Final Report
A Mobile Wireless Electrocardiogram System for Health Care
Facilities
ECE4007 Senior Design Project
Section L01, J and the Three J’s
Joe Richard, Team Leader
John Farner
Jason Fritts
Julian Jaeger
Submitted
December 5, 2007
TABLE OF CONTENTS
Executive Summary ...........................................................................................................3
1. Introduction ..................................................................................................................4
1.1 Objective ..................................................................................................................4
1.2 Motivation ................................................................................................................5
1.3 Background ..............................................................................................................5
2. Project Description and Goals ....................................................................................6
3. Technical Specification ................................................................................................7
4. Design Approach and Details
4.1 Design Approach .....................................................................................................8
4.2 Codes and Standards ..............................................................................................11
4.3 Constraints, Alternatives, and Tradeoffs ...............................................................12
5. Schedule, Tasks, and Milestones...............................................................................14
6. Project Demonstration...............................................................................................15
7. Marketing and Cost Analysis ....................................................................................16
7.1 Marketing Analysis ................................................................................................16
7.2 Cost Analysis .........................................................................................................16
8. Summary .....................................................................................................................17
9. References ...................................................................................................................19
Appendix A .......................................................................................................................21
Appendix B .......................................................................................................................22
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EXECUTIVE SUMMARY
The Mobile Wireless Electrocardiogram (EKG) is a medical telemetry instrument used
by medical personnel to monitor a patient’s EKG signal while allowing the patient to remain
fully mobile and comfortable. This EKG measures small electrical signals from the patient’s
chest caused by the heart beating. Using these signals, doctors and other medical staff can
monitor the patient’s well being and predict future problems. While EKGs are commonly used in
hospitals, many patients would benefit from the ability to perform usual tasks while remaining
monitored on an EKG. In environments such as nursing homes and assisted living centers this
technology would allow the elderly to retain their freedom of movement and still allow nursing
staff to monitor important cases and patients. The wireless data relay ability would allow many
patients to be monitored from a single base station with limited staff. This feature would be
valuable to hospital administrators and management. With a small fleet of these devices an entire
nursing home or hospital floor could be well looked after by a small nursing staff.
This design team has completed a prototype for the mobile wireless EKG unit as well as
developed software for the wireless transmission and base station. The mobile unit can be worn
by the patient, and can measure heart rhythm data, digitize it, and transfer it wirelessly back to
the base station. At the base station the software can store the data and allow medical staff to
view the EKG reading and heartbeats per minute measurement. In further design efforts battery
life could be extended beyond the achieved 3.5 hours, alarms could be added to alert medical
staff of problems, the packaging could be optimized, and a number of other features could be
implemented to increase the unit’s ease of use and appeal to customers.
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Mobile Wireless Electrocardiogram
1.
INTRODUCTION
Electrocardiograms (EKG) have long been used in the hospital environment to diagnose
cardiac arrhythmias and screen for heart disease. However, until recently patients had to be
tethered to the EKG machine by a few feet of wire [1]. The mobile wireless EKG system is an
electrocardiogram with wireless data transmission capabilities, and a monitoring base station
used to receive and monitor the patient’s condition. With the invention of the mobile EKG,
patients can now move freely around their environment, allowing their caregivers to easily
monitor the patient’s status at any location.
1.1
Objective
The objective of this project was to provide hospitals with a wireless electrocardiogram
with the ability to operate away from its monitoring station. Hospitals around the country use
electrocardiograms every day to monitor patients’ heart conditions using stationary bedside EKG
units. Our objective was to allow caregivers the ability to move their patients around their
environments without ever having to disconnect and reconnect EKG wires. Using the same EKG
design used in hospitals today, we added wireless capabilities to the unit by adding an IEEE
802.11g Wi-Fi adapter for data transmission. With the addition of a mobile battery pack, the
EKG base unit is able to travel with the patient wherever they go. We also designed software for
a base station that can receive the EKG unit’s Wi-Fi signal using another IEEE 802.11g Wi-Fi
adapter.
