An ECG Monitoring System Using Conductive Fabric Bahareh Taji, Shervin Shirmohammadi, Voieu Groza, and Miodrag Bolie Sehool ofEleetrieal Engineering and Computer Seienee (EECS) University of Ottawa btajiO
[email protected], {shervin,groza,mbolie}@eees.uottawa.ea Abstract- In this work we evaluated the quality of the ECG signals obtained from conductive fabric dry electrodes. The purpose of this work was to evaluate if lead I ECG collected with dry conductive fabric
electrodes
is suitable
for
applications where
no
special
preparation of electrodes specific for ECG monitoring is allowed. An example of such application is ECG-assisted blood pressure (BP) monitoring device where the user of the BP device should only follow standard procedures of BP measurements. In this paper, we present a system that we developed for the acquisition of ECG signals and their transfer to the PC, and we evaluate the quality of the ECG signals from different electrodes placed at biceps and wrist or touched by fingers. In our experiments we compared signals obtained using gel Ag/AgCI, dry contact electrodes made of golden plates and conductive
fabric-based
electrodes.
Conductive
fabric-based
electrodes are capable of collecting ECG with accuracy comparable to the accuracy of the signal collected by gel electrodes. Keywords-ECG electrodes; ECG monitoring; conductivefabric
I.
INTRODUCTION
Electrocardiography (ECG) signal is the primary diagnostic signal in people with cardiac diseases. ECG monitoring is a standard procedure in current cardiac medicine. Much effort has been spent lately to make ECG monitoring an easy and anywhere-anytime available procedure for people investigated for cardiac problems and especially for those at risk of heart attack or stroke. Cardiovascular Diseases (CVD) are the main cause of death within the population in the ages of 44 - 64 years old, and the second most frequent cause of death of people between 24 and 44 years old [5]. Because of this, both physicians and patients are interested in the results of ECG monitoring research. Previous works to make bio-signals monitoring an affordable, safe and easily available procedure have come up with many commercially available bio-signal monitoring devices. Such systems are mainly composed of three subsystems: the fIrst subsystem consists of sensors that act as interfaces between the patient's body and the device. The second subsystem is the signal processing unit which is the core of the system. Since almost all acquired bio-signals are of small amplitudes (11V to mV level) and low frequencies (0.1 Hz to 1 kHz), they all include conditioning circuits that amplify the signals and fIlter external noises. The last subsystem is the communication unit which allows for connecting the device to a host node that can be a computer or a smartphone.
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The first subsystem of all ECG acquisition devices i.e., the sensors may cause several problems. First of all, since they are in direct contact with the patient's body, in long term use, they can induce skin irritation. Moreover if the sensors are not conveniently and easy to use, they may make the monitoring cumbersome processes which people are not interested to use. Another issue is the time duration they can deliver the same signal quality without degrading, since some electrodes cannot provide the same signal amplitude for a long time and their signal gets attenuated or distorted. In this paper, we propose to use conductive fabric as an ECG electrode which is suitable for long term use and also has a potential to deliver good overall ECG signal quality in chronic use. The ECG signal eventually will be used as a part of blood pressure monitoring device. R-peaks extracted from ECG signals are used in an algorithm for estimating blood pressure (BP). We consider embedding ECG in both home monitoring and ambulatory monitoring BP devices. The user of the horne monitoring BP system would only need to follow regular procedure for measuring BP. This requires that the electrodes that will be used for ECG are dry, flexible and that they do not cause any irritation and they do not require skin preparation. One possible confIguration is having one dry electrode placed in the cuff, like what is pointed out in [1] and another electrode on the BP device in a way that the user will keep pressing it during the measurement. A confIguration with two electrodes on the device (instead of having one under the cuft) is also possible and it will require that the user presses the electrodes with two fmgers from both arms. Besides that confIguration, our BP monitors might be applied for ambulatory monitoring where ECG will be measured every time when BP is measured. This would require different placement of the electrodes - for example one electrode can be under the cuff and another one on the other arm. These applications led us to evaluate the quality of ECG signals at different positions as weil as different electrode confIgurations. We collected ECG signal from one person with regular gel AglAgCI (RedDot) electrodes, conductive fabric and commercial golden plate electrodes. The measurements are done when the electrodes are placed on wrists or biceps on both arms which emulates the ambulatory BP monitoring simulation. In addition, the ECG signal is taken from fmgertips or from one fmgertip and one biceps which emulates the situation when the device is used for horne blood pressure monitoring.
