Download A Low-Cost Long-Life R-R Interval Telemeter with Automatic Gain

T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012)
1
A Low-Cost Long-Life R-R Interval Telemeter
with Automatic Gain Control
for Various ECG Amplitudes
Toshitaka Yamakawa1, 2, *, Genki Matsumoto3, and Toru Aoki4
1
Faculty of Engineering, Shizuoka University
Fuzzy Logic Systems Institute
3
Department of Bioengineering, University of Toledo
4
Research Institute of Electronics, Shizuoka University
2
Abstract — A low-cost R-R interval telemeter with automatic
gain control is the innovative ECG measurement system for both
clinical care and homecare. Highly-reliable R-R interval
detection was achieved by the automatic gain control for
subjects at different ages. The gain is automatically adjusted to
extract R waves at the desired voltage amplitude. The high
accuracy detection with automatic gain control was proved by
the estimation of error occurrence at each subject. The errors of
R-R interval detection did occur at less than 2% probabilities.
The accuracy detection of long-term measurement is also a
crucial issue for the appropriate diagnosis. The results showed
0.752% probabilities of reoccurring errors when 48 hours
measurement was executed. Therefore, a low cost R-R interval
telemeter with automatic gain control is the handheld device
with the high accuracy detection of R waves and the high
reliability of measurement at different ages.
Keywords — Electrocardiography, R-R interval, biomedical
telemetry, automatic gain control
I. INTRODUCTION
Low cost means of vital signs monitoring are demanded
according to that healthcare costs are rapidly increasing. As a
mean for early detection of diseases and of acute symptoms,
long-term monitoring of electrocardiogram delivers
beneficial evidences in both clinical care and homecare. The
analysis of R-R intervals on long-term monitoring gives
diagnostic evidences of arrhythmia [1], chronic obstructive
pulmonary disease [2], Parkinson’s disease [3], and etc.
The development of low cost RR interval telemeter has
been aimed at the simple uses, high mobility, and
high-accuracy detection. This telemeter is regarded as the
consumer friendly device of what can be commercially
available at lower cost and used by any subjects at different
ages. Telemetry system is the supportive monitoring device
since the collected data are able to be transferred as the files
with lower size into hospital’s monitoring systems to obtain
an immediate diagnosis.
The measurement of R-R interval at different sex and ages
can be achieved by the automatic gain control system that is
*
Manuscript received September 1, 2011.
Corresponding author: ([email protected])
capable of adjusting the gain automatically for different
subjects to detect R wave. This innovative program plays an
important role on the high accuracy detection and the quick
adjustment of gain. ECG measurement performed at home is
an ideal financial aid for patients and even for healthy people
as a mean of health promotion and early detection of disease.
As the device used by patients’ hand at home, the important
matter to be considered is lower cost, easier usage, and longer
life with modest measurement performance. In order to
confirm that the proposed system satisfies the above needs,
the experiment was directed for the analysis of the function of
automatic gain control and the accuracy of long term
measurement.
II. SYSTEM DESCRIPTION
The proposed telemetry system consists of two devices: the
R-wave telemeter and the receiver. The R-waves telemeter
measures the electrocardiogram (ECG), detects the R-waves
from the ECG, and wirelessly transmits R-wave occurrences
with 315MHz amplitude modulation. The receiver receives
and demodulates the wireless signal from the telemeter,
calculates the R-R interval, and stores into a PC through the
USB connection. The details of the devices are explained in
following subsections.
A. R-waves telemeter
Circuit Description
Fig. 1(a) and (b) show the block diagram of the proposed
R-wave telemeter and the circuit diagram of the analog
frontend, respectively.
The ECG measured by three electrodes (+, -, GND) is
amplified and conditioned in the analog frontend. In order to
achieve the single power supply of a 3 V battery CR2032, the
common-mode feedback (CMFB) structure is adopted for the
differential signal amplification as shown in the left dashed
rectangle in Fig. 1(b). By using this structure, the output
common-mode voltage of the instrumentation amplifier is set
to Vref what is the reference voltage made by a resistor chain.