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1.2
Motivation
Using current electrocardiograms in hospitals can be a time-consuming and unsanitary
task [2]. Connecting and disconnecting EKG leads from hospital room to hospital room takes up
valuable time that nurses could be using to focus on the patient’s ailment. Changing wires also
introduces bacteria to the patient, further increasing the risk of infection.
By using a mobile EKG, the hospital staff only has to connect wires to the patient once.
This feature will save the caregivers time and decrease the risk of bacteria being introduced to
the patient. The patient will also be “more comfortable in their beds and getting up and around”
since they are free from unnecessary wires [2]. This project was motivated by the realization that
wires are unnecessary given the current state of wireless communication technologies.
1.3
Background
Currently there are a few wireless electrocardiograms being tested on the market.
LifeSync has a mobile EKG system being tested in thirteen hospitals across the country.
LifeSync’s system uses Bluetooth for its wireless capabilities, giving it a range of thirty to forty
feet [1]. The main goal of this device is to “provide a mobile interface to existing EKG monitors
in the hospital” [3].
Phillips provides a wireless “IntelliVue Telemetry System” that can be used to monitor a
patients EKG measurement. This system relies on a 2.4 GHz cellular network using fixed
antennas installed throughout the hospital [4].
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2.
PROJECT DESCRIPTION AND GOALS
The completed project prototype is a mobile electrocardiogram. A patient will wear the
mobile EKG that gathers heart rhythm data, stores the data, and transmits it wirelessly to a
remote base station for analysis by a health care professional. This EKG monitoring system can
be marketed to hospitals, nursing homes, and other health care facilities based on these goals.
-
Safe for user
Reliable EKG data under a variety of circumstances
Base station interface
Easy to use mobile system
Lightweight and small size
Wireless data transfer
Sufficient wireless range
One of the primary goals of the mobile EKG was reliable heart rhythm representation.
The EKG readouts should be comparable to standard EKG units that are currently being used.
The design’s signal digitization was modified to ensure this level of quality, and has exceeded
original design goals. Similarly, the mobile EKG must satisfy the same safety standards as
present systems.
Since the unit will be worn by a patient for the majority of its functional operation, the
EKG interface must be easy to understand. The unit has been proven during testing to be small
and light enough to allow the patient to move unrestricted.
Finally, the advantage of this system is its mobility. The wireless capabilities are the
ultimate determining factor for the range of applications for the system. A moderate wireless
range and battery life allow the patient a level of freedom that is impossible with conventional
EKG systems.
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3.
TECHNICAL SPECIFICATIONS
The goals stated in the project description lead to the quantifiable specifications of the
mobile EKG. Each of these specifications determines the scope of usefulness of the design. The
technical specifications are given below in Table 1.
Table 1. Mobile EKG Technical Specifications.
Proposed
Specifications
≈ 1.25 hours
Final
Specifications
≈ 3.5 hours
≈ 38 m
≈ 45 m
This is a typical range for
802.11g wireless networks.
0.1 W/cm
0.1 W/cm
This is the maximum allowable
power to the EKG electrodes.
< 1.3 kg
1.5 kg
150mm x 115mm
x 80mm
65 Hz
241mm x
161mm x
76mm
22050 Hz
This includes the weight/size of
the eBox, battery pack, EKG
circuit, and packaging.
1000
831
Battery Life
Wireless range
Electrode power
Weight of unit
Size of unit
Resolution
EKG gain
Description
The eBox 2300 uses 15W of
power. Using a 9.6V battery
pack with approximately
2200mAh gives this lifetime.
A sufficient resolution will
ensure accurate analysis.
With heartbeats providing a
signal on the order of a
millivolt, the amplification
circuit must have a gain of
1000.
Safety is of primary importance for the mobile EKG. Adequate isolation from the mobile
pack’s electronics and a minimum amount of power applied to the EKG electrodes help ensure
the patient’s safety.
