cardio-vascular response following exercise response following exercise by: mohamed ali-eid, ... in...
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Cardio-Vascular Response Following Exercise
Final Report for EECE502
By Mohamed Ali Eid Yusr Sabra
Mirna Abou Mjahed
American University of Beirut May 23, 2005
Cardio-Vascular Response Following Exercise
Progress Report for EECE501
By Mohamed Ali Eid Yusr Sabra
Myrna Abou Mjahed
American University of Beirut January 12, 2005
Table of Contents Abstract........................................................................................................ iv List of Figures .............................................................................................. v List of Tables ............................................................................................... vii 1.0 Introduction ............................................................................................ 1 1.1 Background and Overview.............................................................. 1 1.2 Project Objectives ........................................................................... 1 1.3 Report Organization........................................................................ 2 2.0 Literature Review ................................................................................... 4 2.1 The Heart: General Anatomy and Cardiac Cycle ............................ 4 2.2 Heart Signals .................................................................................. 5 2.2.1 Electrical Signal .................................................................... 5 2.2.2 Sound Signal......................................................................... 7 2.2.3 Pressure Signal..................................................................... 8 2.2.4 Impedance Cardiograph........................................................ 8 3.0 Project Approach: Design and Analysis ................................................. 12 3.1 Signal Choice.................................................................................. 12 3.2 ICG Circuit ...................................................................................... 14 3.2.1 Voltage Controlled Current Source. ....................................... 15 3.2.2 Implemented Design. ............................................................. 16 3.3 Computer Analysis.......................................................................... 21 3.3.1 LabView Modules................................................................... 22 3.3.2 Visual Basic Application. ........................................................ 24 4.0 Implementation ...................................................................................... 25 4.1 Implementation of the ICG Circuit ................................................... 25 4.2 Computer Analysis.......................................................................... 26 4.2.1 Implementation of the LabView Modules. .............................. 26 4.2.2 Implementation of the Visual Basic Application..................... 33 5.0 Results and Analysis.............................................................................. 36 5.1 Test Results .................................................................................... 36 5.2 Analysis........................................................................................... 39 6.0 Other Issues........................................................................................... 41 7.0 Conclusion. ............................................................................................ 42 References............................................................................................. 43 Appendix................................................................................................ 45
iii
Abstract Cardio-Vascular Response Following Exercise By: Mohamed Ali-Eid, Yusr Sabra and Mirna Abou Mjahed The correlation between the rate the strength of a heart beat returns to normal
following exercise and cardiovascular health has never been documented.
This report presents the process of developing a device capable of picking up
an impedance signal from the heart and transmitting it into a personal
computer for analysis. The impedance signal is proportional to the stroke
volume (i.e. strength of the heart beat) which allows the device to be used for
research on the correlation between cardiac health and the rate of decrease
of the stroke volume.
iv
List of Figures Figure 1 Basic anatomy of the human heart ................................................ 4 Figure 2 A typical ECG recording of a single cardiac cycle.......................... 6 Figure 3 A typical sound recording of a single cardiac cycle........................ 7 Figure 4 Typical impedance cardiography signal ......................................... 9 Figure 5 Diagrammatic representation of impedance cardiography............. 10 Figure 6 Stages of the design process......................................................... 12 Figure 7 The basic concept governing impedance cardiography................. 14 Figure 8 Voltage Controlled Current Converter............................................ 15 Figure 9 The 4-Electrode arrangement ........................................................ 17 Figure 20 Schematic of the developed circuit for current injection and voltage pickup .......................................................................................................... 17 Figure 11 Input resistance and CMRR vs. Frequency curves...................... 19 Figure 13 XR-2206 oscillator ...................................................................... 20 Figure 14 Schematic showing the implemented circuit of our project. ......... 21 Figure 15 Block diagram showing procedure following signal detection. ..... 22 Figure 16 Amplitude modulation resulting from the followed procedure....... 23 Figure 17 Waveform to demonstrate the application operation.................... 24 Figure 18 Implemented circuit...................................................................... 25 Figure 19 Block diagram showing the transmission of the signal from the PCB to the pc ....................................................................................................... 27 Figure 110 The SCB-68 used to transmit the signal from the PCB onto the PC..................................................................................................................... 27 Figure 20 DAQ-assistant VI module in LabView. It is set to sample at a rate of 100kHz......................................................................................................... 27 Figure 21 AM Demodulation module (Envelope Detection) on LabView...... 28 Figure 22 LabView module used to verify Envelope Detection is functioning effectively. .................................................................................................... 29 Figure 23 Carrier signal ............................................................................... 29 Figure 24 Inputted signal ............................................................................. 30 Figure 25 Modulated signal.......................................................................... 30 Figure 26 Demodulated signal = Inputted signal.......................................... 30 Figure 27 Low pass filter on LabView .......................................................... 31 Figure 28 Configuration of the low pass filter............................................... 31 Figure 29 Differentiation of the demodulated and filtered signal .................. 32 Figure 30 Extraction of peak points onto a text file. ..................................... 32 Figure 31 LabView application for signal processing. .................................. 33 Figure 32 The developed user-interface of our application .......................... 33 Figure 33 Excel output for the Average Rate of Decrease = 0.021.............. 35 Figure 34 VB output for the Average Rate of Decrease = 0.021.................. 35 Figure 35 Electrode placement ................................................................... 36
v
List of Tables Table 1 Results produced upon testing the implemented circuit with a variable resistor ........................................................................................................ 26 Table 2 Date file to be tested on the VB application and Excel worksheet . 34 Table 3 Results of testing project on Mohamed. .......................................... 37 Table 4 Results of testing project on Mirna.................................................. 38
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1.0 Introduction
1.1 Background and Overview
The Nebraska Medical Center at Clarkson and University Hospital reports that
almost one of 2.5 deaths result from cardiovascular disease. Cardiovascular
disease presents a major health and economic burden throughout the world
and especially on developing countries. According to the Australian Institute of
Health and Welfare, by year 2020 heart disease will have grown to become
the leading health problem for the world.
