CARDIAC DIAGNOSING

BY A PIEZOELECTRIC-TRANSDUCER-BASED

HEART SOUND MONITOR SYSTEM

Shinichi Sato

, Takashi Koyama

, Kyoichi Ono

, Gen Igarashi

, Hiroyuki Watanabe

, Hiroshi Ito

Department of Cell Physiology, Akita University Graduate School of Medicine, 1–1–1 Hondo, Akita, Japan

Department of Cardiovascular and Respiratory Medicine, Akita University Graduate School of Medicine

1–1–1 Hondo, Akita, Japan

Mikio Muraoka

Department of Mechanical Engineering, Akita University Graduate School of Engineering and Resource Science

1–1 Gakuen-machi, Tegata, Akita, Japan

Keywords: Piezoelectric transducer (PZT) sensor, Heart sound, Super low frequency sound, Cardiac diagnosis,

Phonocardiographic diagnosis.

Abstract: A variety of diagnosis that auscultation enables is not necessarily conducted thoroughly by all of the

physicians because of its difficulty in judging by means of listening to evanescent sounds with their ears. A

system that displays heart sounds continuously on a computer screen may be convenient for cardiac

diagnosis. Recently, we made a monitor system with employing a piezoelectric transducer (PZT) sensor,

which detected 1

and 2

sound and murmurs clearly. In addition, the sensor was capable of detecting

inaudible low-frequency sounds of below ~20Hz. Using the PZT sensor, we recorded heart sounds at left

second intercostals space in 12 patients susceptible to cardiac diseases in parallel with ECG recording, and

analysed the raw heart sounds and band-pass (20–100 Hz) filtered signal. Second sound in the filtered signal

was completely or often absent and/or a sharp deflection, which appears coincidently with R wave, with a

peak-peak voltage of >20 mV and a duration of <25 ms was missing in the raw sound signal in 90% (9/10)

of the patients diagnosed as having cardiac dysfunctions. Thus, we believe that the PZT-based heart sound

monitor system may contribute to the advance of the phonocardiographic diagnosis of cardiac diseases.

1 INTRODUCTION

The stethoscope was invented by Laennec in 1816

and a diaphragm stethoscope, which was invented

by Bowels in 1894, made a great progress in

stethoscope as it increased the sensitivity in mid-

high frequency range to detect heart sounds (1

and

sound) clearly. However, the diagnosis by

auscultation is not necessarily conducted by all of

the physicians because of its difficulty in judging by

means of listening to evanescent sounds with their

ears. A monitor system that enables us to observe

those heart sounds continuously on a computer

screen may be of help for the phonocardiographic

diagnosis of cardiac dysfunctions.

Monitoring of heart sound using a piezoelectric

transducer (PZT) sensor seems to be an alternative

to the auscultation using a stethoscope. In fact, we

were able to continuously observe the heart sound

signal of an anesthetized small animals on a

computer screen using the PZT sensor (Sato et al.,

2006; Japan patent 4015115; US patent 7174854)

with a custom heart-sound detector circuit (Sato.,

2008). Furthermore, the PZT sensor was capable of

analysing the change in development of cardiac

function during the early postnatal period in mice

(Sato, 2008). Following the animal experiments, we

performed long-term measurements of heart rate of

newborn infants in the neonatal intensive care unit

(NICU) with a PZT sensor that was modified for the

use for the neonates. The study demonstrated that

280

Sato S., Koyama T., Ono K., Igarashi G., Watanabe H., Ito H. and Muraoka M..

CARDIAC DIAGNOSING BY A PIEZOELECTRIC-TRANSDUCER-BASED HEART SOUND MONITOR SYSTEM.

