| Development of a digital ECG telemetry monitoring system with HOLTER function
1 Introduction
Cardiovascular disease is one of the diseases with the highest morbidity and mortality today. Its onset is very accidental and sudden. Some abnormal ECG information only appears in certain special circumstances. Therefore, it is necessary to record and analyze ECG for a long time. ECG monitoring, as an effective means of dynamic monitoring and diagnosis of cardiovascular disease, has been widely used in clinical and home health care. Telemetric ECG monitoring has become the preferred solution in intensive care units and home health care because it brings convenience to patients. Although the traditional analog ECG telemetry system is small in size and low in power consumption, its poor anti-interference ability and large base drift have many adverse effects on the diagnosis of diseases. For this reason, we have developed a digital ECG telemetry monitoring system, which uses frequency shift keying, secondary modulation and demodulation technology, and a high-frequency transmitting and receiving circuit with a phase-locked loop to ensure the reliability and anti-interference of data wireless communication. The transmitter circuit is controlled by a single-chip microcomputer. It can not only filter, remove base drift and compress data from the instrument amplifier ECG signal, but also store the data in a large-capacity flash memory, giving it a HOLTER function, thereby expanding the application range of the transmitter. The receiving part is made into a plug-in for the expansion slot of the PC, and with the ECG monitoring software, it becomes a complete set of home dynamic ECG monitors. It can not only realize the real-time display and analysis and diagnosis of ECG waveforms, but also send abnormal ECG signals to the central monitoring station of the hospital through the network or telephone system for doctors to diagnose and process.
2 Design and implementation of transmitter circuit structure and function
The transmitter circuit can be divided into: ECG amplification, filtering and level adjustment circuit, analog-to-digital conversion circuit, single-chip microcomputer and peripheral control and storage circuit, frequency shift keying and FM transmission circuit, power conversion circuit. Considering the portability and low power consumption requirements of the transmitter, the devices used in the circuit are different from the commonly used devices.
2.1 Design of ECG amplification, filtering and analog-to-digital conversion circuits
Conventional ECG preamplifier circuits use a three-op-amp in-phase parallel differential amplifier circuit to obtain high input impedance and common-mode rejection ratio. We use an integrated low-voltage, low-power instrumentation amplifier INA118 with this structure. The strict matching and calibration of the internal op-amps and resistors give it extremely high performance, a maximum bias current of 5nA, a common-mode rejection ratio greater than 100dB, an adjustable gain of 1 to 1000, a wide power supply voltage range from ±1.35V to ±18V, and a maximum quiescent operating current of 380μA, which is very suitable for battery-powered systems. Its input end also has an overvoltage protection function of up to ±40V. The signal obtained by INA118 enters the next amplifier composed of MAX494 through the DC isolation circuit. MAX494 is a low-voltage micro-power operational amplifier. Its power supply voltage can be from 2.7V to 6V or ±1.35V to ±3V, and the static current is only 150μΑ. It constitutes a secondary amplification and level adjustment circuit to obtain ECG signals in the range of 0 to 5V. After passing through 50Hz double-T notch and second-order low-pass filtering, this signal can be sent to the A/D converter for analog-to-digital conversion. Based on the requirements of power supply, power consumption, and volume, we choose MAX187A/D converter, which has a conversion bit of 12 bits, serial data output, a sampling rate of 75KHz, a power supply voltage of 5V, an operating current of 1.5mA, a built-in sample and hold and a 4.096V reference voltage. The 8-DIP packaging form and three-wire serial interface bring convenience to use. According to the operation sequence of the three-wire interface SCLK, CS and Dout, we use the three I/O ports of the microcontroller to complete the CS chip selection, the output of the SCLK serial clock and the reading of the Dout serial data.
Figure 1 Block diagram of ECG amplification, filtering and analog-to-digital conversion circuit
2.2 Design of microcontroller and peripheral circuits
The application of 89C51 microcontroller saves external program storage and simplifies peripheral circuits. In view of the needs of flash memory address space, we use P1.3~1.6 ports as extended address buses A16~19. After the microcontroller removes the base drift, interference, and compression encoding of the data obtained by A/D conversion, it can be sent to two different places; if it is used as a transmitter, it can be sent to the FSK circuit through the serial port TXD. The serial communication protocol here is specified as: baud rate 4800bps, 8 data bits, 1 stop bit, and 1 check bit; if it is used as a HOLTER, it is sent to the flash memory AM29F016 for storage. The storage space of 29F016 is 2M×8 bytes, which has the advantages of both ROM and RAM. Its embedded algorithm supports online fast byte reading and writing, on-chip or on-page erasing and other functions. Typically, the time to read a byte is less than 200ns, and the time to write a byte is less than 30μs, which can fully meet the real-time storage requirements of low sampling rate A/D conversion data. A 29F016 can be erased and written nearly 100,000 times. In addition, it has the advantages of no data loss during power failure and large capacity, making it an ideal choice for solid-state data storage. The read and write operations of 29F016 are all started through a series of command sequences to start its on-chip algorithm. After writing a certain command sequence, the Data Polling feature provided by it is used to determine whether the internal program is completed or completed correctly [5]. Calculated at a sampling rate of 200Hz, even if the data is not compressed, a 29F016 can store 3 hours of ECG data. If the Hoffman coding algorithm is used, a data compression rate of about 3:1 can be obtained, and nearly 10 hours of ECG data can be recorded at one time, which can basically meet the general ECG data recording requirements.
