A wireless sensor network based on zigbee for ECG monitoring system

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A wireless sensor network based on zigbee for ECG

monitoring system

Adem Alpaslan ALTUN

Selcuk University, Technical Educational Faculty Dep. of Electronics and Computer Systems

Konya, TURKEY altun@selcuk.edu.tr

Nusret BAŞÇİFCİ

Selcuk University

Natural and Applied Sciences Institute Konya, TURKEY

Abstract— This paper describes the development of a remote

monitoring system for ECG signals. Electrical activities obtained from the body are of great help both in the diagnosis of the disease and the follow of the patient. A ZigBee based wireless network via a device integrated into the belt is used to transmit the signals to the system. Thus, analog signal is measured with the multichannel ADC unit of the MSP430F2274 processors; this information is transferred to a device with a USB port by using ZigBee network and finally observation and storing are carried out by being converted to a PC. With this system, signals obtained from the patients can be monitored simultaneously.

Keywords Electrocardiograph; Wireless sensor networks; Zigbee; Telemedicine; Biotelemetry

I. INTRODUCTION

In parallel with the increase in the world population, a considerable increase is being observed in chronic diseases. Health care problems of the aging communities have also become prominent in many countries. Coronary heart disorders hold the first place among the death causes throughout the world. Health care providers plan to simplify the lives of the people suffering chronic disorders such as heart disease by using economical and smart systems. As a result, remote monitoring of the aforementioned patients turns out to be vital and compulsory.

Wireless technologies are used to rapidly exchange images, data, audio, and other information between remote locations. These technologies are currently being applied to improve healthcare around the world [1]. Telemedicine can make available the benefits of new technologies, especially information and communications, in providing medical care. For implementing a health monitoring system, first of all, devices that measure patient's physiological signal should be needed. One user's health monitoring system has a few ZigBee devices to measure one user's physiological data. Even if there are a lot of ZigBee devices nearby, the communication should be accepted between only one user's ZigBee devices. So, the access control should be implemented because there can be a lot of ZigBee devices nearby.

Engineering systems enabling the transmission of a living organism's basic physiological data are defined as

biotelemetry. Biotelemetry is based on the principle of the use of electronic information and communication technologies for health care provision in the cases when the patient and the doctor are away from each other. Biotelemetry refers to the utilization of telecommunication technology for medical diagnosis, treatment, and patient care [1]. Recent technological advances have enabled the introduction of a broad range of telemedicine applications, such as tele-radiology [2, 3], tele-consultation [4], tele-surgery [5], remote patient-monitoring [6, 7], and health-care records management [8] that are supported by computer networks and wireless communication. In parallel with the rapid developments in the wireless communication and networks, biotelemetric systems making use of these also improve. Biotelemetric systems of the healthcare field aim at enabling doctors to be more effective in the treatment processes. In the biotelemetry applications, wireless biotelemetry systems are preferred more than the others as they allow monitoring the living organisms in their natural habitats by not restricting their movements.

Systems where data are transferred to the device through long cables are one of the fields to which wireless communication technology can be applied. Despite some differences, there exists an ECG imaging system generally with 12 cables connected to the patient. One of the major components of the device is electrodes used to receive signals. Electric signals emitted by heart are perceived through electrodes from the skin in this system. Signal obtained through electrodes are transmitted to the device via cables. Frank Wilson defined VR (right arm voltage), VL (left arm voltage) and VF (leg voltage) connections which are from unipolar electrodes in 1934 [9]. These connections were then named as “Wilson’s Derivations”. American Heart Association and British Heart Foundation specified the standard positions and connection types of the V1, V2, V3, V4,

V5, V6 chest connections [10]. The placement of the electrodes

is given in Figure 1 [11].

In 1997, 32 channel EEG signals were transmitted in the 902-928MHz ISM band [12]. In 2004, wireless transmission of ECG signals to a 12m distance took place by means of a receiver and transmitter with a 94MHz – 108MHz operating

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frequency [13]. ECG signals were able to be transmitted to a 30m distance with receivers and transmitters operating at the frequencies of 433MHz and 916MHz in the wireless environment [14]. In 2005, the researchers realized the multichannel biotelemetry system in the 94MHz -98 MHz FM bands [15]. Jasemian et al. [16] describes a project at Aalborg University in Denmark where 10 healthy volunteers had EKG signals continuously monitored by a similar Bluetooth and GPRS system. Proulx et al. [17] presents the design and evaluation of a Bluetooth electrocardiogram sensor that transmits medical data to a cell phone. In addition, there are many studies related to the biotelemetry.

