Development of a low cost fetal heart sound monitoring system for home care application

doi:10.4236/jbise.2009.26055

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Vol.2, No.6, 380-389 (2009)

doi:10.4236/jbise.2009.26055

JBiSE

Development of a low cost fetal heart sound monitoring

system for home care application

Arun Kumar Mittra1, Nitin K. Choudhari2

1MIET, Gondia: Department of Electronics Engineering, Manoharbhai Patel Institute of Engineering and Technology, Gondia (MS),

India; 2Smt. Bhugwati College of Engineering, Nagpur, India.

Email: akmittra@gmail.com

Received 6 January 2008; revised 26 April 2009; accepted 26 June 2009.

ABSTRACT

Variations in fetal heart rate (FHR) is a potential

indicator of stress on unborn in the womb of

mother. In hospitals, FHR surveillance is per-

formed by ultrasound based Doppler equip-

ments. However, recent studies show that fre-

quent exposure to ultrasound radiations is not

recommended for the fetal well-being. Because

of this and many other reasons, these instru-

ments are not recommended for prolonged

home monitoring applications. This work is fo-

cused around development of a prototype sys-

tem for fetal home monitoring application. Pre-

sented system can record the abnormal FHR

and alert the pregnant women to report to a

physician. Recorded data is then processed by

a novel methodology for deriving results of di-

agnostic importance. The instrument has been

tested on pregnant women in the clinical envi-

ronment and has gone through an extensive

clinical trial at local hospitals. The results show

that the technique is suitable and effective for

long-term FHR home monitoring application.

Keywords: Ambulatory Monitoring; Phonocardio-

graphy; Cardiography; Simulation; Signal Processing

1. INTRODUCTION

To diagnose pre-term labor, ambulatory monitoring for

abnormal FHR has proven to be an effective method [1].

Abnormality in fetal heart rate (FHR) is an indicator of

pre-maturity and miscarriage. It is very important to

monitor, such abnormalities in pregnant women, which

are at high risk, with history of miscarriage. These ab-

normalities are unpredictable and may occur at any time,

especially in the case of pre-term labor. The pathogene-

sis of pre-term labor is still poorly understood, however,

the unusual occurrence of pre-maturity and miscarriage

can be largely prevented by the timely diagnosis of

pre-term labor and its arrest with toco lytic medication. If

the unborn heart rate increases very high or drops to a

very low, it calls for urgent attention. In both the cases, it

is obvious that the baby is in stress and special urgent

medical attention is needed. For this reason, the elec tronic

monitoring of the FHR has become one of the most fa-

miliar methods used in the antenatal period [2,3]. The

ultrasound based Doppler instruments are widely used

for this purpose in hospitals, but for varied reasons, they

are not suitable for home monitoring application and

long-term surveillance of unborn [4,5]. It is imperative

to note that these instruments are also invasive in nature.

There is still a gap between existing technologies and

the user requirement for safe, convenient, and reliable

fetal monitoring [6,7 ]. In view of these considerations, a

strong need is felt for the development of a FHR moni-

toring machine which will be non-invasive, cost effec-

tive, simple to operate and which can be used by a preg-

nant woman for prolonged home monitoring application

[8]. Preliminary experiments in this regard have been

conducted [9,10] and a cost effective prototype is de-

veloped for at home long term FHR recording and

monitoring. This instrument can record fetal heart sound

for hours in a standard MP3 format. In case of abnormal

symptoms, the subject visits the physician where the

recorded data can be analyzed with the help of associ-

ated software and computer. The audiovisual display of

fetal heart sound will provide valuable additional infor-

mation to the physician for diagno sis and treatment.

