Proceedings of the 5th International Workshop on Wearable and Implantable Body Sensor Networks, in conjunction with The 5th International Summer School and Symposium on Medical Devices and Biosensors The Chinese University of Hong Kong, HKSAR, China. Jun 1-3, 2008
Influence of contact pressure and moisture on the signal quality of a newly developed textile ECG sensor shirt Saim Kim, Steffen Leonhardt, Nadine Zimmermann, Philip Kranen, David Kensche, Emmanuel Müller, Christoph Quix
Abstract—A newly developed textile integrated sensor shirt, called “ITcares” (Intelligent Textile for CArdio REspiratory Sensing), is presented. Textile integrated ECG sensors are known to be highly depended on the electrode-skin-impedance. Two main influence factors on the skin-electrode impedance are: 1. contact pressure and 2. moisture. Systematic measurements were performed with additional sensors to evaluate the ECG signal quality. Furthermore, signal-to-noise ratios were calculated as a quantitative measure.
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textile-derived ECG [8]-[10]. Thus, several quantitative measurements (see e.g. [8], [11]) have been performed. This paper presents a multi sensor based evaluation of the ECG signal quality derived from the ITcares shirt. Two influence factors that mainly determine the electrode-skin impedance according to [8], [12], [13] were investigated: contact pressure and surface moisture.
I. INTRODUCTION
HE UMIC research cluster was founded to develop a next generation high-speed mobile communication network [1]. As part of this cluster, suitable application scenarios are explored and developed. Medical health care is one of the key scenarios. The HealthNet Vision scenario [2], involves several multi-disciplinary research institutes from textile technology, computer science to medical engineering. The HealthNet Vision describes a near-future patientoriented network that connects the patients with medical professionals as shown in Fig. 1. This kind of homecare scenario is necessary to keep overall health care costs affordable and is therefore actively investigated by research groups around the world [3]-[5]. Key feature is a wireless sensor network that obtains the patients vital parameters (e.g. ECG, respiration rate, and temperature) via an energy-aware, event-driven network [6]. Additionally, peer-to-peer technology is used to ubiquitously distribute the information in a timely fashion to any authorized medical professional independent from distance and location [2],[7]. It is widely known that several factors like micro movements, electrode-skin impedance and electrode positioning have great influence on the signal quality of a Manuscript received February 25, 2008. This research is funded by the cluster of excellence on Ultra High-Speed Mobile Information and Communication (UMIC) of the DFG (German Research Foundation grant EXC 89). Saim Kim and Steffen Leonhardt are with the Philips Chair of Medical Information Technology (MedIT), RWTH Aachen University, D52074 Aachen, Germany (e-mail:
[email protected]). Nadine Zimmermann is with the Institute of Textile Technology (ITA), RWTH Aachen University, D-52062 Aachen, Germany (e-mail:
[email protected]) Philip Kranen and Emmanuel Müller are with the Chair of Computer Science 9 (I9), RWTH Aachen University, D-52056 Aachen, Germany (email:
[email protected]) David Kensche and Christoph Quix are with the Chair of Computer Science 5 (I5), RWTH Aachen University, D-52056 Aachen, Germany (e-mail:
[email protected])
978-1-4244-2253-1/08/$25 ©2008 IEEE
Fig. 1. System setup of the UMIC HealthNet Vision. The patient with the wireless sensor network is shown on the left side and the medical expert on the right side. Both are connected directly and indirectly via the feedback device and the background services.
II. MEASUREMENT SETUP A. Hardware The measurement system configuration is shown in Fig. 2. It is based on an MSP430 microcontroller with built-in 12 bit analog-to-digital converter (Texas Instruments, USA) for the data acquisition. An analog three-axis accelerometer (MMA7260QT Freescale, USA) was attached to the Sternum and used to measure movements of the thorax to detect body posture and general activity of the patient. The ECG was recorded with a newly developed T-shirt, which integrates textile-based electrodes and conductors [14], [15]. Similar systems have been proposed by [11], [16], [17].
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Fig. 2. Measurement setup showing the sensors and the measurement PC.