The ECG, stethoscope, X-ray, ultrasound and stress tests are all examples of
diagnostic techniques used to examine heart functionality. Most of these
devices yield data produced over and in a defined period of time and space
(medical center, on a treadmill, in a physician’s clinic, etc…). We postulate
that effectiveness of diagnosis could be improved by results confined neither
in time nor space. In other words, we propose the development of a device
capable of picking up an appropriate signal from the heart and picking it up
and storing it on any personal computer for later analysis by the physician. It
is hoped that the proposed diagnostic method might detect abnormalities in
the cardiovascular system earlier than conventional methods.
1.2 Project Objectives
The problem was suggested by our supervisor Dr. Nassir Sabah who
specified that our device should be capable of:
1- Detecting a signal that is proportional to the rhythm and the strength of
the heart beat.
2- Transmitting the signal to a personal computer for analysis.
1
3- Calculating the rate of decrease of the impedance cardiograph signal
to normal following exercise.
In the Interim Report we presented in Fall2005-2006 we had included the
storage of the signal as one of the project’s objectives. We chose to modify
this section for feasibility reasons.
Research shows that the correlation between the rate and the strength of a
heart beat as they return to normal following exercise and cardiovascular
health have not been adequately documented. Thus we emphasize that the
signal detected by our device should carry information about the strength of
the heart beat as well as the heart rate. By strength of heart beat we are
referring to the stroke volume of the heart defined as the amount of blood
pumped by the heart into the body. The stroke volume is a well identified
variable parameter correlated to the healthiness of the heart.
As a result, the completion of our project will present a device that has the
potential of investigating a new method for the detection of abnormalities in
the cardiovascular system.
1.3 Report Organization
Literature reviewed throughout the fall semester is presented in Chapter 2 of
the report. It is followed by Chapter 3 which considers our project approach
and describes the various stages of our design. Chapter 4 discusses the
implementations of our design. Chapter 5 includes the results of runs we
carried out on the implemented design. Chapter 6 describes the health,
economic and safety considerations of our design. Our conclusions are finally
presented in Chapter 7.
2
In summary, the report is organized as follows:
Chapter 1.0 Introduction
Chapter 2.0 Literature Review
Chapter 3.0 Project Approach: Design and Analysis
Chapter 4.0 Implementations
Chapter 5.0 Results and Analysis
Chapter 6.0 Health, Safety and Economic Considerations
Chapter 7.0 Conclusion
3
2.0 Literature Review
The purpose of this literature review chapter is to develop a solid background
in the topic of our project. Such a background will allow us to submit a design
whose every stage can be substantiated. We rely on consultations with Dr.
Sabah, the AUB Library services and Internet resources in our search.
2.1 The Heart: General Anatomy and Cardiac Cycle
This section includes a general overview of the functionality of the normal
heart whose basic anatomy is illustrated in Figure 1.
Figure 1 Basic anatomy of the human heart. Retrieved and modified from http://www.cvphysiology.com
As shown in the figure above, the human heart is composed of four chambers
(2 atria and 2 ventricles). Each atrium is separated from its corresponding
ventricle by a valve. Valves are also present between the ventricles and their
corresponding arteries. As a result, the human heart can be thought of as two
separate pumping systems operating within a single organ. The right pump
sends CO2 rich blood to the lungs whereas the left pump sends O2 rich blood
to the body [10’].
The cardiac cycle, movement of blood through the heart, is divided into two
parts: diastole and systole. During the diastole, blood from the body empties
into the right atrium whereas blood from the lungs empties into the left atrium.
4
The pressure developed in the atria due to their filling causes the AV valves1
to open; blood moves to fill 80% of the ventricles. The following contraction of
the atria will allow the rest of the blood to move into the ventricles. During
systole, the ventricles contract and the rise in pressure in these chambers
forces the AV valves to close and pulmonary and aortic valves to open
delivering blood to the lungs and body, respectively. Once the blood leaves
the heart and the pressure drops in the relaxing ventricles, the pulmonary and
aortic valves close [10].
There are a number of signals that can be picked up from heart during its
cardiac cycle. The following section presents an overview of each of these
signals, one of which we will choose for our project.
2.2 Heart Signals
This section of the report is further divided into 4 sections, each dealing with a
different signal that can be picked up during the cardiac cycle.
2.2.1 Electrical Signal
The most common and well-understood signal arising from the heart during
the cardiac cycle is the electrical signal recorded as the electrocardiogram
(ECG). The electrical signal arises from the polarization and depolarization of
the cardiac muscle membrane-the basic concepts behind muscle contraction.
A typical electrical signal wave recorded by an ECG is presented below along
side an explanation of its various sections.
1 Valves separating the ventricles and atria.
5
Figure 2 A typical ECG recording of a single cardiac cycle. Retrieved and modified from http://www.guidant.com/
The P-wave occurs at the contraction of the atria, at the beginning of systole
and end diastole. The QRS- complex occurs at the contraction of the
ventricles, i.e. during systole. The T-wave occurs at the relaxation of the
ventricle, i.e. during end systole [8].
The ECG signal if measured over a period of time will be capable of producing
information about heart rate. Dr. Sabah pointed out that we should check
whether or not the magnitude of the QRS complex varies in proportion to the
heart beat strength.
