CONCURRENT ECG AND AUSCULTATION

SIGNAL TRANSMISSION

THROUGH A SINGLE AUDIO CHANNEL

A bachelor thesis submitted in partial fulfillment of the requirements for the

Diploma in Electrical Engineering of the Electrical and Computer

Engineering Department of the University of Cyprus

Andrea Petsa

ADVISOR

Dr. Constantinos Pitris

Associate Professor

Nicosia, May 2017

2

Acknowledgements

I would like to thank my supervising professor Dr. Constantinos Pitris, firstly for trusting me with

this project, and for his guidance, encouragement and endless patience throughout its completion.

I would like to thank my family for their unconditional love and support, and a special thank you

to my father Adamos Petsas and my uncle Antonis Petsas for their help. I would like to thank my

friends and loved one for supporting me. Lastly, I would like to thank and dedicate this thesis to

Mr. Thasos Polydorou, without whose expertise and wisdom this thesis would not have been

possible.

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ABSTRACT

The goal of this project is to design and improve a basic system for

acquisition of two signals tracking the hearts’ electrical and auscaltatory

activity, namely an ECG signal and a stethoscope signal. The two signals

will be transmitted simultaneously through a single, basic audio channel that

can be found in most everyday personal computers with a soundcard. The

different steps developed in the design process of this prototype will be

demonstrated and elaborated throughout this paper.

The ease of use, multi-purpose function and low cost of such a device makes

it effective for the evaluation and monitoring of patients with known or

suspected heart pathology in a variety of clinical settings. Such a system

could not only prove useful in recognizing and alerting physicians and

caretakers of minor or serious cardiac episodes the moment they happen,

but with appropriate feedback-based learning, could also prevent them. This

can be done by relaying unusual or abnormal patterns to doctors so that they

can suppress the episode clinically.

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Table of Contents

Introduction / Motivation for the Thesis ............................................................................................. 5

CHAPTER 1: The Heart’s Conduction System and the Electrocardiogram ............................................... 6

1.1 Heart Function ........................................................................................................................... 6

1.2 Heart Conduction Cycle .............................................................................................................. 7

1.3 Conduction Waves of the Normal ECG ........................................................................................ 7

1.4 Time Intervals and Heart Beat .................................................................................................. 11

CHAPTER 2: Auscultation and Heart Sounds ......................................................................................... 12

2.1 The Stethoscope and its Function................................................................................................. 12

2.1 Heart Sounds ............................................................................................................................... 13

CHAPTER 3: Remote and Automated Monitoring ................................................................................ 16

CHAPTER 4: Analog Circuit Design ........................................................................................................ 17

4.1 ECG Circuit Elements ................................................................................................................. 17

4.2 Stethoscope Circuit Elements .................................................................................................... 20

4.3 Mixing of the Two Signals.......................................................................................................... 21

CHAPTER 5: Physical Implementation of Prototype .............................................................................. 23

5.1 Breadboard and PERFBoard Implementation ............................................................................ 23

5.2 ECG Electrodes .......................................................................................................................... 24

5.3 Leadwires and Trunk Cable ....................................................................................................... 25

5

5.4 Stethoscope .............................................................................................................................. 25

5.5 Device Enclosure and Outputs................................................................................................... 26

5.6 Notes ........................................................................................................................................ 27

CHAPTER 6: Signals Acquired ............................................................................................................... 29

6.1 Signals Acquired Via Oscillator ..................................................................................................... 29

6.2 Signals Acquired Via Soundcard ................................................................................................... 31

CHAPTER 7: Epilogue ............................................................................................................................ 34

7.1 Ideas for Future Implementation - Improvement ......................................................................... 34

7.2 Conclusion ................................................................................................................................... 35

Appendix .......................................................................................................................................... 36

Bibliography ...................................................................................................................................... 40

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Introduction / Motivation for the Thesis

Each year cardiovascular disease (CVD) causes 3.9 million deaths in Europe and

over 1.8 million deaths in the European Union (EU). More specifically, CVD accounts

for 45% of all deaths in Europe and 37% of all deaths in the EU.