This paper is an initial work that focuses only on acquisition of ECG data and determining quality of ECG signal obtained using electrodes made of conductive fabric. The paper focuses only on collection of ECG signal and integration with blood pressure monitoring will be our future work. The rest of the paper is organized as folIows. Section 11 gives a brief review of all types of ECG electrodes. In Section III, we discuss about ECG based blood pressure monitoring approaches. System design and architecture is presented in Section IV and also device block diagram is iIIustrated in it. Some results and experimental comparisons are given in Section V and the paper concludes in Section VI. 11.
RELATED WORK: ECG ELECTRODES
A. Gel-based electrodes Traditional c1inical ECG electrodes are adhesive gel Ag/AgCI electrodes. The gel applied on these electrodes fonns a conductive path between the skin and an electrode. Due to electrochemical process which occurs between the electrodes and the tissue, real electrical current tlows [12]. Since they have been in use for a long time, most of their principles, characteristics and weaknesses are weil understood. They are adhesive thus could be fixed on the skin easily and show very clean and reliable ECG signals. From the user's point of view, they are problematic due to several reasons as folIows: - The adhesive part and the gel inside them cause skin irritation and contact skin dermatitis if they are in use for a long time. - They need skin preparation in advance like shaving, c1eaning the contact area with alcohol and sometimes using sand paper to remove the dead layer of the skin. - Their most significant drawback is that they cannot be in use for a long time, and as soon as the gel dries out, they must be replaced. That is the reason why they are considered as disposable electrodes. Removing electrodes and applying new ones is an unpleasant process which makes gel electrodes inconvenient. Although they provide a clean ECG signal, they cannot be applicable to a portable ECG monitoring system. Thus, alternatives are being developed to substitute current common electrodes. B.
Dry Electrodes
In contrast to gel electrodes, dry contact electrodes do not need any kind of gel, instead they operate by moisture on the skin, i.e., sweat [12]. Dry electrodes vary from simple stainless steel metal plates to new flexible textiles capable of conducting electrical potential, known as conductive fabric. Dry electrodes exist in two categories of non-contact and contact, discussed next.
1) Dry non-contact electrodes This category mainly consists of capacitive coupled ECG, which works based on coupling between the skin and a metal electrode. Coupling leads to polarization of the electrode, hence causing displacement current tlow.
This is an appropriate approach for long tenn monitoring because there is a layer of insulator between the skin and metal electrode which makes it much more convenient in comparison to gel electrodes which are in direct contact with the body. Insulator together with the electrode and skin form a capacitor through which the signal propagates [5]. As for their electrical model, they are usually simplified and modeled as coupling signals through a small capacitance, around 10's pF. However, in reality there is also a resistance, because whatever the insulator is, it has a non-negligible resistance and the signal is strongly affected by that. To achieve better perfonnance, the capacitance value must be high, therefore the electrode area should be large or the insulator must be chosen from a special material with high dielectric constant [5]. Although capacitive electrodes do not need to be replaced after a while and therefore have some advantages over gel electrodes, they suffer from some drawbacks too. First, signal coming from that kind of sensor is noisier and has less amplitude as what comes from a gel electrode, because there is large impedance between the body and the metal plate of the electrode. Second, this high impedance causes the electrode to act like an antenna for the nonnal noise of the environment [5]. Third, they have high sensitivity to motion artifacts because they are not fixed on the body and any displacement of the ECG source changes the capacitance value. Thus, any undesired body movement changes the distance between electrode and body, thus results in a small changes in potential that can dominate the ECG signal, especially given the small signal amplitude of ECG (1 mV). This is the most significant drawback and needs serious efforts to be taken care of. This unchangeable reality makes their application controversial and challengeable. In recent prototypes employing dry electrodes, bio-signals are amplified by an amplifier embedded in the electrode in order to minimize the noise induced in the path of the sensed signal to the amplifier's input. Such electrodes are made of a standard PCB including an amplifier and called "active electrodes". Active electrodes are sometimes shielded to prevent external interference. All dry non-contact electrodes are prone to be embedded inside a belt or a thin cloth. It is still kind of obtrusive because metal plates, or PCB based electrodes are stiff and they should be very c10se to the body in order to have a good quality ECG signal. Hence the belt must be worn very tightly which is not comfortable. We should mention that many horne monitors use active electrodes.