The Circulation Journal of the American Heart Association published an article
by Simoons M.D and HugenHoltz M.D entitled ‘ECG Changes During and
After Exercise’. The purpose of the authors was to analyze the magnitude and
direction of time-normalized P, QRS and ST sections of the ECG during and
after multistage exercise. The authors found that the magnitude and spatial
orientation of the maximum QRS vector remains substantially constant during
and after exercise [17].
The findings of the article presented above suggest that the electrical signal
ofthe heart is not proportional to the strength of the heart beat. Although the
heart rate can be measured from the ECG, the strength of the heart beat can
not.
6
2.2.2 Sound Signal
After the electrical signal we considered the sound signal of the heart.
Listening to the heart is a diagnostic method used long before the
development of the current stethoscope. The heart produces different sounds
throughout its cycle and a trained physician is capable of identifying faults in
the heart by listening to the sounds it produces.
A typical sound signal is presented below along side an explanation of its
various sections.
Figure 3 A typical sound recording of a single cardiac cycle. Retrieved and modified from http://www.ed4nurses.com
S1 represents the closing of the AV valves after the blood moves from atria to
the ventricles. S2 represents the closing of the aortic and pulmonic valves
after the blood leaves the ventricles. S1 is lower pitched and has a longer
duration than S2 [18].
There are two types of stethoscopes present on the market today: acoustic
stethoscopes and electronic stethoscopes [7]. An electronic stethoscope
appeared to be the ideal solution for our design. Upon literature review and
consultation with Dr. Sabah, however we found that electronic stethoscopes
have their own draw-backs which can be summed up by the fact that they are
very sensitive to impact, manipulation and ambient noise [7].
7
At that point, Dr. Sabah suggested we try to look into other signals that can be
picked up by heart.
2.2.3 Pressure Signal
We came up with idea of using a pressure transducer placed on the chest
above the heart. Changes in the size of the heart during the cardiac cycle
would be perceived as changes in pressure by this transducer, converting the
signal into an electrical one.
After referring to several resources, however, we found that the literature
available on pressure transducers, especially when used in the cardiovascular
field, to be extremely limited.
2.2.4 Impedance Cardiography
The final signal we considered was suggested by Dr. Sabah. In short,
electrodes around the neck and around the waist cause a current of low
magnitude and high frequency to flow through the major vessels connected to
the heart. The resulting changes in impedance provide a rough estimate of
beat-by-beat changes in cardiac output [12].
Impedance of the thorax can be considered to consist of two types of
impedances: 1- time-invariant impedance due to tissues in the thorax and 2-
time-varying impedance due to time variations associated with the cardiac
cycle [12].
Because of the complex structure of the thorax, the origin of the impedance
signal has been extensively studied [15]. In 1952, M.D Bonjer attributed the
impedance change to the changes in the size of both the heart and blood
vessels [2]. A later study done by Patterson proposed that there could be up
to four sources for the ICG (impedance cardiograph) signal: 1- ventricular
8
blood volume and velocity, 2- aortic blood volume, 3- Lung resistivity and 4-
Blood resistivity [21]. This study demonstrated that the use of band
electrodes for current injection and impedance measurements produces an
impedance cardiograph signal that is a representation of all four mentioned
factors, none dominating the rest. It further demonstrated that the use of spot
electrodes placed near the walls of the heart generates an impedance signal
of which 80% is contributed from the ventricular contraction. The figure below
illustrates a typical signal picked up by impedance cardiography.
Figure 4 Typical impedance cardiography signal. Retrieved from [22].
As seen in Figure 4, the change in impedance Z is differentiated into dZ/dt;
the signal used in the analysis. This signal takes into consideration only time-
varying impedances and ignores the constant impedances of time-invariant
tissues of the thorax. The ECG signal is normally recorded along side an
impedance cardiograph for the identification of specific points on the dz/dt
signals used for the calculation of the value of the stroke volume.
9
The variations present in Figure 4 can be explained as follows considering the
use of spot electrodes placed near the anterior walls of the heart in order to
amplify the contribution of ventricular contraction. As mentioned previously in
2.1, the contraction of ventricles is followed by the movement of blood from
the ventricles into the aorta and the pulmonary artery. Since blood is a highly
conductive tissue, its leaving the heart chambers leads to the increase shown
in the recorded signal [21]. Since the signal depends on blood in the
ventricles, Z decreases as the ventricles fill up and increases as they empty.
Impedance cardiography has been employed in the medical field to calculate
stroke volume and cardiac output using Kubicek’s model. Significant
correlations (0.63-0.97) between the stroke volume measured with impedance
cardiography and invasive clinical techniques exist [21]. Nevertheless,
Kubicek’s model has been challenged in a number of papers and is not widely
accepted in the medical community as a reliable method to calculating the
stroke volume [16]. However, though the absolute value of the stroke volume
calculated by Kubicek’s model is controversial, the relative changes of the
stroke volume are valid [3]. In other words, the ICG is proving to be an invalid
signal for the direct measurement of stroke volume but, using spot electrodes
local events such as ventricular volume changes can be traced [21].
Figure 5 Diagrammatic representation of impedance cardiography. Retrieved from [11].
10
Changes in skin temperature and hydration have been suggested as a
common cause to variations in ICG measurements. As mentioned earlier, the
type of electrodes (band or spot) used is significant for determining the
accuracy of the measured signal. Changes in skin temperature and hydration
however have been found to have no significant effect on the impedance
measurements when four electrodes are used – 2 for current injection and 2
for signal acquisition. The use of only two electrodes however does cause
such changes [4].
11
3.0 Project Approach: Design and Analysis
Based on the idea developed in the introduction, we represent the stages of
our design in the figure below.
Figure 6 Stages of the design process
In the sections that follow, we describe the specifications of each stage in
Figure 6.