While statistics show that CVD mortality is now decreasing in nearly all European

countries as a result of downward trends in several key CVD risk factors such as

smoking, alcohol consumption and levels of mean blood cholesterol, as well as

improvements of CVD medical treatment, other CVD risk factors, particularly the

prevalence of overweight/obesity and diabetes, have increased considerably in recent

decades, would indicate that this favorable trend is unlikely to be sustained.

Furthermore, rapid increases in the elderly population of the world and preeminently

Europe are predicted for the coming decades. With the so-called “greying of Europe”,

comes a steady increase in individuals in need of medical care and monitoring.

Therefore, the need for new technologies which provide assistance in tracking and

monitoring patients’ vitals while minimizing cost and resources, now appears more

imperative than ever.

1

Figure 1: Global Aging, 2000 - 2050:

Percentage of Elderly by Region

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CHAPTER 1: The Heart’s Conduction System and the

Electrocardiogram

The heart beats as a result of the generation and conduction of electrical impulses. The

internal cardiovascular pacemaker system is in charge of producing electrical signs which

serve to control and facilitate compression the heart at appropriate time intervals, paired

with characteristic sounds that mark these events. Examining these patterns uncovers an

abundance of information about cardiac regulation as well as pathology, and one of the

methods used to do this is the electrocardiogram (ECG).

1.1 Heart Electrical Function

At each heartbeat, the heart is depolarized to trigger its compression. The electrical signal

produced starts from the top of the heart and travels the bottom.

Current flow around the heart initially starts as current flow within individual heart cells,

and consequently current flow between cells. At depolarization, positively charged Na+

ions enter the cells; at repolarization, positively charged K+ exit the cells. This causes

excess negative charge outside the cell at the start of the cardiac cycle and excess

positive charge outside the cell at the end of the cycle.

Current exits areas that have just been depolarized, to areas that have yet to be

depolarized. Action potentials are fired due to current flow depolarizing neighboring cells,

subsequently depolarizing the heart in its entirety. When every area is depolarized, there

is no difference in charge and no current flow is present. Because different areas of the

heart are depolarized and repolarized at different time points, such spread in current

creates a characteristic pattern with each heartbeat.

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1.2 Heart Conduction Cycle

There are five steps to the heart’s conduction cycle.

1. An action potential is initiated in the SA node and proceeds to the AV node. The action

potential spreads across the atria.

2. Stimulus arrives at the AV node.

3. Stimulus leaves the AV node and travels through the atrioventricular bundle (bundle of

His). There is delay in the depolarization of the AV node.

4. The impulse arrives at the atrioventricular bundles, the left branch depolarizing first and

the right bundle branch following.

5. Stimulus travels to the myocardial cells of the ventricle by means of the Purkinje fibers.

Figure 2: Basic CV schematic showing SA and AV nodes

1.3 Conduction Waves of the Normal ECG

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The electrical action is transmitted all through the body and can be recognized on the

skin, which constitutes the principle behind the function of the ECG. The ECG machine

provides a trans thoracic interpretation of the electrical activity of the heart over a period

of time by processing the electrical signals collected from the skin by electrodes and

producing a graphic representation of the electrical activity of the heart. The basic pattern

of the ECG is

electrical activity in the direction of the ECG lead causes an upward diversion

electrical activity away from the ECG lead causes a downward diversion

depolarization and repolarization deflections occur in opposite directions

Figure 3: Waveform components of a normal ECG tracing

Normal ECG tracing produces a number of characteristic wave entities. The P wave

registers the depolarization in the atria and is followed by atrial contraction (systole). Atrial

systole extends until the Q wave, at which point, the atria relax. A Q wave is the

descending diversion after the P wave. The R wave takes after as an upward deflection,

and an S wave is any descending diversion after the R wave. The Q, R and S waves are

comprised by the ventricular complex and represent depolarization of the ventricles,

followed by ventricular contraction. The T wave takes after the S wave and represents

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repolarization of the heart muscle. Lastly, at times, an extra U wave succeeds the T wave

and is caused by the return of K+ ions into the muscle cells. This marks the return of

cardiac diastole.

Amid the P-R interim (from atrium depolarization to ventricle depolarization), the

compression motivation is transmitted from the SA node, through the atria, to the AV

node, lastly through the conduction system. These diversions can help clinicians’

assessment of the heart. For example, an interval longer than 0.2 seconds may indicate

harm to the AV node’s conduction system.

