Ventilator Testing Based on LabVIEW

introduction

With the development of testing technology and bus technology, automatic testing systems marked by virtual instruments have begun to appear. The so-called virtual instrument is based on the computer and bus system equipment as the hardware platform, and the software realizes the functions that originally needed to be completed by hardware. The user only needs to click the virtual panel of the computer with the mouse to operate, and the automatic testing system of the tested object can be realized. Therefore, in the testing field, there is a saying that "software is the instrument".

The ventilator is an essential rescue device in large hospitals and an important tool for prolonging the patient's life and buying precious time for further treatment. It is suitable for patients with the following conditions: 1. Severe poor ventilation; 2. Severe ventilation disorder; 3. Neuromuscular paralysis; 4. After heart surgery; 5. Increased intracranial pressure; 6. Neonatal tetanus requiring respiratory support when using large doses of sedatives; 7. Asphyxia, cardiopulmonary resuscitation; 8. Respiratory arrest or impending arrest for any reason. It provides respiratory support for critically ill patients with respiratory insufficiency through mechanical devices according to different treatment purposes [1]. With the continuous improvement of electronic and mechanical technology, the performance of ventilators has been improved day by day, and its scope of application has also been expanded and popularized.

At present, the performance indicators of domestic ventilators lag behind those of foreign countries. In order to improve and promote the research of ventilators in China, it is necessary to first establish a ventilator test platform. For this purpose, a ventilator test virtual instrument is built based on LabVIEW . This paper introduces a method for implementing a ventilator test virtual instrument based on LabVIEW .

Graphical language

It is a virtual instrument software development tool based on G language (Graphics Language) launched by NI (National Instrument Company) of the United States . It is currently the most widely used, fastest growing and most powerful graphical software development integrated environment.

A LABVIEW program consists of three parts: front panel, block diagram program, and icon/wiring port. The front panel is used to build the instrument's operation display interface; the block diagram program uses graphical language to control the control objects on the front panel (divided into control quantities and indication quantities); the icon/wiring port is used to define the LABVIEW program as a subroutine, thereby realizing modular programming. Using LABVIEW, designers can easily build a virtual instrument front panel like building blocks.

Realization method of virtual instrument for ventilator testing

The entire test instrument consists of the following equipment: a PC, an acquisition card, and a gas circuit consisting of three pressure sensors and two flow rate sensors.

Figure 1 Framework diagram of virtual instrument for ventilator testing

During the test, an external simulated lung is required. The test system uses NI's data acquisition card PCI-6221, which has 16 16-bit A/D channels and a total sampling rate of up to 250KHz. For the ventilator , the virtual instrument sampling frequency is 1KHz during data acquisition ; the pressure sensor uses the Senchuang 30 INCH-D-4V low-pressure sensor, which has self-calibration, zero point compensation and temperature compensation, a linearity of 0.05%, and a resolution of 3/40inH2O; the flow rate sensor uses the TSI Model 84020× high-precision flow rate sensor, which has temperature compensation, an accuracy of ±2.5% reading ±0.1Lpm, and a response speed of <5ms. Figure 1 is the framework structure diagram of the virtual instrument .

The ventilator test mainly targets four major parameters: pressure, flow, and time (including respiratory rate and inspiratory-expiratory ratio). Other important parameters such as chest and lung compliance, airway resistance, and tidal volume can be estimated by calculation. The test process involves complex mathematical operations, and LABVIEW, as a graphical language, has certain difficulties in software design. This can be achieved through mixed programming with C language or MATLAB. However, MABTLAB is a scripting language, and its running speed is greatly limited. Therefore, in the design of virtual instrument software, a method of mixed programming based on LABVIEW and LABVIEW and C language is adopted.

Mixed programming with C utilizes CIN nodes. CIN is an icon with input and output ports located in the LABVIEW block diagram program window. Users can compile the external code to be called into a format that LABVIEW can recognize and then connect it to this node. When this node is executed, LABVIEW will automatically call the external code connected to this node and pass a specific data structure to CIN. Using CIN technology, users can pass any complex data structure to CIN, and using CIN can achieve higher program efficiency. For specific usage methods, see references [2][3]. The storage format of data in LABVIEW follows the storage format of data in C language, and the two are exactly the same. LABVIEW implements data processing and analysis by calling C language, which greatly simplifies the complexity of the program and speeds up the execution time of the program. Figure 2 shows a schematic diagram of the program complexity, programming efficiency, and execution speed of LABVIEW direct programming and LABVIEW calling CIN node programming. Figure 3 is the entire system software design process. [page]

Figure 2 Program complexity and program size for different programming methods

Programming efficiency, schematic diagram of execution speed

Figure 3 Flowchart of ventilator test virtual instrument software

Ventilator test virtual instrument test results

It is a small, lightweight and rugged device. It can produce very accurate flow, volume and pressure waveforms. It uses bidirectional sensor technology to detect airflow. The VT-Plus has become a commonly used device in ventilator testing.

The ventilator is an intelligent high-end ventilator produced by Siemens [1]. The output state of the Servoi ventilator is measured simultaneously using virtual instruments and VTPLUS to obtain the performance of the virtual instrument .

The sampling rate of each channel of the virtual instrument is 1KHz, which can capture the subtle details of the ventilator when it is working and display them in real time. Figure 4 (a) shows the pressure and flow rate curves obtained when the ventilator is working in PCV mode. The concave pressure in the figure is caused by the leakage caused by manual control. The compensation given by the ventilator can be clearly observed from the flow rate graph . Unlike traditional measuring instruments, the inspiratory airway flow rate and the expiratory airway flow rate are displayed by two curves respectively. Therefore, the flow rate time curve can clearly reflect the instantaneous impact of the ventilator opening valve, the delay fluctuation of the valve closing, and the mutual influence of the two valves when opening. From these waveform characteristics, the performance of the ventilator can be quickly qualitatively analyzed, and the compensation speed of the ventilator , the impact size of the valve opening, and the delay fluctuation size of the valve closing can also be quantitatively obtained . The curve measured by VT-Plus is shown in Figure 4 (b), which shows that some characteristics of the ventilator when working are not measured.

Figure 3 PCV mode measurement pressure and flow curve

At the same time, according to the measured pressure and flow rate time relationship, the real-time tidal volume can be calculated, and parameters such as airway resistance and lung compliance can be calculated. Table 1 is the calculation result of Figure 4.

Table 1 Measurement and calculation parameter values

From the above data and analysis, it can be seen that the performance of this virtual instrument is comparable to that of VT-Plus and can meet the requirements of ventilator testing.

references

Wang Baoguo, Zhou Jianxin. Practical ventilator therapy[M]. Beijing: People's Medical Publishing House

Liu Junhua. Virtual Instrument Design Based on LABVIEW [M]. Beijing: Publishing House of Electronics Industry. January 2003

Liu Junhua. Tutorial on Virtual Instrument Graphical Programming Language LABVIEW. Xi'an: University of Electronic Science and Technology Press [M]. August 2001

Technical Insider[M].Beijing: Hope Electronics Publishing House. January 2001

Zhang Kai, Guo Dong. LabVIEW virtual instrument engineering design and development[M]. Beijing: National Defense Industry Press.