Implantable Cardiac Devices and Electromagnetic Tracking Systems

Are Patients at Risk?

2004

Authors:Chris Holmes – Southern Illinois University at Edwardsville

Amy McKinney – University of Nebraska-Lincoln

Faculty Sponsor:Dr. Hank Grant, EMC Center Director

Glenn Kuriger, EMC Center Assistant Director

Abstract

By definition, an active implantable medical device (AIMD) is a device that is inserted, partially or fully, into a human body or body cavity for permanent use by medical intervention. These devices include equipment such as pacemakers, implantable cardioverter defibrillators (ICDs), pain stimulators, respiration-stimulators, insulin or drug pumps, cochlea implants, and electrocardiogram monitors. Of particular concern are pacemakers and ICDs, which are cardiac implantable devices that treat different heart conditions. According to Faraday’s law, for any time varying magnetic field, a voltage may be induced at the input of an implanted cardiac device, thus creating a surge of excess electricity. These fields may emanate from any electronic device, such as electronic article surveillance system, weapon detectors, or high voltage overhead lines. Hospitals are now utilizing electromagnetic tracking systems during surgery as a more accurate way to view the area being operated. A wireless probe is inserted into the patient and communicates with a base station to relay information through pulsed electromagnetic waves. Hence, these waves could possibly interfere with patients who have implantable cardiac devices if the base station is located too close to the device.

The Center for the Study of Wireless Electromagnetic Compatibility at the University of Oklahoma is conducting in vitro testing to find if there is any possible interaction between implantable cardiac devices and electromagnetic tracking systems. Four questions are under consideration:

1) What degree of interaction, if any, is present between a wireless tracking system base station and various implantable cardiac devises?

2) What is a safe distance between a patient wearing an implantable cardiac device and the base station during transmission?

3) Is there a relationship in the placement of the implantable cardiac device within the body and the corresponding interaction that could occur from exposure?

4) Do some implantable cardiac devices have greater interactions with an electromagnetic tracking system than others?

By using a torso simulator, systematic testing of various degrees of interaction is possible. Due to limited research in the field of medical electromagnetic device interference, a study is warranted to investigate these issues. However, as of the writing of this report, cardiac equipment has yet to be shipped, therefore causing the delay of measurement. As a consequence, it is unwarranted to attempt any speculation on possible results. Hopefully, future research will continue with the aforementioned goals presented in consideration.

Table of Contents

Table of Contents..............................................................................................................................i

Table of Illustrations........................................................................................................................ii

Introduction......................................................................................................................................1

Literature Review............................................................................................................................2

Problem Definition..........................................................................................................................5

Analysis of Problem........................................................................................................................6

Solution Methods Employed...........................................................................................................8

Results............................................................................................................................................10

Recommendations..........................................................................................................................10

Limitations.....................................................................................................................................11

References......................................................................................................................................12

Appendix..........................................................................................................................................a

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Table of Illustrations

Figure 1: 1950's Era Pacemaker................................................................................3

Figure 2: Modern Pacemaker..........................................................................................................3

Figure 3: Left vs. Right Side Orientation.......................................................................................4

Figure 4: Electromagnetic Spectrum..............................................................................................7

Figure 5: Pulsed EM Field..............................................................................................................8

Figure 6: Torso Simulator...............................................................................................................9

Table 1: Test Procedures...............................................................................................................a

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Introduction

One research topic that has received much attention by the academic and commercial

world is that of electromagnetic interference with active implantable medical devices (AIMDs).

Specifically, cardiac devices such as implantable pacemakers and cardioverter defibrillators have

been shown to have potential hazards associated with electromagnetic interference from various

sources such as wireless phones or security systems. The Center for the Study of Wireless

Electromagnetic Compatibility (EMC Center) at the University of Oklahoma has conducted

several studies examining such interactions. In their research, as well as other academic

investigations, interference has proven minimal, if not nonexistent. However, even with these

results, further examination must continue to determine compatibility between new wireless

technology and AIMDs. Medical technology in particular is experiencing rapid technological

advances, which may have benefits for some patients, but hazards for others.