Figure 4: A normal ECG in which duration of waves and their amplitudes are plotted.bachelsjso

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1.4 Time Intervals and Heart Beat

Stage Time [sec]

TOTAL VENTRICULAR SYSTOLE 0.3

Isovolumic contraction 0.05

Maximal ejection 0.1

Reduced ejection 0.15

TOTAL VENTRICULAR DIASTOLE 0.5

Isovolumic relaxation 0.1

Rapid inflow 0.1

Diastasis 0.2

Atrial systole or booster 0.1

GRAND TOTAL (Syst+Diast) 0.8

Figure 6: Time Intervals for each stage of the cardiac cycle

Heart rate is the number of times a heart beats per one minute. To calculate heart rate

using an ECG, one needs to measure the interval between two consecutive QRS

complexes (peaks). The value reciprocal to that interval is the heart’s frequency in cycles

per second, or the signal’s period. A normal, resting heart’s average interval between two

QRS simplexes is about 0.83 sec.

𝑓 =1

𝑇=

1

0.83= 1.2 𝐻𝑒𝑟𝑡𝑧 = 1.2 𝑏𝑒𝑎𝑡𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑

12

𝐵𝑃𝑀 = 1.2 × 𝑏𝑒𝑎𝑡𝑠

𝑠𝑒𝑐 ×

60 𝑠𝑒𝑐

1 𝑚𝑖𝑛= 70 𝑏𝑒𝑎𝑡𝑠 𝑝𝑒𝑟 𝑚𝑖𝑛𝑢𝑡𝑒

Heart rate greater than 100 beats per minute corresponds to the case of tachycardia,

while frequency of less than 60 beats per minute is bradycardia.

CHAPTER 2: Auscultation and Heart Sounds

2.1 The Stethoscope and its Function

Most heart diseases are relating to and reflected by the sounds that the heart produces.

Heart auscultation, defined as listening to the cardiac sound, has been an essential

strategy for the early diagnosis of cardiovascular dysfunction.

Traditionally, auscultation required clinical experience and good auditory skills. The rise

of the electronic stethoscope has prepared for another field of electronic auscultation. The

electronic stethoscope is a device that followed the acoustic stethoscope. Unlike acoustic

stethoscopes, sensors in electronic stethoscopes vary widely.

The simplest method of sound detection is achieved by placing a microphone in the chest

piece. The microphone, mounted behind the stethoscope diaphragm, picks up the sound

pressure created by the stethoscope diaphragm, and converts it to electrical signals. The

electronic stethoscope is also used to electronically filter unwanted frequencies and

background noise, and amplify the sound accordingly. This way, the stethoscope helps

towards a primary evaluation by a qualified clinician, as well as serves as a guide for

further diagnostic testing.

The patient is examined in a supine position, and while sitting and leaning forward. Each

area is systematically auscultated for the sounds created, physiologic splitting, respiratory

variations, as well as any accessory sounds.

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Figure 7: Auscultory sites indicating the heart valve

2.2 Heart Sounds

There are two primary heart sounds, S1 and S2, and two additional sounds, S3 and S4

which can indicate pathology or be normal depending on the patient. The first heart sound,

S1, is caused by the closure of mitral and tricuspid valves in the beginning of the systole.

The second sound, S2, is caused by the closure of aortic and pulmonic valves, concluding

the systole. Time period elapsing between S1 and S2 defines systole (ventricular ejection)

and the time between the S2 and the following S1 defines diastole (ventricular filling).

Following sounds commonly named S3 and S4 may be indicative of pathology.

Figure 8: Primary heart sounds S1 and S2 and diastole (light grey) and Systole (white) of the cardiac cycle

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Figure 9: A recording of a single cardiac cycle juxtaposed with pressure changes during the cycle

Heart sounds are regularly aggravated by different components, which can prohibit their

examination. Heart sound disturbances can be separated into two categories: outside

and inside noises or disturbances. The external disturbances incorporate a wide spectrum

of frequencies brought on by noise created by movement, while signals with inward

starting point aggravations comprise of noises produced by digestive and respiratory

procedures. This is why the advancement of accurate noise removal circuits and

algorithms, which are fit to work in a variety of auditory environments and situations, is of

incredible significance and is the exploration subject of numerous researches.