The basic technique used in the presented instrument

is called fetal Phonocardiography (fPCG) [11,12]. In this

technique a specially designed microphone is placed on

the abdomen of the subject, which primarily detects and

records the fetal heart sound. However, phonocardio-

graphy is extremely suscep tible to ambient noise [13,14,

15]. Unfortunately, the fetal heart activity produces

much less acoustic energy and moreover it is surrounded

by highly noisy environment. This noise has a direct

consequence on the signal that often changes remarkably

from one beat to next, with the additional characteristic

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381

of poor signal to noise ratio (SNR). These unwanted

disturbance signals contribute an additional difficulty in

the detection of the principal fetal heart sounds (S1 and

S2). Hence it is necessary to develop and evaluate sig-

nal-processing technique to improve SNR before ob-

taining reliable time references of the fetal heart sound

signal [16]. In order to improve SNR, and accomplish an

optimal external noise cancellation performance, phono-

cardiographic devices need a very superior and advanced

noise reducer in the preprocessing stage of the instru-

ment. In this work a very effective noise reducer is de-

signed and used in the presented prototype system. The

design is implemented with help of Matlab Simulink and

intensive tests are carried out in order to evaluate the

performance of resultant method. The instrument has

been found effective from 30th week to final term of

gestation with satisfactory level of sensitivity.

The paper is organized as follows: The next section

discusses the hardware of the presented system, used for

the detection and recording of fetal heart sound and am-

bient noise. The following section, describes software

and different signal processing techniques used by the

system. The article then presents a comprehensive com-

puter simulation. The last section deals with experimen-

tal and clinical trial results with conclusions.

2. SYSTEM DESCRIPTION

The Fetal heart sound monitoring system presented in

this study, primarily comprises of two main modules:

1) Detection and Recor ding Module (DRM).

2) Processing and Display Module (PDM).

The DRM is a small hardware, which is placed on the

subject’s abdomen for detection and recording of the

fetus heart sound signal. This module records the ab-

normal FHR and alerts pregnant woman, that it is time to

seek some medical attention.

The PDM is software, developed for physician’s per-

sonal computer. When the subject approaches the physi-

cian along with DRM, recorded data is down loaded and

then processed with the use of PDM, to generate the

results of diagnostic importance.

2.1. Detection and Recording Module (DRM)

It is a small low cost hardware comprising of a specifi-

cally designed acoustic cone, piezoelectric sensor, pre-

amplifiers, power amplifiers, filters and a USB com-

patible MP3 voce recorder. The basic functions of this

module are as follows:

1) Fetal heart sound detection from maternal abdo-

men and recording of the same on one of two

channels in MP3 voice recorder (Abdominal

Channel).

2) External noise detection through an open-airmi-

crophone and recording on another channel (Noise

Channel).

3) Generation of audiovisual indications from fetal

heart sound signal.

Block diagram of DRM with all necessary details is

shown in the Figure 1. It can be seen that the complete

Figure 1. Block diagram of DRM hardware.

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382

system is divided in two basic sections: a) Abdominal

Channel b) Noise Channel.

a) Abdominal Channel: Fetal phonocardiography re-

quires the conversion of mechanical vibration on sub-

ject's abdomen to electrical signal by microphone. Fetus

heart sound is extr emely weak hence it cannot be sensed

properly by putting a sensor immediately on the subject's

abdomen. To overcome this problem a particular acous-

tic cone is developed which is a direct extension of the

chest piece of standard stethoscope. The air enclosed in

the cone acts as a transmission media between the mem-

brane and electro-mechanical transducer device. The

output of the sensor is fed to pre-amplifier for high am-

plification and better no ise rejection. IC LM 381 is used

here for this particular purpose, which raises the signal

from the transducer level to the line level.

It is essential to keep ambient noise as low as possible;

this is carried out by an active low pass filter with

cut-off frequency of 200 Hz. This value of cut off fre-

quency is selected, because most of the fetus heart sound

spectrum lies below this frequency limit. Active filter is

implemented using an easily available operational am-

plifier IC 741 along with suitable resistor capacitor net-

work. IC TBA810 based power amplifier further streng-

thens the output signal from filter, and provides audible

fetus heart sound. I t is then recorded on any one chann el

of MP3 voice-recorder. This amplifier further provides

driving power to audio-visual indicator of the DRM de-

vice.

b) Noise Channel: In fetal phonocardiographic meas-

urement, ambient noise creates major problem at signal

processing stage [17,18]. To overcome this problem,

special signal processing techniques are used in this

study, which require a primary sample of the noise, cre-

ating disturbance in the signal of interest. To facilitate

this, ambient noise is detected through an independent

open-air microphone. After pre-amplification and filter-

ing, noise signals are recorded on another channel of the

memory device.