In the mid 1990’s, the need arose for an independent center to investigate the use of

wireless devices and their potential interaction with AIMDs. In response, Dr. Hank Grant at the

University of Oklahoma created the Center for the Study of Electromagnetic Compatibility

(EMC Center), shortly after his arrival at the university in 1994. Since then, the EMC Center has

investigated wireless interference with devices such as hearing aids, pacemakers, and

implantable defibrillators. In recent years, cardiac devices have received much attention from

the EMC Center, with investigations revealing that while several select models of pacemakers or

ICDs have shown interaction with older style analog wireless phones, newer digital phone

formats such as PCS or GSM are essentially interaction free. However, wireless technology is

always changing, proliferating in both format and use.

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Literature Review

By definition, an active implantable medical device (AIMD) is a device that is inserted,

partially or fully, into a human body or body cavity for permanent use by medical intervention.

These devices include equipment such as pacemakers, implantable cardioverter defibrillators

(ICDs), pain stimulators, respiration-stimulators, insulin or drug pumps, cochlea implants, and

electrocardiogram monitors (Irnich, 2002). Of particular concern are pacemakers and ICDs,

which are cardiac implantable devices that treat different heart conditions. A pacemaker is

utilized in patients experiencing heart failure, which is when heart muscle has become diseased

to the point that it lacks the ability to beat on its own (Ashley, et al, 1998). The device supplies

electric shocks to “pace” the heart either continuously or as needed. A heart “attack” is another

condition altogether. During this time, the heart beats faster than blood can flow, causing the

heart muscle to become starved for oxygen, thus damaging the muscle. An ICD monitors for this

type of activity, then provides a shock of electricity to correct the heart’s rhythm (some cutting-

edge models also function as pacemakers) (Irnich, 2002). ICDs are relatively new compared to

pacemakers, first implanted in 1980 (Cannom and Prystowsky, 2004), as opposed to pacemakers

in 1952 (Hayes and Furman, 2004). However, since the devices use electric signals for both

programming and monitoring of a patient, it can be dangerous if a patient is exposed to any

electric equipment emitting harmful levels of electromagnetic radiation. Figure 1 and Figure 2

demonstrate the extreme change that pacemakers have undergone in the last 50 years.

Pacemakers have now become smaller and more versatile in their use.

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Figure 1: 1950's Era Pacemaker Figure 2: Modern Pacemaker

The causes that attribute to an AIMD to misinterpret a signal can easily be explained.

According to Faraday’s law, for any time varying magnetic field, a voltage may be induced at the

input of an implanted cardiac device, thus creating a surge of excess electricity (Scholten and

Silny, 2001). These fields may emanate from any electronic device, such as electronic article

surveillance system, weapon detectors, or high voltage overhead lines (Witters, et al, 2001).

Some studies have constructed specific “Faraday Cages” to examine this phenomenon (Hedjiedj,

2002). Assuming that the magnetic field is sinusoidal and located in the AIMD range, acting

perpendicularly to the frontal plane of the thorax and is homogeneous, there exists the possibility

that a voltage could be produced. Also, if pacemakers or ICDs are under consideration, the

placement of the device can also greatly influence any induced voltage. Left side implantation

creates an induction loop, versus a right side that typically has an “s-shape” wire lead orientation

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(Scholten and Silny, 2001). Figure 3 pictorially demonstrates the different placement options of

pacemakers within a patient’s chest, and the resulting reference loops that are incurred. Different

types of wireless technologies, primarily found in wireless phones, also attribute to AIMD

interaction.

Figure 3: Left vs. Right Side Orientation

The FDA has found that many different electrically powered medical devices such as

pacemakers and implantable defibrillators can be affected by electromagnetic interference

(Witters, et al, 2001). Both in vitro and in vivo testing has been conducted dealing with this

issue. Studies have focused on GSM (Global System for Mobile Communication) wireless

technology and any interactions that may occur with a cardiac device’s functionality (Barbaro, et

al, 2003b). In vitro results have concluded that when the worst-case scenario is tested (phone at

maximum output and implantable cardiac device at its highest sensitivity) there is some

interaction between certain types of implantable cardiac devices and certain types of wireless

phones. The interaction found could be that the device perceived the electromagnetic waves as a

heart beat, or that the device “fired” when it should not have. However, in all cases, when the

wireless phone was removed and turned off the implantable cardiac device showed no signs of

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being re-programmed (Grant, 1998) (Kuriger, 1998). Based on prior in vitro results, in vivo

testing showed that there were no threatening interactions between specific devices and phones

when in close contact (Barbaro, et al, 2003a). However, it should be noted that in the Barbaro

study, only two phone models and one pacemaker model was examined, so these results may not

hold true for all combinations of pacemakers and wireless technologies.