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Figure 10: Relation between ECG tracing and acoustic cardiographic parameters

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CHAPTER 3: Remote and Automated Monitoring

The present-day vision of the healthcare industry is to improve the level of healthcare and

assistance to individuals at any time and any place on the planet in a more cost-efficient

and patient-friendly way. Enhancing the standard of health service quality is an essential

and consistent need. This significant interest in computational techniques, instruments

and devices ready to give help to patients all the more productively, prompted the

advancement and usage of combinative and intelligent systems. In this setting, integration

of medical information from various sources is key.

This need has rendered the gathering of input data from multiple sources onto a limited

number or even a single device that is also mobile and cost efficient, a beneficial

advancement. In this domain, multi-source patient monitoring evolves into an essential

service. This is intensified by the consideration of patients that are bed-bound or face

mobility issues due to chronic diseases, obesity, cardiovascular problems, etc. in need of

constant monitoring. Therefore, the need to develop instruments that aid such cases while

maintaining a high standard of living is evident.

For increasing patient care efficiency, there arises a need to improve the patient

monitoring devices and make them more mobile. As computers and telecommunications

technologies are advancing, it has become feasible to design more portal monitoring and

tele monitoring systems to collect, record, display in real-time the physiological signal

from the human body topically as well as transmit it to any location. This inspired the

development of telemedicine to a large extent. Telemedicine benefits the patients who

are able to receive health care more efficiently; as well as the doctors who can assist

more patients with the use of such technologies and instruments.

Remote patient monitoring allows individuals to maintain independence, prevent

complications, and minimize financial costs. RPM facilitates these goals by delivering

care right to the home. Additionally, patients and their family members feel comfort

knowing that they are being monitored and will be supported if an emergency arises.

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Figure 10: Major components of a Remote Patient Monitoring System

CHAPTER 4: Analog Circuit Design

The two signals are to be transmitted over a single audio input, therefore they have to be

distinguishable in frequency. The signals are going to be broadcasted through the

computer soundcard, which has a maximum usable bandwidth of 20kHz. This coincides

with the full range of human hearing, which extends from 20-20000 Hz. The ECG signal

has a very low frequency, around 0-100Hz, however the real range of the component of

interest, which is the heart conduction system frequency, is even lower, and does not

exceed 6Hz (at a cardiac arrest inducing 360 BPM). For the stethoscope, a low pass filter

will be used to attenuate frequencies over 10k Hz. This way, there will be clear distinction

between the ECG and the stethoscope frequencies, so that they can be later separated

during the digital processing stage.

4.1 ECG Circuit Elements

The voltage of the heart’s signal is very low, around 1mV (to about ~5 mV). Therefore,

the signal needs to be amplified.

IC1

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The INA121 is a FET-input, instrumentation amplifier. It is low-power and offers a large

degree of accuracy. The INA121 receives the left and right arm and right leg ECG signals

as input. Gain on the INA121 is set by connecting a single external resistor, RG,

connected between pins 1 and 8.

G = 1 +50kΩ

𝑅𝐺

IC2 & IC3

For the Right Leg input, I used the LM741 as IC3, which is a general purpose single

operational amplifier as an inverting amplifier, followed by a second LM741 used as a

voltage follower (unity buffer amplifier).

IC4

IC4 is an 741 OP amp used as a low pass filter attenuates the frequency to lower than

6Hz and amplifies the signal.

f = 1

2𝜋𝑅𝐶=

1

2𝜋×18×103×1,5×10−6 = 6𝐻𝑧

Figure 12: Bode Plot of Frequency Response of Low Pass Filter

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IC5

An inverting 741 OP Amp was used for IC5. Via negative feedback sent back into the

inverting input terminal of the op-amp using a feedback resistor, the relationship

between the output and the inverting input terminal forces the differential input voltage

towards zero.

IC6

To produce a frequency modulated signal, the frequency of the radio carrier is adjusted

in line with the amplitude of the incoming signal. MC1496 is an integrated circuit used

as a modulator, modulating the signal with a specific carrier frequency which is to be set

by the IC. MC1496 was chosen due to its good carrier suppression, adjustable signal

handling and high common mode rejection. The carrier frequency used is 20kHz.

