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Man’s Quest for Immortality:
Simulation of an Implantable Cardioverter Defibrillator
ME 224 Final Project
Andrik Cardenas
John Foglesong
Monica Koepnick
Jon Moncton
November 27, 2001
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Table of Contents
TABLE OF CONTENTS
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SUMMARY
3
INTRODUCTION
4
THEORY
5
WHAT IS AN ICD?
TABLE 1—HEALTHY HEARTBEAT RANGES BY ACTIVITY LEVEL
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DESIGN OF INSTRUMENTATION AND SOFTWARE
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FIGURE 1—X-RAY OF ICD PATIENT
FIGURE 2—CURRENT ICD MODEL
HOW DO ICDS WORK?
SIMULATION OF AN ICD
USER (INPUT) SETTINGS
FIGURE 6—OUTPUT DISPLAY
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RESULTS
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FIGURE 7—EXAMPLE OUTPUT FOR PATIENT
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REFERENCES
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BIOGRAPHICAL SKETCHES OF GROUP MEMBERS
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FIGURE 3—USER INPUTS
SIGNAL GENERATION
FIGURE 3 – SIGNAL GENERATION
COMPARISON CIRCUIT
FIGURE 5—FREQUENCY COMPARISON CIRCUIT
OUTPUT DISPLAYS
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Summary
For this lab, a real-world device, namely the implantable cardioverter defibrillator, is
simulated using classroom principles and learning as well as independent research.
Based on user input, a LabVIEW program compared a heart rate in question with the
appropriate range as determined from user-defined variables. If the heart rate is outside
the normal range, an output signal is sent simulating a defibrillator’s shock to the heart,
which returns the heart rate to a normal rhythm.
The model defibrillator followed predicted results. For heart rates outside the appropriate range,
the resulting output signal returned the heart rate to an optimal pulse. In addition to performance,
the model’s design encompassed usability and readability. A clear panel design afforded both
numeric and graphical output as well as explicit instructions for the user.
Most problems stemmed from the complexities of programming in LabVIEW. Such
dilemmas included generating a signal representative of an actual heart rate and building
a comparison circuit capable of meeting predetermined criteria. All issues were resolved
with the help of the teaching assistant and the lab group’s dedication.
Inherent to the lab, constructing a realistic model of a defibrillator posed challenges.
Although the objective was to create a simplified representation, controllable real-life
factors needed to be accounted for in the design to maintain accuracy and practicality.
Such considerations, within the capabilities of the lab, included age and activity level of
the patient.
All objectives, both tangible and intangible, were met or exceeded. Along with a wellperforming defibrillator simulator, the final project further enhanced understanding of
ME224 principles and real-world applications.
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Introduction
The world of engineering and its applications permeate daily life. A primary objective of
the Mechanical Engineering curriculum is to fully understand and apply in-class
teachings to the world around us to create solutions to everyday problems. One such
field rife with engineering applications is that of biomedical devices. Advanced
technology has been developed to not only prolong but also enrich the lives of those
around us. A striking example is the implantable cardioverter defibrillator (ICD), which
provides immediate electrical therapy to sufferers of recurring heart arrhythmias. For the
final project, this lab meshes academia and the real world. Learning to simulate a realworld device transcends present classroom experience by facing challenges commonly
found in practical applications and forcing critical thinking and use of principles.
Combining key principles learned in ME 224 and research on how such devices operate
enables the simulation of an implantable cardioverter defibrillator. A simulator capable of
accounting for all real-life complexities is beyond the scope of this lab. Therefore, the
principal objective of the lab is to construct a simplified, yet realistic, model of an ICD.
Modeling an implantable cardioverter defibrillator poses many challenges. Because
patient needs vary, all defibrillators are chosen according to the individual. The model
attempts to replicate this selection process by incorporating the most basic patient
characteristics. Specifically, age and activity level are two elements, which dramatically
affect heart rate. As a person ages, heart rate declines. As activity level increases, the
heart must pump more blood to meet the increasing demands on the body, which in turn
raises the heart rate.