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The battery life and range are large enough to ensure that the system can be used in a
variety of locations for a reasonable amount of time. The unit’s relatively small weight and size
cause only small hindrances while moving. Finally, the mobile EKG has proven to be able to
take readings while the patient is moving. With all of these specifications met, the mobile EKG
unit can truly be considered mobile.
The accuracy of the mobile EKG must be comparable to that of existing telemetric
monitoring equipment. The use of an onboard sound card rather than the Phidgets interface board
improved the specifications of the mobile EKG in two ways. First, the gain of the amplification
circuit was originally required to be high enough to obtain a clear heart rhythm reading: 3 to 5
volts for an analog input of the Phidgets interface board. The sound card requires a lower, more
obtainable signal of 1 volt peak-to-peak for its microphone input. The second improvement in
specifications is that the resolution of the heart rhythm signal was vastly increased. The Phidgets
board sampled the heart rhythm at approximately 65 Hz, while the sound card can sample the
signal at frequencies up to 44100 Hz.
4.
DESIGN APPROACH AND DETAILS
4.1
Design Approach
The mobile EKG design consists of two major sections as shown in Figure 1. First, the
analog signal input takes the signal reading from the user, amplifies the signal, filters out any
noise, and converts the signal from analog to digital form [16]. The second part wirelessly
transmits the data back to a computer base station, stores the data, and allows for analysis by
physicians or medical personnel.
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Figure 1. Design flow chart.
The analog signal input can further be divided into
parts consisting of signal measurement, amplification,
signal filtering, safety isolation, and the A/D conversion.
The signal is measured using two electrode leads attached
to the body with a medical adhesive. Pre-prepared
electrode leads were purchased and used for this function.
Figure 2 displays the lead placement on the body. The
Figure 2. EKG lead
.....placement.
leads are named after the appendage to which they are closest (i.e. RA for Right Arm).
The electrodes are attached through shielded wire to the input amplifier. For this
amplifier an AD624AD high-precision instrumentation amplifier has been chosen. The average
voltage measured from the human body is the in the range of 0.1 mV to 1.0 mV [16]. This
amplifier is able to amplify the signal up to 1000 times. An amplification of 831 times was used
for this design. With the signal strength increased, filtering and the A/D conversion became
easier. Figure 3 displays the circuit diagram of the amplifier. [17]
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Measuring the signal from the body and
transferring the signal through wires between the
body and the amplifier creates noise in the signal
that needed to be filtered out. A linear notch filter
was implemented to remove this noise. The average
EKG signal is usually between 3 and 20 Hz [16].
Above these frequencies excess noise is taken in by
the wires from the surrounding environment. Below
Figure 3. Amplifier circuit.
3 Hz the leads will pick up signals from other bodily functions such as breathing and muscle
movement [16]. A passive bandpass filter between 1 and 20 Hz was designed and implemented.
Figure 4 displays the bandpass filter. A
passive filter was designed because of
battery life constraints. As a result the
signal is attenuated by about half, but
because of the amplification this is not a
Figure 4. Passive Bandpass Filter.
problem. Appendix A contains the bode
plot of the filter.
One item not yet addressed is the safety precaution within the circuit to prevent short
circuits from redirecting current back through the user’s chest. Since this device is powered by a
small battery, there is not as much danger as an EKG powered by a 120-volt wall receptacle.
However, it is an important issue that must be addressed. There were two main options to deal
with this problem. The first was to use an optical isolation amplifier between the amplification
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and filtering phase. The second was to use diodes at the input of the signal to disallow any
outbound signals. Due to cost and availability of parts the diode option was chosen.
After amplification and filtering the signal is passed into an analog to digital converter
using a standard microphone cable. The eBox-2300’s sound card was chosen as the A/D
converter for its excellent sampling rate of 44.1 kHz. This sampling rate provided us with the
resolution needed to sample a heart signal. The fact that the sound card was already included in
the eBox, its ease of use, and its excellent sampling rate contributed to it being our choice for an
A/D converter.
