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ORIGINAL ARTICLE
A novel autonomic activation measurement method for stressmonitoring: non-contact measurement of heart rate variabilityusing a compact microwave radar
Satoshi Suzuki Æ Takemi Matsui Æ Hayato Imuta ÆMaki Uenoyama Æ Hirofumi Yura Æ Masayuki Ishihara ÆMitsuyuki Kawakami
Received: 9 August 2007 / Accepted: 28 November 2007 / Published online: 9 January 2008
� International Federation for Medical and Biological Engineering 2007
Abstract We developed a novel method for non-contact
monitoring of stress-induced autonomic activation through
the back of a chair, using a compact 24 GHz microwave
radar (8 9 5 9 3 cm), without large-scale equipment and
placing a heavy burden on the monitored individual.
Following a silent period of 120 s, audio stimuli using a
composite tone of 2,120 and 2,130 Hz sine-waves at
95 dB were conducted for 120 s. From dorsal, LF/HF of
HRV reflecting sympatho-vagal balance was determined
by microwave radar with the maximum entropy method
using eight volunteers (mean age 23 ± 1 years). Mean
LF/HF measured by non-contact and contact (using electro-
cardiography for reference) methods during audio stimuli
increased 34 and 37%, respectively, as compared with
those of the silent period. Maximum cross-correlations
between contact and non-contact measurements aver-
aged 0.73 ± 0.10. Our method appears to be promising
for future monitoring of stress-induced autonomic
activation of operators and may reduce stress-induced
accidents.
Keywords Non-contact � Microwave radar �Heart-rate variability � Audio stimuli �Autonomic activation � Safety precaution
1 Introduction
Using a ceiling attached microwave antenna, we have
proposed a system to monitor the respiratory rates of
subjects on a bed through a thick comfortable bed covering
the subject [15]. Min et al. [6] has reported a non-contact
method to capture the respiratory motion of a subject by the
Doppler ultrasound.
To monitor the autonomic activation induced by mental
stress, without placing any burden on the monitored indi-
vidual, we developed a non-contact autonomic monitoring
method using a 24-GHz compact microwave radar. We
have previously reported non-contact methods to monitor
heart and respiratory rates in experimental animals exposed
to toxic materials or under a hypovolemic state to determine
pathophysiological condition of the subject, such as expo-
sure to toxins or shock induced by hemorrhage [2, 4, 5].
Single photon emission tomography (SPECT) with radio-
isotope (99mTc-FBPBAT) is good for mapping the
autonomic nervous system, but is impractical for autonomic
activation monitoring due to the need for large-scale
equipment [10]. Using continuous electrocardiography
(ECG) with conventional electrodes, rhythmic components
of heart-rate variability (HRV) can be assessed using power
spectral analysis and modifications in autonomic activities
induced by mental stress have been reported in HRV power
spectra [12, 14]. However, long-term electrocardiographic
S. Suzuki � T. Matsui (&) � H. Imuta � M. Uenoyama �H. Yura � M. Kawakami
Department of Management Systems Engineering,
Tokyo Metropolitan University, 6-6 Asahigaoka,
Hino, Tokyo 191-0065, Japan
e-mail: tmatsui@cc.tmit.ac.jp
H. Yura
NeTech, Inc., Sakado 3-2-1, Kanagawa 213-0012, Japan
e-mail: info@netech.jp
M. Ishihara
Division of Biomedical Engineering, National Defense Medical
College, Research Institute, Namiki 3-2, Tokorozawa,
Saitama 359-8513, Japan
123
Med Biol Eng Comput (2008) 46:709–714
DOI 10.1007/s11517-007-0298-3
monitoring using electrodes places a heavy burden on
monitored individuals.
To determine human stress while driving or operating
equipment, we monitored human autonomic activation
induced by stressful sound using non-contact measurement
of HRV with a 24-GHz compact microwave radar, which
can easily be attached to the rear surface of back of a chair
without using either radioisotope or electrodes.
2 Non-contact autonomic activation measurement
system
We designed a non-contact autonomic activation mea-
surement system for non-contact measurement of HRV.