The 1st-order variable-gain low-pass filter was designed to
reduce the high-frequency noise (>38Hz). The gain is defined
by the resistor ratio R2/RC where RC is the resistor value of the
variable resistor. Here, RC is controlled by the automatic gain
T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012)
control program which is embedded in the microcontroller, as
described later. Though this LPF reduces the amplitude of the
R-waves, the sufficient amplitude is obtained since it is
adequately amplified due to the automatic gain control. The
output common-mode voltage of the analog frontend is set to
Vref, since the positive nodes of OPAs are connected to Vref.
Here, Vref of 2.49 V was obtained by a resistor chain. The
power dissipation of the analog frontend was kept under 1.2
mW by adopting the following low power ICs: INA122 for
the instrumentation amplifier, MCP6042 for OPAs, and
MCP4011 for the variable resistor. R1 of 1MΩ, R2 of 1MΩ,
C1 of 1µF, and C2 of 4.7nF were used in this prototype
fabrication.
The output of the analog frontend was connected to an
input node of a MSP430F2011 microcontroller as shown in
Fig. 1(a). This input node is observed by the built-in
comparator, and the comparator outputs the positive pulse
during the input signal is lower than 1.5 V. In other words, the
amplified ECG signal whose amplitude is larger than 0.99 V
(=Vref-1.5) is extracted as an R-wave candidate. The R-wave
candidates are treated by the embedded program as described
in following subsections.
The transmitter module modulates the detected R-wave
signal with 315 MHz On-Off-Keying (OOK) modulation, and
transmits it through an antenna wire within the output power
of 10 mW.
Digital Filter and the R-Wave Detection Program Mounted
on the Microcontroller
A quasi-band-pass filtering program and the R-wave
detection program were installed in the microcontroller as
shown in Fig. 1(a) by “BPF” and “R or not?” blocks.
The BPF program counts the pulse width of the R-wave
-candidate signal which is obtained by the comparator, and
forwards the signal only where the pulse width is in the range
from 3 ms to 50 ms in order to compress the noises (e.g. HF
noise originated in the poor hysteresis of the comparator,
wider ECG pulses such as P or T waves).
The R-wave detection program counts the pulse interval
(assuming R-R interval) of the BPF output. Only where the
Automatic Gain Control Scheme
The amplitude of ECG can be reduced due to the electrodes
alignment, the condition between skin and the electrodes, and
the subjects’ age. In order to maintain reliable R-wave
detection even under the above situations, an automatic gain
control (AGC) function is installed. The AGC mode is
activated when the button mounted on the circuit board is
pushed for more than 2 seconds.
Fig. 2 shows an example of the proposed AGC flow scheme.
The gain of the LPF in the analog frontend is controlled by RC
value which is chosen from 32 steps between 0~50kΩ. In the
begging of the AGC mode, the RC value is set to 16*Rstep (the
half value of maximum RC) where Rstep=50kΩ/32. If the R
wave is detected with this gain, the RC value is changed to the
half between maximum RC and previous RC (e.g.
RC=24*Rstep) in order to reduce the gain. Then, if the R wave
is not detected with the new gain, the RC value is changed to
the half between the previous RC and the RC value of two
times before (e.g. RC=20*Rstep) in order to raise the gain.
After this process is repeated 6 times as shown in Fig. 2, the
final gain is decided. This AGC scheme finds out the
appropriate gain within 6 steps due to the limitation of
available RC value. Since the R-wave detection procedure and
the RC modification only take about 1.5 s and 1 ms,
[0]
[1]
[2]
[3]
[5]
[4]
[6]
50kΩ
(b)
Fig. 1. (a)The block diagram of the R-waves telemeter and (b)the circuit
diagram of the analog frontend.
1
2
1
2
32 steps
(a)
interval is in the range from 200 ms to 1500 ms, the R-wave
-candidate signal is forwarded to the following blocks as the
detected R wave.
The above limitation was settled by the basis of clinical
cardiology [4] and by the healthy various-aged volunteers’
ECG characteristics measured by the frontend circuitry since
this system was premised on the application for the
monitoring of healthy volunteers in this study. The values are
easily modified through the connector equipped on the circuit
board, in the case of the diseased subjects.