Using the input age and the resultant heart rates, the LabVIEW program can be
constructed to check a patient’s current heartbeat against the above rates based on a
chosen activity level. After the comparisons of the input heart rate and the one
determined by the user, the program determines if a shock should be given. This shock
will continue until the heart rate of the “patient” is optimized for the given conditions.
The LabVIEW program interacts with the user with a panel displaying current heart rate,
the user inputs, and any actions that the ICD is taking, in both a graphical and digital
format. To record the actions of the ICD, this panel displays the heart rate as it changes
through time, much like the paper tape generated in conventional medical equipment.
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Theory
The heart acts as a natural pacemaker, controlling the rate of the heart’s muscular
contractions. Acting as an electrical timing device, the heart pumps blood under the wide
range of demands encountered in daily life, from running up the stairs to lying on the
couch watching TV. Everyone’s heart speeds up or slows down under different
conditions and may on occasion skip a beat. Such deviations are relatively trivial and
fleeting. However, sometimes the heart’s electrical system malfunctions and serious
rhythm disorders result. Such deviations may be life threatening without external
defibrillation to shock the heart back into a normal rhythm. A lengthier period of time
between the onset of ventricular tachycardia and treatment can increase the chances for
loss of consciousness and death. Therefore, the sooner treatment can be administered, the
sooner the heart can return to a normal rhythm. Advanced biomedical technology has
allowed for implantable cardioverter defibrillators (ICDs), which provide electrical
therapy immediately as needed.
What is an ICD?
For people who suffer from excessively rapid heartbeats called ventricular tachycardia or
ventricular fibrillation, ICDs have become the dominant treatment method. These small
devices are implanted in the chest wall to continuously monitor heart rhythm and deliver
precisely calibrated electrical shocks when needed to control abnormal heartbeats or
arrhythmia. The devices can also terminate other types of arrhythmias by means of
pacing. However, unlike pacemakers, which work to keep the patient’s heart rate
sufficiently high in one or both heart chambers, these implantable defibrillators slow
down or halt excessively rapid heart rates that arise specifically in the ventricles. The
aim is to prevent ventricular fibrillation, a state in which the ventricles contract in a
completely unsynchronized, or quivering manner that is insufficient to cause heart muscle
contraction and the pumping of blood. This total lack of rhythm results in cardiac arrest,
which can be fatal within minutes if there is no emergency intervention.
The ICD is a small, battery-driven self-contained device that is implanted under the skin,
usually near the left collarbone. One or two floppy thin wires, or leads, run from the ICD
through veins to the chambers within the heart. It continuously monitors the heart
rhythm and treats rapid heartbeats with electrical therapy. Below, Figure 1 shows an
implanted defibrillator with its leads to the immediate left of the device.
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Figure 1—X-Ray of ICD Patient
Current ICDs are much different than the first models of the 1980s. Although still
encased in titanium, average weight and size have decreased dramatically. Typically,
ICDs weigh approximately 132 grams with a displacement of 83 cc, half of its original
size. The most common size of the ICD is 88 mm in height, 18 mm in width, and 63 mm
in length. Most leads are either made of silicone or polyurethane. Projected longevity is
just over 6 years. Figure 2 below illustrates one of the most common models currently
used.
Figure 2—Current ICD Model
How do ICDs work?
William Batsford, M.D. offers a detailed technical explanation of how ICDs work in the
Yale Medical School Heart Book.
“ICDs use one or more of three basic modes of operation: antitachycardia pacing,
low-energy cardioversion (a shock that restores normal heart rhythm), and defibrillation.
Most devices in use today employ high-energy defibrillation for ventricular tachycardia
or fibrillation.
“Antitachycardia pacing short-circuits the rapid ventricular rhythms by sending
brief bursts of impulses to the heart muscle at a pace faster than the already accelerated
ventricular rate. The aim is to depolarize the heart muscle at the right moment,
interrupting the abnormal rhythm and thereby halting the tachycardia. The tachycardia
ceases with in a few seconds, with no pain and little stress to the patient. A device can
safely induce antitachycardia pacing hundreds of times per day if necessary, with little
drain on its power source...