The system consists of a prototype compact microwave
radar (TAU GIKEN Co., Yokohama, Japan), control unit
for the microwave radar, an A/D converter ADA 16-32/
2(B)F (CONTEC Co., Tokyo, Japan) and a personal
computer. The compact microwave radar with an output
power of 10 mW incorporates an oscillator unit and
microwave antenna unit (a quadrupole plane antenna,
1.1 9 1.2 cm) in a small rectangular box (8 9 5 9 3 cm)
and generates a stable microwave signal at 24 GHz with an
output power of 10 mW. The antenna gain is 10dBi and the
diffusion angle is approximately 40�. Microwave (24 GHz)
was adopted to achieve high spatial resolution in order to
monitor the small body surface movements induced by
heartbeats. In our previous studies, 1.215-GHz radar was
used in cardiac and respiratory monitoring; we adopted the
24-GHz radar in order to achieve higher space resolution
required to HRV determination. Microwave radar output
was transferred to the personal computer through the
control unit by the A/D converter (Fig. 1). Without con-
ducting ECG recording by use of conventional electrodes,
the non-contact autonomic activation measurement system
is designed in order to assess the rhythmic components of
HRV, the heartbeat intervals were derived from the peak
intervals of the compact-size microwave radar output sig-
nal using a general-purpose analysis software Bimutus II
(KISSEI COMTEC Co., Nagano, Japan). Power spectra of
the time series of heartbeat intervals were calculated using
a maximum entropy method (MEM) with MemCalc soft-
ware (GMS Co., Tokyo, Japan). MEM offers a higher
spectrum resolution and shorter sampling duration than
those of fast Fourier transform (FFT). The powers of low-
frequency components of HRV (LF 0.04–0.15 Hz) have
been shown to estimate mainly sympathetic activities and
the powers of high-frequency components of HRV (HF
0.15–0.4 Hz) reflect parasympathetic activities. LF/HF can
thus be used as a parameter indicating sympatho-vagal
balance [8].
3 Testing of the non-contact autonomic activation
measurement system under audio stimuli
The system was tested using eight healthy male volunteers
with a mean age of 23 ± 1 years (range 22–25 years).
Subjects sat on a chair with headphones on and the com-
pact microwave radar was attached to the rear surface of
the back of the chair with a 30-mm spacer and about
60 mm left of the spine at around the level of the fourth
intercostal space, where cardiac induced skin surface
motion is larger than that of V5 position of precordial ECG
in our pilot study using laser distance meter (unpublished).
Subjects wore a 1-mm thick cotton T-shirt and the mesh
chair back is made of 2 mm thick polyester plastic. The
length from compact microwave radar to chair back was
30 mm (Fig. 2), in pilot study, the compact microwave
Controller
Bio. Amp.
Contact Measurement System
compact microwave radar
Electrode
Non-contactautomatic activationmeasurement system
PC
A/DConverter
Headphone
SubjectRR
IntervalsMEM LF, HF,
LF/HF
Non-Stimuli Stressful
AudioStimuli
compact microwave radar
e
t
Fig. 1 Schematic diagram of
apparatus for non-contact
monitoring of autonomic
activation
710 Med Biol Eng Comput (2008) 46:709–714
123
radar functioned well up to 50 mm. We did not give sub-
jects any instructions on breathing, such as, holding the
breath. We asked subjects to sit still leaning their head back
against the back of a chair.
Following a silent period of 120 s, audio stimulus
comprising a composite tone of 2,120 and 2,130 Hz sine-
waves at 95 dB was conducted using the headphone for
120 s. The tone range used in clinical nystagmus test
induced by audio stimuli (from 250 to 3,000 Hz at 95 dB)
was adopted for safety precautions [7]. Around 2,100 Hz is
within the most sensitive frequency domain of human
audible field. The beat tone of complex sounds causes
discomfort and alpha brain wave reduction to humans [1].
Power spectra of heartbeat intervals, as LF (0.04–
0.15 Hz), HF (0.15–0.4 Hz) and LF/HF, were calculated
using MEM with the MemCalc software. As a reference,
pectoral ECG was monitored along the non-contact moni-
toring, the output signals from both of the contact and non-
contact system were sampled through A/D converter with
the same sampling rate of 100 Hz. In real time, the
microwave radar output signal was displayed on the liquid
crystal display of the controller of a microwave radar, it
was also shown on the graphic terminal of a personal
computer. The power spectra of HRV (i.e., LF, HF and
LF/HF) for RR intervals derived by ECG were also cal-
culated using MEM. Cross-correlations of non-contact-
derived LF, HF and LF/HF with contact-derived LF, HF
and LF/HF were examined using statistic add-in software
for Excel (SSRI Co., Tokyo, Japan).