3.1 Choice of Signal
Chapter 2 included a literature review of all the signals that can be recorded
from the heart. Among the signals discussed (electrical, sound, pressure and
impedance) the impedance signal was selected for recording. We justify our
choice by revisiting each signal.
Electrical Signal: It was presented in Section 2.2 that the ECG is not
proportional to the strength of the heart beat (i.e. to the stroke volume). This
renders the signal not useful for our purpose.
Sound Signal: Though the sound signal is proportional to the strength of the
heart beat, the picking up of this signal presents a set of difficulties we may be
12
capable of avoiding by choosing another signal. Among these preventable
difficulties is the sensitivity of the microphone required to pick up the signal as
mentioned in Section 2.3. Though such sensitivity is needed to detect the
sound signal of the heart, it also causes it to be highly susceptible to ambient
noise. Movement of patient, breathing of patient, background noise, etc…
present only a few examples of what contributes to ambient noise.
Filtering might come up as a suggested solution to this noise. It however
would require additional signal processing at this stage of the design process.
Consulting with Dr. Sabah, he suggested we continue to look into other
signals that might require less processing in the first stages.
Pressure Signal: The lack of literature on the use of pressure transducers in
cardiovascular applications as mentioned in Section 2.4 led us back to Dr.
Sabah. Dr. Sabah explained that the pressure signal would not only be
difficult to pick up but would yield results that are not very accurate. As a
result we opted to overlook the pressure signal.
Impedance Signal: The impedance signal produced due to changes in the
volume of the tissues of the thorax was the final signal we looked into and the
one we selected. As mentioned in Section 2.5, impedance cardiography –
which measures impedance changes of the chest - produces a signal
proportional to the stroke volume. Even though this signal’s accuracy in
determining the exact value of the stroke volume has been refuted in a
number of articles, the relative changes of the stroke volume it measures
have been widely accepted.
13
Furthermore, the picking up of the impedance signal is expected to present
fewer obstacles than other signals since it depends on the injection of a
constant current followed by the measurement of the resulting voltages.
Since the impedance signal provided solutions to problems presented by
earlier visited signals, it was selected for recording; thus establishing the first
stage of the design process.
3.2 ICG Circuit
As shown in Figure 6, the second stage we set to complete was the
development of a circuit for the measurement of impedance changes in the
thorax. Figure 7 below illustrates the basic idea behind impedance
cardiography.
Figure 7 The basic concept governing impedance cardiography. Results can be compared to the variations seen in Figure 5.
The constant current normally applied is an AC current of the frequency 1-100
KHz and amplitude 0.8-4mA [15], [21], [22], [3].
The use of AC current is preferred over DC current since a DC current
allowed to flow through the skin for a period of time would cause electrolysis
of the blood and chemical burns [6]. Furthermore, the threshold for perception
14
of alternating current depends on its frequency. Currents with frequencies
between 1 and 100 KHz have a perception threshold greater than 10mA (rms)
[5]. The applied current in impedance cardiography is thus safe to use if
maintained at mentioned levels.
3.2.1 Voltage Controlled Current Source
The main component of our circuit as discussed in the previous section is a
current source which provides a constant current of around 1mA at a
frequency between 1 and 100 kHz. The first circuit we looked into is a typical
voltage to current converter circuit shown in Figure 8.
Iin
IL
.
We
L =I
The
In o
Figure 8 Voltage Controlled Current Converter. Retrieved from [23]
performed the necessary analysis on the circuit and found that,
2IC
in
31
2
AV
RRR
gain on IC2 is given by the equation 1+=A2IC
3R50
rder to achieve a gain , we chose 1=A2IC ΩM10=R 3
15
Setting , it followed that the generated current should be ΩK1=R=R 21
6L 10Vinx=I A.
During the first stages following the construction of the presented circuit, we
carried out the testing using AC and DC inputs. The results of the tests were
highly irregular, irreproducible and inaccurate.
3.2.2 Implemented Design
After our attempts with the voltage controlled current source on which we
experimented for over a month, we worked out the design of a custom circuit
compatible with our purpose. The first issue we identified was the range of
values expected at the load.
Cardiograph Impedance vs. Skin Impedance
Unless designed to do otherwise, a circuit intended to detect cardiac
impedance inevitably picks up skin impedance too. Documented studies
record skin impedance to be in the order of Meg-ohms, under DC conditions,
[9] and base cardiac impedance in the range of 25.15 ± 1.74 ohms [14].
These values indicate that in order to accurately record changes in cardiac
impedance, skin impedance should be eliminated.
Electrode Selection
As mentioned earlier, band electrodes and spot electrodes yield different
results upon use for current injection and voltage measurement. Because our
objective is to capture a signal which is most representative of changes in the
ventricles, our design makes use of spot electrodes [21]. Furthermore, the
article demonstrates that the use of the 4-electrode arrangement instead of
the 2-electrode arrangement allows the elimination of skin impedance from
the measurement.
16
As a result, we opted to use the 4-electrode arrangement (2 for current
injection and 2 for voltage recording) as shown in Figure 9. Such an
arrangement eliminates complexities in our circuit and allows us to design for
a load in the order of hundreds of ohms instead of mega ohms.
.
Description of the Designed Circuit
3 2
1
.
Figure 10 Schematic of the developed circuit for current injection and voltage pickup- Section 1:
This section op
with a constant
that Rload is in
ohms; as a res
through the load
Figure 9 The 4-Electrode arrangement
erates as a simple current source and provides our load
current when . It was previously demonstrated LoadZR >>1
the order of hundred ohms with variations in the order of
ult, if R1 is chosen to be >50x Rload, the resultant current
would have a magnitude1
inload R
V=I .