2)
Dry contact electrodes
The simplest dry contact electrode is a metal disc in direct contact to the skin in order to acquire any bio-potential-based signal such as ECG or EMG. Although performance of such metal disc electrodes is comparable to the gel electrodes after a few minutes due to sweat and humidity of the skin, their usage is still limited because of stiffness and the fact that they still need to be embedded in a wearable belt. In addition, they can cause skin irritation especially if the user is wearing the belt for a long time and they are very sensitive to motion artifact.
We should mention that the same technology of active electrodes is applicable for dry contact electrodes as weil as non-contact ones. The newest and most innovative electrode for horne health care monitoring devices is conductive fabric. It is indeed one type of dry contact electrode, and its advantage is that it is just like an ordinary fabric. Conductive textiles can be provided by using various kinds of conductive yarns including silver coated nylon (AgNy), stainless steel yarn (SSt), and silver coated copper (AgCu) [8]. The conductive fabric we use in our device is a flexible, thin, soft, light weighted and stretchy textile made of silver coated nylon. It is appropriate for any application dealing with electrical potential transmission, e.g. ECG. It potentially introduces a new generation of wearable bio-potential electrodes which are more comfortable and easy to use in comparison to what is already available [6]. Compared to the above electrodes, our proposed electrode does not need electrode replacement, guarantees delivering constant signal quality for long time, prevents skin irritation, is not stiff and uncomfortable and detects ECG which is comparable to ECG detected by gel electrodes. Besides that, unlike gel electrodes, it does not need any skin preparation and due to its soft and thin texture it is has the potential to be embedded in any desired shape and even in a shirt.
C. Electrodes used in commercial devices Comrnercially available ECG monitoring systems mostly use gel sensors, especially if they are designed for clinical use. In contrast, horne health care devices mainly employ dry electrodes embedded in a belt or cloth and the user should wear the belt around the chest tightly. Table 1 compares some examples of comrnercially available ECG monitoring systems due to the type of ECG electrode they apply. TABLE I
COMPARISON OF COMMERC1AL PROTOTYPES OF ECG MONlTORlNG SYSTEMS
Device
Vendor
Type of Electrode
Heart scan Heartcheck Easy ECG MAC EASI ECG TruVue
Omron CardioComm Solutions Favorite Plus General Electric Philips Biomedical Systems
Dry direct contact Dry direct contact Dry direct contact Gel Gel Gel
There are also many prototypes of ECG monitoring systems introduced in the literature, such as devices introduced in [9], [7], [13] and [lO]. Table 2 compares the aforementioned devices in terms of type of ECG electrodes they apply. TABLE 2 COMPARlSON OF SOME PROTOTYPES A VAILABLE IN LITERATURE Reference Number [9] [7] [13] [10]
Electrode type Non-contact Capacitive Gel Dry contact embedded in a shirt Dry non-contact integrated in cloth and bel!
III.
ECG ASSISTED BLOOD PRESSURE MONITORS
One of the most comrnon methods for non-invasive blood pressure measurement is the oscillometric method [4]. Since blood pressure is an important vital sign implying one's heart health, many BP metering devices are designed specifically for horne health care purposes. Such devices enable people who are prone or susceptible to high blood pressure monitor their blood pressure ups and downs. In conditions such as obesity, arrhythmia and posture change arterial amplitude sensed by the cuff is not distinct [1]. Besides, blood pressure measured by this method strongly depends on the place where user applies the cuff and how tight it is wrapped around the bicep or wrist [4]. Researchers around the globe are trying to fmd a more reliable method of blood pressure measurement which is not affected by aforementioned factors and relies on signals with higher consistency, such as ECG. Here we refer to two of them which are presented in [1] and [4]. Authors in [4] propose a blood pressure measurement method based on oscillometric method and apply ECG for two purposes. First they use the pulse transit time (pTT, the time delay between R-peaks and photoplethysmographic (PPG) pulses) to estimate the standard deviation of systolic and diastolic blood pressure. In their device, PPG sensors are touched by the patients with their index fmgertips and ECG sensors are installed on the monitor such that they are touched by patients' palms. The researchers propose using ECG as an indicator to make sure that the patient is at rest and thus blood pressure measurement is valid. To do this, they suggest measuring the signal to noise ratio of the ECG signal, and if it is below a given threshold, the BP measurement continues, otherwise the measurement is not validated and the process stops [4]. In another approach, Ahmad et al. [1] proposed a method for blood pressure estimation that detects R-peaks of ECG and employs them to improve the oscillometric pulse peak detection and thus to provide a better estimation of the blood pressure. Furthermore, R peak information is used to find out the maximum amplitude of oscillometric pulses which are interleaved between consecutive R-peaks. Then the maximum amplitude algorithm is applied. Ahmad et al. [1] developed a prototype to implement ECG assisted BP estimation. In this prototype they employ two ECG electrodes, both made of conductive fabric: one is embedded in the cuff which is wrapped around the bicep and the other one is in shape of a wristband and applied on user's wrist. IV.