Quantitative data are expressed as mean ± SD. Statis-
tical analysis was performed using statistic add-in software
for Excel. Sample size was determined to achieve sufficient
assurance for paired t test for relatively uniform subjects.
All study protocols were reviewed and approved (seven
votes in favor versus none against) by the institutional
committee on human studies (Faculty of System Design,
Tokyo Metropolitan University, Tokyo, Japan). Informed
consent was obtained from all subjects.
4 Results
When subjects sat on the chair, the compact microwave
radar output through the control unit showed a cyclic
oscillation, corresponding to ECG (Fig. 3). In both non-
contact (compact microwave radar) and contact (ECG as
reference) measurements, the HRV parameter, LF of a
subject, reflecting mainly sympathetic activation, showed a
peak during cessation of audio stimuli (Fig. 4a). Mean LF
of eight subjects measured by non-contact and contact
methods during audio stimuli increased by 62 and 65%,
Body
Front Back
Clothes(Cotton)
D=30mm
Transmission
Compactmicrowaveradar
Backrest of the chair(Nylon mesh)
SpatiumIntercostale IV
Body
Front Back r
Fig. 2 Setting position of a compact 24-GHz microwave radar to
measure cardiac activity from the dorsal of subjects
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6
0.001.00
2.003.00
4.005.00
6.00
0 1 2 3 4 5 6
Time (sec)
Time (sec)
Rad
ar: N
on-C
onta
ct (
V)
EC
G: C
onta
ct (
V)
(a)
(b)
Fig. 3 A compact microwave
radar output (lower) showing a
cyclic oscillation that
corresponds to the cardiac
oscillation measured by ECG
(upper)
Med Biol Eng Comput (2008) 46:709–714 711
123
respectively, compared with those of the silent period
before audio stimuli [Fig. 4b; non-contact measurement
995 ± 775 m s2 (silent period), 1,617 ± 1,036 m s2 (dur-
ing audio stimuli), p \ 0.0001; contact measurement
781 ± 904 m s2 (silent period), 1,287 ± 1,359 m s2 (dur-
ing audio stimuli), p \ 0.0001]. Cross-correlation of LF
between non-contact and contact measurements in a same
subject showed a maximum value of 0.89 (Fig. 4c) and
maximum cross-correlation values in LF averaged
0.73 ± 0.14 in the eight subjects.
HF of a same subject, reflecting parasympathetic acti-
vity did not show any distinctive change during audio
stimuli (Fig. 5a) and mean HF of eight subjects increased a
very little during audio stimuli in both non-contact and
contact measurements [Fig. 5b; non-contact measurement
1,249 ± 626 m s2 (silent period), 1,424 ± 474 m s2 (dur-
ing audio stimuli), p \ 0.0001; contact measurement:
854 ± 595 m s2 (silent period), 945 ± 801 m s2 (during
audio stimuli), p \ 0.01]. Cross-correlation of HF between
non-contact and contact measurements in a same subject
showed a maximum value of 0.39 (Fig. 5c) and maximum
cross-correlation values of HF averaged 0.64 ± 0.15 in
eight subjects.
LF/HF of a same subject, reflecting sympatho-vagal
balance, exhibited a peak during audio stimuli (Fig. 6a).
Mean LF/HF of eight subjects measured by non-contact
and contact methods during audio stimuli increased by 34
and 37%, respectively, as compared with the silent period
before audio stimuli [Fig. 6b; non-contact measurement
0.86 ± 0.50 (silent period), 1.16 ± 0.59 (during audio
stimuli), p \ 0.0001; contact measurement 1.23 ± 1.18
(silent period), 1.68 ± 1.72 (during audio stimuli),
p \ 0.0001]. Cross-correlation of LF/HF between contact
and non-contact measurements of a same subject showed a
maximum value of 0.81 (Fig. 6c) and maximum cross-
correlation values of LF/HF averaged 0.73 ± 0.10 in the
eight subjects. Without using radioisotope or electrodes,
stress-induced autonomic activation was monitored.