Though the time lag of the R-wave can be caused by the
signal processing in the microcontroller, it is less than few
tens of microseconds since the 16 MHz master clock of the
microcontroller is fast enough. In addition, the delay hardly
fluctuates in each R wave. Therefore the delay does not affect
the values of R-R interval practically.
1
2
1 step
1 step
R wave
: True
R wave
: False
R wave
: True
R wave
: True
R wave
: True
R wave
: False
Gain
: Down
Gain
: Up
Gain
: Down
Gain
: Down
Gain
: Down
Gain
: Up
0Ω
2
Fig. 2. An example flow of the proposed AGC scheme. The black cells and
the gray cells show the selected RC values and the previous RC values,
respectively.
T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012)
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respectively, the whole ACG procedure is finished within 10
seconds.
Energy Reduction Techniques
An energy reduction technique is applied on the
microcontroller as well as the low-power circuit design of the
analog frontend. The microcontroller operates under a
stand-by mode whose power consumption is 0.3 µW when the
output of the built-in comparator is low. Additionally, the
wireless transmitter which dissipates the largest power in the
telemeter is shut down when the R wave is NOT detected. The
measured power consumptions when the R wave is detected
and when the R wave is not detected are 13 mW and 62 µW,
respectively. This allows more than 620 hours of operation
time with a CR2032 coin-shaped battery.
The timing delay of several tens of microseconds occurs at the
beginning of the transmission due to the boot-up time of the
transmitter. However, the measured R-R interval is not
influenced since the receiver counts the interval of the falling
edge of a transmitted R wave as mentioned in following
subsections.
B. Receiver
Circuit Description
The wirelessly transmitted signal is received by the
on-board antenna printed with the meander pattern. The
received signal is demodulated by the 315 MHz OOK
receiver module, and the demodulated R wave is forwarded to
the built-in comparator of the R5F21294SNSP
microcontroller. The comparator output is observed by the
microcontroller’s built-in counter which counts the interval of
the falling edges with the millisecond-order precision. The
hexadecimal number of the counter value is converted to the
decimal number with 4 digits as an R-R interval, and it is sent
to the PC through the USB connection. The whole electric
power is supplied by the USB power source of 5V DC.
III. FABRICATION AND EXPERIMENTAL RESULTS
A. Low-cost Fabrication of the Proposed System
Fig. 3 shows the photographs of the fabricated R-R
interval telemetry system. The components of the proposed
R-waves telemeter was mounted on a 30 mm×50 mm 4-layer
printed circuit board (PCB) as shown in Fig. 3(b). In Fig. 3(a),
the red, yellow, and black cables are the connecting wires of
the ECG electrodes. The receiver was realized on a 80 mm×
40 mm 2-layer PCB as shown in Fig. 3(c). The antennas were
realized by a meander-patterned metal line on the PCBs.
By using the low-cost electric components and the PCB
manufacturing technology suitable for mass production, the
volume cost of the proposed system, excluding the initial
costs of the PCB fabrication, was reduced to less than
US$100 / a system.
B. R-R interval Measurement Accuracy
The measured R-R intervals by using the proposed
telemetry system were compared with the actual R-R
intervals. In order to obtain the correct R-R intervals, the
bio-signal amplifier (AB-611J, Nihon Kohden), the digitizer
(a)
(b)
(c)
Fig. 3. (a) A general view and (b) a top view of the PCB of the R-waves
telemeter, (c) a general view of the receiver. The PCB of the telemeter was
put in a plastic case in order to maintain electrical isolation from human
body.
(PowerLab 4/26, AD Instruments), and the software
(LabChart 6, AD Instruments) for medical application were
used for measurement and analysis of ECG signal. During the
measurement of a 24 year-old male subject for 30 minutes, the
errors of the R-R interval measured by the proposed method
were maintained less than 3.2 ms and the average error was
1.3 ms. Even though the above errors are caused due to that
the proposed telemetry system counts the intervals of the
falling edges of the R wave whereas the ECG monitor counts
the intervals of the peaks, the millisecond-order errors are
ignorable in the medical diagnosis [1]-[4].
