Accepted for presentation at the 18th IEEE International Symposium on Computer-Based Medical Systems, Dublin, June 23-24, 2005.
A Wearable ECG-recording System for Continuous Arrhythmia Monitoring in a Wireless Tele-Home-Care Situation
Rune Fensli Agder University College, Faculty of Engineering and Science, Grimstad, Norway
[email protected]
Einar Gunnarson Hospital of Buskerud, Department of acute medicine, Drammen, Norway
Torstein Gundersen Sørlandet Sykehus HF, Medical department, Arendal, Norway
Abstract New wireless technology for tele-home-care purposes gives new possibilities for monitoring of vital parameters with wearable biomedical sensors, and will give the patient the freedom to be mobile and still be under continuously monitoring and thereby to better quality of patient care. This paper describes a new concept for wireless and wearable electrocardiogram (ECG) sensor transmitting signals to a diagnostic station at the hospital, and this concept is intended for detecting rarely occurrences of cardiac arrhythmias and to follow up critical patients from their home while they are carrying out daily activities.
1. Introduction Advanced monitoring solutions using telecommunicating technologies are used for remote ECG diagnosis, and The American College of Cardiology (ACC) and The American Heart Association (AHA) have published guidelines for ambulatory electrocardiography[1]. The use of telecommunications for remote diagnosis is growing rapidly, and there are several products and projects within mobile ECG recording using Internet solutions, Bluetooth technology, cellular phones, WAP-based implementations and wireless local area networks, WLAN. A remote diagnosis system integrating digital telemetry has been developed, using a wireless patient module, a homecare station and a remote clinical station[2]. Traditionally 24/72 h ECG-recording systems like “Holter-monitoring” can today use built-in mobile telephones to send information to the hospital[3], but is mostly used with a recording unit that physically has to be carried to the hospital for analyzes. Several ongoing international projects where wireless sensors are used within the framework of a standardized Body Area Network (BAN) are focusing on improving the patient’s ability to freely move around in a daily situation while being monitored by a wearable system. Sachpazidis et al[4] are trying to develop a robust platform for real-time monitoring of patients staying in their home transmitting data to doctors working at the hospital. This @HOME concept aims at measuring several vital parameters, within a BANframework. Several authors describe solutions based on sensors using a wireless Bluetooth communication protocol and a standard PDA[5], [6]. Jovanov et al.[7] propose the use of a Personal Area Network (PAN) with wireless intelligent sensors to perform data acquisition, and Kong et al. suggested a broadcast of the ECG signals over the Internet[8].
Our new concept has several advantages compared to existing solutions. It is easy to use and requires no technical skills to operate. The ECG-sensor is a compact electronic electrode which easily can be replaced by the patient himself; just stick it to the chest. The ECG is continuously recorded with a built-in automatic alarm detection system, and the system can give early alarm signals even if the patient is unconscious or unaware of cardiac arrhythmias. With this solution, only one lead is required for the ECG recording. This is accomplished by using a compact “double-electrode” with no wires connected, as this electrode is equipped with a wireless transmitter and battery supply for several days of continuous usage. This new concept was developed as an arrhythmia detection system for long-time ECG-monitoring, and ambulatory electrocardiography, designed as an alternative to the conventional Holter monitoring systems. The solution can be described as a continuous event recorder[9],[10]. With the use of this system, it is possible to make an easier and more cost efficient ambulatory ECG recording compared to existing solutions on the market, and the patient can be continuously monitored in his home-situation doing daily activities. This paper describes the implementation of and experiences with this new system for wireless monitoring.
2. Methodology Our concept for a wireless, continuous event recorder for ECG-signals is based on the construction of a new ECG sensor. The sensor includes two electrical contact points applied directly to the patient’s skin with the use of sticky, conducting hydrogel, and they are directly connected to electronic circuits for amplifying the signal and with a wireless transmission of the recordings to a receiver integrated as a component within a Hand Held Device (HHD). The HHD will be the “intelligent” unit for analyzing and temporarily saving the recorded signals. This unit uses a standard telecommunication facility, GPRS (General Packet Radio Service), for sending an alarm signal together with the measured ECG-recordings to a remote WPR Internet connected server. The doctor at the hospital uses a special remote WPR client installed on a standard PC as a Clinical Diagnostic Station (CDS). Trained personnel will thus be able to evaluate the ECG-recordings and for diagnosing the conditions detected, and follow up the patients accordingly. 2.1. System configuration The wireless sensor is sticky and attached to the patient’s chest. It will continuously measure and wirelessly transmit sampled ECG-recordings by the use of a built-in RF-radio transmitter. The RF-radio receiver converts the ECG-samples by the use of a microcontroller before transmitting the ECG-samples to a standard personal digital assistant (PDA) with a RS232 connector. It is used a small plastic enclosure for the receiver with the same size as the PDA which is a Fujitsu-Siemens Pocket LOOX 700 using Microsoft Windows Mobile Software 2003 for Pocket PC. It is programmed in C# based on .net compiler for Smart Device Applications. The PDA is equipped with a CF-slot GSM/GPRS module RTM-8000 from Audiovox, and controlled by the software the PDA will automatically connect to the GPRS mobile network and transmit necessary data to an Internet connected server, which is shown in Fig. 1.