17
The initial circuit we tested consisted of Section I only and a variable
resistor in place of the load. Even though the circuit proved to be functional
and accurate, when it was set to inject 1 mA current the picked up voltage
was in the order of mV. Sections II and III were added to the circuit in
order to avoid resolution complications when the picked up signal is
introduced into the computer.
- Section II:
This section consists of the op-amp LM741 in the unity-gain configuration.
The op-amp exhibits high input impedance and thus when placed in
parallel with the body’s relatively low impedance is assumed to allow the
following section to amplify the voltage with minimal current consumption.
- Section IV:
The final section, also built from the op-amp LM741, amplifies the picked
up voltage 10 times before it is measured or entered into the personal
computer.
- Component Values:
• R1 > 50x RLoad R1 = 3.3kΩ
• For V10=VRR
=V inin2
3o and ΩK1=R 2 ΩK10=R 3
- ILoad :
Knowing that1
inLoad R
V=I , and that we are designing for ΩK3.3=R1
mA1=~ILoad pk-pk at a frequency 1KHz-100KHz; we found that Vin should
be set to 3V pk-pk . The frequency of Vin and thus that of ILoad was
selected in accordance with the operation curves of the LM741 presented
in Figure 11.
18
Figure 11 Input resistance and CMRR vs. Frequency curves
Taking into consideration: a- Safety/Health limitations b- High input resistance
condition and c- High CMRR condition, we set Vin and thus ILoad to work at
10KHz.
Oscillator Circuit of the Input Voltage
In order to avoid the usage of a function generator or AC power supply and
allow easier usability, we replaced Vin by an oscillator circuit based on the
XR2206 IC shown in Figure 12.
19
Figure 12 XR-2206 oscillator.
Component Values:
From the XR2206 datasheet we determined that:
• A typical value for R is 10KΩ when a signal of 10 KHz is to be
generated (frequency was confirmed from the datasheet to cause <1%
distortion).
• Fµ01.0=K10.K10
1=
fR1
=C⇒RC1
=f
• The output has an amplitude of 60mV per KΩ of R3 for
an output of 3V pk-pk.
ΩK25=R⇒ 3
• The oscillator was followed by a DC blocking capacitor in order to
eliminate the DC offset which could be harmful for the user.
20
Schematic of Full Circuit
The figure below shows the schematic of the implemented circuit including all
components and values.
Figure 13 Schematic showing the implemented circuit of our project.
3.3 Computer Analysis
The previous section 3.2 described the stages and final outcome of the design
process of the circuitry responsible for detecting impedance variations. The
schematic in Figure 13 shows I/O’s labeled TB, FB and OUT. The TB and FB
are the nodes through which the body is connected. From the TB (To Body),
alligator wires inject current into the user’s body; the resulting voltage
variations are then picked up and read from the FB (From Body) alligators.
The signal is amplified and inputted into the computer through the OUT.
The processes that follow were carried out on LabView as is described in the
figure below and further explained in the sections that follow.
21
3.3.1 LabView Modules
.
C
Th
to
si
At
de
Ev
si
hi
Figure 14 Block diagram showing procedure following signal detection
ircuit/PC Interface
e Engineering labs are equipped with a variety of alternaives that allow a pc
read from an external signal. Regardless of the method used however, the
gnal to be studied would have to be sampled at a defined rate.
this point we define the various frequencies occurring at the stages of our
sign.
• Current injected into the body: Designed to have a frequency of 10KHz
• Impedance changes: Have a frequency proportional to that of the heart
rate. For a healthy young person, the frequency is considered to not
exceed 200beats/min ≈ 4Hz.
• Picked up signal to be inputted into the computer: Composed of a
modulated signal. The carrier frequency is that of the input, 10 KHz and
the anticipated of 4Hz.
en though the signal we mean to pick up is of 4Hz, it is carried by a 10KHz
gnal and as a result sampling needs to be done taking into consideration the
gher frequency.
22
By Nyquist, . We choose . signals f2>f KHz100=f10=fs
Demodulation
The procedure described above is depicted in the figure below.
Figure 15 Amplitude modulation resulting from the followed procedure.
Since , the signal could be retrieved by demodulating our signal
using Envelope Detection [13].
signalc ω<<ω
Low-Pass Filtering
The envelope detector followed by a low pass filter in order to suppress any
high frequency components in the retrieved signal. The low pass filter was
designed to have a corner frequency of 15Hz. Such allows no loss of power
from the low frequency.
Outputs
Impedance cardiography as explained in Chapter 2 records the changes in
impedance relative to the cardiac cycle. This result is achieved by drawing the
waveform after the filtering stage. Another typical output of impedance
cardiography is the dZ/dt signal resulting from the differentiation of the just
recorded. As was clarified in Section 2.2.4, the differentiated signal produces
waveforms that are more uniform than those produced when tracing . The
dZ/dt waveform can be obtained and simulated from the output of the
differentiation module added subsequent to the filter.
Z∆
Z∆
23
3.3.2 Visual Basic Application
One of the objectives of our project as stated in Chapter 1 is to develop an
application that allows the user to identify the rate at which his stroke volume
is decreasing back to normal. This outcome is fulfilled by the development of
a software application which accepts as an input the data points resulting from
the dZ/dt waveform and outputs the calculated rate of decrease.
Figure 16 Waveform to demonstrate the application operation
Considering the waveform of Figure 16, the VB application was designed in
order to accomplish the following:
• Discard the mid section as noise
• Detect the peaks marked in red as the only true peaks
• Calculate the rate of decrease between peak 1 and peak 2
• For a repetitive wave, calculate the average rate of decrease of peaks
The described user-interface based application was developed using Visual
Basic and embedded the LabView modules of Section 3.3.1 for signal
processing.
