CARDIAC BEAT DETECTOR

A Novel Analogue Circuitry for the First Heart Sound Discrimination

Shinichi Sato

Department of Physiology, Akita University School of Medicine, 1-1-1 Hondo, Akita, Japan

Keywords: Non-invasive, piezoelectric transducer (PZT), analogue circuit, heart rate, respiration sounds, mice.

Abstract: Cardiac beat detector, which is an analogue circuitry installed in a novel non-invasive system for measuring

heart rate in mice by using a piezoelectric transducer (PZT) sensor, performs an critical role in detecting the

first heart sound (S1) in heart sounds. The PZT sensor detects heartbeat vibration and converts it to an

electrical signal, namely the heart sounds. The measurement in intervals of S1s in the heart sounds is

required to calculate heart rate, however, it is not simple because a S1 is a vibrating signal and has multiple

peaks, which fluctuate in interval and in magnitude. In addition, respiration sound noise, which has

frequency components similar with that of S1, makes S1 detection difficult and complex. The cardiac beat

detector made it possible to overcome these problems by transforming multi-peaked S1 signal into a quasi-

digital pulse. This technique is also available for the use in humans. Thus, the cardiac beat detector would

contribute to the progress in the non-invasive heart rate measurement when it is installed in various, novel

phonocardiogram-based equipments for the use in the fields of clinical and basic science in medicine.

1 INTRODUCTION

In experiments using small animals such as mice, a

clip ECG electrode is often used for ECG recording

(Yamada et al., 2001). However, investigators often

encounter the problem with ECG signal

deterioration or instability during long recording due

to the hairy limbs and drying up of electrolytic paste

between the limbs and the clip electrodes

particularly in small animals. Moreover, there is an

undeniable possibility that the pain induced by the

electrode attachment might activate the sensory

neurons and influence on the physiological state

even in an anesthetized animal (Sato, 2007).

To overcome these problems, we recently

developed a non-invasive cardiorespiratory monitor

system for small animals using a piezoelectric

transducer (PZT) sensor, which converts cardiac

beats into an electrical signal when a small animal

was simply placed on it (Sato et al., 2006; US patent

7174854). The PZT cardiorespiratory monitor

enables stable and long measurement of heart rate of

sleeping or anesthetized animals. Only placing an

animal on the PZT sensor is required to monitor

heart rate, and therefore, it gives no pain to animals.

To calculate the heart rate, it is required to detect the

first heart sound (S1) in a heartbeat signal detected

by the PZT sensor. However, it is not simple to

detect S1 constantly by distinguishing from noises of

a frequency range similar to that of S1 because a S1

is composed of multi-peaked vibrating signal

(Rangayyan and Lehner, 1988) and its magnitude

decreases in anesthetized animals and humans

(Manecke et al., 1999). A cardiac beat detector,

which is made of a custom-designed analogue

circuitry for S1 detection, was strikingly effective

for detecting S1 and the second heart sound (S2) and

for computing heart rate with a simple

microprocessor algorithm.

2 METHOD

2.1 PZT Cardiorespiratory Monitor

The PZT cardiorespiratory monitor system consists

of a PZT sensor device and a main unit, which

contains two band-pass filters, a cardiac beat

detector, a breathing movement detector,

microprocessors and a temperature controller for the

PZT sensor device. The sensor device consists of a

disk-shaped thin PZT placed in a hole cut in a

copper plate and covered by 0.5 mm-thick insulating

sheets, which all were mounted on an electronic

controlled heater (Sato et al., 2006) (Fig. 1).

136

Sato S. (2008).

CARDIAC BEAT DETECTOR - A Novel Analogue Circuitry for the First Heart Sound Discrimination.

In Proceedings of the First International Conference on Bio-inspired Systems and Signal Processing, pages 136-140

DOI: 10.5220/0001069501360140

 SciTePress

Figure 1: Block diagram of PZT cardiorespiratory monitor.

2.2 Signal Separation by Filters

Heart sound and breathing movement signals were

separated from the PZT output signal by filters with

pass band of 280-1000 Hz and 0.4-2.6 Hz for heart

sounds and breathing movements, respectively, as

shown in Fig. 2.

time (s)

0.1

0.04

0.1

0.5

-0.1

-0.04

-0.1

-0.5

0.1

-0.1

PZT

signal(V)

filter 1

output(V)

filter 2

output(V)

ECG(V)

Thermistor

output(V)

0 0.2 0.4 0.6

0.8

Figure 2: PZT output signal, Heart sounds, breathing

movement signal, ECG and thermistor airflow sensor.