A silver-coated polyamide yarn was knitted with spandex to make it stretchable. The positions of the electrodes were chosen according to the L1 lead of Einthoven’s triangle. This configuration was selected because it is widely used in clinical research and also quite immune to artifacts due to small movements of the torso [8]. Two 5 cm x 5 cm patches were applied to the inside of the T-shirt as the bipolar ECG electrodes. A larger third patch with a size of 8 cm was used for the driven right leg circuit. It was positioned on the lower right side of the T-shirt. The positions of all electrodes are marked in Fig. 3. Besides the ECG electrodes the same material was also used to manufacture the textile conductors. One end was stitched with a conductive yarn to the electrode and the other end had a metal push button for connection to the measurement electronics. The textile conductors were applied to the outside of the T-shirt, see Fig. 3.
Fig. 4. The amplification and filtering chain of the ECG circuit. The total gain of the system is set to 1000. The signal path has a bandpass characteristic with a cut-off frequency of 0.16 Hz and 100 Hz. A notch filter at 50 Hz is used to reduce power line noise.
A differential pressure transducer MPX5050DP (Freescale, USA), in combination with a neonatal blood pressure cuff (SoftCheck®, USA) as a pressure pad, was used to measure the contact pressure of the textile electrode. The pad was placed directly on top of one of the ECG electrodes and held in place with a wide rubber band. The length of the rubber band was length adjustable. Thus, the contact pressure could be varied. All sensors were sampled at 250 Hz with the internal 12 bit ADC. B. Software The data was sent from the data acquisition system over a serial connection and processed on a PC using Labview (National Instruments, USA). A program was developed to visualize and store the data to a Labview data file. Additionally, an annotation function was implemented to facilitate data processing and validation. Several push buttons allow the user to set the measurement variables which are used to automatically generate file names. The signal processing and evaluation was also performed with Labview integrated tools. C. Measurement procedure Most of the measurements were performed while sitting. The procedure can be divided into nine phases: 1. 2. 3. 4. 5. 6. 7. 8. 9.
Fig. 3. The ITcares shirt. The positions of the inner electrodes are marked by the dotted lines. The textile conductors are running on the outside.
A block diagram of the ECG amplification and filtering circuit is illustrated in Fig. 4. The input impedance of the op amp is 1 MΩ. Standard ECG cables were used to connect the electronics with the T-shirt.
Normal breathing Holding breath (inflated lungs) Normal breathing Holdings breath (deflated lungs) Normal breathing Standing up Normal breathing Sitting down Normal breathing
Additionally, the contact pressure of the textile electrodes was increased gradually between 0.3 kPa and 6 kPa in order to assess its influence on the signal quality of the ECG. The lower value represents the contact pressure that is created by elasticity of the T-shirt itself. The range of the values was chosen to compare the results to previous measurements of the electrode impedance as investigated in [13]. Secondly,
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the ECG electrodes and reference electrode were alternately moisturized resulting in four different dry/wet combinations. In contrast to previous evaluation, no hydro-gel was used to reduce the skin-electrode impedance [8], [12]. Instead, a water pump was used to spray fine water mist onto the electrodes. Thus, the electrodes were just slightly moist instead of being thoroughly wet –simulating light sweating of the user. Reference Measurements were performed with standard adhesive gel electrodes (GE Marquette, USA) and clip-on Ag/Ag-Cl electrodes (GVB-geliMED, Germany). The gel electrodes were attached to the same position as the textile electrodes. The clip-on electrodes were attached to the extremities. Before each measurement, the pressure transducer and accelerometer were zero-calibrated to reduce the influence of drift.
SNR =
ES EN
Reference
SNR in decibel
Gel electrodes Clip on Ag/AgCl
30.7 27.3
Signal strength (mV)
0,1 0,05 0 -0,05 -0,1 0
1
2 Time (s)
3
Fig. 5. Close up view of an ECG recorded with the ITcares shirt with a contact pressure of 6 kPa and moistened electrodes. The morphology of the ECG components is visible.