24
4.0 Implementation
Following the design process, this chapter reports the implementation of each
stage. This chapter is divided into the subsequent sections:
a- Implementation of the ICG Circuit
b- Implementation of the LabView Modules
c- Implementation of the Visual Basic Application
The testing, results and analysis of the operation of these components are
presented in Chapter 5.
4.1 Implementation of the ICG Circuit
The design of the circuit was explained in detail in Section 3.2.2. Its
implementation included transferring the schematic onto a PCB board as
shown in Figure 17.
Figure 17 Implemented circuit.
25
Verification of the Implemented Circuit
The circuit was tested on a variable resistor in place of the load before actual
application on the body. Results of tests are shown in Table 1.
RLoad Ω VLoad Expected VLoad Measured ILoad %Change Vout* Amplifier Gain
25 Ω 25mV 23mV 0.933mA - -0.22V x9.7
28 Ω 28mV 26mV 0.935mA 0.21% -0.25V x9.6
33 Ω 33mV 31mV 0.939mA 0.43% -0.29V x9.4
40 Ω 40mV 38mV 0.940mA 0.11% -0.36V x9.4
* Vout is the voltage picked up following the 10x amplifier Table 1 Results produced upon testing the implemented circuit with a variable resistor.
The recorded current is demonstrated to be constant upon changing resistive
values. Furthermore, the amplifier gain is noted to be ~ 9-10. Both findings
indicate proper functioning of the implemented circuit.
4.2 Computer Analysis
As described in Section 3.3, the computer analysis stage is divided into two
units: a- LabView Module and b- Visual Basic Application. The LabView
Module is further divided into the stages shown in Figure14.
4.2.1 Implementation of the LabView Modules
This section of the report describes the implementation of the LabView
modules and demonstrates their integration into a unified process.
Circuit/PC Interface
During the first two weeks of testing, we relied on the NI Elvis as a means of
transferring the detected signal onto the pc. Later, we used the SCB-68
connected to a DAQ card through a serial port. The SCB-68 is smaller, lighter
than and as reliable as the NI Elvis.
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Figure 18 Block diagram showing the transmission of the signal from the PCB to the pc.
Input/output channels of the SCB-68.
The DAQ
Through
device (S
process.
as explai
Figure 20
Figure 19 The SCB-68 used to transmit the signalfrom the PCB onto the PC.
assistant is a VI module of LabView linking it to the SCB-68.
the DAQ assistant we were able to configure the data acquisition
CB-68 in this case) and as a result control the data acquisition
We set the DAQ assistant and specified its sampling rate to 100 kHz
ned in Section 3.3.1.
DAQ-assistant VI module in LabView. It is set to sample at a rate of 100kHz.
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Demodulation
As was shown in Section 3.3.1, before the impedance variation signal can be
retrieved, the acquired signal should be demodulated. We developed a
LabView module which uses the Envelope Detection technique in order to
retrieve the sought signal.
Waveform builder
Figure 21 AM Demodulation module (Envelope Detection) on LabView.
The AM Demodulation scheme of Envelope Detection was built using the two
LabView VI’s: peak detect and waveform builder. The peak detect can be set
to identify either valleys or peaks; for our purpose we set it to ‘peaks’. The
waveform builder reconstructs the signal using three variables:
• Peak values: retrieved from the output of the peak detect
• Start time: set to ‘0’
• Frequency of the signal: from theory, the frequency of the demodulated
signal is the same as that of the carrier frequency [13]. The frequency
was set to 1/0.001 = 10kHz.
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Verification of the Developed Module
In order to verify that the AM demodulation scheme used is valid, we tested
the developed module on known input and carrier signals.
(1)
(2)
Figure 22 LabView module used to verify Envelope Detection is functioning effectively.
Figure 22 represents the experiment we performed in order to verify that our
Envelope Detection module is functional. The labeled VI’s (1) and (2) are sine
wave generators whose peaks and amplitudes can be regulated. As can been
seen, VI (1) functions as the carrier signal of frequency 10 kHz and VI (2)
functions as the input signal of frequency 5Hz. The waveforms produced are
shown.
Figure 23 Carrier signal
29
Figure 24 Inputted signal
Figure 25 Modulated signal
Figure 26 Demodulated signal = Inputted signal
30
A comparison between the waveforms in figures 24 and 26 demonstrates that
the demodulation scheme is correct and thus can be safely implemented in
our design.
Low-Pass Filtering
The recovered signal was then passed through a low-pass filter with a corner
frequency of 15 Hz as explained in Section 3.3.1. The figures below show the
Low-Pass VI module on LabView along with its adjusted properties to suit our
operation.
Figure 27 Low pass filter on LabView
Figure 28 Configuration of the low pass filter
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As shown in Figure 28, the low pass filter was chosen to have the Butterworth
topology of the 5th order. The cutoff frequency was set to 15 Hz.
Outputs
Section 3.3.1 identifies two essential outputs of the LabView module,
• dZ/dt signal:
The dZ/dt signal was obtained by differentiating the demodulated and
filtered signal as shown in the figure below.
Figure 29 Differentiation of the demodulated and filtered signal.
• Data points for Visual Basic Analysis:
The amplitude and time of occurrence of the peak points of the dZ/dt
waveform were extracted onto a text file to be later read by the developed
Visual Basic Application. The figure below demonstrates how this objective
was achieved.
Figure 30 Extraction of peak points onto a text file.
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This marks the finish of the signal processing carried out on LabView. Figure
31 shows the system as a whole functional unit.
Figure 31 LabView application for signal processing.
4.2.2 Implementation of the Visual Basic Application
As specified in the design Section 3.3.2, the interface linking the user with
LabView as well as the calculation of the rate of decrease of the dZ/dt peaks
were performed on Visual Basic.
Figure 32 presents the outcome of our VB application as seen by the user.