Base station for Mobile telephone INTERNET Wireless transfer of encountered ECG-alarm situations
The patient is wearing the WPR wireless ECG-sensor
WPR Internet connected server GPRS/ GSM
The Hand-Held device receives ECG-signals and uses automatic arrhythmia detection algorithms
Remote WPR Client at the hospital
The patient can use a web-based system to contact the doctor and read the encountered ECG-findings
The Doctor at the Hospital can make diagnositc evaluations of the recorded ECG-signals
Figure 1. The figure shows the principal components of the wireless ECG-system. 2.1. Wireless ECG sensor functionality The sensor measures ECG-signals with a sampling frequency of 500 Hz. The signal is digitalized with 10 bit resolution, and continuously transmitted to a receiver-module in the HHD, with the use of a modulated RF-radio link where we use the RF-transmitter CC1050 from ChipCon, operating at 869.700 MHz. 2.1. The HHD with RF-receiver and arrhythmia algorithms The HHD has implemented an arrhythmia algorithm based on the non-linear transformation for R-wave detection with adaptive threshold published by Sun et al.[11], with a documented true detection rate of 99, 2%. In order to compensate for a higher sampling frequency than used by Sun el al., we have based the detector on a 6-point detection and use the average of 2 points in the 3-power non-linear transformation. It is implemented several alarm criteria where the doctor in a setup configuration can define the actual alarm limits; this includes bradycardia, tachycardia, and arrhythmia defined as variations in RR intervals. If an abnormal ECG activity is encountered, the HHD will store 1 minute of the ECGrecordings and then transmit the recordings to the WPR server by the use of GPRScommunication. In addition the HHD will calculate Heart Rate (HR) and variations in the RR interval, and averaged values together with maximum and minimum values are calculated every one minute. These values are stored in a status-file which regularly is transmitted to the WPR server as an XML-file. The wireless sensor and the HHD are shown in Fig. 2. 2.3. CDS with a web-based client The WPR server consists of a Microsoft Server 2003 with an SQL-database and a webbased application developed on a Microsoft .net platform. The server is placed in a secure
zone in a local area network (LAN). The signals from the HHD are transmitted using a filetransfer-protocol (FTP), and the recorded data are stored in the database. In order to access the web-application for the clinician, it is established an encrypted Virtual Private Network (VPN) tunnel between the server and the Firewall at the perimeter of the hospitals LAN as shown in Fig. 1. A standard PC with a browser can be used for the CDS. In order to store the actual ECG-recordings in a standardized format, we have implemented the Medical Waveform Format, an open standard proposed by Hirai et al.[12]. 2.4. Patient application A survey study showed that the patients wants a quick feedback from the doctor at the hospital[13], and it is therefore developed a web-based patient application which the patient from his home can access by the use of a standard Internet browser and an encrypted VPNtunnel. The patient can thus read messages and necessary drug prescriptions from the doctor and can send messages to the doctor questioning the actual follow-up.
Figure 2. Pictures shows to the left the sensor applied to a test person’s chest while holding the assembled HHD. To the right the different parts of the HHD, the printed circuit with the receiver unit and battery, connected to the PDA with an RS232 cable.
3. Results
Figure 3. The picture show a typical ECG-recording from the receiver unit connected to the PDA, and with a sensor position corresponding to V2-V3 on the left side of the chest to a test person. The figure capture is from a PC connected to the receiver unit.
Technical test of the system design is performed during March 2005, and the Fig 3 shows the ECG-signal recorded at the HHD. The actual curve shape will be dependant on the exact position of the sensor, and in the figure this position is similar to the standard ECG-positions V2-V3 on the left side of the chest. The screenshots in Fig.4 shows the clinical application on the CDS. To the left the doctor can overview the latest alarm recordings and what time they occurred. To the right a typical alarm recording is shown. The doctor is able to change the scale-factor for the ECG-curves both in timescale direction (X-axis) and in amplification (Y-axis). Information from the regular status-information is retrieved from the database and processed as a trend-analyze for 24 hours variations of RR-interval. The doctor can choose the desired time-interval, and the graphs can be printed out for documentation. In a separate text-box the doctor can make his comments to the actual recorded curves and to the alarm-conditions detected by the system.
Figure 4. The pictures show screenshots from the clinical application on the CDS. To the left is a tabulated overview of the latest alarm recordings, and to the right is displayed an ECG-curve where the doctor can scroll to the right for viewing the whole sequence of 1 minute of ECG-recordings. The curve can be enlarged for better viewing the curve details. The point of alarm is given with the actual time indication. The system is under implementation at the hospital for the first clinical trials during spring 2005. Preliminary informal assessments indicate that the functionality from the cardiologist’s point of view is very useful. A study of the functionality and benefits from the patient’s point of view is carried out as a part of the system trials.
4. Discussion and Conclusion The ECG-signal obtained differs in some ways from a standard lead-I recording as it only use two electrodes that are placed close to each other. It is therefore, necessary to further investigate the use of this recording principle for arrhythmia diagnostic purposes. On the other hand, the recordings in our system are supposed to be comparable to the recordings from an implantable loop recorder used by Krahn et al [14] who used the Medtronic ILR, and
found that this technology is a powerful tool in arrhythmia diagnosis. Even though proper clinical trials are clearly needed to verify our hypothesis, it therefore seems reasonable to assume that our new ECG-monitoring system will be able to, reliably, detect rarely occurrences of cardiac arrhythmias, and thus make correct diagnosis even under situations where the patient has the ability to carry out daily activity including physical exercise, body wash and normal work.
5. Acknowledgement The study is supported from Norwegian Research Council as a MEDKAP-project, and is done in close cooperation with WPR medical AS, Norway.
6. References [1] [2] [3] [4]
[5] [6] [7] [8] [9]
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