SYSTEM ARCHITECTURE
A. ECG Electrode Considering the above-discussed advantages and attractive features of conductive fabric, we opted for that for our device. Conductive fabric is a thin flexible electrode which could easily be used in shape of a wristband to deliver the most convenience to the user. Our results show that conductive fabric is an acceptable alternative for other types of ECG electrodes and can potentially address the comrnon problems of ECG monitoring devices and improve ECG monitoring
significantly in terms of convenience and long term usability. We use conductive fabrics in the inner side of two ordinary wristbands: one for the left wrist and one for the right one.
Fig. 3 shows a block diagram of the entire device and how different modules are connected.
As for our device, we use medical grade silver plated 92% nylon and 8% dorlastan stretchable conductive fabric. The thickness of the conductive fabric we apply is 0.50 mm and its surface resistivity is less than 10hm/D. Fig. 1 shows the conductive fabric we use as our ECG electrodes.
To acquire data by this device, first one needs to write a control code sequence in C and load it into microprocessor. The debugger environment which is specifically designed for many Texas Instruments microprocessors, including the one we are using (CC2531F256), is IAR Embedded Workbench. The code is written in a way that starts data acquisition after user pushes a start button. It sends the digitized data from the device to the PC through MPU's radio frequency (RF) module and antenna while it is capturing data. For our tests, the user wears the electrode, in case we are using the fabric wristbands, or touches the golden plates on the device, in case we are using them as test electrodes. Then we start the device by pushing the start button. ECG waveform appears on the screen of the PC by running a Matlab code.
Fig. 1. Conductive Fabric made of silver coated nylon filaments B.
Data Acquisition (DAQ)
Analog to digital conversion (ADC), two stages of amplifiers and three stages of filters (high pass, low pass and notch) are available on the system's data acquisition module. Analog input feeds the first stage of an operational amplifier and then gets filtered by high and low pass filters and also notch filter. A 60 Hz notch filter is used in our design to remove the power line noise. After the filtering stage, the input feeds the second amplifier stage, before being digitized using the ADe. The ADC employed in our device is ADS8320, Texas Instruments. The resolution is 16 bit and maximum sampling rate is 100 KS/sec. In addition to aforementioned analog part, there is digital part which is fed by the output of ADe. The master processing unit (MPU), shown in Fig. 2, is the main controller for the entire system. It consists of the CC2531F256 microprocessor from Texas Instruments, 32 Kbyte serial RAM memory and all of the hardware resources needed to build a 802.l5.4 based wireless control node [11]. The CC2531F256 also contains 256KB of in system programmable flash memory, hosting the embedded software that controls the system. The same chip is used as a slave processor in many of the system's functional modules. Also visible in Fig. 2 are an on-board chip antenna and a ribbon cable connector that may be used to connect the device to embedded software programming resources. Cable for connection to software development environment
On-board chip antenna
Fig. 2. Master processing unit(MPU) ECG Board
ECG Electrode
MPU
Fig. 3. Device Block Diagram
C. Test Procedure
V.
EXPERIMENTAL RESUL TS
Our objective is to evaluate what type of electrode and where on the body to use it in order to obtain an ECG signal suitable for applying in blood pressure measurement methods like what we discussed in section III. Thus we designed our testing method as folIows. First we performed ECG measurement of one person from wrists with conductive fabric. In all cases we also performed ECG measurements with regular gel Ag/AgCI (RedDot) electrodes which is used as a reference. Next, a similar experiment is performed in which the electrodes were on biceps. Then we took ECGs from fmgertips by two golden plates and fmally we show the results in case we use one golden plate for one hand's fmgertip and fabric electrode on opposite hand's bicep. This is the desired way of taking ECG for our device because we can take ECG from the fabric embedded inside the cuff as one electrode and one golden plate installed on the device as the other one. This enables us to design BP monitoring device without parts and therefore making it user friendly. We need to make sure that ECG detected in this way is suitable for our purpose. We should not miss any R-peaks and also we should not detect any extra peak, due to severe distortion. Missing an R-peak or detecting a fake one both result in incorrect results in ECG-based blood pressure monitoring. We should mention that we did not do any kind of skin preparation before data aquisition. Each measurement took 10 seconds. Fig. 4-a depicts ECG waveform taken from wrists with Ag!AgCl gel adhesive electrodes, while Fig. 4-b shows ECG taken from wrists by applying conductive fabric, and Fig. 4-c shows ECG with gel electrode on the left wrist and conductive fabric on the right wrist. As it could be seen, ECG taken with gel electrodes, as we expected is stable. It has constant P, R and T wave amplitudes while ECG taken with fabric has variation in amplitudes. Besides it is more affected by noise.