Figure 32 The developed user-interface of our applicationOnce the user presses on [View My ICG], LabView traces the dZ/dt signal and
presents him/her with the waveform. The user then enters what he identifies
as the highest true peak and is provided with the rate of decrease of the
detected peaks.
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Verification of the Rate Calculation Algorithm Used
In order to verify that the developed program yields accurate results, we
inputted the same data file into the VB application and onto an excel
worksheet-both performing the same operation.
Peak
Amplitude Time
6.17908
0.70332
5.59894
1.49804
5.77482
2.27946
5.57361
3.03126
5.65430
3.76570
5.68387
4.48001
5.12075
5.16028
5.75937
5.83882
5.42946
6.51269
5.63750
7.19072
5.95035
7.86649
Table 2 Date file to be tested on the VB application and Excel worksheet
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Figure 33 Excel output for the Average Rate of Decrease = 0.021
Figure 34 VB output for the Average Rate of Decrease = 0.021
The matching results of the two applications serve to demonstrate the
effectiveness of the developed Visual Basic application.
The complete code for “Healthy Heart” is included in the Appendix.
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5.0 Results and Analysis
This chapter of the report includes waveforms, data files and calculation
outputs generated upon the comprehensive testing of the project.
5.1 Test Results
Test 1: Mohamed Eid
The four electrodes were applied onto Mohamed. Two electrodes were placed
on his neck and the remaining two electrodes on the base of his sternum.
Figure 35 Electrode placement
Once Mohamed was connected to the circuit which in turn connects to the
SCB-68, the VB/LabView application was run. The obtained results are shown
in the table below.
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Mohamed Eid
dZ/dt
Amplitude &
Time Data
Files
Rate of
Decrease
Table 3 Results of testing project on Mohamed.
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Test 2: Mirna Abou Mjahed
Mirna Abou Mjahed
dZ/dt
Amplitude
& Time
Data Files
Rate of
Decrease
Table 4 Results of testing project on Mirna
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5.2 Analysis
Evaluation
Our project was initially set to perform the following processes:
a- Pick up impedance change signal
b- Store the signal
c- Transmit the signal to pc
d- Analyze the signal
Half way through the Spring semester however it was obvious that some
modification should be made. At the sampling rate we were using (100kHz),
the storage device intended for our project would either be too time
consuming to build or too expensive to buy. Not wanting to deviate from the
main aim of the project and after consultation with Dr. Sabah, we opted to
modify our objectives and move on to the following, more essential stages.
The performed modification did not interfere with the main intention of our
project: the design and implementation of a device that detects impedance
changes proportional to the stroke volume to the cardiac cycle and
determines the time it takes for the stroke volume to return to normal following
exercise.
The results presented in Tables 3 and 4 of Chapter 5 demonstrate our
success in delivering the requirements we established eight months ago.
Room for Improvement
No project is ever complete for there is always room for improvement.
Throughout the different stages of our progress, we kept note of all the areas
that could benefit from improvements.
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• The most obvious improvement involves the implementation of storage
and thus achieving portability of the device.
• Development of a more thorough algorithm for the calculation of the
rate of decrease of the stroke volume. The application could be
designed in a way that requires no user input.
• The project we are delivering is distinguished by its modularity i.e., it
could be easily developed and expanded into providing the user with a
number of output variables such as EKG, heart rate, etc…
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6.0 Other Issues
It is essential for any engineering design and implementation projects to take
into consideration not only the technical aspect of the work but also its social,
health and economic impacts.
The target community for our implemented impedance cardiograph device is
the medical community. We have worked to present the health services with a
cheap and easy to use device that could be used for research on the
correlation between cardiac health and the rate of decrease of the stroke
volume.
The device we have implemented is safe to use. Though it functions by
injecting a constant current into the patient, the current’s amplitude and
frequency are well below approved safety limits.
On a more materialistic note, our design is pleasantly inexpensive and
economical. Circuit components cost no more than 20,000LL, whereas the
signal analysis and calculations require no more than the availability of the
developed executable program on the pc. The most expensive unit of our
project is the connection of the PCB to the PC and includes the cost of the
DAQ card and SCB-68 data acquisition unit. These expenses could be
eliminated once portability is achieved.
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7.0 Conclusion
We conclude this report with a summary of the presented material. This report
begins by describing the main idea of our design and its objective. Following a
comprehensive research and analysis we selected to record the impedance
signal since it proved to be proportional to stroke volume. We then moved to
designing and implementing the various units of the project including:
impedance cardiograph circuit to pick up the impedance signal, transmission
of the signal onto the pc, signal processing and analysis.
As a product of our work, we present a device which picks up a signal
proportional to the stroke volume, analyzes it and presents its results to the
user through a user-friendly application.