However, it was found that the presence of

respiration sound noise, which was produced by

airflow in airway, disturbs the detection of S1 (S2)

when the magnitude of S1 (S2) declined in

anesthetized mice (Fig. 3; upper trace). In addition,

airway secretion produced marked, large-amplitude

respiratory noise (Fig. 3; lower trace). These

respiration-related noises were hardly possible to be

removed by a filter because the frequency

component of them were similar to that of S1; the

period of vibrating signal of S1 (Ts) and respiration

sound noise (Tr) was ranged between 1.4 and 4.0 ms

(average = 2.4 ms, n = 50) and between 3.5 and 7.7

ms (average = 5.4 ms, n = 50), respectively. The

frequency components of both S1 and respiration

sound noise fluctuated, and therefore, they ranged

widely and overlapped each other.

time (ms)

filter 1

output(mV)

200

100

150

-50

0.2

-0.2

0.2 0.4 0.6 0.8

time (s)

filter 1

output(V)

Figure 3: Representative traces of respiration sound noise

(open circles; upper trace) and large-amplitude respiratory

noise (open triangles; lower trace). Arrows and

arrowheads indicate S1 and S2, respectively.

2.3 Cardiac Beat Detector

Since it was difficult to accomplish effective

removal of the respiration sound noise from heart

sounds by a filter because they have similar

frequency components and fluctuate, we have

developed a novel cardiac beat detector circuit,

which consists of two diode detectors connected to a

differential amplifier, an AC amplifier, and a

hysteresis comparator (Fig. 4a). This circuit has

three functions; (1) S1 emphasizing, (2)

transforming S1 into a quasi-digital pulse and (3)

automatic threshold controlling (Sato et al., 2006).

2.3.1 S1 Emphasizing Function

As described above, frequency components of S1

are at slightly higher range than those of respiration

sound noise, although the both components

fluctuate and overlap in part. To overcome the

fluctuation, the cardiac beat detector was designed

to emphasize always a higher frequency sounds

over relatively lower frequency sounds. The S1

emphasizing function is produced by the

combination of two diode detectors, which work as

envelop detectors, and a differential amplifier (Fig.

Figure 4: Function of the cardiac beat detector.

CARDIAC BEAT DETECTOR - A Novel Analogue Circuitry for the First Heart Sound Discrimination

137

4a).

The two-diode detectors produce positive and

negative envelopes with ripples when a sine wave is

input. The voltage of the positive envelope during

the declining phase (V(t)) is determined by a time

constant RC as

V(t) = V

p e

−t/RC

(1)

where R and C are the resistance and the capacitance

of the diode detectors and t is the time after a time of

positive peak in the input and V

p is the voltage of the

peak.

Output voltage difference between the two diode

detectors at t = T/2 (V(T/2)) is calculated as

V(T/2) = V

p (1 + e

−T/2RC

) (2)

where T is the period of the input signal. Therefore,

the higher input signal frequency, the larger voltage

difference the diode detectors output. In fact, output

voltage difference for an input sine wave of higher

frequency (Vs(t

+Ts/2); Fig. 4b) is larger than that

of lower frequency (Vr(t

+Tr/2); Fig. 4c).

This voltage difference appears equally in the

differential amplifier output. Responses of the diode

detectors to an input of a synthesized waveform,

which consisted of alternating 4 cycles of a 500Hz

sine wave (artificial S1) and 10 cycles of a 100Hz

sine wave (artificial respiration sound), are shown in

Fig. 4d and e. The artificial S1 is enhanced as

compared to the artificial respiration sound when

amplitudes of both inputs are almost the same (Fig.

4d), while the artificial S1 is largely enhanced when

it is slightly larger than the artificial respiration

sound in input signal (Fig. 4e). Fig. 4d demonstrates

that the diode detectors have the S1 emphasizing

function, while Fig. 4e shows an additional

contribution of a rectifying property of diode, which

abruptly reduces its resistance to the signal that

exceeds about 0.3V, to the S1 emphasizing function.

Fig. 5 shows an example of quasi-digital pulses

output from the differential amplifier when a real

filtered heart sound signal was input ((A); Fig. 4a).

The amplitude ratio of the S1 signal (filled circle) to

the respiration sound noise (open circles) was

enhanced from 3-fold in the input (broken lines;

upper trace) to 10-fold in the output (lower trace)

(Fig. 5). In addition, the cardiac beat detector

combines the multi peaks of S1 into a quasi-digital

pulse, which is helpful for the comparator to detect

S1 easily. The unique combination of these effects

enabled the emphasizing of S1 of higher frequency

over the respiration sound noise of lower frequency,

thus enabling a great improvement in the S/N ratio

of the quasi-digital pulse.

-2

diff-amp

output(V)

filter 1

output(V)

100500

time (ms)

Figure 5: S1 emphasizing function of cardiac beat detector.

Figure 6: Automatic threshold adjustment controlled by

the cardiac beat detector.