“Low-energy cardioversion and defibrillation work differently from
antitachycardia pacing. A device employing low-energy cardioversion to counteract
ventricular tachycardia does so by delivering a mild shock to the heart muscle, in the
range of 0.5 to 2 joules. The shock depolarizes a small section of the ventricle, breaking
the abnormal rhythm causing the tachycardia…
“A low-energy shock, however, will not stop ventricular fibrillation, in which
many currents flow through the heart muscle in a chaotic fashion. When an implantable
defibrillator senses such a dangerous arrhythmia, it defibrillate just like the external
electronic defibrillator used to revive patients in emergency care for cardiac arrest.
Defibrillation requires a minimum 10-to-15 joule shock, but most devices deliver 30
joules to allow a margin of safety. Devices typically can give up to five shocks, pausing
between each one to sense if the arrhythmia has been checked. This sequence can be
repeated 100 times over the lifetime of the defibrillator.”
Simulation of an ICD
The lab employs these principles to simulate a simplified ICD. Using the aid of
LabVIEW, a simplified model can be constructed which compares the patient’s input
heart rate with the appropriate range. If the input heart rate falls outside of the range
defined by user inputs of age and activity level, an output signal of higher frequency will
be sent resembling the defibrillator’s shock to return the heart to a normal rhythm.
Otherwise, if the heart rate is in the appropriate range, the program exits since no external
defibrillation is necessary.
Because a heart rate, measured in beats per minute, is in itself a frequency, a generated
sine wave with its own predetermined frequency can supply a simulated heart rate.
Converting the input heart rate from beats per minute to cycles per second enables
comparison to the simulated heart rate, i.e. the generated sine wave, in LabVIEW. If a
cutoff frequency is chosen for the sine wave, i.e. the upper limit of a normal heart rate is
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defined, LabVIEW can be used to compare the converted input heart rate frequency
against the selected cutoff frequency. Any input frequency above the cutoff, and,
therefore, much faster than a normal rhythm, is subject to an output signal of a much
higher frequency to shock the heart back into a normal pulse.
To determine if a shock is necessary to return the heart to a normal rhythm, the normal
rhythm must first be defined. Through research, a normal range has been defined for
three levels of activity—high, medium, and low—based on the input age. High levels of
activity correspond to a maximum heart rate of 220 minus the input age.
HRMax  220  Age
(1)
Moderate levels fall within 60-75% of the maximum heart rate. For this lab, the heart
rate for a moderate level of activity will be defined as follows.
HRModerate  0.60 * HRMax
(2)
Low levels generally relate to a resting heart rate of 70 beats per minute. Hence, for all
ages,
HRMin  70
(3)
With the normal heart rates by activity level defined, corresponding ranges for each must be set.
Below, Table 1 shows the healthy heartbeat ranges for all activity levels.
Activity Level
High
Medium
Low
Maximum Heartbeat (beats/min)
220 – Age
60% of Max
85
Minimum Heartbeat (beats/min)
100
80
60
Table 1—Healthy Heartbeat Ranges by Activity Level
Ascertaining healthy ranges for heart rates now gives way to constructing a working
LabVIEW program, which can monitor and adjust a given heart rate. The program’s logic
and creation is further discussed in the Design of Instrumentation and Software section.
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Design of Instrumentation and Software
This experiment could have been designed and run with conventional circuitry.
However, while the complexity would have been much greater, the reliability would have
decreased drastically due to inherent flaws in physical electronics. LabVIEW was used
to model all the components in this experiment because of its accuracy, and ability to
provide a complete working environment for the user, and the programmer. LabVIEW
was more effective for this system because the experiment deals with a closed-loop
feedback system. In this system, the only component that could easily be taken out of
LabVIEW is the signal generation. However, since LabVIEW is already equipped with
this capability, it was beneficial for error reduction to incorporate that aspect into the
program.