Fig. 1. The placement of electrodes for the standard 12-lead ECG

In this study, remote transfer and record of the patient ECG data is performed using ZigBee based wireless network technologies.

II. MATERIALS AND METHODS

ECG is widely used as one of the most simple and effective methods of continuously monitoring the heart for tele-healthcare and conventional medical care. Although it has been used widely for heart rate monitoring however, it has some uncomfortable aspect when people without have sufficient medical knowledge use this device. For instance, at least 3 electrodes will be required for heart rate measurement and user has to know the proper placement of each electrode [13].

A typical ECG sensor must have the following features [18]:

* Capacity to sense the low-amplitude signals between 0.05mV and 10mV,

* Very high input impedance > 5 Mega-ohms, * Very low input leakage current < 1 micro-Amp, * Flat frequency response of 0.05-150 Hz, * High common mode rejection ratio

ECG signals carry a great deal of importance with respect to their evaluations by being recorded constantly during the follow of the heart diseases, determining the appropriate diagnosis and treatment, follow of the applied treatment and specifying the abnormalities and complications that can come up. Therefore, processing, storing and transmitting the ECG signals over digital communication networks occupy an important place in the modern clinical applications.

Having realized the shortcomings of Wi-Fi and Bluetooth technologies in certain applications, producers mentioned the

wireless networks that can organize themselves as it is the case in ZigBee in 1998 for the first time. Afterwards, this was standardized as 802.15.4 by IEEE in May, 2003 [19]. The objective of ZigBee is to provide a low-power, low-cost, reliable network protocol to the devices or sensors that can be required especially for remote use.

ZigBee is the first industrial standard WPAN technology that uses the 2.4GHz ISM band. The transmission distance varies between 10 and 75m depending on the power output and environmental factors [20]. ZigBee devices can save energy by switching to the sleep mode when necessary depending on the flow of the transmitted data [21]. As a consequence, battery life can last for months (more than 2 years) together with the sleep mode periods lasting for hours. This feature promotes ZigBee to a rather high position in the wireless sensor networks used for following and control. Recently, Zigbee is frequently preferred especially in the telemetry applications [22].

III. EXPERIMENTALRESULTS

Wireless technologies used in the medical networks for the transmission of the data have an effective and decisive role in the system. ZigBee is actually a rather suitable technology for wireless sensors and it is preferred in applications at an increasing ratio. ZigBee is better than Bluetooth as regards to its network coverage. Its most preeminent feature when compared to the others is its low power requirement. ZigBee can last for months and even for years at the equivalent battery powers. This feature is an exclusively desired situation in the medical sensor networks.

In this study, the transmission of the ECG signals through IEEE 802.15.4 (ZigBee) structure in a general-purpose wireless measurement system is performed. Tapuz Medical’s ECG belt and eZ430rf480 development kit including MSP430F2274 microprocessor is used for wireless transmission of ECG data [23]. The development kit and ECG belt are shown in the Figure 2 and Figure 3, separately.

Fig. 2. eZ430-RF2480 Development Kit (Emulator Board and Target Board)

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Fig. 3. Tapuz Medical Technology Ltd. Belt

Tapuz Medical Technology Ltd. developed a universal ECG electrode belt, which has six ECG electrodes molded into the structure [24]. The electrode positions correspond to the positions where the chest electrodes would be placed for a normal 12-lead ECG. Fitting sockets for the leads onto the electrodes are provided for.

Carrying the ECG signal in a way free of network noise becomes possible as the measurement system is powered by a battery. Since it is a low power signal, ECG requires a high sensitivity in the measurement system. In addition, there is no need for rapid sampling and carrying for the pulses to be monitored properly. In the experiments carried out with MSP430F2274 that can make 200 Kbits sampling per second, it has been seen that the measurement system can respond to the requirement of 20 kHz bandwidth. As a result, ZigBee has reached to the 250 Kbits/s data flows that are the ZigBee’s communication limit. ECG signals are transferred with such a system when the characteristics of this general-purpose measurement system are taken into consideration.

For the ECG signals to be transferred, firstly they have to be elevated. For this purpose, a low-cost ECG amplifier circuit is prepared. ECG signals are received through the ECG belt placed on the body have been connected to the amplifier circuit with two channels and elevated. ADC10 10-bits ADC block included in MSP430F2274 processor is used to convert the amplified analog signal to digital information. Therefore, an analog signal is converted into a digital with ADC, obtained digital information is processed with MSP430F2274 series processor and it is sent via the channel assigned at RF (radio frequency) with CC2480 receiver/transmitter chip in accordance with the ZigBee protocol. Signal on the RF channel is detected through CC2480 series chip on another sensor network and transmitted to MSP430F2274 series processor to which it is connected by conducting demodulation and decoding. Processor transmits the received information array to the computer to which it is connected over USB. Program loaded in the computer enables the signal to be monitored by making the information array received via USB drawn on the screen. Flow chart of the wireless measurement system is seen in the Figure 4.