Hospitals are now utilizing electromagnetic tracking systems during surgery as a more

accurate way to view the area being operated. This new technology is an upgrade from the

optical systems that have been widely used for many years (Poulin, 2002). This technology

provides the surgeon with a 3D “roadmap” of the patient as well as the exact location of the

probe within the body. The system uses an emitter to produce an electromagnetic field that is

received by a base station. Some stations are capable of converting this information into six

degrees of freedom (Poulin, 2002), allowing the surgeon to view the patient in ways never before

capable with optical systems. The possibility of interference with AIMD patients requires

examination.

Even though there is no major threat to the general public, there are a substantial number

of AIMD patients who could be affected by an electromagnetic interference during surgery.

Approximately 110,000 Americans (eighty percent of which are 65 years of age or older) are

fitted with pacemakers each year (Business Word, 2003). In 2003, it was expected that nearly

130,000 implantable defibrillators were to be implanted worldwide (Cannom, 2004). These

statistics justify the need for further research in the field of wireless electromagnetic interactions.

Problem Definition

Hospitals are now utilizing electromagnetic tracking systems (ETS) during surgery as a

more accurate way to view the patient’s area being operated on. This new technology is an

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upgrade from the optical systems that have been widely used for many years. Using GSM

wireless signals, this technology provides the surgeon with a 3D “roadmap” of the patient as well

as the exact location of the probe within the body. Optical systems are unable to provide this

level of precision. A wireless probe is inserted into the patient and communicates with a base

station to relay information through electromagnetic waves. Hence, these waves could possibly

interfere with patients who have implantable cardiac devices if the base station is located too

close to the device. The EMC Center is conducting in vitro testing to find if there is any possible

interaction. Four questions are under consideration:

1) What degree of interaction, if any, is present between a wireless tracking system base

station and various implantable cardiac devises?

2) What is a safe distance between a patient wearing an implantable cardiac device and

the base station during transmission?

3) Is there a relationship in the placement of the implantable cardiac device within the

body and the corresponding interaction that could occur from exposure?

4) Do some implantable cardiac devices have greater interactions with an

electromagnetic tracking system than others?

Due to limited research in the field of medical electromagnetic device interference, a study is

warranted to investigate these issues.

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Analysis of Problem

Electromagnetic fields can be either continuous or pulsed. The ETS uses pulsed signals

to carry digital information in the GSM wireless format at either 800 MHz or 1900 MHz (See

Figure 4). Pulsed electromagnetic fields, such as in Figure 5, have the capability to create “false

heart signals” which can cause AIMDs to misinterpret data. This creates inherent problems

when in close proximity to AIMDs. However, depending on which cardiac device is under

consideration, the pulsed field may have varying effects. The possibility exists that a pacemaker

will be more sensitive to an exposure than an ICD due to their respective functions. Pacemakers

continuously pace a heart based on measured readings that can be easily interfered with during

exposure to pulsed electromagnetic fields. In contrast, an ICD shocks an erratic heart back into a

normal rhythm, which requires the device to charge before treatment may be administered.

Thus, in order for a pulsed electromagnetic field to become dangerous, the ICD must be

subjected to the field for a longer period of time compared to a pacemaker. Therefore, with more

hospitals utilizing ETS technology, the hazards to AIMDs patients cannot be ignored.

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Figure 4: Electromagnetic Spectrum

Figure 5: Pulsed EM Field

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Solution Methods Employed

All testing will be conducted at the EMC Center on the University of Oklahoma’s campus.