User (Input) Settings
Utilizing LabVIEW’s front panel, the user is allowed to dictate the initial conditions of
the simulation. With case structures and numerical inputs, the user is required to input
Age, Starting Heart Beat, and Activity Level. These variables are used to produce the
original heartbeat, as well as define the parameters for system operation.
Figure 3—User Inputs
User Inputs
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The Starting Heartbeat is used to determine the starting heartbeat of the system. Age and
Activity Level are used to determine the maximum and minimum values for a normal
heartbeat. Table 1, in the preceding Theory section, shows the relationship between these
two input parameters and the values used to drive the system.
Signal Generation
The internal sine wave function generator .vi in LabVIEW was utilized to produce a sine
wave using the input of the starting heartbeat. The frequency of the sine wave was
calculated using a Boughman Frequency Estimator provided in LabVIEW. This was
done to simulate the input that would be received from sensors and conditioned into a
form to be used in a real ICD. Our signal generation produces what would actually be the
signal from physical sensors.
Figure 3 – Signal Generation
Comparison Circuit
After the second part of the program generates a signal, a comparison must be made
between the input heartbeat and the adequate upper and lower limits determined by the
specified age and activity level. The comparison circuit uses a while loop that runs when
the heartbeat is not optimized. For the purposes of this experiment, an optimized
heartbeat is defined as one that is the average of the upper and lower acceptable limits.
The ICD model will continue to “shock” the patient until the heart rate is at the optimal
level. A series of Boolean operations are used as the logic for the comparator. These
operations determine what the system is doing. Every time the while loop runs, the beat
is increased or decreased by one. In this manner, the heartbeat approaches the optimal
value, but does not overshoot it. This type of comparator circuit eliminates the
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complexity of a system that determines the desired value by oscillating the response and
narrowing in on the desired value.
Figure 5—Frequency Comparison Circuit
There are four inputs to the comparison circuit; the upper and lower boundaries for the
heartbeat, the user’s input heartbeat, and the number of the loop that the system is
currently in (this is used in the incrementing or decrementing the heartbeat).
Output Displays
The last important part of the model is to provide the user with easily readable and
understandable output. This output display for this model was designed so that a user
could quickly read visual cues off the screen and effortlessly relate them to the response
of a patient being treated by an ICD.
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Figure 6—Output Display
Several features of the Output Display are especially noteworthy for the user. To the
right of Activity Intensity are three lights, labeled Optimal, Normal, and Abnormal,
corresponding to the present state of the patient’s heart rate. For example, if an input
heart rate exceeds the appropriate range, the red Abnormal light will illuminate until the
defibrillator model “shocks” the heart rate into the normal range. At this point, the green
Normal light will turn on. The defibrillator model continues adjusting the heart rate until
the optimal heart rate is achieved, hence, lighting the green Optimal light. These lights
act as an easily detectable visual cue providing the user an immediate assessment of his
current heart rate. Located above the Pulse Chart, a fourth light, labeled “Shock”,
illuminates whenever the defibrillator model sends an output signal to adjust the heart
rate.
Another feature, especially helpful to the user, is the Pulse Chart, which tracks current
pulse as well as the healthy upper and lower pulse limits based on user input. Using this
graph, users can visually trace the defibrillator model’s effect on pulse and approach to
the optimal heart rate.
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An example of the running defibrillator model’s output is found in the Results section.
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Results
When the first testing took place, most of the problems occurred with the system’s logic.
Once these problems were resolved with in-depth logic tables and analysis of the model,
results were extremely consistent and positive. The model of the ICD works completely
and effectively. To enhance the program’s usability and guarantee accurate, reasonable
results, two instructions are included on the front panel for the user, ensuring that the
correct procedure is followed every time the program is run. When this procedure is
followed, the model works correctly.
The desire to not only guide the heart rate into a specific range, but to achieve an optimal
range only slightly complicated the program, while providing more feedback for the user.
The program operates according to all of the specified parameters and interacts with the
user in an easy, straightforward manner.
