Design of ECG Remote Monitoring System

Abstract: A powerful and low-cost ECG remote monitoring system is developed to realize remote medical treatment of heart disease patients. The front end of the system adopts FPGA control to collect and store the human body's electrocardiogram (ECG) signals, transmit the ECG data to the computer through the serial port, and realize the display and remote transmission system of ECG signals under the LabVIEW software platform. Most of the functions are implemented by software design to facilitate subsequent function expansion. The experimental results meet the expected goals. The system can be widely used in hospitals, communities, homes, and outdoor monitoring to help doctors diagnose patients in time.
Keywords: FPGA; ECG signal; LabVIEW; DataSocket

The heart is the power unit of human blood circulation, heart fluctuations are a sign of life, and heart rhythm is an important sign of human physiological state. Today, cardiovascular disease has become one of the common diseases with a very high incidence rate, which greatly threatens people's lives. Designing a reliable, low-cost, and small-sized ECG acquisition and remote transmission system is of great significance for the detection and prevention of cardiovascular diseases. This system uses FPGA to implement the hardware system for ECG signal acquisition, and uses the LabVIEW development platform to implement the processing of ECG signals, including the use of DataSocket technology to implement the remote monitoring system. The system expands ECG monitoring from beds and hospitals to communities and families for remote monitoring, which is of great significance for solving the imbalance in the distribution of medical resources, reducing patient referral expenses and fatigue, etc.

1 System composition
The whole system consists of three parts: ECG signal acquisition, ECG signal serial port transmission, and ECG signal processing. Its structural block diagram is shown in Figure 1. The front-end hardware design first acquires ECG signals, transmits them to the computer through the serial port, and then processes them under the LabVIEW software system.

1.1 System Hardware Design
The system hardware refers to the acquisition, off-chip storage, and serial port transmission module of ECG signals controlled by FPGA. The front end uses standard leads to collect ECG signals of the human body, and then sends them to the A/D converter after passing through the signal conditioning circuit of the circuit board. The A/D sampling controller is designed in the FPGA using the hardware description language (VHDL). The off-chip storage (SRAM) controller and the serial port communication controller control the signal to pass through the A/D conversion, store it in the off-chip SRAM, and transmit it to the computer through the serial port, and store it in the off-chip SRAM, so that the data can be resent if the network fails to send it once. Figure 2 is the hardware structure block diagram of the system.

The method of obtaining an electrocardiogram relies on electrodes in contact with the limbs, which are called "leads". This system introduces the human body standard lead ECG signal through the front-end electrode, and performs necessary amplification and interference removal on the ECG signal. At the same time, the ECG signal introduced must be amplified and filtered by the signal conditioning circuit before it can be A/D converted. The signal conditioning circuit is shown in Figure 3. Among them, the right leg drive technology has a better effect on suppressing AC signal interference. The preamplifier amplifies the weak (slowly changing) ECG signal of the input circuit and reduces the interference signal to a minimum.
The bandpass filter circuit with a passband frequency of 0.05 to 100 Hz designed by the system separates the useful components of the ECG signal from the collected signal. The output amplifier will further amplify the ECG signal, so that the ECG signal output by the signal conditioning circuit can meet the scale range of the A/D conversion module used in this system. Since the ECG signal is easily interfered by the 50 Hz power frequency of the mains, a notch circuit, that is, a band-stop filter, is used here to filter out the interference. Finally, the processed ECG signal is sent to the input end of the A/D converter. The ECG signal acquisition board designed in this system is a single-channel acquisition board, which is suitable for many patients with heart diseases.

1.2 LabVIEW software system
LabVIEW is a professional test software development platform launched by NI. Its features include graphical programming environment, independent software design and function design, powerful network function, support for multiple common network protocols, strong openness, etc. LabVIEW software is the leader of graphical programming environment. Using this software for principle research, design, testing and implementation of instrument systems can greatly improve work efficiency.
1.2.1 Serial communication
Virtual instrument software architecture VISA (Virtual Instrument Software Architecture) is essentially the general term for I/O interface software library and its specifications. It is the standard uniformly adopted by the VXIplug&play system alliance. VISA is a single interface library for controlling VXI, GPIB, RS232, PXI, PCI and other types of instruments on the LabVIEW development platform. It is a high-level API that uniformly encapsulates other bus driver functions. Test hardware devices can be directly accessed through VISA.
The nodes used for serial communication in LabVIEW are actually VISA nodes. VISA nodes form a sub-palette separately, including 8 nodes, located in the function palette → "data communication" → "protocol" → "serial port" sub-palette, which realizes the configuration, reading and writing of the serial port, serial port interruption, setting the I/O buffer size, clearing the I/O buffer and other operations. However, it should be noted that before using the serial port, the VISA driver software should be installed.
1.2.2 Display and storage of ECG waveforms
Waveform display refers to drawing continuous waveforms of the received ECG data for expert analysis. The basic graphic display controls in LabVlEW are all located on the Controls → All Controls → Graph sub-template. Graph and Chart are the two most basic elements in the LabVIEW graphic display function. These two elements can be called "graphs", but they are very different in LabVIEW. Generally speaking, Chaa is called "record chart", which displays data in real time, point by point (or multiple points at a time) in the coordinate system, reflecting the changing trend of the measured physical quantity, which is similar to the display method of traditional analog oscilloscopes and waveform recorders. Graph is a post-processing of the collected data, first obtaining all the data to be displayed, and then organizing these data into the required graphics according to actual requirements. And display it at one time. Therefore, this system uses Chart for real-time display and Graph for waveform playback.
The time and transient characteristics of the signal are measured by Express VI to measure the period and frequency of the signal. Using the write measurement file, the data can be stored in a text file. When playback is required, the read measurement file is used to play back the data. In this way, the real-time display, storage, playback, printing and other functions of the ECG signal can be realized, and a database can be further established. The test interface is shown in Figure 4.