Figure 13: Typical Modulator Circuit using the MC1496

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Figure 14: Modulation of a wave

IC8

The ICL8038 IC is a waveform generator circuit, capable of producing high accuracy

waveforms. The frequency (or repetition rate) can be selected externally from ~0 to

300kHz. In this circuit, this is done via a variable resistor (VR8) and the frequency is set

to 20kHz to be modulated with the amplified ECG signal.

4.2 Stethoscope Circuit Elements

IC9, IC10, IC11

The stethoscope signal as generated from the stethoscope and amplified by the

electronic microphone is inputted into IC9, which is a low noise microphone preamplifier

cascaded together with IC10, a second order low pass filter. IC11 is an amplifier. All of

the OP amp circuits are implementing 741 integrated circuits.

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4.3 Mixing of the Two Signals

IC7

The next step is to combine the two signals from the ECG and the Stethoscope. There

are many ways to achieve this, but in this case, another 741 was used in an inverted

fashion as a simple 2-channel audio mixer.

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Figure 15: Final Device Schematic

23

CHAPTER 5: Physical Implementation of Prototype

5.1 Breadboard and PERFBoard Implementation

During the initial testing stage, after each individual step was implemented, it was

control-tested using an oscilloscope. Since there is a relatively large number of

variables in the circuit, each value was calculated beforehand and tested afterwards.

It was decided that a breadboard circuit board was to be used for the very initial

implementation stage. The main advantage of a breadboard, especially in this project,

was the fact that its solderless nature allows for more experimentation and even room

for errors that can be resolved instantly. Therefore, it was beneficial to use one and

assess how different given sets of parameters performed in order to determine the most

favorable combination, as well as examine how different integrated circuits performed

compared to one another and reach a decision regarding which ones would be used.

Another advantage taken into consideration was the easy access to power wherever

needed in the circuit. Due to the size of the project, additional breadboards were used,

in order to extend surface area.

Figure 16: Complete breadboard prototype of the device.

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To reserve the prototype developed on the breadboard accessible for future analysis,

improvements and additions, it was decided that a copy would be implemented using

PERFBoard. PERFBoard is a semi-permanent solution, and not as easily modifiable as

a breadboard. However, the circuit elements are considerably more secure than those

on a breadboard, whose unsteady nature makes them prone to disconnections

throughout the circuit, something which is difficult to pinpoint and correct afterwards.

Instead of being straightforwardly removable, the discrete components on a PERFBoard

are soldered in their designated positions.

Figure 17: The PERFBoard-Implemented Prototype

5.2 ECG Electrodes

Modern biomonitoring electrodes used in ECG applications most often incorporate a

plastic substrate that is covered with a silver/silver chloride ionic compound as a

sensor which converts the ion current to electron current. Electrolyte gel is applied

25

between the electrode and tissues. On the opposite side of the electrode is conductive

metal attached to a lead wire connected to the instrument.

5.3 Leadwires and Trunk Cable

The leadwires used were the Philips M1672A 3-lead Leadwires (clip) and the trunk

cable used was the Philips M1669A 3-lead ECG Trunk Cable (12-pin).

Figure 18: Leadwires used for the ECG signal acquisition

5.4 Stethoscope

The stethoscope used is an Omron Sprague Rappaport Stethoscope, an affordable

multipurpose instrument used for the detection of a wide range of heart and lung

sounds. The diaphragm is used to detect faint, high-pitched. respiratory sounds and

components of certain cardiac sounds. The diaphragm attenuates Iow-pitched

frequencies, making high-pitched frequencies easier to recognize. The bell is to detect

low-frequency cardiac sounds, such as first and second heart sounds (S1 and S2). A

plastic junction was used to splice the two eartips together, creating a good environment

for the sound to be more clearly audible through the input, as well as reducing outside

soundwaves.

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5.5 Device Enclosure and Outputs

It was important to choose an enclosure that was large enough to accommodate the

circuit in its entirety. LEDs were added to indicate the 12V, -12V power supply and the

0V. Even though the aim of this project is to ultimately acquire the mixed signal of both

the ECG and the stethoscope, a total of five outputs were chosen in order to track the

signal and monitor it along its path.