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References
[1] Biopac Systems Incorporation at http://www.biopac.com [2] Bonjer, Van Den Berg, Dirken (1952) The Origin of the Variations of Body Impedance Occurring during the Cardiac Cycle. Circulation. Vol 4. [3] Chiang C.Y, Hu W.C, Shyu L.Y., Portable Impedance Cardiography System for Real-Time Noninvasive Cardiac Output Measurement. Proceedings of the 26th Annual International Conference of the IEEE/EMBS. IEEE. 1997. [4] Cornish B.H., Thomas B.J., Ward L.C. (1998) Effect of Temperature and Sweating on Bio-impedance Measurements. App. Radiol. Isot. Vol 49, No 5/6, 475-476. [5] Dalziel (1972) Electric Shock Hazard. IEEE Spectrum [6] Dr. Nassir Sabah lecture notes for EECE 601S and 602S [7] Grenier M.C., Gagnon K., Genest J., Durand J., Durand L.G. (1998) Clinical Comparison of Acoustic and Electronic Stethoscopes and Design of a New Electronic Stethoscope. Excerpta Medica Incorportation. [8] Guidant Corportation at http://www.guidant.com [9] Rosell Javier, Colomnias J, Riu P, Arany R, Webster J. (1988) Skin Impedance from 1Hz to 1MHz. IEEE Transactions on Biomedical Engineering. Vol 35, No. 8. [10] Johnson, R. (2002). Biology. New York: McGraw-Hill Companies [11]Kim D.W, Baker L.E, Pearce J.A, Kim K.Y (1988) Origins of the Impedance Change in Impedance Cardiography by a Three Dimensional Finite Element Model. IEEE Transactions on Biomedical Engineering. Vol 25, No 12. [12] Kubicek WG. (1970) Physiological correlates of the cardiac thoracic impedance waveform. American Heart Journal. 791519-23. [13] Oppenheim A, Willsky A (1997) Signals and Systems. USA: Prentice-Hall International Inc. [14] Parulkar GB, Jindal GD, Padmashree RB, Haridasan GG, Dharani JB. Impedance cardiography in mitral valve disease. J Postgrad Med 1980;26:155-61 [15] Patterson R.P (1989) Fundamentals of Impedance Cardiography. IEEE Engineering in Medicine and Biology Magazine.
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[16] Raaijmakers E., Faes J.C., de Vries P., Heethaar R. (1997) The Inaccuracy of Kubicke’s One-Cylinder Model in Thoracic Impedance Cardiography. IEEE Transactions on Biomedical Engineering. Vol 44. No1. [17] Simoons M.L., HugenHoltz P.G (1975) Gradual Changes of ECG Waveform During and After Excersise. Circulation Journal of the American Association. Vol 52. [18] The Heart Sound Tutor retrieved from http://www.ed4nurses.com [19] The Nebraska Medical Center at http://www.nebraskamed.com/ [20] Tsunami D., McNames J., Colbert A., Pearson S., Hammerschlag R. Variable Frequency Bio-impedance Instrumentation. Proceedings of the 26th Annual International Conference of the IEEE/EMBS. IEEE. 2004. [21] Wang Y., Haynor R., Kim Y. (2001) A Finite-Element Study of the Effects of Electrode Position on the Measured Impedance Change in Impedance Cardiography. IEEE Transactions on Biomedical Engineering. Vol. 48, No.12. [22] Woltjer H.H., Bogaard H., Bronzwaer G.F, de Cock C., de Vries P. (NA) Prediction of pulmonary capillary wedge pressure and assessment of stroke volume by non-invasive impedance cardiography. American Heart Journal. Vol 134 No. 3 [23] Zibb Corporation, http://www.zibb.com [24] Zheng Z., Huang Z., Huang Z., Yang S., Liao Y., High efficiency external counterpulsation apparatus and method for controlling same. US Patent 6863670
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Appendix
Program Code of “Healthy Heart”
Dim size As Double Dim Rate(10000) As Double Private Sub cmdEnter_Click() Open "Time.txt" For Input As #1 Open "Amp.txt" For Input As #2 Dim sizeTime As Double Dim sizeAmp As Double Dim temp As Double Dim i As Double Dim max As Double max = txtHTP.Text 'Calculates the number of entries in the Time file retrieved from labview Do While (Not EOF(1)) Input #1, temp sizeTime = sizeTime + 1 Loop Close #1 'Calculates the number of entries in the Amplitudes file retrieved from labview Do While (Not EOF(2)) Input #2, temp sizeAmp = sizeAmp + 1 Loop Close #2 'Checks if the Time and Amplitude files have equal entries 'If the files do not contain the same number of entries, program terminates If sizeTime <> sizeAmp Then MsgBox ("Error in input files, Please try recording again") End Else size = sizeTime End If ReDim Time(size) As Double ReDim amp(size) As Double Open "Time.txt" For Input As #1 Open "Amp.txt" For Input As #2 Dim temp1, temp2 As Double 'Fills out arrays with acceptable peak amplitudes along with their time values 'Arrays contain zero entries for all outside of range peaks
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For i = 0 To size - 1 Input #1, temp1 Input #2, temp2 If i = 0 Then Time(0) = temp1 amp(0) = temp2 Else If (i <> 0 And temp2 < max) Then Time(i) = temp1 amp(i) = temp2 Else If (i <> 0 And temp2 >= max) Then Time(i) = 0 amp(i) = 0 End If End If End If Next i Close #1 Close #2 sizedivide = size 'Counts the number of zero entries in the arrays For i = 0 To size - 1 If amp(i) = 0 Then sizedivide = sizedivide - 1 End If Next i ReDim amp2(sizedivide) ReDim Time2(sizedivide) Dim j As Single i = 0 j = 0 'Removes the zero entries in the arrays Do While (i < size And j < sizedivide) If amp(i) <> 0 Then amp2(j) = amp(i) Time2(j) = Time(i) i = i + 1 j = j + 1 Else If amp(i) = 0 Then i = i + 1 End If End If Loop
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'Calculates the average rate of decrease Dim AmpDiff, TimeDiff, RateSum As Double For i = 0 To sizedivide - 2 AmpDiff = amp2(i) - amp2(i + 1) TimeDiff = Time2(i + 1) - Time2(i) Rate(i) = AmpDiff / TimeDiff Next i For i = 0 To sizedivide - 2 RateSum = RateSum + Rate(i) Next i Dim FinalRate As Double FinalRate = RateSum / sizedivide txtrod.Text = FormatNumber(FinalRate, 3) End Sub ' This exits the program Private Sub Exit2_Click() End End Sub ' This section connects the application to labview Private Sub ICG_Click() Shell ("ICG.exe") End Sub
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