2.3.2 Automatic Threshold Controlling

Function

The quasi-digital pulse (Fig. 5; lower trace) output

from the differential amplifier was fed into the AC

amplifier. The AC amplifier lowers the baseline of

the differential amplifier output (quasi-digital pulse)

to the negative direction when the magnitude of the

quasi-digital pulse becomes larger (Fig. 6a), while

the baseline approaches 0V when the pulse height

declines (Fig. 6b). These responses of the AC

amplifier to the change in magnitude of the quasi-

digital pulse act as an automatic threshold control,

which help comparator to detect the S1 signal with a

higher sensitivity (Fig. 6).

2.4 Heart Rate Calculation by

Microprocessor Program

The cardiac beat detector improved the incidence of

S1 detection by removing the influence of

respiration sound noises, however, large-amplitude

respiratory noises, which were elicited by an airflow

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in airway with airway secretion, still remained and

induced errors in S1-S1 interval detection for heart

rate calculation. Discrimination between S1 and S2

is also required for the heart rate calculation. I made

a microprocessor program to overcome these

problems. The major algorithms adopted in the

program are; (1) to calculate the correct HR by

selecting four S1-S1 intervals of less error from

eight consecutive intervals and (2) to set a non-

detection period of 75 ms after a S1 (or S2) for the

discrimination of S1 from S2 (Sato et al., 2006).

Figure 7: Comparison between the heart rate calculated by

the PZT system and ECG for 30 min. Output of the PZT

system (lower trace) and the heart rate calculated from the

R-R intervals in ECG (upper trace) (a), their difference

plot (b) and cross-correlogram (c). Black bar in (a)

indicates the duration where large-amplitude respiratory

noise appeared. Lower trace in (a) is lowered to show

almost complete agreement between the two traces.

3 RESULT

Heart rate output from D/A converter in the PZT

system (PZT-HR) and that calculated from ECG

reading (ECG-HR) averaged over every 1 s were

compared using 6 anesthetized adult C57BL/6 mice.

The PZT-HR and the ECG-HR were highly

correlated (Fig. 7a). Difference plot between them

also showed good correlation (Fig. 7b) even during

the period when large-amplitude respiratory noise

appeared (open circles; Fig. 7b). The difference plot

demonstrated the highly reliable detection of HR by

the PZT system; 96.2% (1,729/1,798) of total points

fell within ±2 SD of the mean value. The PZT-HR

also closely followed a rapid decrease in HR at a

rate of 33 b/m/s (arrow in Fig. 7a). Cross-correlation

coefficient between PZT-HR and ECG-HR was

0.995±0.003 (mean±SD, n = 6; Table 1, Fig. 7c).

Table 1: Correlation between PZT-HR and ECG-HR.

mouse r difference (%)

1 0.995 1.9±0.5

2 0.990 1.8±1.7

3 0.992 2.9±1.6

4 0.997 1.3±0.6

5 0.999 2.4±1.3

6 0.997 2.1±0.6

4 DISCUSSIONS

Since the high-frequency component of S1 is

comprised of multi peaks of vibrating signal, the

program code for heart rate calculation would be a

complex one in the case without the use of the

cardiac beat detector although recent developments

in digital signal processing of the phonocardiogram

have been reported (Durand and Pibarot, 1995;

Wang et al, 2001). All intervals between peaks of S1

and S2 in addition to respiration sound noises, which

are all composed of multi-peaked signal and

fluctuate in interval and/or in magnitude, should be

measured quickly and the initial point of the S1

should be properly identified almost instantaneously

during each heart cycle of less than 100 ms. In

contrast to such considerably complex digital signal

processing, making the quasi-digital pulse from

vibrating S1 signal with enhancing S/N ratio using

the cardiac beat detector ensures the easier digital

conversion of the S1 signal for the heart rate

calculation.

In conclusion, the present study demonstrated

that the cardiac beat detector has a performance

suitable for the non-invasive detection of S1 in the

heart sounds of small animals. It should be noted

that the cardiac beat detector is available not only for

anesthetized small animals but also unanesthetized

animals and humans at sleep or rest. Indeed, the PZT

system can be applied to unanesthetized newborn

mice (Sato et al., 2007), human infants (Sato et al.,

2006) or bedridden patients after some alteration to

the sensor construction. As the cardiac beat detector

greatly reduces the program code for S1 detection, it

would help us to create novel phonocardiogram-

based equipments for a wide range of fields in

clinical and basic sciences in medicine.

CARDIAC BEAT DETECTOR - A Novel Analogue Circuitry for the First Heart Sound Discrimination

139

ACKNOWLEDGEMENTS

I wish to thank Prof. Kyoichi Ono, Prof. Nobuya

Inagaki and Prof. Katsuya Yamada for helpful

advice and suggestions.

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