DOI: 10.5220/0003288702800283

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 280-283

ISBN: 978-989-8425-37-9

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

the PZT sensor is quite non-invasive and practically

available as a cardio-respiratory monitor for use in

the NICU (Sato et al., 2010). Recently, we have

made a new heart sound monitor system with

employing a PZT sensor (patent pending), which can

be used as a stethoscope and detected 1

and 2

sound and murmurs as clearly as an electronic

stethoscope did. In addition, the sensor was capable

of detecting inaudible low-frequency sounds of

below ~20Hz (super low frequency sound). In the

present preliminary study, we recorded heart sounds

at left second intercostals space in 12 patients

susceptible to cardiac diseases using the PZT sensor

in parallel with ECG recording. Then we analysed

the raw heart sounds and band-pass (20–100 Hz)

filtered heart sound signal to find out the differences

between the patients with and without cardiac

dysfunctions.

2 METHOD

2.1 PZT Heart Sound Monitor System

The PZT heart sound monitor system consists of a

PZT sensor, A/D converter (Power Lab 26T,

ADInstruments) and a computer, which contains a

signal analysis software (Chart5, ADInstruments)

(Fig. 1). The first prototype PZT sensor consists of a

plastic cylinder (20 mm outer diameter, 90 mm long

and 1 mm-thick wall) and a disk-shaped thin

ceramic-PZT (20 mm diameter), which was adhered

on one open edge of the cylinder.

Figure 1: A photograph of the first prototype PZT sensor

(upper) and block diagram of the PZT heart sound monitor

system.

2.2 Output Signal of the PZT Sensor

and an Electronic Stethoscope

Although the PZT sensor detects a wide frequency

range of heart sound from a patient including super

low frequency sound of below ~20Hz, first and

second heart sound signals were clearly separated

from the PZT-sensor output signal by filters with

pass band of 20–100 Hz, which were comparable

with heart sounds obtained by an electronic

stethoscope (Littman Model 3100) from the same

patient as shown in Fig. 2.

Figure 2: Heart sounds obtained by the PZT sensor (upper)

and an electronic stethoscope (lower).

2.3 Cardiac Signal Recording

Twelve outpatients who visited to the cardiology

outpatient department (internal medicine) in the

Akita University Hospital were enrolled in this study.

In parallel with ECG recording, cardiac signal

recording by the PZT-sensor was performed at

second, third, fourth and fifth left-intercostals spaces

(apex region) in addition to the second right-

intercostals space of the patients. Each recording

time at these sites was within 10 seconds and stored

into a computer at a sampling rate of 5 kHz. Sound

filtering condition was set by using the Chart5

software for the separation of 1

and 2

sounds

from other lower frequency components.

2.4 Heart Sound Analysis

In the present study, the analysis of heart sounds was

performed with the signals obtained at the left

second intercostals spaces of the patients. The raw

heart sound signal was analyzed with focusing on

the period of the QRS wave of simultaneously

recorded ECG. A sharp deflection, which appeared

coincidently with R wave, was measured by its

peak-to-peak voltage (Vp-p, mV) and its rising or

falling time (ms: duration). While, the 1

and 2

CARDIAC DIAGNOSING BY A PIEZOELECTRIC-TRANSDUCER-BASED HEART SOUND MONITOR SYSTEM

281

sounds in the band-pass filtered PZT sensor signal

were qualitatively scored as “Y” (perceivable) or

“N” (non-perceivable). Regardless of the sound

analysis, cardiac diagnosis of the patients was

performed in a conventional way by a physician in

the cardiology outpatient department.

3 RESULT

According to the result of diagnoses conducted by

the physician in the cardiology department, 10 out of

total 12 patients had following one or several cardiac

dysfunctions; aortic valve regurgitation (AR), mitral

valve regurgitation (MR), pulmonary valve

regurgitation (PR), Aortic valve sclerosis (AVS), or

dilated cardiomyopathy (DCM). In the heart sound

analysis, second sound in the filtered signal [arrows

at middle trace in Fig. 3(a)] was completely or often

absent in 6 out of the 10 patients who had above

described cardiac dysfunctions [middle trace in Figs

3(b) and (d)].