From here the heart signal will enter the second stage of the design. Once Window CE is
loaded on the eBox, a simple batch program will perform four main tasks. First it will run
wavrec.exe, a C++ program which records a 5 second wave file containing the patient’s heart
signal. Next the batch file will run MobileEKG.exe, a C# program which converts the wave file
to a text file containing only x and y data pairs. The batch program will then copy this text file to
the base station using Windows CE file sharing and an 802.11g Wi-Fi network. Finally the batch
program deletes any files created in order to save space on the eBox and restarts itself for another
heart signal sample. At the base station the data is displayed using a C# graphical user interface.
This interface reads the text files created by MobileEKG.exe, graphically displays the heart
signal, and calculates its beats per minute. All of the code is available in the project
documentation.
4.2 Codes and Standards
One of the most fundamental electrical standards for medical equipment is the
International Electrotechnical Commission’s standard IEC 60601-1. The first section of this
standard is referred to as the base standard, and provides requirements for the prevention of
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electrical shock, mechanical, ignition, flammable anesthetics, excessive output and flame hazards
[5].
The hazard most relevant to the mobile EKG, the electrical shock hazard, is outlined in
section three of IEC 60601-1 [6]. Limiting exposure to 25 VAC, 60 VDC, circuit separation, and
proper grounding are all stated by this standard [5]. Product safety guided design decisions
regarding battery size and amplifier design. The battery voltages and subsequent amplifier rail
voltages, have been made as low as possible. An isolating diode circuit was integrated into the
EKG amplifier design for further patient safety.
The second type of IEC 60601 sections are referred to as specific standards. The standard
IEC 60601-2-27 specifies safety requirements and essential performance for electrocardiographic
monitoring equipment specifically [5]. This standard is applicable to EKG equipment used
outside of the hospital environment, but does not apply to home or test usage.
Finally, a standard for wireless LAN connections is the IEEE 802.11g. This standard
provides a mandatory maximum data transfer rate of 54 Mbps [9]. The decision to use a
standardized wireless communication technology ensures the most reliable interface between
different systems within the project.
4.3
Constraints, Alternatives, and Tradeoffs
The analog to digital conversion of the EKG signal is what ultimately determines the
resolution of the heart rhythm signal. The initial design of the mobile EKG used a Phidgets
interface board as the ADC. The signal digitization, however, proved to be far too slow to give
an accurate representation of the heartbeat. The most robust alternative to the Phidgets interface
board was using a sound card as an ADC. Not only did the use of a sound card increase the
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resolution the EKG signal, but also provided for a more streamlined design and a lower
production cost.
Reading the EKG signal from a microphone jack and sound card proved to be a
challenging task from the perspective of software design, and introduced another set of design
tradeoffs. Designing software in the C# language for the embedded Windows CE 6.0 limited our
options for interfacing with the embedded PC’s sound card. Some of the approaches that were
explored but eventually discarded included using DirectX and DirectSound libraries, calling
waveIn APIs, and the use of the dynamic link library, coredll. The simplest solution involved
creating a batch file to execute a pre-made C++ sound recording program. Having access to the
source files of this program allowed for full customization.
Safety is a primary medical standard, which narrowed the options of the amplification
circuit design. To prevent the EKG patient from potentially lethal shocks from the amplification
circuit, an isolation circuit was needed. Our two choices were a diode based isolator or an optical
isolator. Isolating diode circuits provide excellent safety, are widely available, and cost much
less than optical isolators [10]. The isolating diode circuit was chosen for its safety benefits and
availability.
One of the wireless EKG’s goals is to allow the patient to move around their environment
freely. For the patient to freely move around their environment, the EKG’s wireless system must
have a large wireless range. We considered three wireless systems for our project: IEEE 802.11g
Wi-Fi, Bluetooth, and a cellular communication system. The IEEE 802.11g Wi-Fi system gave
us the greatest benefit based on range and ease of setup. During testing, the IEEE 802.11g Wi-Fi
provided a range of 45 meters, and can be easily installed in a hospital using Wi-Fi routers
available at any computer retailer [9]. Bluetooth currently has a range of 9 to 12 meters which is
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not sufficient to cover an entire hospital. A cellular based system would require installing
cellular antennas throughout the hospital and would likely consume the entire budget [4]. For its
wireless range, price, and ease of installation the IEEE 802.11g was chosen as the mobile EKG’s
wireless system.