1.2.3 Heart rate calculation and alarm
LabVIEW has a strong signal processing capability. Since heart rate measurement reflects the speed of heart rate, real-time and accurate heart rate detection is widely used in patient monitoring, clinical treatment and other aspects.
This system first converts the ECG signal into a square wave signal with the same period, and obtains the period of the ECG signal by solving the period of the square wave signal. That is, the array function module provided by LabVIEW is used to retrieve the data of two adjacent sampling points in the ECG signal, subtract them and multiply them by the sampling frequency, so that it is converted into an array composed of derivative values. Set the threshold and compare it with the derivative in the array; if it is greater than the threshold, a high level is output; otherwise, a low level is output, thereby obtaining a series of square waves, so that the R wave can be measured. The time between two adjacent R waves is the heart rate cycle, which is then converted into the number of heart beats per minute. Usually, the human heart rate is between 50 and 150 times/min. Therefore, the upper and lower limits of the heart rate can be set. When the heart rate exceeds this range, the indicator light is on, indicating that the patient is in danger of life.

1.2.4 Remote Transmission of ECG Signals
LabVIEW can provide communication methods such as TCP, browser, RDA and DataSocket. Among them, DataSocket is a new technology based on TCP/IP protocol launched by NI. This technology is for measurement and real-time high-speed data exchange on the Internet, and is used for data exchange between multiple applications in a computer or on the network. Although there are many technologies such as TCP/IP and DDE for sharing data between two applications, these technologies are not used for real-time data transmission. Only DataSocket is a technology used to share and publish real-time data in measurement and automation applications. DataSocket
realizes data communication, and leaves complex data processing and display to the client to complete. The network is only responsible for transmitting data (including control commands), and the test system is finally formed in a C/S mode. In this way, network resources are less consumed and the communication speed is fast, which is suitable for remote measurement and control with high real-time requirements and large data volume.
The working mode of DataSocket is shown in Figure 5. DataSocket Publisher and Datasocket Subscliber are the clients of DataSockel Server, and they share and exchange data through DataSocket Servert. The three can run on the same machine or on three separate machines (depending on the specific situation). Separating the server from the publisher and subscriber improves the security and reliability of the system.

The basic system of DataSocket consists of three parts that work together: server manager (DataSocket Server Manager), server (DataSocket Server) and client (DataSocket API). It also includes Dstp (DataSocket Transfer Protocol), Uniform Resource Locator URL (Uniform Resource DataSocket Servet Managerce Locator) and file format procedures.
DataSocket provides external resource location interface and function call interface, accessing data items in the server through URL. The source address is used when reading data, and the destination address is used when writing data. The data transmission protocol, network computer flag and data buffer variable are indicated in the URL, as shown in Figure 6.

DataSocket supports multiple data transmission protocols, and different URL prefixes represent different protocols or data types. For example, "dstp://HYY/data", where HYY is the server name and data is the name of the data item. It can be seen that DataSocket simplifies network data transmission between applications and computers, simplifies program communication, and improves programming efficiency.

2 System Application and Precautions
This system can be applied to heart patients inside and outside the hospital. The system hardware circuit module is connected to a PC or portable computer through a serial port to display, store, analyze, print and remotely transmit ECG data. Doctors can diagnose the condition of heart patients in a timely manner based on the transmitted ECG data. When implementing remote transmission in the software design part, attention should be paid to the following: Before data transmission, DataSocket Server should be started on the server and client respectively. Establish a connection, specify the URL and control connection method, and then send and read data.

3 Conclusion
This system is different from the traditional ECG monitoring system in that it uses software programming to implement the ECG monitoring system functions, so as to facilitate the expansion of system functions. After experimental testing, there was no distortion in data display and remote communication, and the expected effect was achieved. The use of DataSocket communication technology to achieve remote communication of ECG data has high network resource utilization, large data transmission volume, and can serve multiple monitored patients if the server processing capacity is strong enough. The monitored patients can be located in the hospital or outside the hospital (such as home, office, outdoors, etc.), and real-time ECG monitoring can be achieved as long as there is a PC or portable computer and a network. It can be seen that the system has powerful functions and low costs, providing convenience for many heart patients, enhancing the medical service system, and has high social use value and market prospects.