Increasing the contact pressure improved the signal quality even though the influence was less significant when the electrodes were moist as shown in Table II.
ECG (mV)
D. Signal-to-noise calculation The signal-to-noise ratio was estimated by calculating the signal band energy and out-of-band noise energy as described in [13]. The signal is defined as the energy in the signal band between fHP = 0.16 Hz, the cut-off frequency of the high pass, and fc = 30 Hz, which covers the standard - 3 dB frequency bandwidth for patient monitoring [19]. The rest of the spectrum is approximated as noise. In order to avoid spectral components of respiration, the SNR was calculated during inspirational apnea and expirational apnea according to equation (1)-(3).
TABLE I REFERENCE MEASUREMENTS WITH STANDARD ELECTRODES
(1)
0,15 0,1 0,05 0 -0,05 -0,1 -0,15
Breathing
0
2
4
6
Apnea
Breathing
8 10 12 14 16 18 20 22 24 26 28 30 Time (s)
fc
ES =
∫ S( f )
2
df
Contact pressure (kPa)
7 6,5
(2)
f HP
f∞
EN =
∫
6
5,5
2
S ( f ) df
0
(3) Acceleration (g)
fc
III. MEASUREMENT RESULTS The configuration with moist electrodes had a comparable SNR to the reference electrodes (see Table I). An ECG curve is shown in a close up view in Fig. 5. The morphology of the ECG components is visible. Another ECG curve measured with a contact pressure of 6 kPa and moist electrodes is shown in Fig. 6. A slight baseline drift which is caused by respiratory movements is visible from 0-10 s and again from 20-31 s. During the expiration apnea between 10-20 s is no baseline drift
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Time (s)
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Time (s)
0,04 0,03 0,02 0,01 0 0
Fig. 6. ECG with simultaneously recorded pressure and accelerometer curve. A contact pressure of 6 kPa was applied and the electrodes moistened.
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The derivation of the respiration rate from the acceleration data is considered as a next step.
TABLE II CONTACT PRESSURE AND MOISTURE DEPENDENCY OF SNR IN DECIBEL
Condition Electr. Ref.
Contact pressure in kPa 0,2 1 2 4 6
dry dry 6,8 dry moist 23,1 moist dry 20,4 moist moist 25,4
6,8 19,1 21,2 28,6
23,5 20,2 25,2 28,7
19,5 25,6 28,2 25,7
REFERENCES [1] [2]
18,5 21,3 26,1 25,4
[3] [4]
As expected, the “dry electrode with dry reference” configuration showed the worse signal quality corruption. On visual inspection, the ECG signal was strongly covered by noise. Additionally, the signal was superposed by motion artifacts caused by the respiration movement. This could be seen by comparing the ECG with the simultaneously recorded pressure curve in Fig. 7. Breathing
ECG (mV)
0,1
Apnea
[5] [6] [7]
Breathing
[8]
0,05 0 -0,05
[9]
-0,1 0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Time (s)
Acceleration (g)
[10] 0,04 0,03 0,02
[11]
0,01 0 0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 Time (s)
[12]
Fig. 7. ECG signal with simultaneously recorded accelerometer data. The ECG signal is dominated by noise and respiratory motion artifacts. No additional contact pressure was applied and the electrodes were dry, thus the ECG signal amplitude is smaller compared to Fig. 6, due to the higher skin-electrode impedance.
[13]
IV. DISCUSSION
[14]
The increasing SNR at higher contact pressures can be explained by the decreasing skin to electrode impedance as shown in [13]. The use of water spray improved the SNR similar to the use of hydro-gel without the risk of skin irritation. Yet, the measurements showed that the SNR of one single measurement can differ considerably even though the conditions, i.e. contact pressure and moisture, were kept constant.
[15]
[16] [17]
[18]
V. CONCLUSION AND OUTLOOK It was shown that the textile measurement setup is able to record ECG, contact pressure and acceleration in parallel. A first quantitative measurement of the SNR of the ECG signal in dependency of the contact pressure and moisture were presented. Still, more measurements have to be done for statistical evaluation.
[19]
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