Simulation of pacemakers and implantable defibrillators will be accomplished by way of a torso

simulator and testing equipment which generate and monitor electrical signals. Specifically, an

ECG signal injection system will be utilized to simulate heart activity and signal monitoring

equipment for acquiring pacemaker and ICD signals. All equipment will be set up in and around

the torso simulator.

As can be seen in Figure 6, the torso simulator is a plastic box used to represent the

human trunk cavity. The simulator measures 23” x 16.75” x 6” with a volume of 28 quarts. It is

filled with 0.03 molar saline solution which simulates human body conductivity. Two horizontal

grids placed one below the other support both the pacemaker or defibrillator under consideration

and the tracking system base station. The lower grid, which supports the pacemaker or

defibrillator, is completely submersed in the saline solution. The grids can be moved to different

heights, allowing the testing of different separations distances to be measured. The tracking base

station will be moved to every point on the grid to test various interactions.

Figure 6: Torso Simulator

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One important factor of the test procedure is generating an artificial heart signal.

Generating and monitoring a heart rhythm may be accomplished by placing stainless steel plates,

measuring 50 mm x 50 mm x 2 mm, into the torso simulator. One pair of plates (on the long axis

of the tank) monitor the pacemaker or ICD signal, and the second pair (on the short axis) inject a

simulated heart signal into the tank. The signal received from the torso simulator is first applied

to the input of a differential amplifier designed for ECG signal monitoring. After amplification,

the signal is filtered for clarity before being inputted into a two-channel digital storage

oscilloscope. Through observation of the ECG signal on the oscilloscope, any interaction of the

pacemaker or ICD function with the electromagnetic tracking system base station will be

recorded. A summary of this procedural setup can be found in the Appendix (Table 1).

It is important to note that ICD and pacemaker parameters are typically programmed non-

invasively by means of RF signals or pulsed magnetic fields. This ability of the device to

respond to both internal and external magnetic and RF signals allows the device to be

programmed for optimal clinical benefit as the patient’s needs change. Each pacemaker or ICD

company will supply the appropriate instructions necessary for interrogating and programming

its respective units. The programming features vary widely, but all units provide the control

necessary to establish the common parameters needed. Each unit will be programmed for the

worst-case condition (maximum sensitivity and minimum refractory period).

Results

Unfortunately, due to time constraints, testing was unable to commence as of the writing

of this report. Due to limited research concerning this particular type of wireless interaction, it is

unwarranted to attempt any speculation on possible results. Hopefully, future research will

continue with the aforementioned goals presented in consideration.

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Recommendations

Recommendations can be made for future research of implantable cardiac device

interaction with electromagnetic tracking systems in relation to the general in vitro setup.

Previous research at the EMC Center has not made a distinction between left and right side

placement orientation of pacemakers or implantable defibrillators, and this can have a great

effect on the level of interaction with not only electromagnetic tracking systems, but also

technologies such as wireless phones or security systems. Also, previous research has not

considered constant body temperature in the construction of torso simulators. A constant

temperature of 98.6 degrees Fahrenheit would also have an effect on the conductivity of signals

within the simulator, perhaps improving noise levels and accuracy.

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Limitations

The major limitation involved with this particular study is time. It has proven difficult to

obtain the proper implantable device equipment in the timeframe allowed, thus not allowing the

study to commence. However, delays are sometimes unavoidable factors in any form of

research, particularly ones that involve specialized equipment. Hopefully in time this study will

be completed, providing an answer to the safety issue raised in the use of electromagnetic

tracking systems and implantable cardiac devices.

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References

Ashley, Robert J., et al, 1998, Measurement of Potential Magnetic Field Interference with Implanted Cardioverter Defibrillators of Pacemakers, Electro Int. Conf. Proc. of IEEE, 159-170.

Barbaro, V., Bartolini, P., Calcagnini, G., Censi, F., Floris, M., et al, 2003a, In vitro and in vivo evaluation of electromagnetic interference between wireless home monitoring pacemakers and GSM mobile phones, Proc. Ann. Int. Conf. IEEE Engineering in Medicine and Biology Society, 4, 3602-3605.