O/P1: The stethoscope’s signal after it has been filtered and amplified.

O/P2 & O/P3: These outputs are used for general monitoring of the ECG signal using

the oscillator. O/P2 is the instrumentational amplifier INA121 ‘s output signal, while

O/P3 is the output signal of integrated circuit 5, which was the inverter amplifier.

O/P4: This is the ECG signal as observed in the output of the modulator circuit.

O/P5: This is the final signal. It is the combination of the two signals, the ECG and the

stethoscope’s, after they have been filtered and modified, as it observed in the output of

the mixer circuit.

Figure 19: The completed device and its outputs

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5.6 Notes

Patient safety was the first and foremost aspect taken into consideration when

designing the circuit for this device. For this reason, it was firstly ensured that

there were no ground nodes to create dangerous ground loops with. Additionally,

resistors were added in the beginning of the circuit between the electrode inputs

and the op-amp inputs. This could alternatively or additionally be done with

diodes, or in future development, with optical isolation.

Measurement of the ECG signal can be challenging due to the presence of the

DC offset and various interference signals. This potential can be up to 300mV for

a commonplace electrode. The interference signals include the 50 Hz

interference from the power supplies, motion artifacts due to patient movement,

pulses, pacemaker pulses, other monitoring equipment, etc.

Shielded cable was used for the ECG inputs. Shielded cable ensures minimal

signal interference, especially that of the 50Hz variety, which is inevitably found

in the environment all around the device. This is because cable shielding acts as

an electromagnetic energy interceptor, preventing electrical interference from

reaching the cable’s core conductor and disrupting the data signal.

A challenge when acquiring the stethoscope signal through the microphone was

sound interference from the environment and more notably from the device

operator’s movements in handling the stethoscope which would at times result in

slight tapping noises.

Some of the 50Hz common mode interference was also cancelleded out with the

INA121 instrumentation amplifier, which removes the AC line noise common to

both inputs. Further rejecting line noise is done through the third ECG input

which actively cancels the interference. (Driven Right Leg circuitry).

Normally such a project would require an Analog-to-digital Converter (ADC),

however this was deemed unnecessary since the end signal is to be transmitted

through the computer’s microphone input, and therefore processed by the

computer soundcard, essentially a low-sample-rate, high-resolution ADC in itself.

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Figure 20: General diagram of the project

29

CHAPTER 6: Signals Acquired

6.1 Signals Acquired Via Oscilloscope

While constructing the analog circuit, the signal was monitored throughout its path via

the oscilloscope, making changes where needed. Below are the signals measured at

different stages in the final circuit, as observed and monitored on the oscillator

instrument.

Figure 21: (a) O/P 4 and (b) O/P 3 as shown by the oscilloscope.

Figure 21(b) above shows the oscilloscope depiction of the ECG signal waveform as

captured in O/P 3, which is inverted amplifier IC5’s output displayed on the bottom.

Above it is the frequency modulated signal at 20kHz. Looking closely at the ECG signal,

we can see that while there is a small amount of noise that is to be expected, the P,

QRS, and T peaks are very clearly distinguishable. Between the two QRS peaks there

was a 0.7 sec interval, which means the patient’s (60 y/o male) heart rate at that time

was 1.43 beats per second or about 85 BPM.

30

Figure 22: Heart cycle as shown by the oscilloscope when reading the ECG output

Figure 23: O/P 5 signal (b) O/P 3 signal as shown by the oscilloscope

Figure 23(b) above depicts the ECG’s electrical signal on the bottom of the oscilloscope’s

screen, and above is the O/P 5 signal. 23(a) Is the O/P 5 signal which is the signal

outputted by the final stage from the mixing op amp. It is also the signal which is to be

transmitted through the computer’s audio input. It can be observed that this signal at its

current form has no distinguishable peaks or amplitude changes that point towards a

specific stage in the cardiac cycle. This signal can be later demodulated and further

31

analyzed using a number of programs such as MatLab, LabView, Audacity (audio

processing) and many others.

