Man’s Quest for Immortality:
Simulation of an Implantable Cardioverter Defibrillator
ME 224 Final Project
Andrik Cardenas
John Foglesong
Monica Koepnick
Jon Moncton
December 7, 2001
Table of Contents
Table of Contents 2
Summary 4
Introduction 5
Theory 6
What is an ICD? 6
Figure 1—X-Ray of ICD Patient 7
Figure 2—Current ICD Model 7
How do ICDs work? 8
Simulation of an ICD 8
Table 1—Healthy Heartbeat Ranges by Activity Level 9
Design of Instrumentation and Software 11
User (Input) Settings 11
Figure 3—User Inputs 11
Signal Generation 12
Figure 3 – Signal Generation 12
Comparison Circuit 12
Figure 5—Frequency Comparison Circuit 13
Output Displays 13
Figure 6—Output Display 14
Results 16
Figure 7—Example Output for Patient 16
Conclusion 17
References 18
Biographical Sketches of Group Members 19
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 well-performing defibrillator simulator, the final project further enhanced understanding of ME224 principles and real-world applications.
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 real-world 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.
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.
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 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.
(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.
(2)
Low levels generally relate to a resting heart rate of 70 beats per minute. Hence, for all ages,
(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 / Maximum Heartbeat (beats/min) / Minimum Heartbeat (beats/min)High / 220 – Age / 100
Medium / 60% of Max / 80
Low / 85 / 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.
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.