It is important to note that abdominal channel micro-

phone detects the sound primarily originating from the

maternal abdomen, but these signals get mixed with a

damped version of the external noise. The open air Noise

Channel microphone detects only the ambient noise and

does not contain any traces of fetal heart sound signal.

Photographs of prototype experimental model are shown

in the Figures 2(a) & 2(b).

2.2. Processing and Display Module (PDM)

The PDM is the software part of presented system, made

available in the physician’s personal computer. When

pregnant woman feels some abnormality and the DRM

alerts her, she may go to the hospital with the DRM.

Stored data within the device is used for further proc-

essing and investigation through PDM. Block diagram of

PDM with all necessary signal flow details is shown in

the Figure 3.

A brief description of each signal-processing module

incorporated in the block diagram is given below:

a) Data Acquisition: The output of the MP3 voice re-

corder is directly fed to the Line-in of the multimedia

card, which contains on board signal conditioning, ana-

log to digital conversion and digital signal processing

hardware. Matlab data acquisition toolbox is used to

download both channels of MP3 voice recorder and to

store it in two separate *.wav files. These files carry

noised fetal heart sound signal and external noise sepa-

rately.

b) Adaptive Filter: Adaptive filters track the dynamic

nature of a system and allow elimination of unwanted

part of the signal. In this study adaptive filters are used

for external noise cancellation i.e. removal of external

unwanted background sound signal from the fetal heart

(a) (b)

Figure 2. Photographs of prototype.

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383

Figure 3. Block diagram of PDM.

sound signal. As shown in Figure 3, desired signal d(k),

the one to clean up, comes from abdominal channel,

carries noise n(k) and wanted signal s(k). The Noise

Channel carries only background noise n'(k), is applied

as input signal x(k) of the filter. As long as the input

noise to the filter n'(k) remains correlated to the un-

wanted noise n(k), the adaptive filter adjusts its weights

w(k) to reduce the value of the difference between y(k)

and d(k), this results in elimination of ex ternal noise and

a clean fetal heart signal s(k) will appear on the error

port. The generalized mathematical relationship be-

tween different signals and filter weights can be de-

picted as:

d(k) = s(k) + n(k)

y(k) = Filter{x(k), w(k)}

or y(k) = w(k) . n’(k)

e(k) = d(k) – y(k)

= s(k) + n(k) – w(k) . n’(k)

 s(k)

w(k+1) = w(k) + e(k) x(k)

Notice that in this implementation, the error signal

actually converges to the input data signal, rather than

converging to zero.

c) Band Pass Filter: Adaptive filtering of recorded

signal removes only the external noise from the compos-

ite abdominal signal. A band pass digital filter is de-

signed to limit the signal in 35 Hz < f < 200 Hz fre-

quency band. The selection of lower frequency limit is

based on the experimental result, that a considerable part

of the disturbing maternal heart and digestive sound lies

below this border, while the fetal heart sounds are not

dominantly present there any more. Upper limit is set to

remove maternal respiratory sound and remaining traces

of external noise signals.

d) Phonocardiogram: Fetal Phonocardiograph is a

time versus amplitude plot of fetal heart sound and is

considered as the primary time domain characteristic of

the fetal heart sound signal. It is a graphical representa-

tion of vibration or sound signal detected from the ma-

ternal abdominal wall, caused by the contractile activity

of the fetal heart. The general fPCG wave pattern of the

signal over cardiac cycles may be readily appreciated by

visual inspection and can be used as a potential indicator

of few congenital diseases.

