PORTABLE DEVICE TO MONITOR AUTONOMIC NERVOUS

SYSTEM ACTIVITY THROUGH CLOTHES

Kang Moo Lee, Seung Min Lee, Gih Sung Chung, Hong Ji Lee

Interdisciplinary Program of Bioengineering, Seoul National University, Seoul, Republic of Korea

Yong Gyu Lim

Department of Oriental Biomedical Engineering, Sangji University, Wonjusi, Gangwondo, Republic of Korea

Kwang Suk Park

Department of Biomedical Engineering, College of Medicine, Seoul National University, Seoul, Republic of Korea

Keywords: Capacitive electrode, Autonomic nervous system activity, Portable device.

Abstract: Heart rate variability (HRV) reflects the autonomic nervous system (ANS) activity and HRV parameters are

extracted from electrocardiogram (ECG) signal. However, the conventional ECG electrodes such as

Ag/AgCl electrode make a person uncomfortable due to electrolytic paste or conductive adhesive used for

ensuring good contact between electrode and skin. In this study, the portable device with capacitive

electrodes was designed to monitor ANS activity through clothes. By using capacitive electrodes, ECG can

be measured without direct skin-contact through capacitive coupling between the body and the electrodes.

To evaluate the possibility that the ANS activity can be monitored using the capacitive electrode system,

HRV parameters extracted from the capacitive electrodes were compared with HRV parameters extracted

from Ag/AgCl electrodes. Results showed that there was no significant difference between the HRV

parameters of two different electrodes.

1 INTRODUCTION

It has become clear that autonomic nervous system

(ANS) activity is highly related with pathogenesis of

diseases such as chronic heart failure, ventricular

arrhythmias and sudden cardiac death (Packer M et

al., 1996, PJ Podrid et al., 1990, HV Barron et al.,

1996). Therefore, continuous and long-term ANS

monitoring is important for early detection and

treatment of these diseases. Heart rate variability

(HRV) has been used to monitor the ANS activity

because it reflects the interplay between the

sympathetic and parasympathetic function of the

ANS (U. Rajendra Acharya et al., 2006, Conny M.

A. van Ravenswaaij-Arts et al., 1993). HRV is

usually measured from electrocardiogram (ECG)

recordings. However, ECG measurement using

conventional ECG electrodes such as Ag/AgCl

electrode is not adequate for long-term measurement

because of the conductive adhesive or electrolytic

paste. It makes a person feel uncomfortable and

causes skin irritation. To overcome these problems,

there have been previous studies on ECG recording

without direct skin-contact using a capacitive

method (Lim YG et al., 2006, Lee SM et al., 2010).

ECG can be measured through clothes by the

capacitive coupling between skin and electrode.

Since the impedance between skin and electrode is

increased according to the type and thickness of the

clothes, high input impedance of the electrode is

necessary. Thus, capacitive electrode has the

preamplifier, which has ultra high input impedance.

In this study, a portable and simple device that

permits measuring ECG using the capacitive

electrodes is proposed. We evaluated the possibility

that the ANS activity can be monitored using the

developed portable device by comparing time

domain parameters of the HRV obtained from the

capacitive electrodes with the parameters obtained

from conventional Ag/AgCl electrodes.

276

Lee K., Lee S., Chung G., Lee H., Lim Y. and Park K..

PORTABLE DEVICE TO MONITOR AUTONOMIC NERVOUS SYSTEM ACTIVITY THROUGH CLOTHES.

DOI: 10.5220/0003288502760279

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 276-279

ISBN: 978-989-8425-37-9

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

(a)

(b)

Figure 1: (a) Developed portable device with capacitive

electrodes. (b) A person wearing the developed portable

device.

2 MATERIALS AND METHODS

2.1 Capacitive Electrode

It consists of a pre-amplifier which has ultra high

input impedance, electrode face and plate for

shielding circuit. The ultra high input impedance is

required to obtain good signal quality because of the

impedance between skin and electrode. OPA124

(Texas Instrument Inc.) is used for pre-amplifier.

Also, there is a biasing resistance, 5GΩ, for the path

of bias current. The size of electrode is 12 cm

(4 cm

x 3 cm) and thickness is about 1 cm.

Figure 2: A block diagram of a module for data acquisition.

Figure 3: Experiment protocol to induce the change of

autonomic nervous system.

2.2 Measurement System

Figure 1 (a) shows the developed portable device. It

consists of two capacitive electrodes, conductive

fabric electrodes, and a module for signal processing

and acquisition. The device is fixated on the

wearer’s chest as shown in Figure 1 (b). Figure 2

shows a block diagram of the module. ECG was

filtered by band-pass filter with 0.5-35 Hz cutoff

frequency to remove a respiration and external noise.

The driven signal, which was generated by negative

amplification of averaged signal from two capacitive

electrodes, fed back to the body via conductive

fabric electrode to improve signal to noise ratio

(SNR). The signal was digitized at the sampling rate

of 450 Hz and was transmitted to a computer using

Bluetooth module (Parani-ESD200, SENA).