P<0.0001P<0.0001
0ECG Radar
Audio Stimuli
-1
-0.5
0
0.
1
-150 -100 -50 0 150
Non Stimuli Stressful Audio Stimuli
00 60 120 180 240 300 360
1000
2000
3000
4000
5000
6000
LF
(mse
c2 )
LF
(mse
c2 )
500
1000
1500
2000
2500
3000
Contact
Non-Contact
-1
-5 50 100
Time lag (sec)
Time (sec)
Cro
ss-C
orre
lati
on5
(a) (b)
(c)
Fig. 4 a In both non-contact
and contact (ECG)
measurement, LF of a subject
(reflecting sympathetic
activation) shows a peak during
audio stimuli. b Mean LF of
eight subjects measured by non-
contact and contact methods
during audio stimuli increased
62 and 65%, respectively,
compared with the silent period
before audio stimuli. c Cross-
correlation of LF between non-
contact and contact
measurements of the same
subject
-1
-0.5
0
0.5
1
-150 -100 -50 0 50 100 150- -5
0
1000
2000
3000
4000
5000
6000
HF
(mse
c2 )
0 60 120 180 240 300 360
Time (sec)
Audio Stimuli Contact
Non-Contact
0
HF
(mse
c2 )
500
1000
1500
2000
2500
3000
ECG Radar
Non Stimuli Stressful Audio Stimuli
P<0.001P<0.01
Time lag (sec)
Cro
ss-C
orre
lati
on
(a) (b)
(c)
Fig. 5 a In both non-contact
and contact (ECG)
measurement, HF of the same
subject (reflecting
parasympathetic activity)
changes a very little with the
start of audio stimuli. b Mean
HF of eight subjects increases a
very little during audio stimuli
in both non-contact and contact
measurement, compared with
the silent period before audio
stimuli. c Cross-correlation of
HF between non-contact and
contact measurements of the
same subject
712 Med Biol Eng Comput (2008) 46:709–714
123
5 Discussion
We monitored human autonomic activation induced by
stressful sounds through non-contact measurement of the
heart rate variability using a 24-GHz compact microwave
radar. The radar can be easily attached to the rear surface of
back of a chair. The mean LF, which mainly reflects sym-
pathetic activity during audio stimuli is significantly higher
than the mean LF of the silent period before audio stimuli.
The HF indicating parasympathetic activity changed a very
little during the audio stimuli. This can be attributed to the
activation of the sympathetic nerve system induced by
stressful sounds. Without using radio isotope or electrodes,
our system monitors the sympathetic activation through the
back of a chair. Mental stress affects the autonomic nervous
system [12, 14]. Moreover, there is a relationship between
mental stress and traffic accidents [13]. By issuing auto-
nomic activation early warnings, it may be possible to
prevent traffic accidents and industrial accident.
The long-term monitoring of LF/HF in HRV can be used
as a diagnostic test for sepsis [3]. Moreover, it has been
reported that a reduction of HRV is useful in identifying
septic patients at a risk of the development of multi organ
dysfunction syndrome (MODS) [9]. Our method of non-
contact monitoring for HRV can thus be used not only for
monitoring autonomic activation, but also as a future
diagnostic method for sepsis or as a predictor of MODS,
without touching the patient. Zheng et al. [16] has proposed
a wearable health care system for long-term continuous
vital sign monitoring of high-risk cardiovascular patient.
Our system does not require a special wear and may be
suitable for screening of high-risk patients in health check.
The electromagnetic wave damage has been discussed,
especially in the case of human application. The power
density spectrum (PDS) of our system is in the allowable
range of 0.50 mW/cm2. In frequency range over 3 GHz,
the PDS limit is 1 mW/cm2 according to the guideline for
radio waves by Telecommunication Bureau of the Ministry
of Internal Affairs and Communication in Japan.
Our method appears to be promising for future non-
contact autonomic activation monitoring of people who are
operating equipment or elderly personals with cardiac
disorders, sympathetic activation sometimes triggers fatal
cardiac events [11]. The method allows autonomic acti-
vation monitoring, without placing any burden on the
monitored individuals.
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LF/HF between non-contact and
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