5.
SCHEDULE, TASKS, AND MILESTONES
As shown in the Gantt charts located in Appendix B, there were nine major milestones
that needed to be reached before the unit could begin the final test phase. At least two engineers
were assigned to most of these tasks. This method did not only bring more knowledge to each
task, but it also ensured that separate engineers held each other accountable. All nine tasks were
completed, although some steps took longer than originally planned. Most of the delays were
caused by the late change in hardware.
Table 2. Degrees of Difficulty.
1
2
3
4
5
6
7
8
9
Task Name
Build Amplifier Circuit
Build Filter Circuit
Complete “Hello World” eBox program
Test Amplifier and Filter Circuits
Write program to read from eBox sound card
Combine Circuit with other Hardware
Program Wi-Fi to transmit data
Design Packaging for Hardware
Program Computer Interface
Degree of Difficulty
Medium
Medium
Low
Medium
High
High
Medium
Low
High
The degree of difficulty of each task can be seen in Table 2. They are ranked based on the
time it took for completion, the effort it required, and the delay it caused.
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6.
PROJECT DEMONSTRATION
Most of our specifications stated in the proposal were achieved or exceeded. The only
specifications that were not met were weight and size. After completing the packaging of the
mobile unit, the package was weighed and measured. Table 3 shows the weight and size, as well
as the other specifications.
Table 3. Specifications.
Range
Battery life
Weight
Size
Resolution
Proposed
Actual
38 Meters
1.25 Hours
1.3 kg
150x115x80 mm
65 Hz
45 Meters
3.5 Hours
1.5 kg
241x161x76 mm
22050 Hz
To test the Wi-Fi range of the EKG unit, a test patient walked through the floors of the
Van Leer building in order to see how far the unit could reach, and yet continue its wireless
transmission to a wireless router placed some distance away. After numerous tests the average
range was around 45 meters, which far exceeds our original proposed range.
To test battery life, the EKG unit was turned on and allowed to run until the unit stopped
sending EKG data back to the base station. After three and a half hours the unit shut down. A
three and a half hour battery life was almost three times longer than expected.
Finally, because of the switch from the Phidgets board to the eBox sound card ADC, our
resolution jumped from only 65 Hz to 22 kHz. This massive increase allows for very intricate
data point gathering and recording.
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7.
MARKETING AND COST ANALYSIS
7.1 Marketing Analysis
Electrocardiograms have been available on the market for many years, but only a few
wireless electrocardiograms have ever been produced. Currently there are three wireless EKG
systems available: the LifeSync Wireless ECG System, Phillip’s IntelliVue Telemetry system,
and Transoma Medical’s Sleuth Wireless Electrocardiogram Monitoring System. The LifeSync
and Phillips systems provide a wireless interface for an already existing electrocardiogram
system, whereas the Transoma system is a standalone EKG system with wireless communication
capabilities. The LifeSync system uses Bluetooth for its wireless transmission giving it a range
of only 10 meters [18]. The Phillips and Transoma systems use a cellular network that requires
cellular antennas to be installed throughout a hospital. The Transoma system also requires an
EKG chip with an antenna to be implanted in the patient’s body [19].
The mobile EKG system is a stand-alone product, which does not require an existing
EKG system. The mobile EKG system also does not require an intrusive implantable chip for the
patient; instead it uses traditional EKG electrodes attached to the chest of the patient. The
system may also be sold in packages of 5 to 50 units. An existing EKG machine can cost as
much as $4,500, whereas our product’s price is $637 [20]. The mobile EKG also uses IEEE
802.11g for its wireless capabilities giving it a range of up to 45 meters [9].