Barbaro, V., Bartolini, P., Calcagnini, G., Censi, F., Beard, B., et al, 2003b, On the mechanisms of interference between mobile phones and pacemakers: parasitic demodulation of GSM signal by the sensing amplifier, Physics in Medicine and Biology, 48, 1661-1671.

Business Word, 2003, Pacemaker prices still declining, Hospital Materials Management, 28, i6, p1(4).

Cannom, David S., and Prystowsky, Eric N., 2004, The Evolution of the Implantable Cardioverter Defibrillator, NASPE History Series, 27, 419-431.

Grant, Hank and Schlegel, Robert E., 1998, In Vitro Study of the Interaction of Wireless Phones with Cardiac Pacemakers, EMC Report 1998-2.

Hayes, D.L. and Furman, Seymour, 2004, Cardiac Pacing: How it Started, Where We Are, Where We Are Going, NASPE, 27, 693-704.

Hedjiedj, A., Goeury, C., and Nadi, M., 2002, A Methodological approach for the characterization of cardiac pacemaker immunity to low frequency interferences: case of 50 Hz, 60 Hz, 10 kHz and 25 kHz led disruptions, Journal of Medical Engineering and Technology, 26, no.5, 223-227.

Irnich, Werner, 2002, Electronic Security Systems and Active Implantable Medical Devices, Journal of Pacing and Clinical Electrophysiology, 25, no.8, 1235-1258.

Kuriger, Glenn W., Grant, Hank, and Schlegel, Robert E., 1998, In Vitro Study of the Interaction of Wireless Phones with Implantable Cardioverter Defibrillators, EMC Report 1998-1.

Poulin, Francois and Amiot, L.P., 2002, Interference during the use of an electromagnetic tracking system under OR conditions, Journal of Biomechanics, 35, 733-737.

Scholten, A., and Silny, J., 2001, The interference threshold of unipolar cardiac pacemakers in extremely low frequency magnetic fields, Journal of Medical Engineering and Technology, 25, no.5, 185-194.

Witters, Donald, et al, 2001, Medical device EMI: FDA Analysis of Incident Reports, and Recent Concerns for Security Systems and Wireless Medical Telemetry, IEEE International Symposium on Electromagnetic Compatibility, 2, 1289-1291.

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Appendix

Table 1: Test Procedure

Torso SimulatorFill tank with 1.8 g/l saline solutionMeasure saline properties with YSI Model 33 S-C-T Meter Calibrate probe Measure saline temperature Measure salinity (0.18% to 0.21% acceptable) Measure conductivity (nominal value of 3400 micro-mhos per cm)

Position Plastic GridsPosition pacemaker/implantable defibrillator on bottom grid over (0,0) grid pointAdjust height of top grid such that implantable device is 1 cm below test subject

Waveform GeneratorSet frequency to 66.66 HzSet unit to “Burst” modeSet “Burst Rate” to 550 ms for pacemakers, 650 ms for implantable defibrillatorsConnect cable from “Output” jack on generator to stainless steel plates on opposite ends of torso simulator

Signal AmplifierEnable signal pathSet “Gain/Time Constant” to “X10” and “3.2” for implantable pacemakers and “X100” and “3.2” for pacemakersSet “Input” to “On”Set amplifier filter to 10 KHzSet amplifier gain to 500 mV/cmConnect cable from “Input” jack on amplifier to remaining stainless steel plates on torso simulatorConnect cable from “Aux-Out” jack on amplifier to Channel 1 of oscilloscope

OscilloscopeEnsure vertical menu settings on Channel 1 “Coupling” set to “DC” “Invert” set to “Off” “Bandwidth” set to “Full”

“Offset” set to “0 V” “Fine Scale” as needed “Position” as needed

Ensure vertical menu settings for Channel 2 “Coupling” set to “AC” “Invert” set to “Off” “Bandwidth” set to “Full”

“Offset” set to “0 V” “Fine Scale” as needed “Position” as needed

Ensure horizontal menu settings “Time Base” to “Main Only” “Trigger Position” to “50%”

“Sec/Div” set to “250ms”

Set trigger settings “Type” to “Edge” “Source” to “Channel 1” “Coupling” to “DC”

“Slope” to increasing “Level” as needed

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