6.2 Signals Acquired Via Soundcard

(a)

(b)

(c)

(d)

Figure 24: (a) O/P 1 (b) O/P 5 with stethoscope signal only (c) O/P 4 (d) O/P 5 waveforms as displayed

in Audacity

Images above depict the waveforms at various stages of the circuit and at different

outputs. The software used to record the signals was Sound Recorder, which is

included in most Windows versions. The file was converted to .wav (from .wma or .m4u

depending on the Recorder version) and imported to Audacity, which is an open source,

32

cross-platform audio editor and recorder. Alternatively the recording can also be made

using the Audacity software directly.

The signal depicted in Image (a) shows that of the stethoscope as acquired from O/P 1.

It can be seen that the audio is quite high in amplitude. However in Image (b), which is

the same signal but acquired at the final stage of the analog circuit, O/P 5, the signal is

clearly identifiable as heart sounds.

Figure 25: The heart’s S1 and S2 sounds as seen in the stethoscope O/P 5 signa

From Image it can be seen that even though there is some noise since the signal is

shown as is, with no digital filtering, the S1 and S2 peaks are clearly visible at generally

steady intervals. To illustrate this, the peak distance between two consecutive S1

sounds was measured to be 0.847 seconds. This translates to 1.18 beats per second,

and about 71 BPM, which is consistent with the heart’s average resting beats per

minute for the patient (60 y/o male).

Waveform (c) is the ECG signal produced by modulation. Waveform (d) is the ECG

signal as observed in O/P 5, after the mixer circuit. Both of these signals can be

demodulated using software such as MatLab.

33

CHAPTER 7: Epilogue

7.1 Ideas for Future Implementation - Improvements

With the completion of this project and the knowledge gained it is concluded that there

could be many upgrades, but due to time constraints, they could not be realized within

this setup. It is strongly believed that such a device is of compelling potential, because

of its multifunctionality, low cost and relative simplicity in both components and

processing.

The current project features a 3-lead ECG, but an upgrade from the current would be a

12-lead ECG signal acquisition. A 12-lead ECG gives a more detailed look at the heart's

three areas (anterior, lateral, inferior), and can also identify axis and specific bundle

branch blocks.

Since a key aspect of this project is its dual purpose, this advantage can be diversified

even further to include not only ECG and stethoscope signal acquisition and processing,

but signals from other bioinstruments as well. For example, additional sensors for

acquiring biosensor parameters such, body temperature, as blood oxygen saturation

could be implemented. Oximeters are lightweight monitors that attach to a fingertip to

monitor the patient’s amount of oxygen saturation. Oximeters use light of two different

wavelengths on one side and a sensor on the other side to calculate how much of the

blood is carrying oxygen (hemoglobin). Another addition could be an EEG which,

similarly to the ECG, uses electrode inputs.

The device described in this paper focuses mainly on the analog part of the design.

Further improvements could involve a better filtering mechanism on the digital level, as

well as further processing, firstly to create a higher quality signal, and secondly utilizing

machine learning to extract components of the signal and analyze them. This analysis

can not only provide doctors and caregivers with basic stats such as heart rate,

respiratory rate, and pulse pressure variation, but also use this machine learning to

identify cardiac episodes, arrhythmias, or even falls (especially for elderly patients),

proving extremely useful in remote patient care. This information could be relayed to the

34

clinicians wirelessly, via an app or SMS message, in order to assume action and treat or

care for the patients as needed.

Another idea for a future implementation could be the storage of daily biosignal

information logs online, thus creating a database that serves as an Internet patient file

that doctors can access at any time when reviewing a patient’s history.

7.2 Conclusion

The aim of this project was to design and fabricate an ECG and stethoscope signal

acquisition system that would transmit the signals mixed at different frequencies over a

single audio channel. Most of the project’s goals were met, however, as was elabor

ated above, the possibilities for improvements and additions are vast.

The implementation of this design is relatively simple and low cost, and a device such

as the one developed herein can be found useful in a number of patient environments

and situations, especially in outpatient care and remote monitoring. Multiplexing several

signals over a single channel is of significant importance, especially when considering

the channel over which they are transmitted is located in a device already existing in

most modern households, making this device accessible to a large part of the

population.

35

Appendix

36

37

38

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