e) Spectrogram: fPCG signals posses multiple reso-

nance frequencies. This leads towards the need to de-

scribe the fPCG signals, not only in terms of time but

also in terms of frequency domain, also known as spec-

trogram. It provides distribution of the signal’s energy

or power over a wide band of frequencies.

f) Envelop Generation: In order to find out exact pe-

riodicity of heartbeat, it is proposed to use env elop of the

fetal heart sound signal. Th is envelope encompasses and

traces the peaks of signal under consideration. In this

application it is derived by the method of squaring and

low pass filtering [19]. The resultant envelope signal

reflects the amplitude dynamics of signal on the same

time scale as of the original signal.

g) Burst Generator: After the process of envelope

generation, exact positioning of amplitude burst is car-

ried out by the method of thresholding and relaying.

Whenever amplitude of signal envelops goes above a

pre-defined limit it is relayed over as a maximum value

otherwise it is taken as zero. This process results in a

series of discrete pulses representing occurrences, and

duration of fetal heartbeat s.

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h) FHR Calculation: The last element of the sig-

nal-processing block is for the FHR determination. This

is carried out by counting numbers of discrete pulses in

the signal for a pre-defined time interval.

3. SYSTEM SIMULATION

Processing and Display Module (PDM) described above

is simulated using the Simulink modeling tool of Matlab

version 7. The model is built by interconnecting requi-

site blocks, which are available in the software library

and their parameters are entered while designing them

for simulation. The inputs of the simulating system are

the recorded signals of DRM module, where as the out-

puts are Phonocardiogram, Spectrogram and record of

FHR in Beats Per Minutes (BPM). System simulation of

developed PDM module is shown in Figure 4.

After data acquisition, signals are de-noised with the

help of adaptive filter. Fetal heart sound signals, detected

from mother's abdomen are fetched from corresponding

*.wav file and applied to the desired port of the filter

block. This signal carries fetal heart sound and a damped

version of the external noise. The unwanted external

noise is available in another *.wav file and is applied to

the input port of the filter. De-noised signal comes out

through the error port of the filter block. It should be

noted that the output port is not used in the simulation

process; however this port provides internal feedback for

error calculation in the filter block.

Adaptive filtering eliminates only the external noise.

For suppressing remaining artifacts, digital band pass

filter is used in the simulation. This block allows signals

of the specified range only, while signals of all other

frequencies are attenuated. After the band pass filtering

signals are relatively less distorted and applied to the

time scope block of the module. This block provides the

time-domain response of the fetal heart signal, which is

also called as phonocardiogram. The frequency domain

response i.e. cardio-spectrogram of de-noised signal is

produced through periodogram and vector scope block

icons. The periodogram block computes a non-paramet-

ric estimate of the spectrum. The block averages the

squared magnitude of the FFT computed over windowed

sections of the in put and no rmalizes the spectral average

by the square of the sum of the window samples. The

vector scope block is a comprehensive display tool

similar to a digital oscilloscope. It is used here to plot

frequency-domain response of the de-noised fetal heart

sound signal.

Figure 4. System simulation of PDM.

Figure 5. Sub-system for envelop generation.

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385

Figure 6. FHR calculator sub-system.

The simulation block diagram showing the bottom-

most blocks (refer Figure 4) are related with the FHR

calculation. The first block is for envelope generation

and it is a simulink subsystem as shown in Figur e 5.

This sub system is based on the concept of squaring

and low pass filtering. The input signal is first multiplied

by itself. Squaring the signal effectively modulate the

input by using itself as the carrier wave. This means half

the energy of the signal is pushed towards higher fre-

quencies and half is shifted towards DC. The envelope

can then be obtained by keeping all the DC low fre-

quency energy and eliminating the high frequency en-

ergy. In this sub-system, a simple minimum-phase low

pass filter is used to get rid of the high frequency energy.