Fig. 4. Flow chart of the wireless measurement system

A low-cost and portable hardware has implemented for sensing, transferring/receiving, and storing ECG signals as shown in Fig. 5. A chip CC2430 worked in Zigbee module will digitize ECG signals from Sensor Circuit, and then transfer these data back to Electrode-Antenna that transmits them to display/control end. Finally, the real-time ECG signals are able to display/control end with software on PC.

Fig. 5. Schematic for wireless ECG measurement system

ZigBee Receiver-Transmitter is the unit which performs the transfer of the processed data via RF channel in order to transmit it to another node included in the sensor network and enables it to be detected on the opposite side and to be transmitted to the processor. CC2480 chip is used as the receiver-transmitter circuit. Signal received in CC2480 is elevated by a low-power amplifier and reduced to the intermediate frequency. In intermediate frequency (2MHz), complex I/Q signal is elevated after being filtered and digitized by RF receiver ADC module.

Flow chart’s wireless sensor network node placed at the side of the receiver is connected to a PC over a USB. Therefore, digital data received by CC2480 receiver-transmitter is transmitted to one of any COM ports of the PC over MSP430F2274 processor and a USB. Drawing program loaded in the PC gets the received data array drawn on the screen by scaling it. Splash screen of the program and the measured signal are as seen in the Figure 6.

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Fig. 6. Display screen of ECG data transmission software

Wireless ECG transmission software is a program coded objectively with C# programming language. It is the section of “Port Settings” where is determined over which Com Port the node that is connected to computer and is included in the sensor network is connected and at which data rate we will make communication. Data transmission rate is seen to be 9600 baud at the start. As the data transmission rate of the processor through USB is 9600 baud, this value is used without a change. Another section of the software is the “ECG Data Control” which starts the drawing of the data on the screen and finishes the process. Detailed display images of these two sections are shown in the Figure 6.

With such a system, ECG signals are able to be measured over a single node within the transmission range of the ZigBee technology. This range can be extended thanks to the ZigBee’s capability to operate in a knotted system, which allows the patient to stroll around the room or hospital when his ECG measurements are being followed.

IV. DISCUSION

In this study, mentioned before, we aimed to find some new solutions for wireless biomedical applications. By this aim, an ECG acquisition and wireless transmission system was designed by using ZigBee technology. All system parts are designed, realized, and tested for acceptable performance individually. After the all parts of the system connected to each other, whole system was tested. Then ECG signal was transmitted by Zigbee module to PC, and also ECG signal was drawn graphically by the software run on the PC. As can be shown in Figure 6, ECG graphic on the software screen is quite good. Nevertheless, some improvements can be applied to the system.

V. CONCLUSION

With this study, remote monitoring system over a PC based or mobile device has been developed within the telemetry

systems developing since 1960s. Signal transmission device is microcontroller-based and portable and the software developed for the remote processor device is network connection-based.

In this study, a biological signal can be transferred to any place with an Internet access. Furthermore, depending on the signal transmission power, the signal can be monitored in the same place even without an internet connection owing to the wireless network adapter.

In the health monitoring system, One user's health monitoring system has a few ZigBee devices to measure one user's physiological data. Even if there are a lot of ZigBee devices nearby, the communication should be accepted between only one user's ZigBee devices. So, the access control should be implemented because there can be a lot of ZigBee devices nearby.

Furthermore, receiver application could include advances ECG analysis with more to automatically detect various high risk heart diseases, get early diagnosis and prediction of care demands.

ACKNOWLEDGEMENT

This study has been funded under the thesis project “ZigBee based Mobile Health Monitoring System Design and Application” approved by the Selcuk University (Turkey) in 2010.

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Figure

Fig. 1. The placement of electrodes for the standard 12-lead ECG
Fig. 1. The placement of electrodes for the standard 12-lead ECG p.2
Fig.  2.  eZ430-RF2480  Development  Kit  (Emulator  Board  and  Target  Board)
Fig. 2. eZ430-RF2480 Development Kit (Emulator Board and Target Board) p.2
Fig. 3. Tapuz Medical Technology Ltd. Belt
Fig. 3. Tapuz Medical Technology Ltd. Belt p.3
Fig. 4. Flow chart of the wireless measurement system
Fig. 4. Flow chart of the wireless measurement system p.3
Fig. 6. Display screen of ECG data transmission software
Fig. 6. Display screen of ECG data transmission software p.4

References