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Fig. 4. ECG taken from wrist by various electrodes. (a) wet electrodes,(b) conductive fabric,(c) one wet and one fabric
Fig. 5. ECG taken from biceps by various electrodes. (a) wet electrodes,(b) conductive fabric,(c) one wet and one fabric
P, R, S and T waves are still recognizable and their amplitudes are approximately the same as their amplitude in ECG taken by gel electrodes. Therefore the obtained waveform is acceptable. In case we apply one gel and one fabric electrodes, we observe less variation in amplitudes in comparison to using fabric electrodes on both wrists, but the R-peaks have lower amplitude.
In the sequel as mentioned above, we took ECG with dry contact electrodes and from fmgertips. The electrodes are small golden plates (l8mm x 12mm). Then, we placed one golden plate on fmgertip of one hand and one fabric electrode on the bicep of the other arm. Fig. 6 demonstrates the results. Fig. 6-a shows ECG taken from fmgertips using golden plates, while Fig. 6-b shows the result when fabric electrode is on bicep and opposite hand's fmgertip is on the golden plate. The ECG signal taken with golden plates shows severe tluctuation in R peak amplitude. Since applying force strongly affects the quality of ECG taken by any electrode [2], using golden plates results in very different ECG signals dependent on how much pressure user applies to the plate while recording ECG.
Fig. 5 illustrates the same results for biceps. Fig. 5-a shows ECG wavefonn taken from biceps with AglAgCI electrodes. Fig. 5-b shows ECG taken from biceps with conductive fabric. The last one, Fig. 5-c shows the ECG with gel electrodes on left bicep and conductive fabric on the right one. Results from biceps are consistent with the results we took from wrists.
have quite stable ECG suitable for ECG assisted BP algorithrns and at the same time let the device be comfortable.
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We applied different electrodes in different locations of the body. Our results show that in all cases that we tried, the device was capable of acquiring an ECG signal in a way that R-peaks and also T and P sections are recognizable.
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One possible extension to this work is applying external force to the electrodes by using a wrist or arm cuff equipped with conductive fabric in its inner side and use it as the ECG electrode. To apply external force we need to intlate the cuff up to a certain level and detect the ECG. Since ECG electrodes have better behavior in case they are under pressure [2], we expect to obtain more accurate ECG signal with larger R-peak amplitude if we apply pressure.
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Time(s) (b) Fig. 6. ECG taken [rom finger tips(a) golden plates ,(b) conductive fabric on bicep and golden plate for one finger tip
Applying different amounts of pressure results in different contact area between the plate and finger and this affects skin electrode impedance. Furthermore sweating is another affecting factor in skin-electrode impedance and thus in detected ECG. In contrast, the ECG taken with one golden plate and one fabric electrode shows more stability in R-peak amplitudes, however lots of noise in T and P waves is observed. In all taken sampies in various methods we did not miss any R-peak and we did not detect any extra one. Therefore we can conclude that all combinations are applicable for our purpose in implementing ECG based BP measuring algorithm in this device. For future work, we are planning to explore how external pressure affects the ECG quality and shape. VI.
CONCLUSION AND FUTURE WOR!(
Although much effort has been made in recent years to develop proper ubiquitous vital sign monitoring systems, including commercial ones, there is not any standard system architecture, size or performance and any kind of them has its own issues. One common problem for BP monitoring devices is their lack of robustness when measuring the blood pressure of some c1asses of patients or in some conditions. To mitigate these problems some ECG assisted BP measurement methods were invented. So, ECG electrodes are needed for such monitors; ECG sensors have their own issues, since they are mostly cumbersome and uncomfortable. The objective of this paper is fmding how and with what electrode to detect ECG to
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