2.3 Modulation and transmission method
The serial data sent by the microcontroller serial port is first sent to the FSK frequency shift keying circuit for the first modulation. The frequency shift keying circuit is completed by the integrated phase-locked loop circuit 4046. It changes the resistance value of the peripheral oscillator circuit connected to 4046 through the control of the analog switch by the serial digital signal, thereby obtaining two FSK signals of oscillation frequency corresponding to high and low levels [6], and then completes the frequency modulation transmission through ACMTX16. ACMTX16 is a large-scale integrated single-chip transmitter chip designed for data transmission, wireless paging, etc. It uses PLL synthesis technology internally and only needs an external low-frequency crystal to obtain a stable self-locking high-frequency signal. Its power consumption is extremely low and has a power saving mode. The typical operating current is 2.5mA, the transmission power is adjustable, and the small package of SMD14 fully meets the design requirements.
2.4 Design of power conversion circuit
The transmitter is powered by a No. 5 battery, and the MAX777 DC/DC converter increases the voltage of 1.5V or 3V to 5V. MAX777 is an 8-DIP package with a voltage input range of 1 to 6V, a maximum output current of 240mA, and an efficiency of 82%. In order to provide positive and negative power supplies for the operational amplifier, this 5V power supply is used to generate a 2.5V floating ground. The specific circuit is shown in Figure 2.
Figure 2 Power conversion circuit
3 Circuit design and implementation of telemetry receiving board
The popularization and application of computers have prompted us to make the telemetry receiving part into a computer expansion slot plug-in type. The receiving board contains a demodulation circuit, a serial-to-parallel conversion circuit, and a bus interface circuit, as shown in Figure 3.
Figure 3 Block diagram of telemetry receiving board circuit
The high-frequency modulated signal received by the external antenna is first sent to ACMTR18 for demodulation. ACMTR18 is a single-chip receiving chip corresponding to ACMTX16. It contains VCO and PLL circuits, and the receiving sensitivity is greater than 105dBm. It can receive serial data greater than 50kbps in wireless data transmission applications. The demodulated signal is then sent to a 4046 phase-locked loop for FSK demodulation to obtain standard serial data. The serial-to-parallel conversion is implemented by the 8250 programmable communication interface circuit [6]. After programming the 8250 according to the specified communication protocol, the 8250 can detect the RXD level signal in real time under the drive of the external crystal oscillator clock. When a low level appears on the RXD line, the internal counter is turned on to start the confirmation of the start bit and the reception of serial data. After the conversion is completed, the RDY signal is sent to generate the IRQ2 interrupt in the computer, and the CPU executes the interrupt service program to read the data. The 8250 is isolated and driven by the bus driver LS245. The logic gate circuit provides the chip select signal CS to the 8250.
The control of the receiving board by the computer and the functions of data processing, graphic display, communication, etc. are realized by self-compiled software, and its flow is shown in Figure 4.
Figure 4 Software flow chart
4 Results
This digital ECG telemetry monitoring system has been clinically applied in Tsinghua University Hospital for more than one month, and compared with the existing analog ECG telemetry system in the hospital. The experiment shows that this digital ECG telemetry monitoring system has fully reached the performance indicators of the analog telemetry system, such as the effective distance of indoor telemetry is greater than 30 meters, the received ECG signal is not distorted, and the continuous working time is greater than 24 hours. In addition, this system also has some characteristics that the analog ECG telemetry system does not have:
1) The received ECG signal is highly stable and will not cause baseline drift due to patient activities.
2) Various high-frequency interference signals have no effect on the ECG signal.
3) The receiving circuit automatically checks the ECG serial data. When the received ECG signal has a bit error, the system automatically prompts and marks it. When the bit error rate exceeds the limit, the system stops receiving the ECG signal until it returns to normal.
4) The software can analyze and process ECG signals in real time. When abnormal ECG signals are found, they will be automatically dialed up and transmitted to the hospital monitoring center. The doctor's diagnosis results or medical advice can be fed back to the user in real time.
5) When the transmitter is used as the HOLTER function, it can continuously record 3 hours of raw ECG data. The obtained data can be sent to the computer through the receiving board for ECG waveform display and ECG data analysis.
Due to the complex circuit structure of the transmitter and the large number of application chips, its power consumption is greater than that of the analog ECG transmitter. If the transmitter uses two No. 5 rechargeable batteries, it can work continuously for 24 hours, while the analog ECG transmitter can work continuously for about a week with one 9V laminated battery. The reduction of transmitter power consumption is expected to be achieved through further optimization of the circuit configuration.
5 Conclusion
Clinical use shows that the performance of this digital ECG telemetry monitoring system is better than that of similar analog telemetry systems, and it is an alternative ECG telemetry and monitoring device. This system has passed the inspection of doctors and relevant experts and obtained a national utility model patent. |
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