binary to BCD code subprogram. The main program flowchart is shown in Figure 4. In order to prevent the abdominal pressure steep change caused by occasional causes such as cough, an average filter method was used to reduce the accidental error in data processing. Figure 5 is a flowchart of the Average Filter subprogram. This subprogram uses the average filtering method to process data, that is, the data sampled 10 times in succession is added up after the removal of values and minimum values, and the average number is taken according to the 8 times of sampling values, then the valid sample values are saved in the send buffer.

Figure 4 Main program flowchart

Figure 5 Flowchart of Average Filter Subprogram

Precautions against signal interference

The system is used to monitor abdominal pressure in critically ill patients. Reliability has become the focus of design. The system uses a high pressure sensor to detect pressure changes, so the suppression of interference signals becomes the key to the success of the design. The main considerations in the design are as follows:

(1) In order to suppress the noise effect caused by power voltage fluctuation, the decoupling capacitance, shielding cover and filter circuit are added.
(2) The unused input of each logic circuit chip is connected to the used input or grounded according to the logic relationship, or to a higher level, so as to reduce the influence of external interference signal on the system.
(3) Clock pulse signal configuration is appropriately close to CPU, select short and thick lead.
(4) shielding the weak signal lines; The power cord is separated from the signal cable to prevent line-to-line crosstalk.
(5) Reasonable ground wire layout: Because the signal frequency of this system is 11.0592 MHz, it is digitally separated from the analog, connected to the ground wire at the power end, and maximize the ground area of the analog circuit; The grounding wire should be as thick as possible to prevent the grounding potential from changing with the change of current, unstable the timer signal level of the system, and poor the anti-noise performance of the system.
(6) No signal line forms a loop; Keep the line as short and straight as possible; Minimize excess pore volume; Try to use 45 degree polylines instead of 90 degree polylines.
(7) The clock line should be perpendicular to the I/O port line to reduce the interference of the I/O port line to the clock circuit.
(8) Analog signal input line and reference voltage end as far as possible from digital circuit signal line.
(9) Average filtering is used to reduce accidental errors in software design.

conclusion

This paper designs a dynamic measurement system of abdominal pressure, which uses double-channel catheter, continuous perfusion and continuous measurement to achieve the dynamic measurement of abdominal pressure. The instrument uses AT89S52, a low power 8-bit single-chip computer, to analyze and process the abdominal pressure signal collected by the pressure sensor in real time. The designed peritoneal pressure dynamic measuring instrument with a digital tube display can display the detected peritoneal pressure signal in real time. The measuring instrument improves traditional equipment and methods and does not place more burden on medical personnel who are familiar with traditional equipment. This abdominal pressure meter abandons the traditional method of manual periodic monitoring and uses advanced continuous measurement method, which opens up a new way for monitoring abdominal pressure in critically ill patients.