2.3 Experiment

A 25-year-old healthy male subject wearing the

developed belt on a normal cotton cloth of 680 um

thickness performed the experiment under the seven

steps: 1) rest, 2) playing Tetris game, 3) rest, 4)

mathematical calculations, 5) rest, 6) listening to

classic music, 7) rest as shown in Figure 3. ECG

using the capacitive electrode system was measured

simultaneously with ECG using Ag/AgCl electrodes

for 5 minutes in each state. The Pan and Tompkins

peak detection algorithm was used to detect R-peaks

of ECG (Pan J and Tompkins WJ, 1985), which

were used to calculate RR-intervals. We extracted

the following time domain parameters of HRV to

validate the device: the standard deviation of RR-

intervals (SDRR), the root-mean-square of

successive differences of the RR-intervals (RMSSD),

and the percentage of the successive RR-intervals

differing higher than 50msec (pRR50).

PORTABLE DEVICE TO MONITOR AUTONOMIC NERVOUS SYSTEM ACTIVITY THROUGH CLOTHES

277

Table 1: Time domain parameters of HRV.

Parameters

Electrode

type

Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7

Averaged RR-interval (ms) Capacitive 768.07 753.97 751.75 733.16 771.94 826.37 733.62

Ag/Agcl 768.07 753.97 751.75 733.16 771.93 826.37 733.63

SDRR (ms) Capacitive 57.07 41.91 47.48 49.35 69.17 71.29 58.49

Ag/Agcl 57.08 41.73 47.64 49.36 69.07 71.69 58.61

RMSSD (ms) Capacitive 65.23 56.18 54.39 63.96 86.5 84.59 68.65

Ag/Agcl 65.15 55.75 54.71 63.99 86.24 85.31 68.81

pRR50 (%) Capacitive 50.9 44.08 37.19 52.33 60.39 60.77 55.04

Ag/Agcl 51.67 43.83 38.44 52.83 60.11 60.22 55.28

Heart rate (beats/min) Capacitive 78.12 79.58 79.81 81.84 77.73 72.61 81.79

Ag/Agcl 78.12 79.58 79.81 81.84 77.73 72.61 81.79

SDRR, standard deviation of RR-intervals; RMSSD, root-mean-square of successive differences of RR-intervals;

pRR50, percentage of the successive RR-intervals differing higher than 50msec

3 RESULTS AND DISCUSSION

Figure 4 illustrates the simultaneous ECG recordings

from Ag/AgCl electrodes and capacitive electrodes.

It shows that R-peaks of ECG waveform from the

developed device using the capacitive electrodes are

synchronized to the waveform obtained through the

Ag/AgCl electrodes. Therefore, the ECG obtained

by our device without direct skin-contact is a

reliable waveform for detecting R-peaks and

deriving RR-intervals.

The time domain parameters of HRV obtained

from the developed device with capacitive electrode

had no significant difference with the HRV time

parameters obtained from the Ag/AgCl electrode as

shown in Table 1. Thus, our portable device can be

used to monitor ANS activity through clothes.

Figure 5 shows the changes in HRV parameters at

each step. The RMSSD and pRR50 were high when

the subject was listening to classic music. This

means a shift of the autonomic balance towards a

more parasympathetic dominance. Since our system

can monitor ANS activity without any conductive

adhesive or electrolytic paste, it could be used for

long-term ANS monitoring.

There are several problems that we have to solve

on further study. Firstly, ECG can be deteriorated by

the motion artifacts when the person has large

movements. It is required to reduce the effect of

motion to use this portable device in daily life.

Secondly, the capacitive electrodes have to be

fixated tightly on the chest by the belt. Thus, the

person wearing the device can feel uncomfortable

because of the effect of fastening force of the belt.

Figure 4: Simultaneous ECG recordings from Ag/AgCl

electrodes and capacitive electrodes.

BIODEVICES 2011 - International Conference on Biomedical Electronics and Devices

278

Figure 5: Changes in time domain parameters of HRV at

each step.

Thirdly, the signal quality from the device can be

changed by the thickness of the inserted cloth.

Figure 6 shows the ECG recordings obtained

through our device from the subject wearing a cotton

cloth with different thicknesses of 680 (a), 1370 (b),

2750 (c), and 5560 (d) um. The common mode noise

was increased according to the increase of the

thickness of the cloth. It could be difficult to detect

correct R-peaks from ECG recordings with a cloth

thickness of 5560 um. Thus, HRV parameters for

ANS monitoring could be derived wrongly. Our

developed portable device can be more practical by

solving these problems.

Figure 6: ECG waveforms from the subject wearing a

cotton cloth with different cloth thicknesses.

ACKNOWLEDGEMENTS

This work was supported by the MKE(The Ministry

of Knowledge Economy), under the

ITRC(Information Technology Research Center)

support program supervised by the NIPA(National

IT Industry Promotion Agency) (NIPA-2010-

(C1090-1021-0003 )), in part by MCIE through

the Core Technology Development Program and in

part by Seoul R&BD Program (10606M0209725

and JP090968M0209721), Republic of Korea.

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