The front panel was designed and created to be easy to read and use. The computer
screen does not offer the same interaction as a knob or other physical selector, but the
panel is integral to the program’s success. The user can follow the execution of the
program through completion on the Pulse Chart.
As an example, the following figure demonstrates a realistic situation encountered by the
model defibrillator. The 160 beats per minute heart rate in question belongs to a 30 yearold patient at a moderate level of activity. These inputs correspond to a healthy
maximum heart rate of 140 beats per minute and a minimum heart rate of 80 beats per
minute according to Table 1, as found in the Theory section. Upon running the program,
immediately the Abnormal light illuminates, as does the Shock light, because of the
patient’s excessive unhealthy heart rate. As the program continues running, the Pulse
Chart displays the defibrillator model’s slowing effect on the patient’s heart rate. Once
the heart rate reaches the upper limit of 140 beats per minute, the Abnormal light turns
off, and the Normal light turns on. As the patient’s heart rate approaches the optimal
heart rate of 110 beats per minute, the Shock light turns off, and the Optimal light turns
on. The Pulse Chart in Figure 7 demonstrates the entire process. The chart plots the
heart rate across the time interval required to reach the optimal heart rate.
As validated by the above example, the experiment was completely successful. The end
objective of accurately modeling a realistic implantable cardioverter defibrillator was met
with the aid of LabVIEW. Once the basic components were correctly modeled, some
complexity was added to make the model more realistic and better handle actual
situations addressed by the real world device.
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Figure 7—Example Output for Patient
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References
Batsford, M.D., William. “Pacemakers and Antitachycardia Devices.” Ch 26; Yale
Medical School Heart Book.
Groh, M.D., William J., Foreman, R.N., Lynne D., Zipes, M.D., Douglas P. “Advances
in the Treatment of Arrhythmias: Implantable Cardioverter-Defibrillators.” American
Family Physician, January 15, 1998.
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Biographical Sketches of Group Members
Andrik Cardenas was born in Mexico and raised in Los Angeles. He attended Palisades
Charter High School. He was part of the National High School Institute, which led to his
interest in Northwestern University. He received a Co-op position with United Parcel
Service in 1999. He spent last summer as an intern with Ford Motor Company in
Dearborn Michigan. Currently, he is a senior pursuing a Bachelor’s degree in
Mechanical Engineering. He has accepted a full time position as a design engineer with
Ford Motor Company, which he will start in May 2002.
John Foglesong was born in Indianapolis, Indiana on August 29th 1980. He spent the first
fourteen years of his life there. After which he moved to London, England. Where he
went to high school at the American School in London. It was in high school that he
found his strengths in math and science, although he did also participate in the school
orchestra and choir. From there he enrolled in the McCormick School of Engineering
and Applied Sciences at Northwestern University, where he is currently a senior studying
mechanical engineering. He has spent time working for Lord Corporation, in Erie
Pennsylvania. While he was there he interacted with both engineers and operators in
order to re-write manufacturing instructions. Last summer he was an intern for Eli Lilly
& Co. where he was a member of the equipment development team, specializing in the
design and manufacturing of industry specific unique high-tech equipment. Next year
Mr. Foglesong will return to Indianapolis in order to take a position as a full-time process
engineer for Eli Lilly & Co.
Monica Koepnick came to Northwestern University from San Antonio, Texas. She will
be graduating this December with a degree in Mechanical Engineering and a
Manufacturing Specialization. After interning for Toyota and the Quaker Oats division
of Pepsi Co., she will be joining Quaker Oats this spring in the Supply Chain Senior
Associate Program here in downtown Chicago.
Jon Moncton is currently a fifth year senior pursuing degrees in both Mechanical
Engineering and Cello Performance. With graduation approaching in June, he plans on
entering the engineering industry and finding work before pursuing graduate studies.
Previous work experience includes a summer internship in the mechanical design
department of General Motors Corporation's Electro-Motive Division in LaGrange, IL.
Jon is also and avid member and captain of Northwestern Men's Crew and is planning a
trans-Canada bicycle trip next summer.
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