Table 1: Results of cardiac diagnosis and sound analysis.

patient heart disease 2

sound Vp-p (mV)

1 normal Y 58

2 AR, MR, PR Y -

3 AR, MR, PR N -

4 DCM N -

N 27.6

6 MR, PR Y 10.5

7 MR, PR, DC

Y -

8 normal Y 25.5

9 AVS N 22.5

10 AR, MR, PR N -

11 MR Y 58.4

12 AR N -

Furthermore, a sharp deflection [arrows at top trace

in Fig. 3(a)], which appears coincidently with R

wave in healthier patients, with an average of three

consecutive peak-to-peak voltages (Vp-p) of >20

mV and a duration of <25 ms was missing in the raw

sound signal in 7 out of the 10 cardiac patients [top

trace in Figs 3(b) and (d), Table 1]. Accordingly,

patients who had at least either one of above 2 heart

sound deficits were 90% (9/10) of the patients

diagnosed as having cardiac dysfunctions. Two other

patients were scored as normal with cardiac function

by both the sound analysis and a diagnosis

conducted by the physician.

Figure 3: From top to bottom trace; raw heart sounds,

band-pass filtered heart sounds, and ECG of (a) patient #1

with normal cardiac function and patients (b) #3, (c) #6,

(d) #10 with cardiac dysfunctions as shown in Table 1.

4 DISCUSSIONS

Since two decades ago, many researchers argued the

possibility in analysing cardiac function by using

heart sounds or phonocardiogram signal processing

technique (Rangayyan and Lehner, 1988; Durand

and Pibarot, 1995; Manecke et al., 1999). Electronic

stethoscope, though it is cutting edge equipment and

commercially available these days, it seems that it

still not has an overwhelming advantage against the

conventional stethoscope.

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The striking difference between our PZT heart

sound monitor system and the electronic stethoscope

is the sound detection element; a small vibration

detecting PZT disk adhered on the top of a plastic

cylinder (Fig. 1) directly touch a patient’s chest and

record heart sounds, while heart sounds are detected

by a microphone via the air inside the head of the

electronic stethoscope. The construction of the PZT

sensor is crucial for the detection of super low

frequency sound of below ~20Hz, which may be out

of range for an electronic stethoscope because its

sensor head is diaphragm type that intends to detect

mid-high frequency range sounds.

In the present study, we analysed the raw heart

sound signal that contained the super low frequency

sound, and we found that the raw sound signal of

cardiac patients lacked a sharp deflection that

appears in coincidently with R waves on ECG. We

also found that 2

sound in the filtered heart sound

was missing in most of the cardiac patients.

However, the 2

sounds were often observed in the

heart sound signal recorded at 3

, 4

or 5

(apex)

intercostals spaces. Accordingly, the recording of

heart sound signal with the PZT sensor at left 2

intercostals space seems to be effective for the

diagnosis of patients susceptible to cardiac diseases.

It should be noted that the analysis of the super-low-

frequency sound is likely to be useful for cardiac

diagnosis (see Fig. 3).

As we only found a small part of information

hidden in the low frequency heart sound, we need

further investigate the sounds created by the heart in

cardiac patients with developing a quantitative

method for the cardiac diagnosis including such as

frequency domain analysis. Furthermore, we should

develop a simple and easy to use heart sound

monitor system, which continuously displays the

cardiac signal of a patient and provides us a tool for

visually diagnosing for the use in cardiology

outpatient department in hospitals.

In conclusion, the present study demonstrated

that the PZT-based heart sound monitor system has a

performance suitable for detecting heart sounds

including super low frequency sound. Although our

results are very preliminary and we may need to do a

further comparative analysis with other electronic

stethoscopes, we believe that phonocardiogram-

based analysis with the PZT heart sound monitor

system may provide us a new strategy for the

diagnosis of cardiac diseases.

ACKNOWLEDGEMENTS

This work was supported in part by the Vehicle

Racing Commemorative Foundation, Suzuken

Memorial Foundation, Nakatani Foundation of

Electronic Measuring Technology Advancement and

an intramural grant from Akita University. The

authors thank the staff of the department of internal

medicine at Akita University Hospital and Yu Obara,

Yuta Nakamura and Hideaki Kobayashi for their

contributions to the examination of the prototype

PZT sensor.

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