Output of envelope generator is connected to the relay

block (refer Figure 4). This block allows its output to

switch between two threshold values. Once the relay is

ON, it remains ON until the input drops below the valu e

of the switch-off point parameter and when the relay is

OFF, it remains OFF until the input exceeds the value of

the switch-on point parameter. This block converts the

envelope signal into a series of discrete pulses. These

pulses are fed to FHR Calculator sub-subsystem. Details

of this sub-system are shown in Figure 6, in which the

Counter block increments an internal counter each time

it receives a trigger at the click (Clk) port. A trigger sig-

nal at the reset (Rst) port brings the counter to its initial

state.

Counter output is converted to Beats Per Minute (BPM)

and is then fina l l y di spl aye d on t he o utput device.

4. EXPERIMENTAL TESTING AND

RESULTS

Experimental testing is necessary to verify the reliability

and performance of any system under a developmental

stage. An artificial womb, which give simulated testing

conditions for fetal monitoring system is very appropri-

ate and useful for initial testing in comparison to actual

clinical testing on pregnant women. In this work, an arti-

ficial womb is prepared for simulated performance test-

ing of monitoring systems under study. Through Matlab

signal processing toolbox, simulated signals are gener-

ated for fetal heart sound, maternal heart sound, maternal

respiratory sound and for external noise. After amplifi-

cation these signals are applied to different speakers

placed underneath a rubber balloon filled with water.

DRM hardware under test is placed on the opposite side

of the balloon. This arrangement simulates a fetal heart-

beat passing through amniotic fluid to the wall of the

mother's abdomen. For providing external support, the

complete assembly is housed in a solid wooden tub. It is

then placed in a thick glass envelop for elimination of

outside noise interference. Presented system was initially

tested on above-mentioned artificial womb. This was a

totally subjective test, performed only to check viability

of the instrument.

After satisfactory performance with artificial womb,

system has been tested on the pregnant women in clini-

cal environment. More than 15-fetal heart sound re-

corded samples were taken from different women, who

were between 36 to 40th week of singleton pregnancy.

Recorded data was transferred to personal computer in

*.wav file through multimedia card. Matlab Simulation

discussed above was used to process and display the

recorded sound from wave files. Figure 7 shows signal

waveforms at various stages of the simulation, obtained

from abdominal recording of a pregnant woman at 39

weeks of gestation (Subject No. 1). In these graphs, X-

axis represents the time in seconds whereas Y-axis

represents amplitude of signal in volts. Uppermost wave-

form (Graph a) represents 2 seconds time span of repre-

sentative sample of fetal heart sound, practically re-

corded through the abdominal microphone.

Second waveform (Graph b) is the corresponding ex-

ternal noise, recorded through external microphone. It

may be noted that noise level is very high, which con-

taminates the fetal heart sound to a larger extent. This

representative sample of noise is used as a reference

input for adaptive filters in signal processing stage of

simulation. Third waveform (Graph c) describes the

de-noised signal coming out from the adaptive filter. It

can be observed that external noise is considerably re-

duced and amplitude burst are distinctly visible in the

waveform. In order to further increase the signal to noise

ratio, band pass filters are used. Fourth waveform

(Graph d) is the signal after band pass filtering and this

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386

Figure 7. Simulation output (a) Signals from abdominal channel (b) Signals from external channel (c) Signal after adaptive filtering

(d) Signal after band pass filtering (e) Signal after envelope generator (f) Signal after thresh holding.

is the final phonocardiographic signal, which the instru-

ment provides. Fif th waveform (Graph e) is signal enve-

lope provided by the complex process of envelope gen-

eration. This envelope is then passed through an ampli-

tude thresholding process that in turn converts amplitude

burst of the envelope signal into discrete pulses. The last

waveform (Graph f) indicates the final processed signal,

used for the FHR calculation.

It is believed that a more precise examination of

phonocardiographic signal may be useful for pre-detec-

tion of intra uterine growth retardation and other abnor-

malities of fetal. To facilitate this, a separate time scope

block is provided in the simulation. This block provides

a zoomed-in version of signal for any specified period of

A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389

387

Figure 8. Fetal phonocardiogram.