C. Automatic Gain Control under Various Ages
In order to investigate the automatic gain control of the
telemeter, four various-aged subjects were selected to observe
how accurate a gain was adjusted to detect R waves at
different ages. The subject’s ECG was measured by using the
disposable electrodes (Vitrode Bs-150, Nihon Kohden) in
accordance with Lead II of Einthoven’s low. The study
population comprised four subjects; 5 and 73 year-old
females and 24 and 74 year-old males. All of subjects were
doing a normal daily activity during the measurement. Fig. 4
shows the R-R interval of all of subjects (the five minutes in
the middle of the measurement period were shown in order to
highlight the fluctuation of R-R interval). Even though the
ECG amplitudes varied by the subjects’ age and sex [5], the
R-R
interval
was
successfully
measured
with
millisecond-order precision by the automatic gain control.
The probability of reoccurring errors was estimated to prove
the accuracy and reliability of R-wave detection. The total
numbers of collected data of a 24 year-old male was 2768 for
half an hour. In his case, the error region where the R-R
interval data is regarded as the inappropriate value should be
under 0.3 seconds and over 1.3 seconds. Here, the error
region was determined by considering the average value of
the measured R-R intervals and the expected range of the
heart rate of a healthy subject (based on [4] and [5]). Eight
errors were found within the error region of this subject. The
major reason of errors would be the motion artifacts. Table 1
describes each subject’s results including the number of the
collected data, errors, and the probability of error
occurrences. Here, the probabilities or reoccurring errors
were simply calculated with dividing the number of errors by
the total number of data. As shown in Table 1, almost all cases
have less than 2% probabilities. These minimum probabilities
are negligible since the effect of the errors can be compressed
in the frequency analysis which is frequently used in the
diagnosis of long-term ECG monitoring [6], [7]. Therefore,
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T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012)
the results prove the high reliability of this system even when
subjects’ ages are different.
D. Long Term R-R Interval Measurement
Capability of long term measurement is also a crucial role
on ECG measurement since the patients with acute symptoms
are required to be monitored continuously. In this subsection,
the accuracy of R-R interval detection was analyzed and
quantified when the measurement period was 48 hours. A 24
year-old male was selected and the measurement was
executed during his daily routines excluding bedtime. The
total number of collected data was 268,312, and the error
region was set to be the same region as pervious subsection.
To study the accuracy detection of telemeter, the
probability of reoccurring errors was calculated by the same
method used in previous subsection. Its value turns out to be
0.752%, which can be concluded that the physiological
signals in long term measurement were successfully detected
with minimum errors. Therefore, it is unnecessary for patients
to be restrained at hospital to get a long-term ECG since the
recording can be easily performed at home with lower cost.
IV. CONCLUSION
An R-R interval telemetry system was realized within the
mass-production cost of US$100. The automatic gain control
scheme has been developed for the easy uses and the high
accuracy R-R interval detection toward both clinical care and
homecare for various-aged subjects. The desired functions
and the validity of the system were proved with the subjects
different in age and sex. In addition, the result of long term
measurement was the critical evidence that the proposed
system has high accuracy and high reliability.
TABLE I
THE PROBABILITY OF REOCCURRING ERRORS AT DIFFERENT SEX AND AGES.
Time Period
(min)
Error Region
Total Errors
Total
Probabilities
5 year-old
female
24 year-old
male
73 year-old
female
74 year-old
male
18
30
30
30
<0.3s,
1.3s>
27
2021
1.336%
<0.3s,
1.3s>
8
2768
0.289%
<0.3s,
1.3s>
35
2091
1.674%
<0.5s,
1.5s>
33
1638
2.014%
Fig. 4. R-R interval of 4 subjects at different sex and ages within 5 minutes.
ACKNOWLEDGMENT
This research was partially supported by Adaptable and
Seamless
Technology Transfer
Program through
Target-driven R&D (231Z04347), Japan Science and
Technology Agency.
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