7.2 Cost Analysis
Building the prototype cost about $120 in parts and $19,200 in labor as shown in the cost
calculation in Appendix B. This cost assumes four engineers worked for 160 hours each at a pay
rate of $30 per hour. Including fringe benefits and overhead, the developmental cost of the
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prototype was about $37,386. The cost of parts was relatively low for the prototype because
many of the needed parts are already in stock. The main required parts that were not available
were the AD624AD Precision Instrumentation Amplifier which cost $19.32 [11], and a 5V
Switching Voltage Regulator, which cost $29.95 [13]. Shipping costs, a couple of other smaller
items and packaging made up the rest of the required $120 for building the prototype.
When mass producing the mobile EKG unit, part costs will be higher due to the fact that
other parts need to be ordered for every single unit. Every unit requires an eBox-2300 which
costs about $133 [14]. This will raise the cost of parts to around $253. With assembly and labor
costs being fairly low, each unit will cost $637 to produce. This is assuming 2,000 units can be
sold every year for five years. At a selling price of $725, around $88 or 12.2% profit would be
made on every single unit. Selling 10,000 units over five years would lead to a total profit of
$883,614.
8.
SUMMARY AND CONCLUSION
Currently the EKG unit is in a complete prototype form. It can measure EKG data from a
patient and transfer the data wirelessly through a wireless router to software developed for
analyzing and storing the data. The software can store the data of any number of mobile units
and keep track of various records of patients such as date of birth, height and weight, and
medical history.
Many things could still be optimized or implemented in future work on the device. As
with any mobile unit the battery life should be extended, and the size of the packaging shrunk to
reduce size and weight. A set of alarms could be implemented either to the software or the unit
itself to warn of any medical problems detected by the EKG. The signal could further be filtered
to improve clarity of the signal. It has been suggested that PDA support could be implemented so
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that medical staff could monitor patients away from the base station. Finally, GPS support or
location tracking could allow medical staff to track the location of patients in case of an
emergency.
Before real production on this project could be implemented, the packaging would need
to be optimized and the battery life extended. Another round of testing and design would be
needed before any production could begin.
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9.
REFERENCES
[1]
Life Sync Corp., “Why the LifeSync System ?,” [Sales Literature], [citied 2007 Sep 4]
Available HTTP: http://www.lifesynccorp.com/healthcareproviders/why-lifesync.html
[2]
D. Wood, RN (2004, August). “Wireless ECGs Make for Flexibility, Freedom.” Nurse
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I. Noorzaie, CSE 574 “Survey Paper: Medical Applications of Wireless Networks,”
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[4]
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[5]
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[6]
International Electrotechnical Commission, International Standard IEC 60601-1-1, 2nd
Edition, Geneva: International Electrotechnical Commission, 2000
[7]
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[cited 2007 Sep 13] , Available HTTP:
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[8]
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[9]
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[12]
Phidgets Inc., “Analog Sensors”, [Company Website], [cited 2007 Sep 17], Available
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[13]
RoboticsConnection.com, “5V Switching Voltage Regulator”, [Company Website],
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2xRS-232)”, [Company Website], [cited 2007 Sep 17], Available HTTP:
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7
[15]
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[17]
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[Online Serial] June, 2000, Available HTTP:
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[18]
Commil, “Bluetooth vs. Wi-Fi,” [Company Website], [cited 2007 Sep 4], Available
HTTP: http://www.commil.com/bluetooth_vs.htm
[19]
EKG Machines, “Interpretive EKG/ECG Machines,” [Company Website], [cited 2007
Sep 13], Available HTTP: http://www.ekg-machines.com/ekg-machines.html
[19]
Transoma Medical, “Sleuth Implantable ECG Monitoring System” [Company Website],
[cited 2007 Dec 5], Available HTTP: http://www.transomamedical.com/products.asp
[20]
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Sep 13], Available HTTP: http://www.ekg-machines.com/ekg-machines.html
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APPENDIX A. Filter Bode Plot
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APPENDIX B. Gantt Charts (Old and New)
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Old Gantt Chart
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New Gantt Chart
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