Figure 9. Fetal spectrogram.

time. Figure 8 shows such display for a time interval of

0.8 seconds.

Phonocardiogram is the time domain response of the

fetal heart sound signal. Presented system is in addition;

capable of providing frequency domain response of the

signal, called Spectrogram. It represents the contribution

of every freq uency of the sp ectrum to the power of over-

all signal. Spectrogram of fetal heart sound for a small

time window around 0.55 sec instant of previous illus-

tration is shown in the Figure 9.

In order to support the performance of developed sys-

tem, phonocardiogram signals recorded through proto-

type were compared with signals of simultaneously used

ultrasound Doppler based instrument (Model: Coddle-

Graph–L of Maestros Mediline Systems). In this com-

parative experimentation following parameters were

measured:

N’ = Total number of amplitude bursts detected by

prototype.

N = Total number of amplitude bursts detected by ref-

erence instrument.

M = Total number of missed bursts by the prototype.

F = Total number of false bursts detected by the pro-

totype.

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388

Table 1. Outcome of comparative measurement.

Subject No Number of Amplitude

Burst from Prototype Number of Amplitude Burst

from Reference Instrument Number of Missed

Amplitude Burst Number of False

Amplitude Burst Performance

Index %

1 260 270 10 0 96.30

2 290 286 0 4 98.60

3 274 280 6 0 97.86

4 278 284 6 0 97.89

5 310 302 0 8 97.35

6 294 298 4 0 98.66

7 290 296 0 6 97.97

8 318 324 0 6 98.15

9 292 290 2 0 99.31

10 312 302 10 0 96.69

11 266 280 0 14 95.00

12 302 306 0 4 98.69

13 286 288 0 2 99.31

14 298 304 0 6 98.03

15 314 310 4 0 98.71

16 304 308 0 4 98.70

From these data, performance of the instrument [20]

can be derived with the help of formula:

Performance Index (NMF



)

X 100

This comparative measurement is performed on 16

pregnant women between 36th to 40th week of gestation

age, and average recording duration was stayed limiting

to one minute. It is observed that in most of the cases,

phonocardiographic-based prototype signal quality is

almost at par with the ultrasound Doppler based signals.

Table 1 shows result of these measurements and calcula-

tion of corres p on d i n g perform a nce indi ces.

In light of these recorded values, the overall Perform-

ance Index of the system is found around 97% in corre-

lation of ultrasound based Doppler instrument. This per-

formance value is fairly good for a prototype model and

will certainly improve in commercially advanced sys-

tem.

5. CONCLUSIONS

This work presents development of a very powerful,

non-invasive, portable and low cost battery operated

standalone fetal heart sound recording and monitoring

system that can be used in prevailing home environment.

The hardware of prototype model is of the size 9 X 8 X

4.4 cm and of the weighs 205 grams with 9 V alkaline

battery. Signal recorded through this prototype model

are digitally processed and analyzed on a personal com-

puter. Using enhanced adaptive and band pass filtering

techniques, a remarkable imp rovement in signal to noise

ratio is achieved by the system. Processed signals are

finally used to produce impressive results of significant

diagnostic and clinical importance. Instrument has been

tested on pregnant women with varied gestational period

and also validated by simultaneous measurement with a

standard ultrasonic Doppler device. From the results it

can be concluded that the presented system is viable and

can effectively be used in the development of commer-

cial phonocardiographic-based fetal home care monitor-

ing system.

6. ACKNOWLEDGEMENTS

The fetal heart sound recordings were done at district government

women hospital and at Ratnaparkhi Nursing Home Gondia (M.S.). The

authors of this paper would also like to thank Dr. Shrish Ratnaparkhi

(Gyneacologist), Dr. (Mrs.) Megha Ratnaparkhi (Obstetrician), Prof.

Vijay Chourasia and Prof. (Mrs.) Vijaya Rahangdale for their kind

support in carrying out observations with the help of developed proto-

type instrument. Pregnant women who volunteered to participate in

clinical test are also appreciated for their kind gesture.

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